Gravitational Effects Mechanical Micro Structural Properties Tungsten Heavy Alloys 2006

The Pennsylvania State University

The Graduate School

Intercollege Graduate Degree Program in Materials

GRAVITATIONAL EFFECTS ON MECHANICAL AND MICROSTRUCTURAL

PROPERTIES OF TUNGSTEN HEAVY ALLOYS

A Thesis in

Materials Engineering

by

Louis G. Campbell

© 2006 Louis G. Campbell

Submitted in Partial Fulfillment of the Requirements

for the Degree of

Master of Science

November 2006

The thesis of Louis G. Campbell was reviewed and approved* by the following: Randall M. German Brush Chair Professor in Materials Thesis Advisor Ivica Smid Associate Professor of Engineering Science and Mechanics Barbara A. Shaw Professor of Engineering Science and Mechanics Donald Heaney Senior Research Associate, Engineering Science and Mechanics James P. Runt Chair, Intercollege Graduate Degree Program in Materials Science and Engineering *Signatures are on file in the Graduate School.

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ABSTRACT

Fundamental driving forces for liquid phase sintering behavior of tungsten heavy

alloys are often obscured by gravitational effects such as buoyancy and settling. Past

studies looking at tungsten heavy alloys that have been liquid phase sintered under both

orbital microgravity and ground-based conditions have generated considerable insight

into the basic science of liquid phase sintering. The set of matched microgravity and earth

gravity sintered samples provides unique opportunities for studying gravitational effects

on materials processing, particularly in understanding the fundamental science of liquid

phase sintering. The research also provides knowledge applicable to in-space fabrication

and repair technologies.

In this study on tungsten-nickel-iron, the bulk mechanical properties hardness and

bulk modulus were measured and compared between the earth gravity and microgravity

sintered samples. Microstructural parameters (phase fraction, grain contiguity, and grain

size) were also gathered in statistical quantities using recent advances in computer-aided

quantitative microscopy techniques. Changes in grain size distribution, contiguity, phase

fraction, hardness, and elastic modulus with respect to sintering time, gravity condition,

and liquid phase content were examined.

iv

TABLE OF CONTENTS LIST OF TABLES............................................................................................................ vii LIST OF FIGURES ........................................................................................................... ix ACKNOWLEDGEMENTS............................................................................................. xiii Chapter 1: Introduction ....................................................................................................... 1 Chapter 2: Background ....................................................................................................... 2

2.1 Sintering and Liquid Phase Sintering ................................................................. 2 2.1.1 Microstructural Evolution During Liquid Phase Sintering......................... 3 2.1.2 Material System Properties for WNiFe....................................................... 4

2.2 Prior Work .......................................................................................................... 8 Chapter 3: Procedures ....................................................................................................... 10

3.1 Sample Fabrication ........................................................................................... 10 3.1.1 Shuttle Flight Information......................................................................... 10 3.1.2 Sample Matrix........................................................................................... 11 3.1.3 Sample Fabrication ................................................................................... 12 3.1.4 Sintering Procedures ................................................................................. 13

3.2 Sample Selection............................................................................................... 16 3.3 Metallographic Procedures ............................................................................... 18 3.4 Image Analysis.................................................................................................. 19

3.4.1 Quantitative Microscopy Image Capture .................................................. 19 3.4.2 Analysis Procedure ................................................................................... 22

3.4.2.1 Analysis Script ...................................................................................... 24 3.5 Ultrasonic Testing............................................................................................. 26

3.5.1 Testing Procedures and Equipment .......................................................... 27 3.5.1.1 Longitudinal Wave Testing................................................................... 28 3.5.1.2 Shear (transverse) wave testing ............................................................ 30

3.5.2 Data analysis ............................................................................................. 32 3.5.2.1 Error analysis ........................................................................................ 34

3.6 Hardness Testing............................................................................................... 35 3.6.1 Initial Testing: Determination of Test Procedures.................................... 35 3.6.2 Hardness Specimen Preparation and Testing............................................ 37 3.6.3 Hardness Indent Analysis ......................................................................... 38

Chapter 4: Results ............................................................................................................. 41 4.1 Data Presentation and Distribution Basis ......................................................... 41 4.2 General Observations........................................................................................ 42

4.2.1 Macro Observations .................................................................................. 42 4.2.2 General Features ....................................................................................... 47 4.2.3 Liquid Phase Microstructures ................................................................... 48

4.3 Quantitative Microscopy Results...................................................................... 51 4.3.1 Overall Quantitative Microscopy Results................................................. 52

4.3.1.1 Phase Fractions ..................................................................................... 52 4.3.1.2 Contiguity Results................................................................................. 55 4.3.1.3 Grain Size Results................................................................................. 58

4.3.2 Positional Variations................................................................................. 62 4.3.2.1 Positional Effects on Phase Fractions ................................................... 62

v

4.3.2.2 Positional Effects on Contiguity ........................................................... 68 4.3.2.3 Positional Effects on Grain Size Distributions ..................................... 73

4.4 Mechanical Property Results ............................................................................ 79 4.4.1 Ultrasonic Velocity Results ...................................................................... 79 4.4.2 Hardness Results....................................................................................... 84

4.4.2.1 Overall Hardness Results...................................................................... 84 4.4.2.2 Positional Variations............................................................................. 88 4.4.2.3 Indent Examinations ............................................................................. 93

Chapter 5: Discussion ....................................................................................................... 97 5.1 Gravitational Effects on Microstructures.......................................................... 97

5.1.1 Anomalous Microstructural Data.............................................................. 97 5.2 Comparison with Prior Results ....................................................................... 100

5.2.1 Grain Size Results Comparison .............................................................. 100 5.2.2 Contiguity Results Comparison .............................................................. 104

5.3 Gravitational Effects on Mechanical Properties ............................................. 105 5.3.1 Ultrasonic Modulus................................................................................. 105 5.3.2 Hardness Values...................................................................................... 110 5.3.3 Liquid Phase Hardness............................................................................ 112

5.4 Significance of Positional Gradients............................................................... 113 5.5 Time Effects.................................................................................................... 116

5.5.1 Contiguity and Phase Evolution.............................................................. 116 5.5.2 Grain Growth .......................................................................................... 118

5.5.2.1 Power Law Grain Growth................................................................... 121 5.5.2.2 LSW Grain Growth............................................................................. 124

5.5.3 Hardness Evolution................................................................................. 126 Conclusions..................................................................................................................... 129

Effect of gravity on microstructures ........................................................................... 129 Effect of gravity on mechanical properties................................................................. 129

Future Work .................................................................................................................... 130 Bibliography ................................................................................................................... 131 Appendix A Metallographic Procedures......................................................................... 138

Repreparation Procedures ........................................................................................... 138 Sputter Coating ........................................................................................................... 139

Appendix B Quantitative Image Analysis Details .......................................................... 141 B.1 Sample Images ................................................................................................ 141 B.2 Image Analysis Script ..................................................................................... 143 B.3 Hardness Analysis Script ................................................................................ 152 B.4 Operator Dependence...................................................................................... 153

Appendix C Ultrasonic Signal Analysis ......................................................................... 155 MATLAB Script for Time of Flight Analysis ............................................................ 155

Appendix D Statistical Testing ....................................................................................... 158 D.1 Distribution Similarity .................................................................................... 158 D.2 Sample Similarity............................................................................................ 158

Appendix E Grain Size Distributions ............................................................................. 160 78WNiFe Grain Size Distributions............................................................................. 161

vi

35WNiFe Grain Size Distributions............................................................................. 167 93WNiFe Grain Size Distributions............................................................................. 168

vii

LIST OF TABLES

Table 1: Sample matrix for ground and microgravity sintering experiments on IML-2, MSL-1, and MSL-1R. All samples sintered at 1500°C. ........................................... 11

Table 2: Summary of powder characteristics of component powders used for sample fabrication .Particle size values are measured by laser scattering, and surface area by 5-point nitrogen BET. ............................................................................................... 12

Table 3: Effective sintering times for hold times used in sample sintering cycles........... 15 Table 4: Selected samples compositions and conditions for this research. ..................... 17 Table 5: Calibration factors, error estimates, and resolution limits for quantitative

microscopy by digital image analysis. ...................................................................... 20 Table 6: Quantitative microscopy analysis script sequential description. ........................ 25 Table 7: Densities of component elements [65], theoretical densities of alloys, and

tungsten volume fraction calculated by rule of mixtures.......................................... 27 Table 8: Summary of magnifications, counts, plane coverage and sintering conditions for

quantitative microscopy samples. ............................................................................. 51 Table 9: Numerical summary of phase fraction data for all samples. Values are overall

average, ± numbers are one standard deviation observed between different fields.. 52 Table 10: Contiguity results for all samples including statistical similarity testing across

sintering gravity conditions....................................................................................... 56 Table 11: Numerical summary of 2D grain size distributions. G10, G50, and G90 are the

circular equivalent diameter at the 10th, 50th (median), and 90th percentiles, respectively. .............................................................................................................. 59

Table 12: Number of data points in relative Y positions for phase fraction and contiguity.................................................................................................................................... 67

Table 13: Number of grains per relative Y position in positional grain size data. ........... 74 Table 14: Longitudinal velocities from ultrasonic testing. Velocity values are plus or

minus experimental error. ......................................................................................... 79 Table 15: Shear velocities from ultrasonic testing. A dash indicates no velocity

calculation was possible due to poor signal. Plus or minus values are experimental error........................................................................................................................... 80

Table 16: Hardness data numerical summary, including results of statistical similarity tests. .......................................................................................................................... 85

Table 17: Comparison of prior and current average grain size results. Prior results taken from [71]. ................................................................................................................ 101

Table 18: Comparison of prior and current contiguity results. Prior results taken from [71]. ......................................................................................................................... 104

Table 19: Bulk modulus calculated from ultrasonic longitudinal velocities. ................. 106 Table 20: Shear modulus calculated from ultrasonic shear velocities. ........................... 107 Table 21: Handbook values for bulk and shear modulus of similar tungsten alloys [72].

................................................................................................................................. 110 Table 22: Nominal handbook values for mechanical properties of selected tungsten heavy

alloys [72]. .............................................................................................................. 112 Table 23: Results of statistical testing to determine significance of positional gradients in

hardness, contiguity, tungsten content, and grain size............................................ 115

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Table 24: Metallographic polishing procedure used in preparation of samples in this study. Starting point within the procedure varied depending on the condition of the sample. .................................................................................................................... 139

Table 25: Summary of operator dependence ANOVA testing results on test image set. Operators are labeled A through D, numbers indicate initial test session (1) and after procedure improvement (2, final procedure). Similarity confidences were in excess of 99%..................................................................................................................... 154

ix

LIST OF FIGURES

Figure 1: Schematic of microstructural changes during liquid phase sintering. Reproduced from [2]................................................................................................... 3

Figure 2: Assessed W-Ni binary phase diagram................................................................. 5 Figure 3: Assessed W-Fe binary phase diagram................................................................. 5 Figure 4: Ternary phase diagrams for WNiFe at 1500°C (sintering temperature, top) and

1465°C (bottom) ......................................................................................................... 6 Figure 5: Ternary phase diagrams for WNiFe at 1455°C (near solidus, top) and 1400°C

(solid state, bottom). ................................................................................................... 7 Figure 6: Cold isostatic pressed long compact and sectioned presintered samples used in

this study [52]. .......................................................................................................... 13 Figure 7: Crucible (a) and cartridge (b) assembly used for loading samples into the LIF

for sintering [52]. ...................................................................................................... 14 Figure 8: Left: Pulse generation and signal recording equipment used in time of flight

analysis. Right: Waveform signal and transducer on part. ....................................... 27 Figure 9: Longitudinal ultrasonic signal plots. Top: near-ideal signal. Bottom: typical

signal seen during testing.......................................................................................... 29 Figure 10: Shear wave ultrasonic signal plots. Top: unusually clear signal. Bottom left:

typical signal seen during testing. Bottom right: indefinite signal without clear interpretation. ............................................................................................................ 31

Figure 11: Envelope detection time of flight plots for longitudinal (top) and shear (bottom) wave ultrasonic analysis. ........................................................................... 33

Figure 12: Leco V-100-C1 indenter used in hardness testing. Left: photograph of indenter. Right: schematic view of sample in acrylic mount.................................... 38

Figure 13: Images used for Vickers hardness analysis. Top: mosaic low-mag image used for reference. Arrow at upper left indicates arbitrary zero point.Bottom: image of indent used for image analysis of diagonal lengths of hardness indents. ................. 40

Figure 14: Box plot symbol definitions used on distributed raw data plots. .................... 41 Figure 15: Mosaic images of 1 min (top row) and 15 min (bottom row) 78WNiFe sintered

samples. Left column: 1 gravity. Right column: microgravity. ................................ 43 Figure 16: Mosaic images of 45 min (top row) and 120 min (bottom row) 78WNiFe

sintered samples. Left column: 1 gravity. Right column: microgravity. .................. 44 Figure 17: Mosaic images of 180 min (top row) and 600 min (bottom row) 78WNiFe

sintered samples. Left column: 1 gravity. Right column: microgravity. .................. 45 Figure 18: Mosaic images of sintered 35WNiFe (top) and 93WNiFe (bottom). Left

column: 1 gravity. Right column: microgravity. ...................................................... 46 Figure 19: Representative microstructures of 78WNiFe. (a) Typical structure. (b)

Porosity-rich structure. (c) Liquid-rich structure. (d) Porosity and liquid-rich, also showing varying colors given to porosity by sputtering........................................... 47

Figure 20: Mosaic images of 35WNiFe sintered in 1G (top) and microgravity (bottom). Both samples were etched prior to image capture. ................................................... 49

Figure 21: Backscatter SEM images of 35WNiFe sintered in 1G for 600 min. Left: lower part of sample, showing tungsten grains. Right: upper part of sample showing etched microstructure. .......................................................................................................... 50

x

Figure 22: Backscatter SEM images of upper region of 35WNiFe sample sintered in 1G for 600 min, showing microstructural features. Left: lamellar structure correlating to heavily etched region. Right: border of heavily etched region showing both precipitates and lamellae........................................................................................... 50

Figure 23: Porosity (vapor) phase fraction results for all sintering time and gravity conditions. Top: all times. Bottom: with 45 minute result removed to show relations on 1, 15, 45, 120, 180and 600 m samples. Bars are summary average, and error bars are one standard deviation across all analyzed fields. Percentages are volume percent....................................................................................................................... 53

Figure 24: Phase fraction comparative summary chart for all sintering times and both gravitational conditions............................................................................................. 54

Figure 25: Tungsten content and gravitational effects on porosity fractions for 35W, 78W, and 93W. ......................................................................................................... 55

Figure 26: Contiguity for ground and microgravity sintered 78WNiFe. .......................... 57 Figure 27: Contiguity for 35WNiFe and 93WNiFe in ground and microgravity sintered

conditions.................................................................................................................. 57 Figure 28: Grain size distribution boxplots for 78WNiFe (top), 35WNiFe/93WNiFe

(bottom)..................................................................................................................... 60 Figure 29: Grain size distribution (2D) for 35WNiFe 180 min sintering time. Top: 1G.

Bottom: microgravity................................................................................................ 61 Figure 30: Positional effects on phase fraction for 1-15m sintering times for ground-based

(left) and microgravity (right) sintered 78WNiFe samples. Absent data in 15 minute samples is due to no random fields measured in that particular horizontal band of the sample. ...................................................................................................................... 63

Figure 31: Positional effects on phase fraction for 45-120m sintering times for ground-based (left) and microgravity (right) sintered 78WNiFe samples. ........................... 64

Figure 32: Positional effects on phase fraction for 180-600m sintering times for ground-based (left) and microgravity (right) sintered 78WNiFe samples. Absent data in 180 minute samples is due to no random fields captured in that particular horizontal band of the sample. ............................................................................................................ 65

Figure 33: Positional effects on phase fraction for 35WNiFe (top) and 93WNiFe samples (bottom) for ground-based (left) and microgravity (right) sintered samples. ........... 66

Figure 34: Positional and gravitational effects on contiguity, 1 and 15 min sintering time 78WNiFe................................................................................................................... 69

Figure 35: Positional and gravitational effects on contiguity, 45 and 120 min sintering time 78WNiFe........................................................................................................... 70

Figure 36: Positional and gravitational effects on contiguity, 180 and 600 min sintering time 78WNiFe........................................................................................................... 71

Figure 37: Positional and gravitational effects on contiguity for 35WNiFe (top) and 93WNiFe (bottom).................................................................................................... 72

Figure 38: Positional effects on grain size distributions in 1 and 15 min sintered 78WNiFe 1G and µG samples. ................................................................................. 75


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Figure 41: Positional effects on grain size distributions for 35WNiFe (top) and 93WNiFe (bottom)..................................................................................................................... 78

Figure 42: Ultrasonic velocities for 78WNiFe, showing effect of gravity and sintering time. Top: longitudinal wave data. Bottom: shear wave data. Bars are estimated experimental error. .................................................................................................... 82

Figure 43: Ultrasonic velocities for 35WNiFe and 93WNiFe, showing effect of gravity and tungsten content. Top: longitudinal wave data. Bottom: shear wave data. Bars are estimated experimental error............................................................................... 83

Figure 44: Vickers hardness of 78WNiFe sintered in 1G and 0G conditions. Column is average and bars are one standard deviation of replicate tests on each sample. Top: including all data points. Bottom: excluding data points observed to be in atypical (porous or high-liquid) regions. ................................................................................ 86

Figure 45: Tungsten content effects on hardness. Top: including porous or liquid-rich regions. Bottom: excluding porous or liquid-rich regions. ....................................... 87

Figure 46: Positional variations in hardness for 1 and 15 min sintered 78WNiFe. .......... 89 Figure 47: Positional variations in hardness for 120 and 180 min sintered 78WNiFe. .... 90 Figure 48: Positional variations in hardness for 600 min sintered 78WNiFe................... 91 Figure 49: Positional variations in hardness for 35WNiFe sintered 180 min and 93WNiFe

sintered 120 min........................................................................................................ 92 Figure 50: Slip lines generated from hardness indent, in liquid pool on 1G 600 min

78WNiFe sample. ..................................................................................................... 93 Figure 51: Slip line interaction with tungsten precipitates in 1G 180 min 35WNiFe

sample near top of sample. Top: low magnification of indent and lines. Bottom: higher magnification of slip line interaction with precipitates. ................................ 94

Figure 52: Slip lines in liquid pool adjacent to hardness indent on 1G 15 min 78WNiFe sample. ...................................................................................................................... 95

Figure 53: Slip lines in liquid pool off corner of hardness indent on microgravity 120 min 93WNiFe sample. ..................................................................................................... 95

Figure 54: Slip lines generated by hardness indents on microgravity 180 min 78WNiFe sample. ...................................................................................................................... 96

Figure 55: Overlay of cumulative grain size distribution for 180 min sintered 78WNiFe.................................................................................................................................... 98

Figure 56: Macropore formation in 50WNiFe samples sintered in microgravity for 180 minutes. Top: MSL-1R sample half. Bottom: MSL-1 sample, both halves. ............ 99

Figure 57: Comparison of prior and current grain size distributions measured for 78WNiFe sintered 15 minutes in 1G (top) and µG (bottom).................................. 102

Figure 58: Comparison of prior and current grain size distributions measured for 78WNiFe sintered 120 minutes in 1G (top) and µG (bottom)................................ 103

Figure 59: Ultrasonic bulk (top) and shear (bottom) modulus results for 78WNiFe. Columns are measurement points, bars are estimated experimental error.............. 108

Figure 60: Ultrasonic bulk (top) and shear (bottom) modulus results for 35WNiFe and 93WNiFe. Columns are data points, bars are experimental error........................... 109

Figure 61: Gravitational and sintering time effects on Vickers hardness of 78WNiFe.. 111

xii

Figure 62: Tungsten content and gravitational effects on Vickers hardness of WNiFe. 111 Figure 63: Contiguity and porosity evolution over time for 78WNiFe sintered in 1G and

µG. .......................................................................................................................... 117 Figure 64: Gravitational and sintering time effects on grain size distributions for

78WNiFe................................................................................................................. 119 Figure 65: Gravitational and tungsten content effects on grain size distributions for

WNiFe alloys. ......................................................................................................... 120 Figure 66: Grain size time evolution as average grain size time plot for 78WNiFe. ..... 120 Figure 67: Fitted log-log power law grain growth plots for 78WNiFe sintered in 1G (top)

and µG (bottom)...................................................................................................... 122 Figure 68: Fitted cubic LSW grain growth plots for 78WNiFe sintered in 1G (top) and

µG (bottom). ........................................................................................................... 125 Figure 69: Vickers hardness evolution with sintering time for 78WNiFe...................... 126 Figure 70: Densification of Fe-Cu alloys liquid phase sintered in microgravity. Fe-43Cu

has similar solid volume fraction to 78WNiFe but with lower solubility of solid phase in liquid phase. Reprinted from [77]............................................................. 127

Figure 71: Original image used in demonstration of quantitative microscopy analysis. 141 Figure 72: Image after various stages of processing. (a) after initial thresholding, showing

pores on white spots. (b) after automatic segmentation, red lines at grain boundaries. (c) after correction of phases (all porosity is liquid with polishing debris, noted during image capture). (d) after grain boundary correction. (e) solid (blue) and liquid (red) grain boundaries. (f) segmented grains (green) with dilated solid-solid boundaries (blue), only green areas with upper left inside guard frame box are measured. ................................................................................................................ 142

Figure 73: Grain size distribution (2D) for 78WNiFe 1 min sintering time. Top: 1G. Bottom: microgravity.............................................................................................. 161








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ACKNOWLEDGEMENTS

Funding for this work was provided by the National Aeronautics and Space

Administration under grant NAG8-1452.

The author would also like to thank the following Penn State faculty and students:

Vani Ramabhatt for her assistance in microstructural preparation and analysis.

Dr. Bernard Tittmann and Michael Pedrick for assistance in ultrasonic measurements.

Dr. Christopher Muhlstein and Dr. David Shelleman for assistance in hardness

measurements.

Personal thanks to the faculty, staff, and students at CISP for many years of shared

challenges, triumphs, learning, support and friendship.

And special thanks to my wife Marita, for her unwavering patience and support.

xiv

FRONTISPIECE

1

Chapter 1: Introduction

From 1994 to 1997, 77 tungsten heavy alloy samples were liquid phase sintered in

microgravity in a set of experiments that generated matched sets of ground-based

sintering and identical orbital experiments on the NASA shuttle Columbia. The samples

spanned a variety of tungsten contents, matrix alloys, and consolidation techniques. A

significant volume of prior work on these samples has given fundamental insights into

both liquid phase sintering and microgravity processing of materials.

While there have been significant findings in gravitational effects on liquid phase

sintered microstructures, data in true statistical quantities has not yet been gathered. The

alterations in mechanical properties induced by microgravity processing of materials have

not yet been determined, but have only been extrapolated from microstructural

measurements.

Recent advances in computer-aided quantitative microscopy allow generation of

statistically significant microstructural parameter measurements. Actual mechanical

property measurements, while destructive to the samples, can verify physical property

changes in the material that have only been theorized from prior microstructural

measurements.

The goal of this research was to examine the effect of gravity on microstructures

of liquid phase sintered alloys using statistical amounts of quantitative microscopy data

and to physically measure the effect of gravity on mechanical properties.

2

Chapter 2: Background

2.1 Sintering and Liquid Phase Sintering

Sintering is defined as a thermal process which increases the strength of a powder

mass by bonding adjacent particles via diffusion or related atomic level events [1]. In

practice it is a technique of manufacturing parts by using the flowability and formability

of loose powder masses to get a low density near-net shape form, then heating the form to

a temperature sufficient to form bonds that densify the form, giving it mechanical

strength and any other desired mechanical, thermal, or electrical properties. Many

variables are involved in sintering, including physical and chemical characteristics of the

initial powders, the consolidation method used to make the form, and the precise time,

temperature, and atmospheric profile used to solidify the form.

Sintering is a diffusion based process driven by the high surface area of smaller

particles: the process proceeds by lowering the surface energy of the system by forming

interparticulate bonds. The bonds can grow either by solid-state processes such as surface

and volume diffusion or by mixed-phase processes such as solid-liquid solution-

reprecipitation and vapor transport. Pressure can also be externally applied to enhance the

formation of sinter bonding, and exothermic chemical reactions can also be combined

with other bond growth mechanisms to enhance densification by sintering [2].

Important measures of sintering effectiveness are density and porosity. Density is

often expressed as percent of theoretical, with theoretical density being the density of the

pure material with no voids or pores. Pores are voids that initially started as space

between the formed particles or intraparticulate voids. Increasing density results in fewer

pores or voids. The shape, size, and spatial distribution of these pores within a part has a

large effect on the mechanical properties of the part. Although some functional parts such

as filters are designed to have a controlled size and amount of porosity, generally porosity

is detrimental to the properties of the final sintered product.

Liquid phase sintering is sintering above the temperature where a liquid forms in

the material system, either briefly or persistently. This greatly speeds densification of the

system by accelerating mass transport and providing capillary pressure on the bonding

3

particles equivalent to a large external pressure. The enhanced mass transport in the

liquid phase accelerates both densification and microstructural coarsening (grain growth).

Liquid phase sintering is an important industrial process, and is used to

manufacture articles made from stainless steels, superalloys, sialon, aluminides, tool

steels, titanates, magnetic alloys, tungsten alloys, cemented carbide, thermal alloys,

electrical contactor alloys, and many other bulk or niche-market alloy systems [2].

2.1.1 Microstructural Evolution During Liquid Phase Sintering Prior to liquid phase formation, the form densifies slightly due to solid-state

diffusional transport mechanisms. After liquid phase formation, however, several

reactions occur. Particles rearrange within the liquid, then grow and shrink depending on

their size by a process called Ostwald ripening where smaller particles are consumed

while larger particles grow. After particles grow enough to touch and solution-

reprecipitation has reached steady-state, solid state diffusion proceeds. A schematic of

microstructural changes is shown in Figure 1 [2].

Figure 1: Schematic of microstructural changes during liquid phase sintering.

Reproduced from [2].

4

Grain growth in liquid phase sintering has been studied extensively, as the grain

size of final microstructures relates strongly to mechanical properties of the final part

primarily by the Hall-Petch relation [3]. The classic theory of grain size evolution in two-

phase microstructures by Ostwald ripening was given by Lifshitz, Slyozov, and Wagner

(LSW); however, while this model has been shown to be accurate in predicting grain

growth rates of dilute systems (low volume fraction of coarsening phase) but does not

perform well if volume fractions approach the limit at which solid-solid contacts are

abundant [4]. Several other models and definitions of the limits of these models have

been proposed and explored to correct the errors in LSW theory [2, 4-17]: one notable

model is that proposed by German and Olevsky [8].

Grain growth can be empirically modeled in general by a log-log linear fit,

calculating an intercept and slope which gives a power law fit [18]. This modeling

assumes that grain growth follows the following form and accompanying logarithmic

reduction:

nG kt=

1ln( ) ln( )G t kn

= + Equation 1

where G is average grain size, t is effective sintering time, n and k are constants.

For liquid phase sintering, LSW theory predicts grain growth at isothermal

temperatures using the following relation:

3 30G G tκ= + Equation 2

where G0 is the initial grain size, and κ is a thermally activated parameter.

2.1.2 Material System Properties for WNiFe The WNiFe alloy system has been studied and has phase equilibria diagrams

showing basic solubility relationships for the system. Selected diagrams are included here

for reference on the material system. Binaries are shown in Figure 2 and Figure 3.

Tungsten shows a high solubility in both nickel and iron, indicating a high driving force

for solution-reprecipitation during liquid phase sintering. Selected ternaries are given in

Figure 5 and Figure 5 [19] with alloy compositions marked on the diagrams. While

5

several phase regions exist in the ternary system, the three alloys chosen in this study are

all drawn from the same phase field and undergo similar phase transformations during the

sintering performed in this study.

Figure 2: Assessed W-Ni binary phase diagram.

Figure 3: Assessed W-Fe binary phase diagram.

6

Figure 4: Ternary phase diagrams for WNiFe at 1500°C (sintering

temperature, top) and 1465°C (bottom)

7

Figure 5: Ternary phase diagrams for WNiFe at 1455°C (near solidus, top) and

1400°C (solid state, bottom).

8

2.2 Prior Work

The terms Earth gravity, ground gravity, or 1G used here refer to a gravitational

acceleration of 9.81 m/s2, as in prior works. The terms orbital gravity, space-based,

microgravity or µG refer to a gravitational acceleration of 10-6 m/s2 with transients up to

10-4 m/s2.

A copious amount of prior research has investigated the effect of gravity on liquid

phase sintering tungsten heavy alloys [6-13, 20-58]. A conference presentation in 2005

([26]) summarized key findings of the prior work utilizing this set of samples:

• Compacts densify prior to shape distortion under 1G conditions

• Gravity induces tungsten grain settling, creating bottom to top gradients in grain size,

solid volume fraction, and contiguity

• Grain settling does not occur in microgravity: instead, surface-to-core gradients arise

• Grain settling in 1G conditions induces structural rigidity through skeleton formation

that limits distortion

• Elimination of pores is problematic in microgravity due to lack of buoyant force to

force the pores to migrate out of the material

• Green body homogeneity dominates other factors in controlling densification in 1G

conditions

Microgravity has previously been found to have multiple effects on the sintered

microstructures of tungsten heavy alloys. Microgravity does not lead to homogeneous

microstructures; rather, pores tend to cluster, solid-solid contiguity varies from surface to

core of the sample, and localized agglomerations become more common for both pores

and grains [52]. Other work has also determined critical volume limits for gravitationally

induced settling of grains and successfully modeled the size and extent of the settled

region [30, 31].

Prior work [23] has also determined that various solid fraction WNiFe alloys

sintered for 120 minutes in microgravity had grain sizes measured by lineal intercept to

be smaller than identical samples sintered under Earth gravity, and that grains grew to

larger sizes in the bottom portions of the ground-based sintered samples. The same study

9

also examined the liquid phase near the grains for tungsten content gradients near the

grains, and found none.

Among the open questions remaining from the prior work are confirmation of a

grain growth model given in [8] by generation of statistical quantities of data points to

verify the form of the grain size distribution. The various distribution types discussed in

[8] are similar near the center of the distribution, and vary most near the tails. As the tails

of a probability distribution are by definition low-frequency, gathering enough data

points to accurately fill the tails sufficiently for modeling is challenging.

Other questions that have not been addressed are physical measurements of the

mechanical properties of microgravity sintered tungsten heavy alloys. While considerable

data has been generated on the microstructures, mechanical property data has only been

extrapolated from traditional microstructure-mechanical property relations.

10

Chapter 3: Procedures

3.1 Sample Fabrication

The goal of the microgravity experiments were to study liquid phase sintering without

phase separation, settling, buoyancy forces, or any other gravitationally induced

complications. Gravitational settling is a major factor in liquid phase sintering, leading to

anisotropic properties in the sintered material. By sintering samples under microgravity, it

was hoped that fundamental understandings into the mechanisms of liquid phase sintering

would be developed.

Experiments were proposed by Professor Randall German of the Pennsylvania State

University to study tungsten liquid phase sintering systems [34]. In tungsten-rich liquid phase

sintering systems, commonly known as tungsten heavy alloys (WHA), high density is

achieved through the presence of a second phase that is liquid at high temperatures and has

solubility for tungsten, commonly nickel-iron or nickel-copper. In these systems sintering

must be well controlled to achieve density without undesired shape distortion. At

temperatures above the melting point of the liquid phase, the sample has very little strength

and will slump due to gravity and round off edges due to surface tension effects. Key

variables in sintering this system include the tungsten content, liquid phase alloy

composition, sintering temperature, and sintering time. In the flight experiments, tungsten

content, alloy composition, and sintering time were explored. Sintering temperature was kept

constant to simplify experiments and furnace setup.

3.1.1 Shuttle Flight Information Microgravity sintering experiments were conducted from 1994 through 1997 on three

Columbia shuttle missions STS-65 (July 1994), STS-83 (April 1997), and STS-94 (July

1997). Flight STS-65 was the second flight of the International Microgravity Laboratory

(IML-2). Flight STS-83 was the first flight of the Microgravity Science Laboratory (MSL-1),

and flight STS-94 was an MSL reflight designated MSL-1R. The reflight was necessary due

to problems with Columbia’s fuel cell that shortened the STS-83 mission. Further references

to shuttle flights will be made by mission; IML-2 corresponds to STS-65, MSL-1 and MSL-

1R correspond to STS-83 and STS-94 respectively.

11

3.1.2 Sample Matrix

On the IML-2 flight, five different tungsten contents and three different sintering

times were tested. The liquid phase alloy was 7:3 nickel:iron by weight, which is commonly

used in tungsten heavy alloy processing. On the MSL-1 and MSL-1R flights, lower tungsten

contents and two additional liquid phase alloys, 8:2 and 6:4 nickel:copper by weight were

investigated to examine the effect of tungsten solubility in the liquid phase. A listing and

count of the samples at each condition for both flights is shown in Table 1. The extra

replicates were from a study on sample preparation effects at the 78 wt%, 93 wt%, and 98

wt% tungsten levels [45].

Table 1: Sample matrix for ground and microgravity sintering experiments on IML-2, MSL-1, and MSL-1R. All samples sintered at 1500°C.

Ground Based

7:3 NiFe 6:4NiCu 8:2NiCu

time W wt% 35 50 65 78 83 88 93 98 50 88 50 88

1 min 1 1 1 2 1 1 3 1 1 1 1

15 min 1 1 1 3 1

45 min 1 1 1 1 1 1 1

120 min 7 1 1 3 2

180 min 1 1 1 1 1 1 1

600 min 1 1 1 1 1 1 1

Microgravity

7:3 NiFe 6:4NiCu 8:2NiCu

time W wt% 35 50 65 78 83 88 93 98 50 88 50 88

1 min 2 2 2 3 1 1 3 1 1 1 1

15 min 1 1 1 3 1

45 min 1 1 1 1 1 1 1

120 min 7 1 1 3 2

180 min 2 2 2 1 1 1 1

600 min 2 2 2 1 1 1 1

12

3.1.3 Sample Fabrication For the microgravity sintering missions IML-2, MSL-1, and MSL-1R, two sets of

samples were fabricated. The characteristics of the component powders are given in detail in

[32] for MSL-1 and MSL-1R samples, and the powder characteristics for both flights are

shown in Table 2. It should be noted that all three elemental powders were made at the same

respective production facilities but with different owners, hence the powders are as similar as

possible between the experimental missions..

Samples were physically prepared on earth by cold isostatically pressing mixed

elemental powders of the desired composition at 200MPa into cylinders 12.7mm in diameter

by ~10cm long. The majority of the pressed compacts were then vacuum presintered at

1400°C for one hour to provide handling strength. After presintering, samples were sectioned

into smaller cylinders measuring 12-14mm in height. In order to find the effects of sample

preparation, some selected samples were also uniaxially pressed to similar geometry,

presintered in argon, presintered in hydrogen, or hot isostatically pressed. The samples

examined in this work were all identically prepared by vacuum presintering at 1400°C after

cold isostatic pressing. A photo of the green and presintered parts is shown in Figure 6.

Table 2: Summary of powder characteristics of component powders used for sample fabrication .Particle size values are measured by laser scattering, and surface area by 5-point nitrogen BET.

Mission IML-2 MSL-1,1R

Powder Tungsten Nickel Iron Tungsten Nickel Iron

Supplier GTE Novamet GAF Osram Sylvania

INCO (Novamet)

ISP

Grade - - - M37 123 R1470

D10 (µm) - - - 4.1±0.4 3.8±0.4 2.7±0.4

D50 (µm) 8.0 11.0 6.3 12±1 9.7±0.6 5.9±0.9

D90 (µm) - - - 33±4 8.5±0.6 13±2

BET SA (m2/g) - - 0.56 0.17±0.04 0.42±0.1 0.52±0.1

BET Size (µm) - - - 1.8 1.6 1.5

He Pycnometer Density (g/cm3)

- - - 19.2 8.9 7.9

13

Figure 6: Cold isostatic pressed long compact and sectioned presintered samples

used in this study [52].

3.1.4 Sintering Procedures

Samples examined in the course of this thesis were part of the NASA research project

Gravitational Role in Liquid Phase Sintering, sponsored by the Microgravity Science

Division at NASA. Samples of tungsten heavy alloy were sintered both on Earth under

normal gravity and under microgravity conditions. Samples were sintered in a furnace

designated as the Large Isothermal Furnace (LIF) designed and built by the Japanese Space

Agency (NASDA), which was designed to operate at temperatures in excess of 1500°C with

minimal thermal gradients. Both earth-based and microgravity-based sintering experiments

were conducted in the LIF. Photos of the crucible and cartridge assemblies are shown in

Figure 7.

14

Figure 7: Crucible (a) and cartridge (b) assembly used for loading samples into

the LIF for sintering [52].

Sintering thermal profiles consisted of identical profiles, with only the hold time at

1500°C varying. The thermal profile was heat at 10°C/min to 800°C, hold for 60 min, ramp

at 10°C/min to 1500ºC, hold for the designated hold time (variable), control cool at 3°C to

1420°C guaranteed soak, then furnace cool to ambient temperature. The sintering atmosphere

was vacuum, nominally 1E-4 torr pressure. No actual monitoring of pressure was available in

the LIF, but sealed cartridges were under a vacuum of better than 1E-4 torr prior to flight.

Hold times of 1, 15, 45, 120, 180, and 600 min were applied at the maximum temperature of

1500°C. In liquid phase sintering systems such as W-Ni-Fe, substantial densification only

occurs after liquid phase is formed [1]. In this system, the binary nickel-iron alloy has a

15

solidus of 1440°C; the addition of tungsten depresses this somewhat further, yielding a

solidus of ~1430°C for the solution phase [19]. In the sintering profile used in this work, this

yields an effective sintering time (i.e. time in presence of liquid phase) approximately 30

minutes longer than the programmed hold time. The effective sintering times are summarized

in Table 3.

Table 3: Effective sintering times for hold times used in sample sintering cycles.

Sinter Hold Time Effective Sintering Time

1 min 31 min

15 min 45 min

45 min 60 min

180 min 210 min

600 min 630 min

16

3.2 Sample Selection

In order to generate pertinent data for grain growth, the 78WNiFe samples at six

different sintering times of 1, 15, 45, 120, 180, and 600 minutes in both ground and orbital

sintering conditions were chosen. The effect of tungsten content is also an important factor in

both grain growth and mechanical properties, hence 35WNiFe sintered at 180 minutes and

93WNiFe sintered at 120 minutes in both ground and microgravity were also selected. At

these compositions, samples of identical sintering time were not available, but enough time-

dependent data should be generated from the six-time 78WNiFe study to account for the

sintering time difference in analysis. In addition to sintering time and tungsten content

effects, two samples were briefly examined to look at microstructures within the liquid

phase: these were high-liquid 35WNiFe samples sintered for 600 minutes in microgravity

and Earth gravity.

A matrix of the samples selected for this work indicating sample codes, shuttle

mission, composition, gravitational and sintering time variables is given in Table 4. Samples

for MSL-1 and MSL-1R were prepared from exactly the same basic stock powders and all

initial preparation was done simultaneously. Samples for the IML-2 mission were prepared

several years before MSL-1 and MSL-1R sample preparation.

17

Table 4: Selected samples compositions and conditions for this research.

Sample Code Composition (wt. %)

Time at 1500°C(minutes)

Gravity Condition Mission

Microstructural gravity and time effects

61G 78W-15.4Ni-6.6Fe 1 Ground MSL-1

111R 78W-15.4Ni-6.6Fe 1 Microgravity MSL-1R

9-64 78W-15.4Ni-6.6Fe 15 Ground IML-2

44-69 78W-15.4Ni-6.6Fe 15 Microgravity IML-2


21S 78W-15.4Ni-6.6Fe 45 Microgravity MSL-1

16-65 78W-15.4Ni-6.6Fe 120 Ground IML-2






Tungsten content effects


134R 35W-45.5Ni-19.5Fe 180 Microgravity MSL-1

13-95 93W-4.9Ni-2.1Fe 120 Ground IML-2


Repeatability of microgravity sintering

24-66 78W-15.4Ni-6.6Fe 1 Ground IML-2





Gravitational effects on liquid phase microstructures



18

3.3 Metallographic Procedures

The metallographic procedures used in this study were a refinement and extension of

procedures developed over the years that these samples have been studied, most recently

published in [49]. The vast majority of the samples had been previously examined, and were

already sectioned and mounted. A few samples had to be remounted due to problems with the

original mounting caused by aging. All samples were repolished for this study to a 0.04µm

colloidal silica finish; varying amounts of effort were required depending on the condition of

the sample. All samples were also verified for their identity, according to the sample logs,

and recoded as necessary if the labels had worn or degraded. Details on the final

metallographic preparation on the samples examined in this study are given in Appendix A.

Samples were sputter coated with platinum oxide to enhance contrast, giving a vivid two-

tone blue color to the samples, which without enhancement would be gray on gray with

insufficient contrast for image analysis [59].

The only etching performed in this work was on the two samples selected to study the

microstructures of the liquid phase. These samples were metallographically polished the

same as other samples, but were not platinum sputtered. Instead, these samples were etched

for 90 seconds with a nickel-iron alloy etchant comprising of 60 mL methanol, 15 mL

hydrochloric acid, and 5 grams of ferric chloride (FeCl3). The etchant was applied by static

immersion of the polished surface in the etchant.

19

3.4 Image Analysis

Before and after sputter coating, samples were examined with an inverted stage

Nikon Epiphot 300 optical microscope equipped with a software controlled motorized stage

and a IEEE 1394 24-bit color digital camera at 1280x1024 pixel resolution. Images were

viewed, captured, and analyzed, using Clemex Vision Pro 3.5 (Clemex, Longueuil, Canada)

software installed on a Windows PC. The motorized stage and 1394 camera attached to the

microscope were provided by Clemex. Images were captured at full 1280x1024 pixel

resolution in 24 bit color. The Nikon microscope is equipped with a revolving nosepiece with

five objectives with apparent eyepiece magnifications of 50x, 100x, 200x, 500x, and 1000x.

All references to magnification in this work will be to these apparent eyepiece magnifications

for simplicity. True magnification will vary depending upon the output device and size.

Before quantitative microscopy image capture was performed on sputtered samples, a

reference mosaic image of an unsputtered sample was captured. Mosaic captures use the

computer controlled stage on the microscope to capture adjoining images then reassemble

them into a larger, composite mosaic image. This results in an image of better resolution than

a traditional low-magnification microscope picture, but with a field of view large enough to

see the whole sample. Mosaic images were captured for reference on the samples, to

determine sample size for field selection, and to make qualitative observations on the relative

microstructures of the samples.

3.4.1 Quantitative Microscopy Image Capture

Length calibration of images was performed by capturing images of a calibrated

optical scale (Olympus part number M-0550) with gradations at 0.01mm (10µm). Using the

Clemex software, the number of pixels from edge to edge of the largest number of gradations

possible at the selected magnification was measured 5 times, and the distance calibration in

µm per pixel calculated from this length. This procedure was performed in both X and Y

directions within the image, and it was found that the distance aspect ratio for the images was

1:1, requiring no distortion correction and only a single distance calibration number needed.

Pixel line lengths varied no more than by three pixels over five measurements for any

magnification; given this, an error estimate was made for each calibration. As a relative

measure assuming maximum error, this three-pixel calibration variability results in a 2.9%

20

relative error on a feature measuring 10% of total field width, or a 1.2% relative error on a

feature measuring 25% of total field width.

The linear error estimate is conservative, as the primary measurements used in this

work are based on pixel counts or area. To examine the amount of error expected from the

image analysis, a single cluster of grains on one image was measured several times,

deliberately varying the analysis parameters to the limits of the logical interpretation of the

image (threshold levels, manual edit position, etc). The average area error in pixels was 0.66

pixels on each feature, ranging from 0.33 to 0.83 pixels. This area pixel error was then used

to calculate an error estimate on circular equivalent diameter (see section 3.4.2 below). A

summary of the calibration factors and error estimates for each objective is given in Table 5:

in the table, if calculated pixel resolution error and analysis script parameter variation error

was less than the accepted resolution limit for the objectives used [60] the optical resolution

limit is listed.

Table 5: Calibration factors, error estimates, and resolution limits for quantitative microscopy by digital image analysis.

Eyepiece Mag

Distance Calibration (µm/pixel)

Linear Err Estimate

(µm)

Area Err Estimate

(µm2)

Circ Equiv Diameter Err

Estimate (µm)

Field Area(µm2)

50x 1.776 5.3 2.08 1.63 4134242

100x 0.8905 2.7 0.52 1.1 † 1039388

200x 0.4449 1.3 0.13 0.69 † 259439

500x 0.1784 0.54 0.02 0.42 † 41716

1000x 0.08842 0.27 0.01 0.22 † 10247

†Optical resolution limit for microscope objectives. Unmarked values are calculated errors from digital pixel resolution and operator effects from multiple trials.

Large numbers of images for analysis were desired in order to measure

microstructural parameters on the maximum feasible number of grains. While previous work

had shown fully automatic analysis techniques using this equipment to be possible for grain

size and phase fractions [2], other parameters such as contiguity and connectivity were better

performed using a “quasi-automatic” technique, where most measures were made

automatically but allowing user input to correct common mistakes made by the automatic

21

algorithms. Most errors resulted from artifacts from sample preparation, primarily adherent

polishing particles and staining from liquid trapped in pores. As such, while fully automatic

methods could have allowed analysis of the entire visible sample plane, the number of

images had to be limited to those realistically analyzable by an operator.

The scale of the microstructure greatly influenced the number of images that had to

be captured for each sample. For best accuracy, the minimum feature size should be over 5%

of the total field width, in this case the primary feature of interest was a tungsten grain. Grain

size varied considerably among the samples of different sintering times, and higher

magnifications were required for the smaller grains found in the shorter sintering times. It

was found that 500X magnification was necessary for samples sintered at 1, 15, and one 45

min sample, while 200X was sufficient for sintering times up to 180 min, and 100X was

appropriate for the 600 min samples. The viewable area of the sample is much smaller at

higher magnifications, resulting in more images to cover the same area.

Magnification was selected by performing one full preliminary analysis on a live

image to determine the number and scale of grains within the image. Lower magnifications

were always tried first. If manual corrections could be applied consistently and easily at the

trial magnification, that magnification was chosen: if not, magnification was increased to see

the grain features better.

In capturing images, whenever possible enough images were taken to capture the

entire sample cross-section. This was possible at magnifications of 100X and 200X. Image

capture was performed by programming the automated stage with a stage pattern with

slightly overlapping fields in a shape designed to cover the entire sample and all edge

regions. When possible, automatic focus was used, however better and more consistent

results were obtained using an operator to focus each image. With the automatic stage this

proceeded very quickly, even with operator focusing each image. Captured images were

saved along with calibration data and automatically assigned a filename with incrementing

number from 1 to maximum number of fields.

After image capture of images at 100X and 200X, a list of numbers from 1 to the

maximum number of fields was created, and then randomized using visual basic macros in

Microsoft Excel. The randomized numbers were broken into multiple sets of ten images

each, which was a convenient number of images to be analyzed at one sitting. Each set was

22

analyzed in toto, and enough sets were analyzed until the total number of tungsten grains

exceeded 6,000. The lower limit of 6,000 was chosen as a compromise between maximizing

grain data and minimizing operator time per sample. If additional accuracy or higher grain

counts were desired, additional images not in the completed sets and captured from the same

capture session can be analyzed and the results combined.

For samples imaged at 500X, however, the number of images required to capture the

entire section plane made full coverage capture impractical. A list of ~200 random image

locations was generated using the overall plane dimensions of the sample. Random field

positions were generated using a program written in Microsoft Excel to generate non-

overlapping fields of a given size within a shape approximation of the sample plane section

(rectangle or circle). The coordinate list was then used with the positional tracking stage to

move the field of view to the specified coordinate and capture an image. Captured images

were then randomized and divided into sets as with other samples.

A list of magnifications for each sample and counts of images and grains are included

with the quantitative microscopy results.

3.4.2 Analysis Procedure In describing the procedure used for quantitative image analysis, a few terms should

be defined. These terms are taken from the Clemex Vision PE user’s guide [61], and may

vary somewhat from standard image analysis texts. A Field is a single frame or image. A

Bitplane is a Boolean pixel mask overlaid on a grayscale or color image, indicating regions

on the sample. Object Measures are measurements based on individual (not connected)

regions on a bitplane, and fundamentally include area, length, perimeter, position, etc. Field

Measures are measurements based on the entirety of a field, including overall pixel count

measures such as area or averaged object measures (i.e., contiguity). The fundamental

measures taken by this digital analysis system include X-Y position, pixel count (area),

perimeter, and feret length (Feret’s Diameter, analogous to taking calipers across many

opposing sides at angles spaced 2.5° apart). All other measures are directly or indirectly

based on these fundamental measures.

The primary measures of interest in this research included grain size, contiguity, and

phase fraction. The grain size of objects is occasionally difficult to reduce to a single number

due to shape; however, in this work as in prior works [9, 30, 35-40, 49, 62] the grains are

23

rounded in nature, and a sphere (3D) or circle (2D) is used to approximate their shape. In the

case of spherical or equiaxed round grains, the planar intersect to the grain is circular. A

single parameter known as circular equivalent diameter can thus be used to describe the grain

size. Circular equivalent diameter is related to fundamental image analysis measures by:

πACED 4

= Equation 3

where A is the area of the object. This is a two-dimensional measurement.

The second measure of interest in this work is contiguity. Contiguity is a measure of

the degree of connection in one phase of a multi-phase microstructure. In this case we are

interested in tungsten (or solid) grain contiguity, defined as [63]:

∑∑∑∑

++=

SVSLSS

SSSS SSS

SC Equation 4

where CSS is the solid-solid (or tungsten-tungsten) contiguity, ∑ SSS is the sum of lengths of

all solid-solid boundaries in a given field, ∑ SLS is the sum of lengths of all solid-liquid (or

tungsten-matrix) in a given field, and ∑ SVS is the sum of lengths of all solid-vapor (or

tungsten-pore) boundaries in a given field. The formula in Equation 4 differs from the

traditionally applied stereological formula [38] because here we are using direct length

measurements for contiguity rather than line-intercept counts more suited to manual analysis

of images. For the lengths of these boundaries, which are long, curved objects, a measure

known as String Length was used. This is an area and perimeter-based length measure more

accurate for long, thin objects than a feret-based length. String length is calculated from

fundamental image analysis measures by:

4162 APPthStringLeng −+

= Equation 5

where P is the perimeter and A is the area. Contiguity is basically a field measurement:

although individual boundary lengths are object measurements, the definition of the

parameter requires summation across an entire field. Contiguity measured from planar

microstructural sections is a two-dimensional measurement.

24

The third measure of interest in this work was phase fractions. These measurements

are field-based measurements, counting the total number of pixels in a given bitplane set up

to include an entire phase, either solid (tungsten), liquid (NiFe matrix), or vapor (pores). This

pixel count is then normalized to the total sample area in the field to give a phase fraction.

Because phase fractions are point-count methods, they can be directly treated as the three-

dimensional measurement volume fraction [64].

3.4.2.1 Analysis Script The analysis script developed in [49] was further refined here, based on lessons

learned from that work. The full script was written for Clemex Vision PE v3.5, and is

included in Appendix B along with comments intended to simplify use and sample images of

the stages of the analysis procedure. A summary of the analysis script is given in Table 6.

The amount of manual corrections applied during the analysis raised some concern about

operator dependence: to investigate this, a study was performed with four different operators

on a set of 10 fields (images). The study indicated that no significant difference was

introduced by various operators – details are included in Appendix B.

25

Table 6: Quantitative microscopy analysis script sequential description.

1. Load images automatically from specified directory

2. Threshold (color) to select tungsten grains

3. Threshold (color) to select liquid phase

4. Boolean manipulation on thresholds to clean up spot artifacts

5. Slightly round rough edges of Boolean objects

6. Find pores by Boolean NOR on grains and liquid phase

7. Determine sample area by Boolean OR on solid and liquid, joining separated objects.

8. Remove grains, liquid, pores outside of sample area.

9. Automatic grain segmentation using circular kernel, avoid oversegmentation by limiting sphericity of grains to be segmented

10. Manual edit: correct oversegmentation and missed grain areas

11. Manual edit: correct missing liquid phase areas

12. Manual edit: correct misidentified pores

13. Manual edit: correct un-separated grains

14. Save image: overwrite original, as only changes are bitplanes.

15. Measure tungsten phase field measures on unsegmented tungsten phase

16. Measure porosity field measures

17. Measure sample area (may be less than field area if field at edge of sample) and field position

18. Measure liquid phase field measures

19. Generate solid-solid boundaries: Boolean DIFF of segmented tungsten phase from unsegmented tungsten phase

20. Generate solid-liquid boundaries: Boolean AND on dilated liquid phase and segmented solid phase

21. Generate solid-vapor boundaries: Boolean AND on dilated pore phase and segmented solid phase

22. Remove incomplete solid-solid, solid-liquid, and solid-vapor boundaries at edge of field

23. Measure boundary lengths (S-S, S-L, S-V, L-V) for contiguity

24. Find triple points in solid-solid boundaries by binary manipulation. Separate triple points into three separate boundaries to get one boundary per object for accurate connectivity

25. Expand SS boundaries without overlapping

26. Measure tungsten object measures of grain size, shape, and position

27. Measure tungsten connectivity from count of expanded solid-solid boundaries overlapping each tungsten grain

28. Measure pore size, shape and position

29. Save final image as new file with final bitplanes for later review

30. Save data (Excel 4.0 format)

31. Enter data in relational database

26

3.5 Ultrasonic Testing

Bulk elastic modulus measurements were performed using the ultrasonic time-of-

flight technique. This technique measures bulk or shear modulus from the sound velocity

through a material of known density. The use of C-scan ultrasonic to visualize internal

porosity was evaluated, but the small size of the samples made the testing problematic,

therefore only pulse-echo time of flight was used in this work. The relationship between

modulus and one-dimensional sound velocity in bars is given by:

ρ2Lb vE =

ρ2Sb vG =

Equation 6

Equation 7

where Eb is the bulk modulus, vL is the longitudinal velocity of sound in the material, ρ is the

density of the material, Gb is the shear modulus, and vS is the transverse (shear) velocity of

sound in the material. The longitudinal and shear velocities of sound in the material are

calculated by:

(µs)Flight of Time(mm)Length Test x 2

, =LSv Equation 8

where test length is the length of the test piece along the test axis, and the time of flight is

calculated from the ultrasonic waveform, measured experimentally.

The velocities in Equation 6 and Equation 7 can be measured using ultrasonic

transducers by transmitting an ultrasonic pulse through the sample and record the resulting

signal waveform. Echoes from the opposing face of the sample create peaks in this

waveform, and velocity can then be calculated from the time between peaks and the length of

the sample. Once the velocity is measured, modulus can be calculated using the theoretical

density of the material. The bulk and shear moduli are biaxial elastic properties of the

material, and are the mechanical properties of interest from this testing.

Application of the equations above to determine bulk and shear modulus requires the

density of the material. The densities were calculated from the densities of the component

elements [65] using the rule of mixtures, and are shown in Table 7 along with theoretical

volume percent of tungsten for each WNiFe alloy studied in this work. The use of theoretical

27

densities prevents direct accounting for porosity variations in the sample, but gives a

consistent cross-comparison value.

Table 7: Densities of component elements [65], theoretical densities of alloys, and tungsten volume fraction calculated by rule of mixtures.

Material Density (g/cc) Vol. % Tungsten

W 19.25 -

Fe 7.874 -

Ni 8.908 -

78W-15.4Ni-6.6Fe 15.11 61.2 %

93W-4.9Ni-2.1Fe 17.71 85.5 %

35W-45.5Ni-19.5Fe 10.64 19.3 %

3.5.1 Testing Procedures and Equipment Pulse generation for ultrasonic signal generation was performed using a Matec

TB1000 tone burst PCI card installed on a Windows 98 pentium-based PC. Data collection

was performed using a Tektronix TDS3012B eScope, capable of reading frequency data at up

to 100 MHz and 1.25 x 109 samples per second. The Tektronix was configured to save data

for ultrasonic time of flight data analysis as a comma delimited text file, output to floppy

disk. The tone generation and signal recording equipment is shown in Figure 8.

Figure 8: Left: Pulse generation and signal recording equipment used in time of flight analysis. Right: Waveform signal and transducer on part.

28

Prior to analysis of the tungsten alloy samples, test measurements were made on a

15x10x2.5 cm thick aluminum plate to verify correct operation of the equipment. The

block is used as a local standard in time of flight measurements carried out in this

laboratory.

Preliminary analysis on the samples using available transducers indicated that the

small sample geometry and angled back face could generate indefinite signal waveforms,

making analysis difficult. In order to minimize these small size and geometric effects,

small transducers at high frequencies were used. Distances on all parts were measured

using calipers, and all fixturing/transducer orientation was performed by hand. The small

sample size made replicates difficult, but at least two consistent measurements were

taken on each test piece when possible.

Sample preparation was typically performed on the second half of samples that

had been cut previously for metallographic analysis. If only one half of the sample was

available, the sample was removed from the metallographic mount and cleaned of

polymer residues. All samples had one face hand-polished to a 600 grit finish using

structured alumina abrasive (Trizact 600, Leco Corporation); this was the face used for

contact with the transducer. In interpreting ultrasonic signal plots, multiple peaks

(echoes) are desired, as these give confidence that the peaks are from back wall return

and not from side wall or other interfering geometrical effects [66]. The characteristics of

desired peaks are even spacing and multiple repetitions, allowing measurement of time

between secondary or later peaks rather than the broad and occasionally indefinite

primary peak.

3.5.1.1 Longitudinal Wave Testing For longitudinal velocities, a 20MHz longitudinal transducer with a 3.2mm

sensing diameter (Ultran Group, State College, PA) was used to apply the ultrasonic

pulse and collect signal data. Coupling was done with Sonotech Ultragel II ultrasonic gel.

A pulse width of 0.1µs was used at 20MHz frequency and a high-pass filter set at 10MHz

to receive the data. Typical longitudinal wave ultrasonic signal plots are shown in Figure

9. Most of the longitudinal waveforms from the analysis on the samples showed signals

29

similar to the lower plot in Figure 9, with at least two peaks in addition to the primary,

allowing confidence in final time of flight data.

Figure 9: Longitudinal ultrasonic signal plots. Top: near-ideal signal. Bottom:

typical signal seen during testing.

30

3.5.1.2 Shear (transverse) wave testing

For shear wave velocities, a 20MHz transverse wave transducer with a 3.2mm

sensing diameter (Ultran Group, State College, PA) was used with local honey (slightly

crusty) for coupling. The tone source setup to get the cleanest signal for shear wave

testing was at 10MHz frequency with low and high pass filters set at 10MHz, and a pulse

width of 0.4µm. Typical shear wave ultrasonic signal plots are shown in Figure 10.

Multiple peaks were relatively rare, as seen in the typical signal plot, requiring the

primary peak to be used to determine time of flight and increasing potential for error in

the analysis. Indefinite signal plots occurred fairly often, with peaks that could not be

identified with confidence as back wall echoes rather than side wall or angled reflection

effects, shown in bottom right of Figure 10.

31

Figure 10: Shear wave ultrasonic signal plots. Top: unusually clear signal.

Bottom left: typical signal seen during testing. Bottom right: indefinite signal without clear interpretation.

32

3.5.2 Data analysis

Several techniques can be used in measuring the time between peaks in the

ultrasonic waveform. Three techniques evaluated in this work were peak-to-peak,

correlation, and envelope detection. The peak-to-peak method consists of examining the

intensity-time signal and finding peaks or selected nth zero crossing, and determining the

difference in time. The correlation method uses a mathematical technique of stepping one

section of signal waveform containing a peak over another section containing a peak, and

finding the maximum overlap in frequency space. The envelope detection method

consists of using the Hilbert transform to obtain a signal envelope, and finding time

difference between amplitude peaks in the envelope [67]. All three methods were applied

to the collected data, and it was determined that the envelope detection method proved

the most reliable across all of the collected data.

Calculation of time of flight was performed using an envelope detection

calculation using a custom script written in MATLAB. This technique applies a Hilbert

transform (magnitude of Fourier transform) to obtain a signal amplitude envelope,

allowing more accurate calculation in this case than simple peak-to-peak or nth zero

crossing time measurements [68]. Due to the broad nature of the primary peak in the

signals, times of flight were determined between secondary and tertiary peaks for the

majority of samples when possible. However, some samples had high attenuation and

primary to secondary peak times had to be used.

The envelope detection algorithm used in the MATLAB code consisted of user

selecting regions within the signal waveform that contained the two peaks of interest.

User interaction was required in order to filter the effect of wall effect and other false

peak echoes in indefinite or non-ideal type signal waveforms. Once selected, fast Fourier

transform (FFT) and inverse FFT were applied to the selected regions, and the magnitude

(amplitude) of the signal in phase space was calculated for each discrete time step. Peaks

in the magnitude curves within each selected region were found and subtracted to give

the time of flight in microseconds (µs). Plots of the amplitude curves were displayed to

user to allow verification of proper peak selection.

The script written for analysis using MATLAB is included in Appendix C.

Typical envelope detection plots with calculated values are shown in Figure 11 for both

33

longitudinal and shear wave analysis: longitudinal was normally calculated from

secondary and tertiary peaks, while shear was most often calculated from primary to

secondary peaks.

Figure 11: Envelope detection time of flight plots for longitudinal (top) and

shear (bottom) wave ultrasonic analysis.

34

3.5.2.1 Error analysis The sources of errors in this analysis include uncertainty in the length

measurement and uncertainty in the time of flight calculation. The uncertainty for sample

test length measurements was estimated based on the geometry of the sample. All

samples had one flat, polished face and one back face that was highly curved, slightly

curved, or flat. By several caliper measurements, it was determined that highly curved

surfaces had an uncertainty of ~0.08 mm; slightly curved back surfaces had an

uncertainty of ~0.05 mm; and flat back surfaces had an uncertainty of ~0.04 mm. The

relative magnitude of these errors varied with the overall length of each test piece.

The other source of error, uncertainty in time of flight calculation, was influenced

by many factors including quality of the signal, presence of false echoes from grazing

incidence on side walls, flatness of the back surface, etc. A blanket error estimate of

±0.08 µs was applied to time of flight measurements. This was higher than observed

variations from multiple measurements on some samples, but is approximately the

maximum estimated error from the time of flight calculations. The primary contributor to

this error was indistinct localized envelope peaks: the clarity of the peaks varied by

sample with the quality of the output signal. Errors were calculated for individual

samples by standard error propagation rules, and are presented with the results.

35

3.6 Hardness Testing

Bulk mechanical property measurements were desired from the matched

microgravity and ground-based sintered samples. However, the small size of the samples

precluded traditional tensile testing. Hardness testing was selected as a useful attainable

measure for this study. After considering several available methodologies such as

nanohardness and other instrumented indentation techniques, a relatively high load

Vickers indention test was selected. The tip scale for nanohardness was too fine to look at

bulk properties, and Rockwell type techniques were considered to be too coarse.

3.6.1 Initial Testing: Determination of Test Procedures Initial hardness testing was performed on a sample of 88W-7:3 Ni:Fe sintered in

hydrogen under normal gravity for 10 minutes at 1485°C and quenched. This sample was

fabricated as part of another research project not mentioned in this work and was not part

of the gravitational effects project. It was selected because it was made from similar

starting powders and had a similar grain size to the 1 minute sintered samples. Initial

testing was performed using a Leco M-400-H microhardness tester equipped with a VL-

101 video line measurement system and a standard Vickers indenter. This indenter can

apply 10 to 1000 gram force (g-f) loading. The primary goals of this initial testing were

to determine a method of avoiding mount compression effects, identify an optimum load,

and to gauge the precision of the measurement.

While mounting the specimen in a plastic greatly facilitates both fixturing during

hardness indentation and subsequent microscopic examination, mounting introduces

another material into the system and could potentially produce inaccurate results due to

compression of the plastic. The initial specimen mounting method was to grind flat the

test face and the back face. The test face was hand polished using polycrystalline

diamond to a 3 µm finish using the same equipment and similar polishing procedures to

those in section 3.3. This unmounted set of data was the control for examining alternate

mounting methods. Multiple loads of 100, 300, 500, and 1000 g-f were applied to the

unmounted specimen, and the results recorded. The specimen was then hot compression

36

mounted in mineral-filled diallyl phthalate, resulting in a mount thickness of ~1cm. The

mounted specimen was repolished and retested at the same loads as when unmounted.

It was found that while unmounted, resulting Vickers hardness was independent

of applied loads from 100 to 1000 g-f. However, Vickers hardness showed a strong

inverse relation to indenter force when mounted in diallyl phthalate. Mounting in diallyl

phthalate was found to be inappropriate for this testing. However, as mounting greatly

accelerates both testing and post-analysis, further attempts were made to develop a

mounting method for this testing.

It was also found that a 1000 g-f (1 kg-f) load produced diagonals of ~73µm in

length. Preliminary grain size on the longest sintered samples (600 min) indicated a

maximum grain size of ~100µm. To avoid phase induced local effects on hardness, an

indent size of at least 2.5 times the maximum grain size of all samples was desired,

indicating that higher forces than 1 kg-f were needed.

To meet this load requirement, further testing was performed using a Leco V-100-

C1 indenter, capable of Vickers or Knoop indentation with 0.3-20 kg-f loading set to 15

sec duration, applied by automatic mechanism. This indenter has a movable stage

equipped with X and Y translation by means of micrometers graded in 25 µm (0.001

inch) increments. The V-100-C1 indenter was equipped with a calibrated optical reticle

for measuring indent sizes and calculating Vickers hardness. Calibration of the optical

reticle was performed using an optical scale. A second set of initial tests was performed

on this instrument using a Vickers calibration block certified at 734 HV at 0.3 kg-f load,

which indicated that the instrument was operating correctly.

After verifying correct machine operation with the calibration block at its rated

300 g-f load, exploratory testing was done on the calibration block using loads of 2 to 10

kg-f. Two of the unmounted halves of the replicate samples (samples 31S and 41S, see

Table 4) were selected, and polished flat on both faces. The test face of each sample was

polished to a 3 µm diamond finish. Two thin 38 mm diameter wafers of hot compression

mount transparent acrylic (Struers Specifast) were formed, with thicknesses of 3.8 mm

and 12.2 mm. Vickers indentation tests were performed on samples 31S (78WNiFe,

180m, microgravity) and 41S (78WNiFe, 600m, microgravity) at loads of 2, 5, and 10

37

kg-f; these samples were tested both flat on the stage plate, and placed on top of the

acrylic discs to examine potential mounting effects.

On sample 41S, which had the largest grain size, indents at 10 kg-f were found to

have a diagonal of ~275 µm, substantially higher than the grain size. Five indents at 10

kg-f were made on samples 31S and 41S both unmounted and placed atop the acrylic

wafers of both thicknesses. The results of theses 5 Vickers tests from the mounted and

unmounted condition were compared using a Student’s T-test (see Appendix D) and were

found to be statistically identical, indicating that the use of thin acrylic to mount the

specimens would not adversely affect the results.

3.6.2 Hardness Specimen Preparation and Testing The final hardness testing preparation procedure was to dismount previously

mounted samples by means of judicious application of hammer and chisel, plane the

samples using 600 grit sandpaper (if necessary), then hot compression mount the samples

in transparent acrylic at 180°C and 25MPa for 7m applied heat and pressure, cooling

slowly to avoid shrinkage effects. The amount of acrylic below the test face was 2-4 mm

in depth. After mounting, all edges were broken on 600 grit sandpaper to ensure a planar

bottom surface. The Leco V-100-C1 indenter and a schematic of the prepared sample are

shown in Figure 12.

Recommended practice for Vickers hardness testing [69] is to apply indents

spaced at least 2.5 times the diagonal length from edge to edge. Edge-to-edge spacing

was achieved by using the stage micrometer control, spacing three full turns between

each indent guaranteed at least 3 times the diagonal length between indents. Indents were

applied starting at one arbitrary “corner” of the sample, marked with permanent marker

for later identification, and proceeding in a boustrophedic manner until a regular grid

pattern of 12 to 30 indents were applied, depending on sample size and geometry.

Regions with unusual microstructure (one-phase regions, porous regions, abnormal

grains, etc) were not avoided but were tagged during analysis for later sorting, along with

relative position and index of each indent.

38

Figure 12: Leco V-100-C1 indenter used in hardness testing. Left: photograph

of indenter. Right: schematic view of sample in acrylic mount.

3.6.3 Hardness Indent Analysis Hardness measurements were made using the eyepiece reticle at the time of

analysis to give immediate feedback on testing results. However, it was determined that

the eyepieces available at time of testing were not of the ideal magnification to yield high

accuracy on results-in many cases, the indents were larger than could fit in the field of

view of the eyepiece, and estimates had to be made. To ensure consistency, and to reduce

operator fatigue effects on the data, tested samples were then imaged and analyzed using

the Clemex Vision system used for quantitative microscopy analysis. This analysis also

gives a quantitative measure of the relative position of the indents within each sample,

allowing positional hardness variations to be mapped.

For reference, a mosaic image at 50X apparent magnification was taken.

Positional tracking was performed by setting a zero point at the arbitrary pattern start

(upper left), then using the coordinate tracking stage to measure the centroid of dark area

(indent). Vickers hardness was generated from image analysis by thresholding to capture

2-4 mm

39

the indent, and automatically finding the two longest ferrets at least 45° apart. These

images were taken at 200X apparent magnification, and removal of pores or other dark

interfering features was done by manually editing the bitplanes prior to hardness analysis.

A typical mosaic image and actual indent image are shown in Figure 13. If any unusual

or interesting features were spotted during image capture, further images were taken for

record.

Vickers hardness was calculated using the following equation [69]:

dPHV 4.1854

= Equation 9

where HV is Vickers hardness in MPa, P is the applied load in grams force, and d is the

average diagonal length in millimeters. The analysis script used in Clemex Vision PE is

included in Appendix B. Measured positional data and calculated hardness data was

saved as an Excel spreadsheet and later imported into a MS Access relational database for

correlation with other measured factors.

40

Figure 13: Images used for Vickers hardness analysis. Top: mosaic low-mag

image used for reference. Arrow at upper left indicates arbitrary zero point.Bottom: image of indent used for image analysis of diagonal lengths of hardness indents.

41

Chapter 4: Results

4.1 Data Presentation and Distribution Basis

Numerical data are presented graphically here in several formats. Most graphs use

common conventions, however for viewing distributions of raw data the box plot format

was chosen for its capability of showing both central tendency and distribution width

without assuming a distribution curve such as Gaussian normal. As generation of box

plots can vary from study to study, an explanation of the symbols and interpretation used

in the box plots in this study is given in Figure 14. Details on other figure types will be

listed in the caption or in the referring text if necessary.

All calculated averages, statistics, and graphs of distributed data are calculated on

a number (population) basis unless otherwise stated.

Figure 14: Box plot symbol definitions used on distributed raw data plots.

**

Outliers

75% PercentileThird quartile (Q3)

Median

25% PercentileFirst quartile (Q1)

WhiskersUpper and lower limits:

Q(1,3)±1.5*(Q3-Q1)

Box Width Proportional to Number of Data

Points

42

4.2 General Observations

4.2.1 Macro Observations From examining the mosaic images shown in Figure 15 through Figure 17, some

general qualitative observations were made on the samples. At the 1 min sintering time,

shape distortion was minimal in both ground (1G) and microgravity (µG), with distortion

increasing particularly for microgravity as sintering time increased. Liquid-solid

separation occurred in both samples; for ground-based samples, liquid rose to the top as

expected. In microgravity samples, liquid pools devoid of tungsten particles were

dispersed through the sample. Liquid segregation was very slight in the 1m samples, and

substantial liquid segregation developed only after at least 15 min at sintering

temperature of 1500°C in both ground and space sintered samples.

Pore clusters were seen to develop in microgravity at sintering times above 15m,

while porosity was less obvious in most ground based samples. One exception to this was

the 45 min sintering time. Both the ground and microgravity sintered samples showed

large internal macropores. The size of the macropores may indicate potential

contamination of the raw material, although randomization of the samples during

preparation should have prevented the formation of the same defect in both ground and

microgravity sintered 45 min samples.

43

Figure 15: Mosaic images of 1 min (top row) and 15 min (bottom row)

78WNiFe sintered samples. Left column: 1 gravity. Right column: microgravity.

44



45



46

Figure 18: Mosaic images of sintered 35WNiFe (top) and 93WNiFe (bottom).

Left column: 1 gravity. Right column: microgravity.

For the 78WNiFe samples, differences due to the presence or absence of gravity were significant. While porosity developed in both Earth gravity and microgravity, porosity was generally more pronounced and evenly distributed in the microgravity samples. Tungsten-liquid separation also occurred in both gravitational conditions, although low-solid regions were found at the top of 1G samples and distributed through the microgravity samples. Differences were more pronounced in the lowest solid content sample (35WNiFe), with all tungsten grains falling to the bottom of the sample during sintering, leaving the sample mostly liquid. Microgravity sintered 35WNiFe had globally distributed but locally clustered tungsten grains. No obvious phase separation was noted

47

in 93WNiFe, although microgravity sintered 93WNiFe showed some central clustering of liquid pools and pores.

4.2.2 General Features

Several features were common to nearly all samples. The majority of all

78WNiFe samples were a typical connected grain LPS microstructure. Localized

exceptions were rich in either liquid phase, porosity, or both. Representative images of

general microstructure and these typical features are shown in Figure 19. Large amounts

of porosity were often associated with liquid-rich regions, but not always. Pores were

bounded by both solid and liquid.

(a) (b)

(c) (d)

Figure 19: Representative microstructures of 78WNiFe. (a) Typical structure. (b) Porosity-rich structure. (c) Liquid-rich structure. (d) Porosity and liquid-rich, also showing varying colors given to porosity by sputtering.

48

4.2.3 Liquid Phase Microstructures The samples of 35WNiFe sintered for 600 minutes in 1G and microgravity were

etched to look at differences in the liquid phase caused by gravity. Images of the etched

samples are shown in Figure 20. The 1G sintered sample showed a dramatic response to

etching, while the microgravity sintered 35WNiFe sample showed almost no response to

etching. Small, isolated areas on the microgravity sintered sample did show some signs of

attack, and were confirmed by higher magnification optical microscopy to be similar to

the etched regions in the 1G sample.

After etching, the 1G sample was taken for examination on a Philips XL30 SEM

equipped with EDAX Genesis EDS. The region that etched more heavily in the 1G

sintered 35WNiFe sample was full of tungsten-rich lamellae and precipitates; the regions

that did not etch showed a smooth feature, although tungsten was also detected by EDS.

SEM images are shown in Figure 21 and Figure 22.

49

Figure 20: Mosaic images of 35WNiFe sintered in 1G (top) and microgravity

(bottom). Both samples were etched prior to image capture.

50

Figure 21: Backscatter SEM images of 35WNiFe sintered in 1G for 600 min.

Left: lower part of sample, showing tungsten grains. Right: upper part of sample showing etched microstructure.

Figure 22: Backscatter SEM images of upper region of 35WNiFe sample

sintered in 1G for 600 min, showing microstructural features. Left: lamellar structure correlating to heavily etched region. Right: border of heavily etched region showing both precipitates and lamellae.

51

4.3 Quantitative Microscopy Results Field and objects counts for quantitative microscopy data are given in Table 8,

along with sample codes, compositions, sintering times, and apparent magnifications

used. Phase fraction and contiguity are field measurements, giving one overall value per

field. Grain size is an object measurement, with one measurement per grain.

Table 8: Summary of magnifications, counts, plane coverage and sintering conditions for quantitative microscopy samples.

Sample Code

Composition (wt. %)

Sinter Time (min)

Gravity Apparent Mag

Field Count

Grain Count

Sample Plane

Coverage

61G 78W-15.4Ni-6.6Fe 1 1G 500X 70 6766 4.6%

111R 78W-15.4Ni-6.6Fe 1 µG 500X 40 5382 2.4%

24-66 78W-15.4Ni-6.6Fe 1 1G 500X † † †

39-68 78W-15.4Ni-6.6Fe 1 µG 500X † † †

11S 78W-15.4Ni-6.6Fe 1 µG 500X † † †

9-64 78W-15.4Ni-6.6Fe 15 1G 200X 20 7670 6.5%

44-69 78W-15.4Ni-6.6Fe 15 µG 200X 15 6167 10.0%

71G 78W-15.4Ni-6.6Fe 45 1G 200X 30 7389 10.6%

21S 78W-15.4Ni-6.6Fe 45 µG 500X 169 6374 10.4%

16-65 78W-15.4Ni-6.6Fe 120 1G 200X 70 7257 21.9%

45-70 78W-15.4Ni-6.6Fe 120 µG 200X 40 6377 11.0%

81G 78W-15.4Ni-6.6Fe 180 1G 200X 50 6066 21.0%

131R 78W-15.4Ni-6.6Fe 180 µG 100X 12 6449 23.4%

31S 78W-15.4Ni-6.6Fe 180 µG 200X † † †

91G 78W-15.4Ni-6.6Fe 600 1G 200X 130 6165 54.2%

141R 78W-15.4Ni-6.6Fe 600 µG 100X 40 8419 51.8%

41S 78W-15.4Ni-6.6Fe 600 µG 100X † † †

13-95 93W-4.9Ni-2.1Fe 120 1G 200X 50 7040 76.0%

49-100 93W-4.9Ni-2.1Fe 120 µG 200X 50 7308 16.0%

84G 35W-45.5Ni-19.5Fe 180 1G 200X 60 1205 25.0%

134R 35W-45.5Ni-19.5Fe 180 µG 200X 60 795 26.2%

† Not analyzed

52

4.3.1 Overall Quantitative Microscopy Results

4.3.1.1 Phase Fractions A numerical summary of field based data including phase fractions (tungsten,

liquid, and porosity) and tungsten grain contiguity is given in Table 9. Charts of porosity

fraction are shown in Figure 23. The large amount of porosity on the 45 min samples

obscures relative values between the other samples: the bottom plot in Figure 23 shows

the 1, 15, 120, 180, and 600 min data scaled appropriately. A combined chart of phase

fractions is shown in Figure 24 for comparative examination on the 78WNiFe samples.

Phase fraction results for compositional variations (35W, 78W, and 93W) are shown in

Figure 25. As phase fraction are point count methods (area percent), they can be directly

treated as a 3D measurement (volume fraction) [64].

Table 9: Numerical summary of phase fraction data for all samples. Values are overall average, ± numbers are one standard deviation observed between different fields.

Composition Sinter Time (min)

Gravity Tungsten vol. %

Liquid vol. %

Porosity vol. %

1G 58.0 ± 0.1 41.9 ± 0.1 0.03 ± 0.00 78W-15.4Ni-6.6Fe 1

µG 57.2 ± 0.2 42.8 ± 0.2 0.00 ± 0.00

1G 62.0 ± 1.1 38.2 ± 0.7 0.04 ± 0.00 78W-15.4Ni-6.6Fe 15

µG 56.8 ± 1.3 42.1 ± 1.0 1.20 ± 0.17

1G 47.4 ± 0.7 36.8 ± 0.5 15.8 ± 1.1 78W-15.4Ni-6.6Fe 45

µG 46.0 ± 0.1 37.0 ± 0.1 16.9 ± 0.2

1G 56.3 ± 0.4 42.9 ± 0.3 0.85 ± 0.04 78W-15.4Ni-6.6Fe 120

µG 55.1 ± 0.3 44.1 ± 0.2 0.85 ± 0.05

1G 53.9 ± 0.3 46.1 ± 0.3 0.01 ± 0.00 78W-15.4Ni-6.6Fe 180

µG 53.6 ± 1.5 46.2 ± 1.4 0.41 ± 0.10

1G 52.1 ± 0.2 46.9 ± 0.2 1.03 ± 0.07 78W-15.4Ni-6.6Fe 600

µG 56.2 ± 0.6 43.3 ± 0.5 0.70 ± 0.05

1G 94.0 ± 0.4 5.3 ± 0.2 0.53 ± 0.03 35W-45.5Ni-19.5Fe 180

µG 95.1 ± 0.4 4.8 ± 0.1 0.06 ± 0.00

1G 13.4 ± 0.1 86.8 ± 0.4 0.12 ± 0.00 93W-4.9Ni-2.1Fe 120

µG 14.4 ± 0.1 85.8 ± 0.2 0.14 ± 0.01

53

Gravity/Time Effects on Porosity: 78WNiFe

0%

5%

10%

15%

20%

1 15 45 120 180 600

Sintering Time (min)

Per

cent

Por

osity

GroundMicrogravity

Gravity/Time Effects on Porosity: 78WNiFe, No 45m Data

0.0%

0.2%

0.4%

0.6%

0.8%

1.0%

1.2%

1.4%

1.6%

1 15 120 180 600


Perc

ent P

oros

ity

GroundMicrogravity

Figure 23: Porosity (vapor) phase fraction results for all sintering time and

gravity conditions. Top: all times. Bottom: with 45 minute result removed to show relations on 1, 15, 45, 120, 180and 600 m samples. Bars are summary average, and error bars are one standard deviation across all analyzed fields. Percentages are volume percent.

54

Gravity/Time Effects on Phase Fractions: 78WNiFe

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1G µG 1G µG 1G µG 1G µG 1G µG 1G µG

1 1 15 15 45 45 120 120 180 180 600 600

Per

cent

Pha

se (

vol.

%)

Tungsten % Liquid % Porosity %

Gravity

Sinter Time(min)

Figure 24: Phase fraction comparative summary chart for all sintering times

and both gravitational conditions.

55

Tungsten Content Effects on Porosity

0.0%

0.1%

0.2%

0.3%

0.4%

0.5%

0.6%

0.7%

0.8%

0.9%

1.0%

180 180 120 120

35W 78W 78W 93W

Sintering Time (min) and Tungsten Content (wt.%)

Perc

ent P

oros

ity

GroundMicrogravity

Figure 25: Tungsten content and gravitational effects on porosity fractions for

35W, 78W, and 93W.

4.3.1.2 Contiguity Results Contiguity results are summarized numerically in Table 10. The data are

displayed graphically in Figure 26 for 78WNiFe and in Figure 27 for 35WNiFe and

93WNiFe. The contiguity results in the table are overall averages across all fields. One

standard deviation of the field-to-field results for each sample was used as the spread of

the data. Statistical testing (see Appendix D) was performed to determine if there was a

significant difference between microgravity and Earth-gravity sintered samples for a

given composition or sintering time.

For 78WNiFe, the expected result was that contiguity would be higher in the 1G

sintered samples. However, this is not universally true, as results show that at 1 min, 180

min, and 600 min the microgravity samples have a higher contiguity. Statistical analysis

of the data showed very strong dissimilarities across most sintering times for 78WNiFe

except for the 15 minute sintered sample, which showed similarity (albeit with a low P-

value). The 93W sample showed no difference in contiguity between microgravity and

Earth gravity.

56

Table 10: Contiguity results for all samples including statistical similarity testing across sintering gravity conditions.


Gravity Contiguity %

05.0t Similarity

1G 10.1 ± 1.6 78W-15.4Ni-6.6Fe 1

µG 13.3 ± 2.0

Dissimilar P<0.001

1G 17.1 ± 4.5 78W-15.4Ni-6.6Fe 15

µG 14.8 ± 2.2

Similar P=0.060

1G 12.4 ± 2.1 78W-15.4Ni-6.6Fe 45

µG 7.4 ± 1.7

Dissimilar P<0.001

1G 12.1 ± 3.3 78W-15.4Ni-6.6Fe 120

µG 9.1 ± 1.2

Dissimilar P<0.001

1G 9.9 ± 2.1 78W-15.4Ni-6.6Fe 180

µG 17.0 ± 3.7

Dissimilar P<0.001

1G 9.9 ± 4.3 78W-15.4Ni-6.6Fe 600

µG 11.2 ± 1.6

Dissimilar P=0.008

1G 3.4 ± 2.0 35W-45.5Ni-19.5Fe 180

µG 2.5 ± 2.8

Similar P=0.083

1G 34.0 ± 2.8 93W-4.9Ni-2.1Fe 120

µG 33.8 ± 3.6

Similar P=0.726

57

Cont

igui

ty

Time (min)Gravity

60018012045151µG1GµG1GµG1GµG1GµG1GµG1G

0.25

0.20

0.15

0.10

0.05

0.00

Gravitational and Sintering Time Effects on Contiguity for 78WNiFe

Figure 26: Contiguity for ground and microgravity sintered 78WNiFe.

Cont

igui

ty

Comp (wt%)Gravity

93W-4.9Ni-2.1Fe35W-45.5Ni-19.5FeµG1GµG1G

0.4

0.3

0.2

0.1

0.0

Gravitational and Tungsten Content Effects on Contiguity

Figure 27: Contiguity for 35WNiFe and 93WNiFe in ground and microgravity

sintered conditions.

58

4.3.1.3 Grain Size Results Grain size numerical results are summarized in Table 11. Also reported in Table

11 are results of statistical testing on significance of similarities of grain size distributions

(see Appendix D). Across the sintering time groups, microgravity had a significant effect

on grain size distributions for all of the 78WNiFe samples and the low-tungsten

35WNiFe samples. Gravity had a less significant effect on grain size distribution in the

93WNiFe sample: the P-value of 0.1 indicates there is probably no significant difference

between grain size distributions for 93WNiFe sintered in microgravity or Earth gravity.

Individual plots of all 2D grain size distributions are included in Appendix E. Composite

data for 78WNiFe, 35WNiFe, and 93WNiFe are shown in Figure 28 as box plots.

For low-tungsten 35WNiFe alloys, grain sizes were remarkably different (Figure

30). Two distinct morphologies and sizes of particles were noted on examination: the

smaller grains responsible for the peak near 10µm in Figure 29 were found in the liquid

phase, consistent with shape of precipitates formed on cooling from liquid. The larger

tungsten grains were at the bottom of the sample, and are consistent with observed shapes

of other rounded grains in other samples. No tungsten precipitates were found near larger

grains. The microgravity sample showed the larger, rounded tungsten grain morphology:

the finer tungsten grains seen in the 1G sample were observed in isolated, infrequent

locations, mainly associated with grain boundaries in the NiFe matrix.

Differences between Earth-gravity and microgravity grain size values listed in

Table 11 are larger than the estimated experimental error for all values except for 180

minute sintered 78WNiFe and for the G10 value for 15 minute sintered 78WNiFe. The

fact that distributions tested as dissimilar but the numerical differences at the three points

chosen for comparison are within experimental error of each other suggests that the grain

size distributions at 180 minutes for 78WNiFe in the two different gravitational

conditions are actually similar.

59

Table 11: Numerical summary of 2D grain size distributions. G10, G50, and G90 are the circular equivalent diameter at the 10th, 50th (median), and 90th percentiles, respectively.

Composition Time (min)

Gravity Grain Count

G10 (µm)

G50 (µm)

G90 (µm)

205.0χ

Similarity

1G 6766 5.8 13.2 20.3 1

µG 5382 3.1 10.6 18.3

DissimilarP<0.001

1G 7670 8.4 17.8 27.6 15

µG 6167 8.6 16.9 24.8

DissimilarP<0.001

1G 7390 10.1 20.6 30.3 45

µG 6374 8.0 18.8 29.6

DissimilarP<0.001

1G 7257 14.1 29.5 44.9 120

µG 6377 12.0 26.7 40.5

DissimilarP<0.001

1G 6066 14.3 30.8 46.9 180

µG 6449 14.8 30.5 46.6

DissimilarP<0.001

1G 6165 19.2 46.4 72.2

78WNiFe

600

µG 8419 22.6 45.3 67.1

DissimilarP<0.001

1G 1205 3.9 7.0 27.8 35WNiFe 180

µG 795 10.8 27.6 43.3

DissimilarP<0.001

1G 7040 14.6 34.1 54.9 93WNiFe 120

µG 7308 13.7 32.7 53.5

Similar P=0.098

60

Circ

. Equ

iv. D

iam

eter

(µm

)

Time (m)Gravity

600180120451511GµG1GµG1GµG1GµG1GµG1GµG

120

100

80

60

40

20

0

Gravity and Sintering Time Effects on Grain Size (2D): 78WNiFe

Circ

ular

Equ

ival

ent

Dia

met

er (

µm)

W%, TimeGravity

93W, 120 min35W, 180 min1GµG1GµG

120

100

80

60

40

20

0

Gravity and Tungsten Content on Grain Size (2D)

Figure 28: Grain size distribution boxplots for 78WNiFe (top),

35WNiFe/93WNiFe (bottom).

61

Grain Size (2D) Histogram: 35WNiFe 1G 180m

0

50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80 90 100

Circular Equivalent Diameter (µm)

Freq

uenc

y Co

unt

0%

25%

50%

75%

100%

Cum

ulat

ive

%

Grain Size (2D) Histogram: 35WNiFe µG 180m

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100


Freq

uenc

y Co

unt

0%

25%

50%

75%

100%

Cum

ulat

ive

%

Figure 29: Grain size distribution (2D) for 35WNiFe 180 min sintering time.

Top: 1G. Bottom: microgravity.

62

4.3.2 Positional Variations

Investigation of gravity-induced anisotropy was performed by using the positional

data gathered simultaneously with all quantitative microscopy measurements through the

use of the automated sample stage. To see positional effects for samples with different

heights, fractional distance from either the bottom for ground sintered samples or an

arbitrary edge for microgravity was calculated, and separated into 8 bins. Data were

grouped after separation into these bins, and is presented here. Missing data in the charts

indicate that the random sample fields did not fall into the 12.5% arbitrary bin size; this is

most common for smaller samples or the largest size fields at the lowest used

magnification (100X).

4.3.2.1 Positional Effects on Phase Fractions While quantitative results for phase fractions showed little overall variation

between 1G and µG samples for 78WNiFe, positional variations (shown in Figure 30,

Figure 31, and Figure 32) were more distinct. As can be seen from the data, overall phase

fraction differences between 1G and µG are slight, but gravitational sedimentation of

tungsten in 1G samples creates a liquid-rich area at the top. Porosity also tends to rise in

1G samples, while it stays near center in µG samples.

Phase fraction data for low and high solid content samples are shown in Figure

33. The low-solid 35WNiFe sample distinctly shows the gravitationally induced settling

of the grains at the bottom of the sample: the remainder of the sample above the lower

portion is over 97% liquid. The microgravity sintered 35WNiFe sample showed a fairly

constant ratio of phases with respect to position, although tungsten content is higher away

from the top and bottom edges of the sample. The high-solids 93WNiFe sample showed

little variation in phase fraction, although variation was slightly higher in microgravity

than in Earth gravity.

63

Positional Effects on Phase Fractions78WNiFe 1G 1 min

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000

Relative Distance from Bottom

Phas

e Fr

actio

n

Tungsten LiquidPorosity Theor. W%

Positional Effects on Phase Fractions78WNiFe µG 1 min

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000

Relative Distance from Arbitrary Edge

Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n


Figure 30: Positional effects on phase fraction for 1-15m sintering times for ground-based (left) and microgravity (right) sintered 78WNiFe samples. Absent data in 15 minute samples is due to no random fields measured in that particular horizontal band of the sample.

64


0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n


Figure 31: Positional effects on phase fraction for 45-120m sintering times for ground-based (left) and microgravity (right) sintered 78WNiFe samples.

65


0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n


Positional Effects on Phase Fractions78WNiFe µG 600m

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n


Figure 32: Positional effects on phase fraction for 180-600m sintering times for ground-based (left) and microgravity (right) sintered 78WNiFe samples. Absent data in 180 minute samples is due to no random fields captured in that particular horizontal band of the sample.

66


0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n



0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.125 0.250 0.375 0.500 0.625 0.750 0.875 1.000


Phas

e Fr

actio

n


Figure 33: Positional effects on phase fraction for 35WNiFe (top) and 93WNiFe samples (bottom) for ground-based (left) and microgravity (right) sintered samples.

67

As phase fraction and contiguity are field based measurements, they were

calculated from the same fields. The number of fields (data points) represented by the

summary charts for both phase fraction and contiguity are given in Table 12.

Table 12: Number of data points in relative Y positions for phase fraction and contiguity.

Phase/Contiguity Count at Rel. Y Position Composition (wt. %)

Sintering Time (min.) Gravity 0.125 0.25 0.375 0.5 0.625 0.75 0.875 1.0

78W 1 µG 5 5 3 3 2 9 3 10

78W 1 1G 11 10 4 15 5 5 10 8

78W 15 µG 3 3 1 2 1 3 0 2

78W 15 1G 5 1 5 0 2 3 1 3

78W 45 µG 10 25 16 14 19 24 18 16

78W 45 1G 4 2 2 5 6 3 3 4

78W 120 1G 10 11 14 5 5 13 2 0

78W 120 µG 5 6 6 8 2 7 2 4

78W 180 µG 3 2 2 0 1 2 0 2

78W 180 1G 6 5 5 11 6 6 5 5

78W 600 µG 6 6 2 6 5 5 5 5

78W 600 1G 17 14 20 11 21 17 12 2

35W 180 µG 2 5 12 3 4 10 4 3

35W 180 1G 11 2 4 8 2 4 4 0

93W 120 1G 5 7 5 7 2 11 7 6

93W 120 µG 6 4 5 7 12 5 4 7

68

4.3.2.2 Positional Effects on Contiguity Contiguity also varied through the samples with position. Contiguity was

calculated similarly to the positional effects on phase fraction, dividing the sample

arbitrarily into 8 bins from bottom (lower part of sample during sintering) to the top.

Box plots of contiguity positional variation are shown in Figure 34, Figure 35,

and Figure 36 for 78WNiFe and in Figure 37 for 35 and 93WNiFe. These show

distribution of contiguity across multiple values within each positional band. For the time

study data on 78WNiFe, the general trend was that ground-based samples had higher

contiguity than microgravity samples for sintering times over 1 minute. The contiguity in

ground-based samples was also higher near the bottom of the samples, decreasing

towards the top. The low-solid sample of 35WNiFe sintered under earth gravitational

conditions showed higher contiguity than microgravity at the bottom of the sample, but

just above the settled grain region showed nearly zero contiguity. Above that region,

contiguity increased mainly due to the clustering of precipitation-formed tungsten in

chains along NiFe grain boundaries.

The high-solid sample 93WNiFe showed no significant positional variations in

contiguity, although the spread of measured contiguity increased towards the top of the

1G sample, and had a drop near the top of the sample in the µG sample.

69

Relative Y Position

Grai

n Co

ntig

uity

1.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.001.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.00

Ground Microgravity

78WNiFe Positional Contiguity - 1 min

Relative Y Position

Grai

n Co

ntig

uity

1.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.001.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.00

Ground Microgravity


Figure 34: Positional and gravitational effects on contiguity, 1 and 15 min

sintering time 78WNiFe.

70

Relative Y Position

Grai

n Co

ntig

uity

1.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.001.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.00

Ground Microgravity


Relative Y Position

Grai

n Co

ntig

uity

1.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.001.000.750.500.25

Ground Microgravity




71

Relative Y Position

Grai

n Co

ntig

uity

1.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.001.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.00

Ground Microgravity


Relative Y Position

Grai

n Co

ntig

uity

1.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.001.000.750.500.25

0.25

0.20

0.15

0.10

0.05

0.00

Ground Microgravity




72

Relative Y Position

Grai

n Co

ntig

uity

1.000.750.500.25

0.10

0.08

0.06

0.04

0.02

0.00

1.000.750.500.25

Ground Microgravity


Relative Y Position

Grai

n Co

ntig

uity

1.000.750.500.25

0.425

0.400

0.375

0.350

0.325

0.300

0.275

0.250

1.000.750.500.25

Ground Microgravity

93W Positional Contiguity - 120 min

Figure 37: Positional and gravitational effects on contiguity for 35WNiFe

(top) and 93WNiFe (bottom).

73

4.3.2.3 Positional Effects on Grain Size Distributions Grain size showed some positional trends as well as the expected growth with

increasing sintering time. Box plots showing positional effects on grain size distributions

are shown in Figure 38, Figure 39, and Figure 40 for the 78WNiFe samples and in Figure

41 for 35WNiFe and 93WNiFe. The number of grains represented by each box in the

charts is summarized in Table 13, and box width is proportional to the countof data

points.

Ground sintered samples in general show a larger grain size for all sintering times,

except in regions at the top of the sample where the grain size decreases to below that of

the microgravity samples due to liquid segregation effects. The segregation effect is most

pronounced in the 35WNiFe sample, where outside the sedimented grain zone, tungsten

grains are small precipitates rather than larger particles. Gradients in grain size were

noted in 78WNiFe for 45, 120, and 600 min sintering times. The 93WNiFe sample

showed no significant positional variation in grain size distributions in either

microgravity or Earth gravity.

74

Table 13: Number of grains per relative Y position in positional grain size data.

Grain Count at Rel. Y Position Composition (wt. %)

Sintering Time (min.) Gravity 0.125 0.25 0.375 0.5 0.625 0.75 0.875 1.0

78W 1 µG 52 150 154 49 104 148 116 22

78W 1 1G 380 184 46 177 107 114 122 75

78W 15 µG 553 683 409 431 265 1309 512 1220

78W 15 1G 1054 970 593 1489 534 633 831 662

78W 45 µG 1164 1469 568 649 943 841 0 533

78W 45 1G 1749 645 1747 493 814 1258 408 556

78W 120 1G 688 975 519 645 743 1209 899 696

78W 120 µG 1327 1341 2595 2063 40 4 0 19

78W 180 µG 1020 1241 1003 1159 672 1057 1069 36

78W 180 1G 691 1038 1135 1298 413 1108 253 441

78W 600 µG 1856 1202 1078 481 281 541 73 937

78W 600 1G 717 543 654 1269 883 679 733 588

35W 180 µG 813 1215 672 1728 1347 1391 983 270

35W 180 1G 860 716 923 1147 1154 801 468 96

93W 120 1G 647 936 830 883 302 1678 1060 704

93W 120 µG 854 567 696 1074 1762 712 647 996

75

Relative Y Position

Circ

. Equ

iv. D

iam

eter

(µm

)

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

35

30

25

20

15

10

5

0

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

Ground Microgravity

78WNiFe Grain Size Positional Trends - 1 min

Relative Y Position

Circ

. Equ

iv. D

iam

eter

(µm

)

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

50

40

30

20

10

0

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

Ground Microgravity


Figure 38: Positional effects on grain size distributions in 1 and 15 min

sintered 78WNiFe 1G and µG samples.

76

Relative Y Position

Circ

. Equ

iv. D

iam

eter

(µm

)

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

50

40

30

20

10

0

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

Ground Microgravity


Relative Y Position

Circ

. Equ

iv. D

iam

eter

(µm

)

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

90

80

70

60

50

40

30

20

10

0

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

Ground Microgravity




77

Relative Y Position

Circ

. Equ

iv. D

iam

eter

(µm

)

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

80

70

60

50

40

30

20

10

0

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

Ground Microgravity


Relative Y Position

Circ

. Equ

iv. D

iam

eter

(µm

)

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

120

100

80

60

40

20

0

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

Ground Microgravity




78

Relative Y Position

Circ

. Equ

iv. D

iam

eter

(µm

)

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

120

100

80

60

40

20

0

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

Ground Microgravity


Relative Y Position

Circ

. Equ

iv. D

iam

eter

(µm

)

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

90

80

70

60

50

40

30

20

10

0

1.00

00.

875

0.75

00.

625

0.50

00.

375

0.25

00.

125

Ground Microgravity


Figure 41: Positional effects on grain size distributions for 35WNiFe (top) and

93WNiFe (bottom).

79

4.4 Mechanical Property Results

4.4.1 Ultrasonic Velocity Results The calculated ultrasonic velocities are given in Table 14 for longitudinal wave

testing and in Table 15 for shear wave testing. Velocity values are averages plus or minus

the estimate of experimental error. For most shear wave testing and some longitudinal

wave testing replicates were not possible due to poor quality of signal. Some shear wave

had signal quality poor enough to preclude analysis of the data. As with hardness,

ultrasonic testing was impossible on the 45 min 78WNiFe samples due to macroporosity.

Table 14: Longitudinal velocities from ultrasonic testing. Velocity values are plus or minus experimental error.

Sample Code

Composition (wt.%)

Sinter Time (min)

Gravity Test Type Longitudinal

Velocity (mm/µs)

24-66 78W-15.4Ni-6.6Fe 1 1G Longitudinal 5.15 ± 0.30

61G 78W-15.4Ni-6.6Fe 1 1G Longitudinal 5.53 ± 0.35

111R 78W-15.4Ni-6.6Fe 1 µG Longitudinal 5.57 ± 0.45

11S 78W-15.4Ni-6.6Fe 1 µG Longitudinal 4.57 ± 0.24

39-68 78W-15.4Ni-6.6Fe 1 µG Longitudinal 5.18 ± 0.35















80

Table 15: Shear velocities from ultrasonic testing. A dash indicates no velocity calculation was possible due to poor signal. Plus or minus values are experimental error.

Sample Code

Composition (wt.%)

Sinter Time (min)

Gravity Test Type

Shear Velocity (mm/µs)

24-66 78W-15.4Ni-6.6Fe 1 1G Shear 2.62 ± 0.10

61G 78W-15.4Ni-6.6Fe 1 1G Shear 2.86 ± 0.0.11

39-68 78W-15.4Ni-6.6Fe 1 µG Shear -

111R 78W-15.4Ni-6.6Fe 1 µG Shear 2.55 ± 0.12

11S 78W-15.4Ni-6.6Fe 1 µG Shear 2.46 ± 0.08

9-64 78W-15.4Ni-6.6Fe 15 1G Shear 2.85 ± 0.15

44-69 78W-15.4Ni-6.6Fe 15 µG Shear -

16-65 78W-15.4Ni-6.6Fe 120 1G Shear 3.16 ± 0.23

45-70 78W-15.4Ni-6.6Fe 120 µG Shear 2.74 ± 0.12

81G 78W-15.4Ni-6.6Fe 180 1G Shear -

31S 78W-15.4Ni-6.6Fe 180 µG Shear 2.55 ± 0.09

131R 78W-15.4Ni-6.6Fe 180 µG Shear 2.25 ± 0.09

91G 78W-15.4Ni-6.6Fe 600 1G Shear 2.71 ± 0.10

141R 78W-15.4Ni-6.6Fe 600 µG Shear 2.57 ± 0.11

41S 78W-15.4Ni-6.6Fe 600 µG Shear 2.69 ± 0.10

84G 35W-45.5Ni-19.5Fe 180 1G Shear 2.67 ± 0.12

134R 35W-45.5Ni-19.5Fe 180 µG Shear -

13-95 93W-4.9Ni-2.1Fe 120 1G Shear 3.11 ± 0.10

49-100 93W-4.9Ni-2.1Fe 120 µG Shear 2.65 ± 0.10

81

Insufficient replicate test data were available for statistical testing of differences

between gravitational conditions. This limitation was imposed by the small sample size,

which restricted all testing to be in a single location on the test piece, limiting the utility

of replicate testing on the same sample. Replication by multiple sample was available for

at 1 minute (2 ground, 3 microgravity), 180 min (1 ground, 2 microgravity), and 600

minute (1 ground, 2 microgravity). Individual sample results are given in the tables

above; averages within samples are replicate tests on the same sample.

Plots of the effects of sintering time and gravity on longitudinal and shear wave

velocities are shown in Figure 42 for 78WNiFe, showing sintering time and gravitational

effects, and Figure 43 for 35WNiFe and 93WNiFe, showing the effects of tungsten

content and gravity. For 78WNiFe samples sintered in Earth gravity showed higher

velocities at all sintering times, although estimated experimental error is larger than the

observed differences. For 35WNiFe and 93WNiFe, differences were again within

experimental between gravitational conditions.

82

Gravity/Time Effects on Ultrasonic Longitudinal Velocity78W-15.4Ni-6.6Fe

4.0

4.5

5.0

5.5

6.0

6.5

1 15 120 180 600


Long

itudi

nal V

eloc

ity (m

m/µ

s)

GroundMicrogravity

Gravity/Time Effects on Ultrasonic Shear Velocity78W-15.4Ni-6.6Fe

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

1 15 120 180 600


Shea

r Ve

loci

ty (m

m/µ

s)

GroundMicrogravity

Figure 42: Ultrasonic velocities for 78WNiFe, showing effect of gravity and

sintering time. Top: longitudinal wave data. Bottom: shear wave data. Bars are estimated experimental error.

83

Gravity andTungsten Fraction Effects on Ultrasonic Longitudinal Velocity

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

35W-45.5Ni-19.5Fe 93W-4.9Ni-2.1Fe

Composition (wt.%)

Long

itudi

nal V

eloc

ity (m

m/µ

s)

GroundMicrogravity

Gravity andTungsten Fraction Effects on Ultrasonic Shear Velocity

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

35W-45.5Ni-19.5Fe 93W-4.9Ni-2.1Fe

Composition (wt.%)

Shea

r Ve

loci

ty (m

m/µ

s)

GroundMicrogravity

Figure 43: Ultrasonic velocities for 35WNiFe and 93WNiFe, showing effect of

gravity and tungsten content. Top: longitudinal wave data. Bottom: shear wave data. Bars are estimated experimental error.

84

4.4.2 Hardness Results

4.4.2.1 Overall Hardness Results

Hardness data showed considerable scatter, as shown in Table 16 and Figure 44.

Due to the macropores present in the 45 min sintered 78WNiFe, hardness testing was not

performed. Replicates were available for 1 minute sintered samples at both 1G and µG

conditions: these are included in the averages and standard deviations in the table.

Statistical t-tests on difference of means was applied to determine if there were

significant differences between the hardness of microgravity sintered samples and Earth

sintered samples (see Appendix D). For 78WNiFe, sintering times of 1, 180, and 600

minutes had no significant difference in mechanical properties: however, intermediate

sintering times of 15 and 120 minutes showed significant differences in mechanical

properties between the two gravitational conditions. The 35WNiFe and 93WNiFe solid

content samples also showed statistically significant differences between gravitational

conditions, although the 93WNiFe is just barely significant at the 95% confidence level.

During hardness testing observations were made on the locations of the indent and

the microstructure in the area of the indent. Inhomogeneities in the microstructure were

not avoided, but were identified with the indent index for later use. The inhomogeneities

for 78WNiFe were either liquid-rich areas or liquid pools, or porous areas with pores

visible either on the surface or under the surface after indentation. Typical regions for

78WNiFe and 93WNiFe are regions without porosity and without excessive liquid.

Obviously the 35WNiFe typical region is not liquid-free: in this case, typical regions

were any non-porous region.

Filtering the data to typical regions only on all samples resulted in reduced scatter

in the hardness values, summarized in Table 16. Statistical testing was also performed to

determine if a significant difference was caused by gravity. Using typical data only,

gravity has a significant effect on hardness of 78WNiFe at sintering times of 15, 120, and

600 minutes. Gravity also had a significant effect on 93WNiFe and 35WNiFe.

In all cases, using filtered data or all data, when gravity had a significant effect

the 1G sintered samples had a higher hardness than microgravity sintered samples. This is

interesting in the low-solids 35WNiFe sample, where the settling of the grains causes the

sample to be pure solidified matrix for the top 80% of visible sample area, yet the overall

85

hardness of the 1G sample is higher than the hardness of the microgravity sample, which

has a distributed tungsten grain microstructure.

Table 16: Hardness data numerical summary, including results of statistical similarity tests.


Gravity HV10kg-f All

Regions

HV10kg-f Typical Regions

Only

05.0t

Similarity All Regions

05.0t Similarity

Typical Regions Only

1G 266 ± 17 270 ± 11 78W-15.4Ni-6.6Fe

1

µG 270 ± 22 273 ± 13

Similar P=0.870

Similar P=0.286

1G 268 ± 17 272 ± 7 78W-15.4Ni-6.6Fe

15

µG 251 ± 16 257 ± 7

Dissimilar P=0.004

Dissimilar P<0.001

1G 266 ± 10 267 ± 11 78W-15.4Ni-6.6Fe

120

µG 231 ± 25 242 ± 10

Dissimilar P<0.001

Dissimilar P<0.001

1G 247 ± 59 277 ± 15 78W-15.4Ni-6.6Fe

180

µG 266 ± 15 269 ± 10

Similar P=0.211

Similar P=0.116

1G 237 ± 52 264 ± 16 78W-15.4Ni-6.6Fe

600

µG 231 ± 30 242 ± 10

Similar P=0.647

Dissimilar P<0.001

1G 233 ± 51 238 ± 47 35W-45.5Ni-19.5Fe

180

µG 158 ± 15 154 ± 10

Dissimilar P<0.001

Dissimilar P=0.001

1G 294 ± 7 294 ± 7 93W-4.9Ni-2.1Fe

120

µG 289 ± 6 289 ± 6

Dissimilar P=0.04

Dissimilar P=0.041

86

Gravity/Time Effects on Vickers Hardness78W-15.4Ni-6.6Fe, with liquid/porous regions

175

200

225

250

275

300

325

1 15 120 180 600


10kg

f Vic

kers

Har

dnes

s (M

Pa)

GroundMicrogravity

Gravity/Time Effects on Vickers Hardness78W-15.4Ni-6.6Fe, no liquid/porous regions

175

200

225

250

275

300

325

1 15 120 180 600


10kg

f Vic

kers

Har

dnes

s (M

Pa)

GroundMicrogravity

Figure 44: Vickers hardness of 78WNiFe sintered in 1G and 0G conditions.

Column is average and bars are one standard deviation of replicate tests on each sample. Top: including all data points. Bottom: excluding data points observed to be in atypical (porous or high-liquid) regions.

87

Tungsten Content Effects on Vickers Hardnessincluding porous/atypical regions

† 180m sinter‡ 120m sinter

100

125

150

175

200

225

250

275

300

325

35W† 78W† 78W‡ 93W‡

Tungsten Content (wt%), balance 7:3 Ni:Fe

10kg

f Vic

kers

Har

dnes

s (M

Pa)

GroundMicrogravity

Tungsten Content Effects on Vickers Hardnessexcluding porous/atypical regions

† 180m sinter‡ 120m sinter

100

125

150

175

200

225

250

275

300

325

35W† 78W† 78W† 93W‡

Tungsten Content (wt%), balance 7:3 Ni:Fe

10kg

f Vic

kers

Har

dnes

s (M

Pa)

GroundMicrogravity

Figure 45: Tungsten content effects on hardness. Top: including porous or

liquid-rich regions. Bottom: excluding porous or liquid-rich regions.

88

4.4.2.2 Positional Variations

Positional variations on hardness were determined similarly to positional effects

on microstructural analysis, using the X and Y coordinates relative to upper left generated

simultaneously by the analysis routine. As the number of measurements was lower than

the quantitative microcopy analysis, only four bins were used for positional divisions.

Plots of variation of measured hardness with position including data from all regions are

shown in Figure 46 through Figure 49.

For 78WNiFe 1G sintered samples, the general trend was for hardness to be

higher in the bottom portions of the sample, tracking with the increased contiguity

(Figure 34 through Figure 36) and solid fraction (Figure 30 and Figure 31) also observed

in the bottom of the sample. No significant variation in hardness was observed at the

shortest sintering time of 1 minute for either ground-based or microgravity sintered

samples.

89

Relative Y Position

HV

_10k

gf

1.000.750.500.25

320

300

280

260

240

220

200

180

1601.000.750.500.25

µG 1G

78WNiFe Hardness Positional Trends - 1 min

Relative Y Position

HV

_10k

gf

1.000.750.500.25

290

280

270

260

250

240

230

220

210

200

1.000.750.500.25

µG 1G


Figure 46: Positional variations in hardness for 1 and 15 min sintered

78WNiFe.

90

Relative Y Position

HV

_10k

gf

1.000.750.500.25

300

280

260

240

220

200

180

1601.000.750.500.25

µG 1G


Relative Y Position

HV

_10k

gf

1.000.750.500.25

300

250

200

150

100

1.000.750.500.25

µG 1G


Figure 47: Positional variations in hardness for 120 and 180 min sintered

78WNiFe.

91

Relative Y Position

HV

_10k

gf

1.000.750.500.25

300

250

200

150

1.000.750.500.25

µG 1G


Figure 48: Positional variations in hardness for 600 min sintered 78WNiFe.

The 1G sintered low-solids 35WNiFe sample showed interesting variations with

position (Figure 49). The indents in the lower 25% of the sample were all within the

settled grain region. Between 25% and 50% of the height, indents were all in the region

directly above the settled grains. Indents above 50% of the relative height were all in a

pure liquid region, yet showed higher hardness than the settled region. With respect to

position, hardness does not track directly with phase fraction (Figure 33) for 35WNiFe.

The high-solids 93WNiFe samples showed no significant trend with sample position in

either 1G or microgravity conditions (Figure 49).

92

Relative Y Position

HV

_10k

gf

1.000.750.500.25

325

300

275

250

225

200

175

150

1.000.750.500.25

µG 1G


Relative Y Position

HV

_10k

gf

1.000.750.500.25

310

300

290

280

270

1.000.750.500.25

µG 1G


Figure 49: Positional variations in hardness for 35WNiFe sintered 180 min

and 93WNiFe sintered 120 min.

93

4.4.2.3 Indent Examinations After hardness testing was performed, the indents were examined both in the

process of measuring the diagonals of the indents and for general features. Slip lines were

found to be generated by the indents, visible in the liquid phase near the indents. The slip

lines occurred on several indents on each sample at each condition. The most dramatic

instance of this was found around an indent in a liquid pool on 78WNiFe sintered for 600

minutes in Earth gravity, shown in Figure 50. The slip lines were observed both in Earth

gravity (Figure 50, Figure 51, Figure 52) and microgravity (Figure 53 and Figure 54) at

multiple sintering times. Lines were observed to pile up on incidence with tungsten grains

(Figure 50, Figure 53) and with tungsten precipitates in low-solid samples (Figure 51).

Figure 50: Slip lines generated from hardness indent, in liquid pool on 1G 600

min 78WNiFe sample.

94

Figure 51: Slip line interaction with tungsten precipitates in 1G 180 min

35WNiFe sample near top of sample. Top: low magnification of indent and lines. Bottom: higher magnification of slip line interaction with precipitates.

95

Figure 52: Slip lines in liquid pool adjacent to hardness indent on 1G 15 min

78WNiFe sample.

Figure 53: Slip lines in liquid pool off corner of hardness indent on

microgravity 120 min 93WNiFe sample.

96

Figure 54: Slip lines generated by hardness indents on microgravity 180 min

78WNiFe sample.

97

Chapter 5: Discussion

5.1 Gravitational Effects on Microstructures

Gravity has a significant effect on the microstructures of liquid phase sintered

tungsten heavy alloys. It induces positional anisotropy in liquid fraction, contiguity, and

grain size. The comparisons in grain size and contiguity indicate that microgravity

sintered samples have smaller grains than comparable 1G sintered samples, verified by

statistical significance testing. This has been reported previously for limited subsets of

the samples examined here [23, 70].

The gravitational effect on grain size is strongly influenced by tungsten content.

Grain size was larger for 1G sintered samples with high solid fractions, but the difference

was not statistically significant. Low solid fraction samples showed that 1G sintered

35WNiFe had much smaller grain size and broader distribution than identical

microgravity sintered 35WNiFe.

5.1.1 Anomalous Microstructural Data One anomalous point in the grain size data was the 180 minute sintered 78WNiFe

sample. While statistical testing shows that the grain sizes are dissimilar between 1G and

µG, the numerical data is nearly identical. A plot of the cumulative grain size (circular

equivalent diameter) for 180 minute sintered 1G and µG 78WNiFe is shown in Figure 55.

It can be seen from the figure that the microgravity sintered sample has a slightly larger

grain size, although the width of the distribution and sizes at the upper size of the

distribution are quite similar. Statistical testing using chi-squared test for independence

on the two distributions did indicate that the two had significantly different grain size

distributions.

One possible cause for this is experimental effects: the microgravity sample had

12 fields examined at 100x apparent magnification, while the Earth gravity sample had

50 fields examined at 200x apparent magnification. The higher magnification gives better

resolution (less error) at the smaller sizes of the grain size distribution: this may have

skewed the microgravity sample towards the higher end.

98

Gravity Effects on Grain Size: 180m Sinter Time

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50 60 70 80


Freq

uenc

y %

1G 180mµG 180m

Figure 55: Overlay of cumulative grain size distribution for 180 min sintered

78WNiFe.

A correction and verification for this is possible. A replicate sample of 180 min

microgravity sintered 78WNiFe is slated for quantitative microscopy, but has not yet

been completed at the time of this writing. This second microgravity sample will be

analyzed at the higher magnification, and the results compared.

A second piece of anomalous data from quantitative microscopy is the massive

macropores in 45 min sintered 78WNiFe. These macropores may be due to initial sample

forming and presintering. It is possible that one preform had higher dissolved gas content

than others, releasing this gas to form the spherical large gas pore observed in the sample.

While samples were randomized after sectioning from preforms, it is possible that

samples in both gravitational conditions came from the same preform.

Macropore formation in microgravity was not limited to the 45 min 78WNiFe

samples examined here. The composition 50W-35Ni-15Fe was also sintered in

microgravity for 180 min. Due to the shuttle reflight (MSL-1 and MSL-1R), two sintered

replicates were available for this condition. Both samples, even from different shuttle

flights and sintering runs, showed a single, giant macropore formed after sintering (see

Figure 56).

99

Figure 56: Macropore formation in 50WNiFe samples sintered in microgravity

for 180 minutes. Top: MSL-1R sample half. Bottom: MSL-1 sample, both halves.

100

5.2 Comparison with Prior Results

Most of the samples examined in this study had been analyzed previously for

contiguity, grain size, and phase fraction. Most of this data was available in only

summary form: data for direct sample-to-sample comparisons were available for the 15

minute and 120 minute 78WNiFe samples in both gravitational conditions [71].

5.2.1 Grain Size Results Comparison Numerical comparison values for grain size are reported in Table 17. Good

agreement was seen in grain size for all samples except for the 15 minute 1G sample. The

15 minute 1G 78WNiFe showed larger average grain size and a broader distribution in

this study compared to the prior analysis. The consistency of current 1G 15 min results

with prior results suggests differences in the analysis. Prior analysis used an older more

primitive image analysis program (ca 1992) that required full manual segmentation of

each grain for measurement, compared to the quasi-automatic analysis used in this study.

This may be a source of difference between prior data and current data, however if the

difference was due to technique the differences should be systematic, not limited to one

data set. In the current analysis, the only difference in the current study between the

analysis on 1G 15 min and µG 15 min 78WNiFe was the operator. Operator dependence

was shown to be negligible for this technique on trial analysis samples (see Appendix B),

and a review of the raw data did not find any unusual observations.

Chi-square testing for similarity of grain size distributions could not be performed

due to the lower number of data points in the prior analysis. Overlays of grain size

distributions as frequency plots are presented in Figure 57 for 15 min and Figure 58 for

120 min 78WNiFe. Overlays show good agreement on grain size distributions, and even

in the 15 min 1G 78WNiFe sample the prior grain size distribution could be a subset of

the current distribution. Grain size histograms smoothed out as expected with the larger

numbers of data points in this study.

101

Table 17: Comparison of prior and current average grain size results. Prior results taken from [71].

Grain Size (µm) Prior

Grain Size (µm) Current 78WNiFe Condition

Average Std. Dev. Average Std. Dev.

15 min 1G 16.28 6.06 17.97 7.24

15 min µG 16.3 5.73 16.45 6.32

120 min 1G 27.86 11.12 29.48 11.94

120 min µG 26.09 9.78 26.81 10.68

102

78WNiFe Grain Size 1G 15 minPrior and Current Data

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60

Grain Size (µm)

Freq

uen

cy (

Cu

rren

t)

0

10

20

30

40

50

60

70

80

90

100

Freq

uen

cy (

Pri

or)

Current Prior

78WNiFe Grain Size µG 15 minPrior and Current Data

0

100

200

300

400

500

600

700

800

0 10 20 30 40 50 60

Grain Size (µm)

Freq

uen

cy (

Cu

rren

t)

0

5

10

15

20

25

30

35

40

45

50

Freq

uen

cy (

Pri

or)

Current Prior Figure 57: Comparison of prior and current grain size distributions measured

for 78WNiFe sintered 15 minutes in 1G (top) and µG (bottom).

103

78WNiFe Grain Size 1G 120 minPrior and Current Data

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40 50 60

Grain Size (µm)

Freq

uen

cy (

Cu

rren

t)

0

10

20

30

40

50

60

Freq

uen

cy (

Pri

or)

Current Prior

78WNiFe Grain Size µG 120 minPrior and Current Data

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40 50 60

Grain Size (µm)

Freq

uen

cy (

Cu

rren

t)

0

5

10

15

20

25

Freq

uen

cy (

Pri

or)

Current Prior Figure 58: Comparison of prior and current grain size distributions measured

for 78WNiFe sintered 120 minutes in 1G (top) and µG (bottom).

104

5.2.2 Contiguity Results Comparison Contiguity results from past and current work are summarized in Table 18. A

substantial difference is noted from past to current values, with current values in all cases

smaller than prior work. Some of this may be attributed to difference in analysis

technique: prior analysis was performed using a line-intercept method, while current

work measured the lengths of the physical boundaries between phases.

Contiguity can be predicted for liquid phase sintering systems based on dihedral

angle and solid fraction [62]: predicted results agree more closely with prior results than

with current results. The contiguity prediction assumes no shape accommodation [2].

Shape accommodation was observed in 78WNiFe, indicating that the contiguity

prediction model may not be directly applicable, although application of the model to

prior data indicated a good match on the same samples.

To resolve the issue of variation in contiguity, the same images should be

analyzed using point-count stereological techniques rather than the physical boundary

length measurements. This would serve as an accuracy check on the measurements

themselves. With the images corrected and saved in digital format, an analysis routine

can be written for this purpose using the image analysis software with a minimum of

required operator time and user interaction.

Table 18: Comparison of prior and current contiguity results. Prior results taken from [71].

Contiguity Prior

Contiguity Current 78WNiFe Condition

Average Std. Dev. Average Std. Dev.

15 min 1G 18.50 5.12 17.1 4.5

15 min µG 23.03 3.75 14.8 2.2

120 min 1G 18.27 3.02 12.1 3.3

120 min µG 20.92 4.17 9.1 1.2

105

5.3 Gravitational Effects on Mechanical Properties

Gravity had a significant effect on mechanical properties of liquid phase sintered

tungsten-nickel-iron. Samples sintered in Earth gravity had higher hardness and

ultrasonic velocities than samples sintered in microgravity. Gravity induced bottom-to-

top gradients in hardness for 1G sintered 78WNiFe and 93WNiFe,with higher hardness at

the bottom of the 1G samples. The low tungsten content 35WNiFe sample sintered in

Earth gravity showed higher hardness in the grain-free liquid region than in the settled

tungsten grain region or in the homogeneous microgravity sintered sample.

5.3.1 Ultrasonic Modulus

Ultrasonic bulk modulus measurement results were calculated, and are

summarized in Table 19 and Table 20. Graphical results are shown in Figure 59 for

78WNiFe. Although ground-sintered materials look to have higher bulk and shear

modulus than microgravity sintered samples, the experimental error may be large enough

to obscure the difference. The consistency of the difference across samples and sintering

time, however, indicates that the higher modulus values for 1G sintered samples may be

significant. Bulk and shear modulus results for 35WNiFe and 93WNiFe are shown in

Figure 60; the only significant difference between microgravity and Earth sintered

samples was in 93WNiFe, which had a higher shear modulus in 1G than in microgravity.

106

Table 19: Bulk modulus calculated from ultrasonic longitudinal velocities.

Sample Code

Composition (wt.%)

Sinter Time (min)

Gravity Test Type Bulk

Modulus (GPa)

24-66 78W-15.4Ni-6.6Fe 1 1G Longitudinal 401 ± 47

61G 78W-15.4Ni-6.6Fe 1 1G Longitudinal 463 ± 59

111R 78W-15.4Ni-6.6Fe 1 µG Longitudinal 468 ± 76

11S 78W-15.4Ni-6.6Fe 1 µG Longitudinal 316 ± 33

39-68 78W-15.4Ni-6.6Fe 1 µG Longitudinal 406 ± 55















107

Table 20: Shear modulus calculated from ultrasonic shear velocities.

Sample Code

Composition (wt.%)

Sinter Time (min)

Gravity Test Type

Shear Modulus

(GPa)

24-66 78W-15.4Ni-6.6Fe 1 1G Shear 103 ± 8

61G 78W-15.4Ni-6.6Fe 1 1G Shear 123 ± 10

39-68 78W-15.4Ni-6.6Fe 1 µG Shear -

111R 78W-15.4Ni-6.6Fe 1 µG Shear 98 ± 10

11S 78W-15.4Ni-6.6Fe 1 µG Shear 91 ± 6

9-64 78W-15.4Ni-6.6Fe 15 1G Shear 123 ± 13

44-69 78W-15.4Ni-6.6Fe 15 µG Shear -

16-65 78W-15.4Ni-6.6Fe 120 1G Shear 151 ± 22

45-70 78W-15.4Ni-6.6Fe 120 µG Shear 113 ± 10

81G 78W-15.4Ni-6.6Fe 180 1G Shear -

31S 78W-15.4Ni-6.6Fe 180 µG Shear 98 ± 7

131R 78W-15.4Ni-6.6Fe 180 µG Shear 76 ± 6

91G 78W-15.4Ni-6.6Fe 600 1G Shear 111 ± 8

141R 78W-15.4Ni-6.6Fe 600 µG Shear 100 ± 8

41S 78W-15.4Ni-6.6Fe 600 µG Shear 109 ± 8

84G 35W-45.5Ni-19.5Fe 180 1G Shear 76 ± 7

134R 35W-45.5Ni-19.5Fe 180 µG Shear -

13-95 93W-4.9Ni-2.1Fe 120 1G Shear 172 ± 12

49-100 93W-4.9Ni-2.1Fe 120 µG Shear 124 ± 10

108

Gravity/Time Effects on Bulk Modulus78W-15.4Ni-6.6Fe

250

300

350

400

450

500

550

600

650

1 15 120 180 600


Bulk

Mod

ulus

(GPa

)

GroundMicrogravity

Gravity/Time Effects on Shear Modulus78W-15.4Ni-6.6Fe

50

70

90

110

130

150

170

190

1 15 120 180 600


She

ar M

odul

us (G

Pa)

GroundMicrogravity

Figure 59: Ultrasonic bulk (top) and shear (bottom) modulus results for

78WNiFe. Columns are measurement points, bars are estimated experimental error.

109

Gravity/Tungsten Fraction Effects on Bulk Modulus

50

100

150

200

250

300

350

400

450

500

550

35W-45.5Ni-19.5Fe 93W-4.9Ni-2.1Fe

Composition (wt.%)

Bulk

Mod

ulus

(GPa

)

GroundMicrogravity

Gravity/Tungsten Fraction Effects on Shear Modulus

50

70

90

110

130

150

170

190

35W-45.5Ni-19.5Fe 93W-4.9Ni-2.1Fe

Composition (wt.%)

She

ar M

odul

us (G

Pa)

GroundMicrogravity

Figure 60: Ultrasonic bulk (top) and shear (bottom) modulus results for

35WNiFe and 93WNiFe. Columns are data points, bars are experimental error.

110

Handbook values for bulk and shear modulus are not available for these particular

alloys, however similar alloys have been tabulated [72] and are shown in Table 21. Bulk

modulus values measured here are substantially higher than the tabulated values, by an

approximate factor of 2. Shear moduli are in a similar range to the handbook values for

90WNiFe and 80WCuNi, but are higher than that listed for 93WNiFe.

Table 21: Handbook values for bulk and shear modulus of similar tungsten alloys [72].

Alloy Composition(wt.%)

Bulk Modulus (GPa)

Shear Modulus (GPa)

93W-5Ni-2Fe 234-282 124-140

90W-7Ni-3Fe 224-286 120-142

80W-13Cu-7Ni 175-205 90-105

5.3.2 Hardness Values A boxplot of all Vickers hardness test results for 78WNiFe is shown in Figure 61.

Hardness for 35WNiFe and 93WNiFe is shown in a similar plot in Figure 62. A listing of

nominal handbook mechanical properties for some similar tungsten heavy alloys [72] is

shown in Table 22. The values measured in this study were slightly lower than the

handbook values for 93WNiFe. No direct data were available on 78WNiFe or 35WNiFe,

neither of which is commercially produced tungsten alloys. The lower tungsten content

tungsten-copper alloy included for reference in Table 22 is not ideal for comparison, as

the solubility of tungsten in copper is negligible compared to nickel-iron, giving different

mechanical properties in the final product due to lesser grain growth.

111

HV

_100

00

TimeGravity

6001801201511GµG1GµG1GµG1GµG1GµG

300

250

200

150

100

Gravity and Sintering Time Effects on Vickers Hardness

Figure 61: Gravitational and sintering time effects on Vickers hardness of

78WNiFe.

HV

_100

00

CompositionGravity

93W-4.9Ni-2.1Fe35W-45.5Ni-19.5FeMicrogravityGroundMicrogravityGround

325

300

275

250

225

200

175

150

Gravity and Tungsten Content Effects on Vickers Hardness

Figure 62: Tungsten content and gravitational effects on Vickers hardness of

WNiFe.

112

Table 22: Nominal handbook values for mechanical properties of selected tungsten heavy alloys [72].

Alloy Composition (wt.%)

Vickers Hardness

Tensile Strength (MPa)

Young’s Modulus (GPa)

93W-5Ni-2Fe 300-340 780-1100 325-355

90W-7Ni-3Fe 250-310 770-955 310-360

80W-13Cu-7Ni 330-370 630-770 245-255

5.3.3 Liquid Phase Hardness The higher hardness of the liquid-rich phase of 1G sintered 35WNiFe can be

explained by examination of the microstructure: microscopy indicated that fine tungsten

precipitates had formed in this region, resulting in a lamellar microstructure for some

portions and clear, large NiFe grains with grain boundary precipitates of tungsten.

Precipitates or a lamellar microstructure would interfere with slip systems in the alloy,

resisting deformation and resulting in a higher strength. The micrographs of indent-

generated slip lines interacting with these microstructural features demonstrate this effect.

It is interesting that near the settled region, this lamellar microstructure is not

observed in the liquid phase, indicating that the formation of the fine precipitates does not

occur when tungsten grains are nearby. The microgravity sintered 35WNiFe sample

shows very little of this lamellar microstructure-only in very isolated regions does it exist,

compared to the top 30-40% of the sample for 1G sintered 35WNiFe.

The two precipitate formations are due to variation in tungsten precipitate

nucleation: the finely divided lamellar structure is typical of homogeneous nucleation,

while the larger precipitates at grain boundaries are due to heterogeneous nucleation.

Lamellar structures are not seen near any existing tungsten grains – etching reveals that

they form in bulk only far from the settled grain region in 1G sintered 35WNiFe. Within

the diffusion distance of WNiFe above the solidification temperature of the matrix alloy,

extant tungsten grains serve as preferred heterogeneous nucleation sites, effectively

pulling dissolved tungsten out of the liquid matrix during cooling before solidification of

the NiFe matrix. In the absence of tungsten grains, nickel-iron grain boundaries can serve

as the heterogeneous nucleation site.

113

5.4 Significance of Positional Gradients

Positional gradients were noted in nearly all samples examined in this study in

mechanical properties, microstructural properties, or both. ANOVA single factor testing

was performed using Minitab to determine if significant difference in average values

were observed at each of the relative positions presented earlier as results. ANOVA was

performed using the measured property as the response and the relative positions as a

factor. The results of the ANOVA tests along with approximate trend direction (if any)

are given in Table 23.

The three microstructural parameters tended to have similar significance for

trends: for a given sample, if contiguity, phase fraction, or grain size showed a significant

positional trend, all three parameters tended to have significant trends. Microgravity

samples did show significant differences between relative positional levels, but the

direction of the trends varied. Some microgravity samples showed significant differences

in data but no definite trend. Samples sintered in Earth gravity showed significant

microstructural trends as well, and the trend direction was in most cases negative – i.e.

lower contiguity, tungsten fraction, and grain size towards the top of the sample.

Significant hardness trends existed in both microgravity and Earth-gravity

samples, but only when excluding atypical regions from the data. Although this reduces

the number of data points available for statistical testing, enough data was retained to

perform valid statistical testing, albeit with less confidence than the microstructural data.

For microgravity sintered samples, only the 600 min 78WNiFe sintered sample showed a

significant trend, which was positive – examination of the low-magnification mosaic

image indicated several regions high in liquid and porosity in the lower half of the

sample, which can cause the observed trend in hardness. For 1G sintered samples, the

significant trends in hardness were uniformly negative except for the 35WNiFe, which

had a positive trend as discussed previously.

With decreasing grain size, hardness should increase: however, for 1G sintered

samples the hardness, grain size, contiguity, and tungsten content trends are apparently

parallel, indicating the reduction in tungsten content and contiguity is sufficient to

114

overcome the strengthening effect of smaller grains, resulting in lower hardness. Similar

parallel trends are not observed in microgravity sintered samples, although for several

samples a decrease in hardness and/or contiguity, tungsten content, and grain size occurs

near the middle of the sample. The most pronounced sample with the center decrease in

measurements was the 93WNiFe microgravity sample, which showed significantly

depressed contiguity and hardness in the center, but with no definite trend in tungsten

content or grain size. The 93WNiFe ground-based sample contrasts sharply with the

microgravity, with no significant trends observed across any measurements.

115

Table 23: Results of statistical testing to determine significance of positional gradients in hardness, contiguity, tungsten content, and grain size.

78WNiFe

HV (all) HV (typ. only) Contiguity W Fraction Grain Size

Gravity Time (min) P-val Trend P-val Trend P-val Trend P-val Trend P-val Trend

µG 1 0.153 V 0.162 / 0.009 \ 0.000 \ 0.000 \

µG 15 0.257 V 0.317 ^ 0.098 \ 0.036 \ 0.044 —

µG 45 NA NA NA NA 0.000 V 0.000 V 0.541 \

µG 120 0.558 — 0.057 \ 0.020 ^ 0.425 / 0.756 V

µG 180 0.209 V 0.538 V 0.001 V 0.000 V 0.000 V

µG 600 0.069 / 0.019 / 0.368 V 0.023 V 0.000 V

1G 1 0.053 \ 0.175 \ 0.001 \ 0.000 \ 0.000 V

1G 15 0.196 \ 0.003 \ 0.009 \ 0.001 \ 0.230 —

1G 45 NA NA NA NA 0.099 / 0.061 V 0.000 \

1G 120 0.492 \ 0.587 \ 0.000 \ 0.000 \ 0.000 \

1G 180 0.366 \ 0.028 \ 0.024 \ 0.000 \ 0.001 \

1G 600 0.000 \ 0.014 \ 0.000 \ 0.000 \ 0.000 \

35WNiFe and 93WNiFe

HV (all) HV (typ. only) Contiguity W Fraction Grain Size

W wt% P-val Trend P-val Trend P-val Trend P-val Trend P-val Trend

µG 35W 0.193 V 0.326 V 0.883 — 0.128 — 0.007 \

1G 35W 0.036 / 0.025 / 0.002 — 0.000 \ 0.000 \

µG 93W 0.012 V 0.012 V 0.000 V 0.000 — 0.117 V

1G 93W 0.227 \ 0.259 \ 0.222 / 0.064 V 0.931 —

Bold indicates significance (α=0.05) NA = testing not possible due to macropores

Trend Types (approximate) with respect to distance from bottom or arbitrary edge: — = flat V = low in center ^ = high in center / = positive \= negative

116

5.5 Time Effects

5.5.1 Contiguity and Phase Evolution Contiguity and porosity were observed to vary considerably over time, with

changes altering with gravitational condition during sintering. Time series plots of

contiguity and porosity are shown in Figure 63. Contiguity was observed to rise sharply

between 1 and 15 min of isothermal sintering time at 1500°C, then decrease. Samples

sintered in microgravity diverged considerably from the 1G samples – contiguity for µG

samples dropped below that of 1G for 45-120 minutes isothermal time, then increased

above that of the 1G samples at 180 min, then seemed to reach a steady state.

Microgravity and Earth gravity sintering produced similar contiguities at 600 minutes,

with µG samples having slightly but significantly higher contiguity than 1G samples.

Recall that statistical testing indicated significant contiguity differences between

gravitational conditions at all sintering times except for 15 min of isothermal time. After

long isothermal times, microgravity contiguity is higher than 1G contiguity, while at

shorter times 1G contiguity exceeds µG contiguity. The 180 min isothermal sintering

time microstructural data did show some other anomalous characteristics, and this

contiguity data should be verified before speculating on a possible mechanism for the

variation.

Porosity evolution showed a sharp increase in both conditions at short sintering

times of 15 and 45 minutes (including the macropores formed at 45 minutes), then

reached steady state in both gravitational conditions after 120 minutes of isothermal time.

Except for the 45 minute macropore sample, microgravity sintered samples showed

higher porosity than 1G samples at all times, supporting prior work on pore elimination

being problematic in microgravity [23, 52, 73-76]. The substantially higher porosity in

the 1G sintered sample is probably a sampling effect: on review of the data, the randomly

chosen sample fields had more fields within the macropores than the matched

microgravity sample did.

117


Avg

Con

tigu

ity

6005004003002001000

0.18

0.16

0.14

0.12

0.10

0.08

0.06

ShortGravµG1G

78WNiFe Contiguity Evolution


Ave

rage

Por

e Fr

acti

on

6005004003002001000

0.12

0.10

0.08

0.06

0.04

0.02

0.00

ShortGravµG1G

78WNiFe Porosity Evolution

Figure 63: Contiguity and porosity evolution over time for 78WNiFe sintered

in 1G and µG.

118

5.5.2 Grain Growth At all times for 78WNiFe, grain size was statistically dissimilar between 1G and

µG samples. Changes in grain size distributions are given by a box plot in Figure 64. The

width of grain size distributions is generally larger for 1G sintered 78WNiFe than for

microgravity for isothermal sintering times over 1 min, and the largest difference is in the

upper tail of the distribution. From the positional gradient analysis, this is due to larger

grain sizes in 1G near the bottom of the sample.

The effect of tungsten content on grain growth in WNiFe is shown in Figure 65.

In microgravity, grain size was larger with higher tungsten content at given sintering

times. In Earth gravity, the trend was similar between 78W and 93W, but settling effects

caused both an enlargement and a reduction in grain size for 35WNiFe. Grains in the

settled region were larger than their microgravity counterparts, but precipitate formation

in the high-liquid region resulted in large numbers of very small grains. Overall, grain

size was smaller for 1G 35WNiFe (but larger in the settled region) than for µG 35WNiFe,

and smaller than 1G 78WNiFe.

The general trend for grain growth is given as an average plot for 78WNiFe in

both gravitational conditions in Figure 66. Both gravitational conditions showed similar

behavior. Grains grew faster in 1G at short sintering times, but at isothermal times of 180

and 600 minutes microgravity grains were larger than 1G grains on an average basis.

119

Isothermal Sintering Time (min)

Circ

. Equ

iv. D

iam

eter

(µm

)

60018012045151

120

100

80

60

40

20

0

60018012045151

Ground Microgravity

78WNiFe Grain Size Time Evolution

Figure 64: Gravitational and sintering time effects on grain size distributions

for 78WNiFe.

120

Circ

ular

Equ

ival

ent

Dia

met

er (

µm)

Time (min)W (wt%)

Gravity

18012078W35W93W78W

1GµG1GµG1GµG1GµG

120

100

80

60

40

20

0

Gravity and Tungsten Content Effects on Grain Size (2D)

Figure 65: Gravitational and tungsten content effects on grain size

distributions for WNiFe alloys.

Sintering Time

Avg

Cir

cula

r Eq

uiva

lent

Dia

met

er (

µm)

6005004003002001000

50

40

30

20

10

ShortGravµG1G

Grain Size Evolution (2D) for 78WNiFe

Figure 66: Grain size time evolution as average grain size time plot for

78WNiFe.

121

5.5.2.1 Power Law Grain Growth Grain growth can be empirically modeled in general by a log-log linear fit,

calculating an intercept and slope which gives a power law fit [18]. This modeling

assumes that grain growth follows the following form and accompanying logarithmic

reduction:

nG kt=

1 lnln( ) ln kG tn n

= + Equation 10

where G is average grain size, t is effective sintering time, n and k are constants. The

grain growth rate was examined by applying a linear fit to a log-log plot of the measured

grain size data for 78WNiFe sintered at 1, 15, 45, 120, 180, and 600 minutes. Recall, the

thermal cycle used for liquid phase sintering added 30 minutes of time at liquid phase, or

an effective sintering time of isothermal hold time + 30 minutes. Fitted line plots are

given in Figure 67.

122

ln(EffSinterTime)_1G

ln(C

ED)_

1G

6.56.05.55.04.54.03.53.0

4.00

3.75

3.50

3.25

3.00

2.75

2.50

S 0.0710141R-Sq 97.8%R-Sq(adj) 97.2%

78WNiFe Power Law Grain Growth - Earth Gravityln(CED)_1G = 1.359 + 0.3830 ln(EffSinterTime)_1G

ln(EffSinterTime)_µG

ln(C

ED)_

µG

6.56.05.55.04.54.03.53.0

4.00

3.75

3.50

3.25

3.00

2.75

2.50

S 0.0941440R-Sq 97.1%R-Sq(adj) 96.4%

78WNiFe Power Law Grain Growth - Microgravityln(CED)_µG = 1.033 + 0.4419 ln(EffSinterTime)_µG

Figure 67: Fitted log-log power law grain growth plots for 78WNiFe sintered

in 1G (top) and µG (bottom).

123

The power law fit the time series data for 78WNiFe well for both 1G and µG

sintering conditions, resulting in the following predictive equations for grain growth:

2.61 34.7G t= Earth Gravity 2.26 10.3G t= Microgravity

Equation 11

The larger growth exponent for 1G sintering conditions indicates that grains grow faster

in 1G than in µG. The settling induced by gravity shortens the distance between particles,

as indicated by higher solid contents for the lower 2/3 of the sample in 1G. Shorter

distance between tungsten particles allows faster grain growth due to diffusion effects.

According to Fick’s first law, for a fixed amount of time more net flux can occur over a

short distance than over a long distance.

The power law model fits well, but some deviations are apparent at short and long

sintering times of 1 and 600 isothermal minutes in both gravitational conditions. Equation

10 is a simplification of a more fundamental kinetic model of grain growth in liquid

phase sintering given by [23]:

0n nG G kt= + Equation 12

which includes G0 as the initial grain size. LSW theory is based on this kinetic behavior

and predicts an exponent of n=3. Power law exponents by regression (without intercept)

are not equal to 3, indicating LSW theory may not wholly predict the grain growth in this

material, as discussed in [8, 23, 46, 47, 73] among others.

124

5.5.2.2 LSW Grain Growth For liquid phase sintering, LSW theory predicts grain growth at isothermal

temperatures using the following relation:

3 30G G tκ= + Equation 13

where G0 is the initial grain size, and κ is a thermally activated parameter. Plots of the

curve fits are given in Figure 68. The LSW cubic equation fit the data extremely well,

resulting in the following values:

( )33 1.7 131.8G t= + Earth Gravity

( )33 14 150.3G t= − + Microgravity Equation 14

with all grain sizes in micrometers. The negative value for G0 is interesting in the

microgravity case, as it should be the starting grain size. The 1G value for G0 is almost

equal to the BET size of the starting tungsten powder, which is appropriate as surface

area reduction is the driving force for sintering and the LSW model. With negative G0,

the physical realities of the LSW constants do not make sense – a barrier mechanism to

grain growth is probably present, requiring this simple empirical fit to take on non-

physical numbers to adjust. The κ values for the two gravitational conditions would

indicate that microgravity sintering has less of an energy barrier than 1G, but with one

nonphysical constant it may have less meaning.

No substantial deviations from the model are noted at any particular sintering

times for either gravitational condition. For LSW theory, the model is derived based on

physical principles, and the fact that the model fits well but with non-physical parameters

indicates an assumption in the model may have been violated or a correction factor is

missing.

It should be noted that all grain sizes used for this modeling is 2D average grain

size: more accurate regression using whole distributions should be performed once the

grain size distributions have been converted to a 3D basis.

125

EffectiveSinterTime_1G

CED

^3_

1G

7006005004003002001000

90000

80000

70000

60000

50000

40000

30000

20000

10000

0

S 2521.36R-Sq 99.4%R-Sq(adj) 99.3%

78WNiFe LSW Grain Growth - Earth GravityCED^3_1G = 5 + 131.8 EffectiveSinterTime_1G

EffectiveSinterTime_µG

CED

^3_

µG

7006005004003002001000

90000

80000

70000

60000

50000

40000

30000

20000

10000

0

S 1185.57R-Sq 99.9%R-Sq(adj) 99.9%

78WNiFe LSW Grain Growth - MicrogravityCED^3_µG = - 2760 + 150.3 EffectiveSinterTime_µG

Figure 68: Fitted cubic LSW grain growth plots for 78WNiFe sintered in 1G

(top) and µG (bottom).

126

5.5.3 Hardness Evolution Along with microstructural parameters, hardness also showed variations with

sintering time and gravitational sintering condition. A plot of hardness with respect to

sintering time for 78WNiFe is shown in Figure 69. Hardness showed a decrease, then an

increase, then another decrease over long time for both gravitational conditions. Hardness

measurements were found to be significantly different with respect to gravitational

condition for isothermal sintering times of 15, 120, and 600 minutes. The decrease in

hardness with sintering time was sharper and more rapid for sintering in microgravity

than in Earth gravity. Hardness was also higher in 1G sintered samples for all sintering

times over 1 minute. The time evolution trend for hardness is very similar to that

observed for contiguity in 78WNiFe, and is the inverse of porosity evolution.


Avg

HV

_10k

gf

6005004003002001000

280

270

260

250

240

GravityGroundMicrogravity

Hardness Evolution for 78WNiFeIncludes Typical Regions Only

Figure 69: Vickers hardness evolution with sintering time for 78WNiFe.

127

The severe drop in hardness after short sintering times in microgravity sintered

samples is very similar to a drop in density observed in Fe-Cu microgravity liquid phase

sintering experiments across similar time scales [77]. This density drop is shown in

Figure 70. In terms of solid and liquid percent, the Fe-43Cu system has very similar solid

and liquid fraction (~40 vol. % liquid phase) to 78WNiFe, but with different solubility. A

decrease in density was observed from sintering times of 5 and 20 minutes isothermal

time, followed by an increase at 65 minutes isothermal time. Similar time evolution of

proportional phenomena (density and hardness) for liquid phase sintering systems of

substantially different phase solubility values indicates this is a mechanical phenomenon.

Recall, the first stage of liquid phase sintering after liquid phase formation is

rearrangement, where prior solid-solid boundaries are broken and particles move to

accommodate the forces generated by liquid phase capillary pressure. This has been

previously reported on these WNiFe samples in studies on distortion and dimensional

anisotropy [46, 47]. The drop in hardness over time maps approximate boundaries of the

rearrangement time window for this system, and the parallel behavior of density through

initial stages of liquid phase sintering indicates a common behavior between the two

liquid phase systems.

Figure 70: Densification of Fe-Cu alloys liquid phase sintered in microgravity.

Fe-43Cu has similar solid volume fraction to 78WNiFe but with lower solubility of solid phase in liquid phase. Reprinted from [77].

128

In contrast, the time scale observed for rearrangement densification for Co-Cu

microgravity liquid phase sintered samples was much shorter [73]: densification

increased after sintering times of 2.5 minutes isothermal time. The solid phase in this

system has a substantially higher solubility for the liquid phase than either W-NiFe or Fe-

Cu, resulting in faster penetration of extant grain boundaries and faster onset of the

rearrangement phase. Coring was noted in the final Co-Cu solid phase particles,

indicating high intersolubility of phases. This coring was not observed to be present in

the WNiFe system studied here.

Corresponding gravitational data for either of the Co-Cu or Fe-Cu system

microgravity experiments is not readily available. In the case here with 78WNiFe, the

drop in hardness was not nearly as sharp during rearrangement, explained by gravity

forcing particle contact/rearrangement. Without the external force inducing grain rotation

and rearrangement, the rearrangement phase of liquid phase sintering lasts longer,

resulting in lower mechanical properties in microgravity processed materials for the same

processing time.

129

Conclusions Effect of gravity on microstructures • Gravity has an effect on the liquid phase microstructure as well as the solid phase

microstructure. • Gravity has a significant effect on grain contiguity

o For 78WNiFe, grain contiguity was higher in 1G sintered samples for sintering times of 15 to 120 minutes

o For 78WNiFe, grain contiguity was higher for microgravity sintered samples for sintering times of 1, 180 and 600 minutes

o For 35WNiFe and 93WNiFe, gravity had no significant effect on grain contiguity

• Gravity has a significant effect on grain size o For 78WNiFe, grain sizes were significantly larger in 1G sintered samples

than microgravity sintered samples at all sintering times over one minute • Gravity induced bottom-to-top gradients in phase fraction, contiguity, and grain size

for 78WNiFe and 35WNiFe, but not in 93WNiFe

Effect of gravity on mechanical properties • Gravity alters the ultrasonic longitudinal and transverse velocity in 78WNiFe

o Ultrasonic velocities and corresponding bulk and shear moduli were higher in ground sintered samples than in microgravity

• Gravity has a significant effect on Vickers hardness of 78WNiFe, 35WNiFe, and 93WNiFe

o Microgravity sintered samples had lower hardness than samples sintered in Earth gravity

o Gravity induced bottom-to-top gradients in Vickers hardness for 78WNiFe for sintering times of 180 and 600 minutes

o Gravity induces bottom-to-top gradients in Vickers hardness for 93WNiFe and 35WNiFe

93WNiFe 1G sintered had slightly higher hardness at the bottom of the sample

93WNiFe µG sintered had lower hardness in the center of the sample than at top and bottom

35WNiFe 1G sintered had higher hardness in liquid region than in settled grain region

o Gravity reduces the period of softening due to grain rearrangement in initial phases of liquid phase sintering

130

Future Work

• Convert grain size distribution to 3D using Saltykov or Cruz-Orive method, identify

distribution type, calculate grain growth rate and correlate results to existing grain

growth models (LSW, German-Olevsky, etc.)

• Complete quantitative microscopy on microgravity sintering repeatability samples to

examine reproducibility of microstructures in microgravity sintering, and confirm 180

minute 78WNiFe anomalous data point

• Re-analyze for contiguity using stereological line-intercept method for direct

comparison with prior analysis results.

• Perform statistical interaction analysis among the microstructural parameters and

between microstructural parameters and hardness

• Perform further mechanical testing on samples by micro-compressive crush test, or

instrumented large indenter type testing to get stress-strain data for microgravity

processed materials

• Expand extensive microstructural quantification to WNiCu samples in test matrix at

6:4 Ni:Cu and 8:2 Ni:Cu, to measure matrix solubility effects and potentially model

systems other than tungsten heavy alloys

131

Bibliography

1. R.M. German, Powder Metallurgy Science. 2nd ed. 1994, Princeton, NJ: Metal

Powder Industries Federation.

2. R.M. German, Sintering Theory and Practice. 1996, New York: John Wiley & Sons.

3. R.W. Hertzberg, Deformation and fracture mechanics of engineering materials. 1996: John Wiley & Sons, Inc.

4. P.W. Voorhees, "Ostwald ripening of two-phase mixtures." Annu. Rev. Mater. Sci., vol. 22, pp. 197-215, 1992.

5. H. Frydrych, M. Sowa, I. Olszewska, et al., "Evaluation of the structure of sintered materials with participation of the liquid phase." Rudy i Metale Niezelazne (Poland), vol. 43 no. 9, pp. 444-449, 1998.

6. R.M. German. "Grain agglomeration and coalescence in liquid phase sintering." Advances in Powder Metallurgy and Particulate Materials, Metal Powder Industries Federation, 105 College Rd. East, Princeton, NJ 08540-6692, USA 1996. Washington, DC; USA.

7. R.M. German and Y. Liu, "Grain agglomeration in liquid phase sintering." Journal of Materials Synthesis and Processing (USA), vol. 4 no. 1, pp. 23-34, 1996.

8. R.M. German and E.A. Olevsky, "Modeling grain growth dependence on the liquid content in liquid-phase-sintered materials." Metallurgical and Materials Transactions A (USA), vol. 29A no. 12, pp. 3057-3067A, 1998.

9. J. Liu and R.M. German, "Densification and shape distortion in liquid-phase sintering." Metallurgical and Materials Transactions A (USA), vol. 30A no. 12, pp. 3211-3217A, 1999.

10. J. Liu, Y. Liu, and R.M. German, "Simulation of percolation structure of grain bonding in liquid-phase sintering by three-dimensional grain structure reconstruction." Metallurgical and Materials Transactions A (USA), vol. 31A no. 12, pp. 3187-3193A, 2000.

11. J. Liu, A. Upadhyaya, and R.M. German, "Application of percolation theory in predicting shape distortion during liquid-phase sintering." Metallurgical and Materials Transactions A (USA), vol. 30A no. 8, pp. 2209-2220, 1999.

12. Y. Liu, R.G. Iacocca, and R.M. German. "A challenge to liquid phase sintering concepts." Advances in Powder Metallurgy and Particulate Materials--1995. Vol. 1,

132

Metal Powder Industries Federation, 105 College Rd. East, Princeton, NJ 08540-6692, USA 1995. Seattle, Washington; USA.

13. P. Lu and R.M. German, "Multiple grain growth events in liquid phase sintering." Journal of Materials Science (USA), vol. 36 no. 14, pp. 3385-3394, 2001.

14. A. Mortensen and C.S. Marchi. "Capillary forces in liquid phase sintered structures and the analogy with foams and emulsions." HTC-2000 (High Temperature Capillarity) Third International Conference, Welding Research Institute of Osaka University, Mihoga-oka 11-1, Ibaraki-shi, Osaka, 567, Japan 2001. Kurashiki; Japan.

15. A.N. Niemi, "Microstructural Evolution and Control in Liquid-Phase Sintered Alloys." Diss. Abstr. Int., vol. 43 no. 9, pp. 85, 1983.

16. A.B. Rodriguez and J.G. Sevillano. "Viscoplastic Flow of High Density W--Ni--Fe Alloys During Liquid-Phase Sintering." Tungsten & Tungsten Alloys--1992, Metal Powder Industries Federation, 105 College Rd. East, Princeton, New Jersey 08540-6692, USA 1993. Arlington, Virginia; USA.

17. P.W. Voorhees, "Coarsening in binary solid-liquid mixtures." Metallurgical Transactions A, vol. 21A no. 1, pp. 27, 1990.

18. J. William D. Callister, Materials Science and Engineering, An Introduction. 1985, New York: John Wiley & Sons, Inc.

19. ASM Handbook Vol. 3 - Alloy Phase Diagrams. ASM Handbook 2003 [cited 2006 July 15, 2006]; Available from: http://products.asminternational.org/hbk/.

20. C. Binet, K.L. Lencoski, D.F. Heaney, et al., "Modeling of Distortion after Densification during Liquid-Phase Sintering." Metallurgical and Materials Transactions A. Vol. 35A, no. 12, pp. 3833-3841A. Dec. 2004, vol., 2004.

21. R. German, "Space study of gravitational role in liquid-phase sintering." Ind. Heat., vol. 62 no. 2, pp. 52-54, 1995.

22. R.M. German, "Microstructure of the gravitationally settled region in a liquid-phase sintered dilute tungsten heavy alloy." Metall. Mater. Trans. A, vol. 26A no. 2, pp. 279-288, 1995.

23. R.M. German, A. Griffo, and Y. Liu, "Gravitational effects on grain coarsening during liquid-phase sintering." Metallurgical and Materials Transactions A (USA), vol. 28A no. 1, pp. 215-221A, 1997.

24. R.M. German, R.G. Iacocca, J.L. Johnson, et al., "Liquid-phase sintering under microgravity conditions." JOM (USA), vol. 47 no. 8, pp. 46-48, 1995.

133

25. R.M. German, Y. Liu, and R.G. Iacocca, "Microstructure quantification procedures in liquid-phase sintered materials." Acta Materialia (USA), vol. 47 no. 3, pp. 915-926, 1999.

26. R.M. German, S.J. Park, and J.L. Johnson. "Critical Learning from Microgravity Sintering of Tungsten Alloys: Implications for Extraterrestrial Fabrication and Repair." Sintering 2005, 2005. Grenoble, France.

27. A.M. Gokhale, A. Tewari, and R.M. German, "Effect of gravity on three-dimensional coordination number distribution in liquid phase sintered microstructures." Acta Materialia (USA). Vol. 47, no. 13, pp. 3721-3734. 8 Oct. 1999, vol., 1999.

28. A.M. Gokhale, A. Tewari, and R.M. German, "Effect of gravity on three-dimensional coordination number distribution in liquid phase sintered microstructures." Acta Materialia (USA), vol. 47 no. 13, pp. 3721-3734, 1999.

29. D.F. Heaney and R.M. German. "New grain growth concepts in liquid phase sintering." Advances in Powder Metallurgy and Particulate Materials-1994. Vol. 3. Compaction, Sintering, and Secondary Operations, Metal Powder Industries Federation, 105 College Road East, Princeton, NJ 08540-6692 1994. Toronto, Ont; Canada.

30. D.F. Heaney, R.M. German, and I.S. Ahn, "The gravitational effects on low solid-volume fraction liquid-phase sintering." Journal of Materials Science, vol. 30 no. 22, pp. 5808-5812, 1995.

31. D.F. Heaney, R.M. German, and I.S. Ahn. "The Gravity Effect on Critical Volume Fraction in Liquid Phase Sintering." Advances in Powder Metallurgy & Particulate Materials--1993. Vol. 2. Compaction, Sintering, and Secondary Operations, Metal Powder Industries Federation, 105 College Rd. East, Princeton, New Jersey 08540-6692, USA 1993. Nashville, Tennessee.

32. R.G. Iacocca and P. Downs, Characterization of the powders used for the MSL-1 tungsten heavy alloy samples. 1995, National Aeronautics and Space Administration. p. 11.

33. R.G. Iacocca, P. Downs, and R.M. German. "Three dimensional microstructures of sintered materials." Advances in Powder Metallurgy and Particulate Materials--1997. Vol. 3, Metal Powder Industries Federation, 105 College Rd. East, Princeton, NJ 08540-6692, USA 1997. Chicago, IL, USA.

34. R.G. Iacocca and R.M. German. "Experimental Design for Liquid Phase Sintering in Microgravity." Advances in Powder Metallurgy & Particulate Materials--1993. Vol. 2. Compaction, Sintering, and Secondary Operations, Metal Powder Industries Federation, 105 College Rd. East, Princeton, New Jersey 08540-6692, USA 1993. Nashville, Tennessee; USA.

134

35. R.G. Iacocca, R.M. German, and C. Kidd. "Microstructural analysis of tungsten heavy alloy samples sintered in 1-G conditions: preparation for shuttle flight." 1994 International Conference and Exhibition on Powder Metallurgy and Particulate Materials, Metal Powder Industries Federation, 105 College Road East, Princeton, NJ 08540-6692, USA 1994. Toronto, Ontario; Canada.

36. R.G. Iacocca, Y. Liu, and R.M. German. "Microstructural examination of tungsten heavy alloys sintered in a microgravity environment." Advances in Powder Metallurgy and Particulate Materials--1995. Vol. 1, Metal Powder Industries Federation, 105 College Rd. East, Princeton, NJ 08540-6692, USA 1995. Seattle, Washington; USA.

37. G. Liu and H. Yu, "Stereological characterization of particle contiguity." Journal of Microscopy, vol. 181 no. 1, pp. 82-87, 1996.

38. J. Liu and Z.Z. Fang, "Quantitative characterization of microstructures of liquid-phase-sintered two-phase materials." Metallurgical and Materials Transactions A, vol. 35A no. 6, pp. 1881-1888A, 2004.

39. J. Liu and R.M. German, "Grain boundary sliding and component shape distortion during liquid-phase sintering." Metallurgical and Materials Transactions A (USA), vol. 32A no. 8, pp. 2087-2095A, 2001.

40. J. Liu and R.M. German, "Microstructural parameters related to liquid-phase sintering." Metallurgical and Materials Transactions A (USA), vol. 31A no. 10, pp. 2607-2614A, 2000.

41. Y. Liu and R.M. German, "Contact angle and solid-liquid-vapor equilibrium." Acta Materialia (USA), vol. 44 no. 4, pp. 1657-1663, 1996.

42. Y. Liu, D.F. Heaney, and R.M. German, "Gravity induced solid grain packing during liquid phase sintering." Acta Metall. Mater. (USA), vol. 43 no. 4, pp. 1587-1592, 1995.

43. Y. Liu, R.G. Iacocca, J.L. Johnson, et al., "Microstructural anomalies in a W-Ni alloy liquid phase sintered under microgravity conditions." Metall. Mater. Trans. A (USA), vol. 26A no. 9, pp. 2484-2486, 1995.

44. P. Lu and R.M. German. "Presintering effects on tungsten heavy alloy liquid phase sintering." PM2TEC 2001: 2001 International Conference on Powder Metallurgy & Particulate Materials, Metal Powder Industries Federation, 105 College Rd. East, Princeton, NJ 08540-6692, USA 2001. New Orleans, LA; USA.

45. P. Lu, R.M. German, and R.G. Iacocca, "Presintering effects on ground-based and microgravity liquid phase sintering." Metallurgical and Materials Transactions A (USA), vol. 32A no. 8, pp. 2097-2107A, 2001.

135

46. E.A. Olevsky and R.M. German, "Effect of gravity on dimensional change during sintering. I. Shrinkage anisotropy." Acta Materialia (USA), vol. 48 no. 5, pp. 1153-1166, 2000.

47. E.A. Olevsky, R.M. German, and A. Upadhyaya, "Effect of gravity on dimensional change during sintering. II. Shape distortion." Acta Materialia (USA), vol. 48 no. 5, pp. 1167-1180, 2000.

48. R. Raman and R.M. German, "A mathematical model for gravity-induced distortion during liquid-phase sintering." Metall. Mater. Trans. A (USA), vol. 26A no. 3, pp. 653-659, 1995.

49. J. Shen, L. Campbell, P. Suri, et al., "Quantitative microstructure analysis of tungsten heavy alloys (W-Ni-Cu) during initial stage liquid phase sintering." International Journal of Refractory Metals and Hard Materials, vol. 23 no. 2, pp. 99, 2005.

50. A. Upadhyaya and R.M. German. "Effect of gravity on microstructure and macrostructure of tungsten heavy alloys." 1998 International Conference and Exhibition on Powder Metallurgy and Particulate Materials, Metal Powder Industries Federation, 105 College Rd. East, Princeton, NJ 08540-6692, USA 1998. Las Vegas, NV; USA.

51. A. Upadhyaya and R.M. German. "Effect of microstructural parameters on distortion in liquid phase sintered W-Ni-Cu alloys." Advances in Powder Metallurgy and Particulate Materials--1996. Vol. 5, Metal Powder Industries Federation, 105 College Rd. East, Princeton, NJ 08540-6692, USA 1996. Washington, DC; USA.

52. A. Upadhyaya and R.M. German, "Gravitational effects during liquid phase sintering." Materials Chemistry and Physics, vol. 67, pp. 25-31, 2001.

53. A. Upadhyaya and R.M. German, "Shape distortion in liquid-phase-sintered tungsten heavy alloys." Metallurgical and Materials Transactions A (USA), vol. 29A no. 10, pp. 2631-2638A, 1998.

54. A. Upadhyaya, R.G. Iacocca, and R.M. German, "Gravitational effects on compact shaping and microstructure during liquid-phase sintering." JOM (USA), vol. 51 no. 4, pp. 37-40, 1999.

55. X. Xu, A. Upadhyaya, R.M. German, et al., "The effect of porosity on distortion of liquid phase sintered tungsten heavy alloys." International Journal of Refractory Metals & Hard Materials (UK), vol. 17 no. 5, pp. 369-379, 1999.

56. S. Ye, Y. He, and J.E. Smith, Jr., "Dihedral angle measurement in microgravity liquid phase sintered microstructures." Journal of Materials Science (USA), vol. 38 no. 2, pp. 377-388, 2003.

136

57. W. Yi, D.F. Heaney, R.G. Iacocca, et al., "Space shuttle offers insights into liquid phase sintering." Met. Powder Rep. Vol. 49, no. 11, pp. 8-10. Nov. 1994, vol., 1994.

58. W. Yi, R.G. Iacocca, D.F. Heaney, et al., "Linking microstructure and macrostructure during LPS under gravity." Metal Powder Report (UK), vol. 55 no. 7-8, pp. 10-12, 2000.

59. A. Upadhyaya, B. Ozkal, and R.M. German, "Application of interference layering for metallography of sintered alloys." P/M Science & Technology Briefs, vol. 1 no. 2, pp. 17-21, 1999.

60. M.W. Davidson. Concepts and Formulas in Microscopy - Resolution. [cited 2006 July 30]; Available from: http://www.microscopyu.com/articles/formulas/formulasresolution.html.

61. Clemex Vision PE/Lite 3.5 User's Guide. 2003, Montreal, Canada: Clemex Technologies Inc.

62. J.L. Johnson, A. Upadhyaya, and R.M. German, "Microstructural effects on distortion and solid-liquid segregation during liquid phase sintering under microgravity conditions." Metallurgical and Materials Transactions B (USA), vol. 29B no. 4, pp. 857-866B, 1998.

63. E.E. Underwood, Quantitative Stereology. 1970, Reading, MA: Addison-Wesley.

64. J.C. Russ and R.T. Dehoff, Practical Stereology. 2nd ed. 1999, New York, NY: Plenum Press.

65. M. Winter. Physical properties of tungsten; Physical properties of nickel; Physical properties of iron. 2006 [cited 2006 February 4]; Available from: www.webelements.com.

66. Y. Bar-Cohen and A.K. Mal, ASM Handbook Vo. 17 - Nondestructive Evaluation and Quality Control, in ASM Handbook. 2001, ASM International.

67. B. Tittmann and M. Pedrick, E Mech 523 Lab Lecture. 2005, Pennsylvania State University: University Park, PA.

68. M. Pedrick and B.R. Tittmann. "Ultrasonic micrometer position indicator with temperature compensation." IEEE UFFC-S Ultrasonics Symposium, 2004. Montreal, Canada.

69. J.L. Hay. ASM Handbook Vol. 8 - Instrumented Indentation Testing. ASM Handbook 2003 [cited 2005 Oct 25, 2005]; Available from: http://products.asminternational.org/hbk/.

137

70. Y. Liu, "Three dimensional grain size distribution and grain structure reconstruction in liquid phase sintering," Ph. D. thesis, Pennsylvania State University, 1999.

71. R.G. Iacocca and P. Downs, Initial Characterization of Post-Flight WHA Samples from IML-2. 1995, Pennsylvania State University.

72. CES Selector v4.5. 2004, Granta Design: Cambridge.

73. Y. He, S. Ye, and J. Naser, "Effects of composition and sintering time on liquid phase sintered Co-Cu samples in microgravity." Journal of Materials Science, vol. 35, pp. 5973-5980, 2000.

74. Y. He, S. Ye, J. Naser, et al., "Initiation of metamorphosis of porosity during liquid-phase sintering in microgravity." High Temperatures High Pressures (UK), vol. 32 no. 5, pp. 613-620, 2000.

75. Z. Xue, J.G. Vandegrift, A. Nadarajah, et al. "Fate of pores in liquid phase sintered metal compacts processed in microgravity." 7th International Symposium on Experimental Methods for Microgravity Materials Science, Warrendale, PA: Minerals, Metals & Materials Society 1995. Las Vegas, Nevada; USA.

76. Z. Xue, J.G. Vandegrift, and J.E. Smith, Jr., "Morphological instability of pores in microgravity processed liquid phase sintered metal compacts." Microgravity Sci. Technol. (Germany), vol. 8 no. 2, pp. 112-117, 1995.

77. Z. Xue, S.L. Noojin, J.G. Vandegrift, et al., "Microgravity liquid phase sintering and pore evolution." Materials Transactions, JIM (Japan), vol. 37 no. 5, pp. 1084-1090, 1996.

78. E.L. Crow, F.A. Davis, and M.W. Maxfield, Statistics Manual. 1960, New York: Dover Publications, Inc.

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Appendix A Metallographic Procedures

Repreparation Procedures

The same basic polishing procedure was applied to each sample, although the

starting point within the procedure varied depending upon the initial condition of the

sample. All samples previously cut were sectioned using a diamond wafering blade on a

Leco VC-50 wafering saw. Samples sectioned in this study were cut using a bonded

alumina abrasive wafering blade on a Struers Accutom-5 precision wafering saw.

Previously mounted samples were either hot compression mounted phenolic resin or cold

mounted using epoxy resin. Samples remounted for this study were mounted in

transparent compression mounted acrylic (Struers Specifast) using a Struers ProntoPress-

20 hot mounting press. While the acrylic did not give any advantage as far as edge

retention or filling of pores, it allows the sample height to be observed during polishing

and is a faster preparation than cold-set transparent epoxy mounting.

Polishing on SiC papers was performed by hand using silicon carbide papers

provided by Leco. All steps following the silicon carbide grinding were performed using

a Struers RotoPol-4 automatic polishing system. All diamond suspensions were

polycrystalline, purchased from Struers (Westlake, OH). Colloidal silica was also

provided by Struers (product designation OP-S) and was an alkaline suspension,

providing slight chemical attack on the tungsten samples in this study. The polishing

procedure is summarized in Table 24, along with descriptions of the polishing platens.

An alcohol/glycol based extender/lubricant was used for all diamond polishing, and small

amounts of water were used to extend the colloidal silica suspension. During the last

minute of the final colloidal silica step, water was applied at a flow sufficient to puddle

on the moving cloth, and the force was reduced to 5N. This was found to significantly

reduce the amount of adherent colloidal silica on the samples after polishing, resulting in

much fewer imaging artifacts in subsequent analysis.

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Table 24: Metallographic polishing procedure used in preparation of samples in this study. Starting point within the procedure varied depending on the condition of the sample.

Step Abrasive Size Platen Platen Description

Force Time

1 SiC 320 grit Paper Sandpaper Hand 30-90s

2 SiC 600 grit Paper Sandpaper Hand 30-90s

3 Diamond 9 µm Plan Hard ridged woven nylon

25N 2m

4 Diamond 9 µm Dac Hard napless flat woven nylon

25N 2m

5 Diamond 3 µm Dac Hard napless flat woven nylon

25N 3m

6 Colloidal Silica ~0.04 µm Chem Soft porous synthetic

25N 4-6m

Cleaning of samples between each polishing step consisted of stroking the sample

with soapy cotton ball under running water, then vigorously rinsing with anhydrous

alcohol (denatured ethanol) blowing dry with compressed air. After the final step, in

order to remove adherent colloidal silica particles, a similar procedure was performed for

longer times, after soaking the sample in soapy water immediately. If allowed to dry onto

the sample surface, colloidal silica particles were found to be very difficult to remove.

Due to previous research, the initial conditions of the samples examined in this

study varied considerably, and not all samples began at the same point in the polishing

procedure. Each sample was treated individually and polished according to its mount

condition, cleanliness, planarity, and sample thickness. The final step on all samples prior

to image capture and analysis was step 6 in Table 24.

Sputter Coating

Following metallographic preparation and initial mosaic capture, images were

sputtered with platinum using an SPI Sputter Coater (Structure Probe Inc, West Chester,

PA). This sputter coating applies a platinum/platinum oxide interference layer that

induces a color contrast between the tungsten and nickel-iron phases in the material when

140

viewed in reflective light (brightfield imaging). Sputtering was performed in partial

pressure dry air, with a pressure ~0.2 torr. The sputtering current was controlled by small

changes in partial pressure to 18±2mA, and the time for sputtering was 80-160s

depending on how quickly the sample developed the desired level of contrast. This

method has been used extensively in previous research in order to create enough contrast

for digital image analysis [49, 59].

The resulting image from brightfield optical microscopy has tungsten phase as

dark blue, nickel-iron as light blue, and pores as black, white, or brown depending on

their depth and whether they have been epoxy infiltrated. This color contrast is essential

to digital image analysis; without this step there is not enough difference between

tungsten and nickel-iron for selection of different phases within an image analysis

program. The sputter coating also highlights any water stains or residual polishing media

(colloidal silica) that is left on the sample. While these are easily identifiable as artifacts,

every effort must be made to have the sample surface as clean as possible prior to

platinum sputtering.

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Appendix B Quantitative Image Analysis Details

B.1 Sample Images

Typical sample images from the quantitative image analysis process are shown in

Figure 71 and Figure 72. To aid in identification of artifacts, notes were taken during

image capture: primary possibility of confusion was mistaking white stains for pores:

while occasionally difficult to discern on captured images this can be determined easily

by altering focus (looking for feature above or below sample plane) during image

capture.

Figure 71: Original image used in demonstration of quantitative microscopy

analysis.

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(a) (b)

(c) (d)

(e) (f)

Figure 72: Image after various stages of processing. (a) after initial thresholding, showing pores on white spots. (b) after automatic segmentation, red lines at grain boundaries. (c) after correction of phases (all porosity is liquid with polishing debris, noted during image capture). (d) after grain boundary correction. (e) solid (blue) and liquid (red) grain boundaries. (f) segmented grains (green) with dilated solid-solid boundaries (blue), only green areas with upper left inside guard frame box are measured.

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B.2 Image Analysis Script

Written for Clemex Vision PE version 3.5, Lou Campbell Modified from script developed by Lou Campbell and Junwu Shen in 2004 [49]. Tested and verified by Lou Campbell, Junwu Shen, Guneet Sethi, Tom Walker, and Vani Ramabhatt 2005. Program auto-analyzes all images in set directory in full auto run mode. Does not require stage pattern, but can be modified to run from stage pattern if desired. Select and organize files for analysis prior to analysis. PROLOG 001 Comment ' Modify lines PROLOG003 and FIELD251 for guard frame per ' sample. ' Modify lines FIELD002, FIELD185, FIELD287 for proper file ' locations. 002 Set Process Frame to 0,0 1280x1024 pixels Set Process Frame to 0,0 905x724 µm 003 Set Guard Frame to 127,1 650x470 µm Set Guard Frame to 180,1 920x664 pixels 004 012 Comment ' Image source: Optical, 180s (3x60s) Pt sputtered after OP-S, ' 16-19mA, sample face ~8mm under post (scribe mark). Aperture at ' first darkening, light at Photo, NCB11.

General Instructions for Operators Sample based limits for guard frames, to handle image edge effects on object measures Instructions for sample prep (Pt sputter) and illumination setup on Nikon Epiphot 300

FIELD 001 Hide => All 002 Load Image '*' File: *.cxi Path: I:\CISP IaFiles\Images\NASA Microgravity\84G\84G set 1 003 Clear => All 004 Copy Image -> Viewer 2 005 Smooth x1 006 Guard Frame As Process Frame 007 008 Comment ' Initial thresholds for tungsten grains. BP2-Full grains (all dark blue ' phase). BP3-Liquid Phase. Pores found by subtraction. 009 Color Threshold -> BPL2 Hue: start = 171°, delta = 85° Saturation: 0%..100% Intensity: 18..168 Pause On Run 010 Color Threshold -> BPL3 Hue: start = 157°, delta = 119° Saturation: 0%..100% Intensity: 163..255 Pause On Run 011 Swap Viewer 2 <-> Image 012 013 ' Clean up selected grains and liquid 014 Copy BPL2 -> BPL1

Auto-load images in full auto run mode: modify this line to change directory of images. Preliminary smoothing to improve thresholding-preserve initial image Select color (HSI) for tungsten grains Select color (HSI) for liquid phase. Better to slightly overselect. Restore original image Clean up spot artifacts and artifacts below 5 pixels in size

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015 Copy BPL3 -> BPL9 016 Copy BPL1 -> BPL2 017 Copy BPL9 -> BPL3 018 Hide => BPL1, BPL9, BPL10, BPL11, BPL12 019 Chord Size BPL2 -> None Diameter = 5 020 Invert BPL2 -> BPL2 021 Trap BPL2 -> None 4x4 022 Invert BPL2 -> BPL2 023 Chord Size BPL3 -> None Diameter = 4 024 Invert BPL3 -> BPL3 025 Trap BPL3 -> None 4x4 026 Invert BPL3 -> BPL3 027 028 ' Get rid of spots within grains 029 Dilate CIRC x2 => BPL2 030 (BPL2 DIFF BPL3) -> BPL2 031 (BPL2 NOR BPL3) -> BPL4 032 Copy BPL4 -> BPL5 033 Dilate CIRC x1 => BPL5 034 Transfer (BPL5 SEL BPL3) -> None 035 (BPL2 OR BPL5) -> BPL2 036 Chord Size BPL2 -> None Diameter = 5 037 Invert BPL2 -> BPL2 038 Trap BPL2 -> None 4x4 039 Invert BPL2 -> BPL2 040 041 ' More grain/liquid cleaning 042 Copy BPL2 -> BPL4 043 Erode CROSS x1 => BPL4 Extend 044 Prune Branch => BPL4 045 Zone CIRC x1 => BPL4 046 Chord Size BPL4 -> None Diameter = 5 047 (BPL4 DIFF BPL3) -> BPL2 048 Copy BPL3 -> BPL5 049 Erode CROSS x1 => BPL5 Extend 050 Prune Branch => BPL5 051 Zone CIRC x1 => BPL5 052 Chord Size BPL5 -> None Diameter = 5 053 (BPL2 OR BPL5) -> BPL6 054 Closing CROSS x2 => BPL6 Extend 055 (BPL3 AND BPL6) -> BPL3 056 Chord Size BPL3 -> None Diameter = 5 057 Invert BPL3 -> BPL3 058 Chord Size BPL3 -> None Diameter = 5 059 Invert BPL3 -> BPL3 060 Closing CIRC x1 => BPL3 Extend 061 062 Comment ' Pores can be white, brown, or black depending on size and ' sputter behavior. ' Find pores by 1-Solid-Liquid and clean up, pores on BP3, original ' liquid on BP12 ' Find Sample Area (BP10) by large closing on SOLID+LIQUID. 063 Copy BPL3 -> BPL12 064 Copy BPL3 -> BPL9 065 Hide => BPL11, BPL12 066 (BPL2 NOR BPL3) -> BPL3 067 Chord Size BPL3 -> None Diameter = 7 068 Closing CIRC x1 => BPL3 Extend

Remove tiny grain spots Further cleanup of grains and liquid regions-smooth edges for better segmentation. Find pores by Boolean NOR on grains and liquid phase

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069 Invert BPL3 -> BPL10 070 Closing OCT x8 => BPL10 Extend 071 Fill => BPL10 072 Erode CROSS x2 => BPL10 Extend 073 Separate HEX => BPL10 074 Chord Size BPL10 -> None Diameter = 42 075 Square Grid 1x1 -> BPL11 Overall Grid Dimensions 1280 x 1024 pixels 905 x 724 µm 076 Dilate CROSS x1 => BPL11 077 Transfer (BPL10 SEL BPL11) -> BPL10 078 Erode CIRC x30 => BPL10 Extend 079 Dilate CIRC x30 => BPL10 080 Hide => BPL10 081 082 ' Delete non-sample area pores, liquid, and grains. 083 (BPL3 AND BPL10) -> BPL3 084 Chord Size BPL3 -> None Diameter = 7 085 Zone CIRC x1 => BPL2 086 (BPL2 DIFF BPL9) -> BPL2 087 (BPL2 DIFF BPL3) -> BPL2 088 (BPL2 AND BPL10) -> BPL4 089 (BPL9 AND BPL10) -> BPL9 090 Copy BPL4 -> BPL11 091 Hide => All 092 Show => BPL3, BPL4, BPL9 093 Comment ' Have cleaned grains on BP4 and BP11, cleaned pores on BP3, ' uncleaned ' grains on BP1, sample area on BP10, cleaned liquid on BP9. 094 095 ' Automatic Segmentation of tungsten grains to BP5 096 ' Sequential reconstruction by pixel size 097 Reconstruct CIRC x11 => BPL4 098 Object Transfer BPL4 -> BPL5 Sphericity less than 0.8 099 Reconstruct CIRC x23 => BPL5 100 Object Transfer BPL5 -> BPL6 Sphericity greater than 0.8 101 Reconstruct CIRC x37 => BPL5 102 Object Transfer BPL5 -> BPL8 Sphericity greater than 0.7 103 (BPL6 OR BPL8) -> BPL6 104 Reconstruct CIRC x53 => BPL5 105 Object Transfer BPL5 -> BPL8 Sphericity greater than 0.7 106 Separate CIRC => BPL5 107 (BPL5 OR BPL6) -> BPL5 108 (BPL5 OR BPL8) -> BPL5 109 (BPL5 OR BPL4) -> BPL5 110 111 ' Correction of segmentation-quasi auto 112 Clear => BPL1, BPL2, BPL4 113 Hide => All 114 Show => BPL2, BPL5 115 Pause Edit Lasso BPL2 Join incorrectly cut grains with red Line tool.

Determine sample area by excessive closing on combined solid and liquid phases. Find if non-sample area touches edge: if it does, it is mounting media and image is on edge of sample. Store found sample area on BP10. Remove grains, liquid, pores outside of sample area. Done with initial cleanup and sample area bitplane determination Automatic segmentation-circular kernel, stepwise by size (smallest first) and isolation. High sphericity grains do not need further segmentation-isolate these and do not oversegment. Final separation, only on largest and most irregular objects Combine intermediate segmenting steps Manual edit step: correct oversegmentation and missed grain areas

146

Draw unselected grain areas with red Lasso tool. 116 Transfer (BPL5 SEL BPL2) -> BPL7 117 (BPL2 OR BPL7) -> BPL7 118 (BPL9 DIFF BPL7) -> BPL9 119 (BPL9 DIFF BPL5) -> BPL9 120 Closing CIRC x3 => BPL7 Extend 121 (BPL5 OR BPL7) -> BPL5 122 (BPL3 DIFF BPL5) -> BPL3 123 124 Clear => BPL2, BPL4 125 Hide => All 126 Show => BPL2, BPL9 127 Pause Edit Lasso BPL2 Draw additional liquid regions in Red with Lasso tool. 128 (BPL9 OR BPL2) -> BPL9 129 (BPL5 DIFF BPL9) -> BPL5 130 (BPL3 DIFF BPL9) -> BPL3 131 132 Clear => BPL2, BPL4 133 Hide => All 134 Show => BPL2, BPL3 135 Pause Edit Lasso BPL2 Draw additional pore areas in red (BP2). Draw replacement pores in pink (BP4). Delete false pores on BP3 if necessary. 136 (BPL3 OR BPL2) -> BPL3 137 Transfer (BPL3 SEL BPL4) -> None 138 (BPL3 OR BPL4) -> BPL3 139 (BPL5 DIFF BPL3) -> BPL5 140 (BPL9 DIFF BPL3) -> BPL9 141 142 Clear => BPL2, BPL4 143 Copy BPL5 -> BPL6 144 Hide => All 145 Show => BPL2, BPL3, BPL4, BPL5 146 Pause Edit Draw BPL2 Draw lines cutting grains in red (BP2). Draw areas to subtract from solid and add to liquid in pink (BP4) with Lasso tool. 147 (BPL5 DIFF BPL2) -> BPL5 148 (BPL5 DIFF BPL4) -> BPL5 149 (BPL9 OR BPL4) -> BPL9 150 151 Chord Size BPL5 -> None Diameter = 6 152 Copy BPL5 -> BPL6 153 Disconnect HEX => BPL6 154 Reconstruct CIRC x1 => BPL6 155 Chord Size BPL6 -> None Diameter = 6 156 (BPL9 DIFF BPL6) -> BPL9 157 (BPL3 DIFF BPL6) -> BPL3 158 (BPL3 DIFF BPL9) -> BPL3

Apply edits Manual edit: correct missing liquid phase areas Apply edits Manual edit: Correct pores (most pores are actually polishing artifacts, mainly dirt) Apply edits Manual edit: Segment unsegmented grains, also can switch areas from solid to liquid if missed in prior edit. Last edit prior to analysis, can hand-edit other bitplanes here as well if necessary. Apply edits Clean up tails and sloppy drawing, final segmentation on grains to avoid diagonal connections

147

159 Copy BPL11 -> BPL7 160 (BPL7 OR BPL6) -> BPL7 161 (BPL7 DIFF BPL3) -> BPL7 162 (BPL7 DIFF BPL9) -> BPL7 163 Closing CIRC x1 => BPL7 Extend 164 ' Regenerate sample area from manually edited bitplanes 165 (BPL5 OR BPL3) -> BPL10 166 (BPL10 OR BPL9) -> BPL10 167 Closing OCT x5 => BPL10 Extend 168 Comment ' Remove small porosity on edge of sample area, clean up other ' phases. 169 Invert BPL10 -> BPL8 170 Dilate CIRC x1 => BPL8 171 Transfer (BPL3 SEL BPL8) -> BPL8 172 Chord Size BPL8 -> None Diameter = 15 173 (BPL3 OR BPL8) -> BPL3 174 Trap BPL3 -> None 3x3 175 (BPL3 AND BPL10) -> BPL3 176 (BPL6 AND BPL10) -> BPL6 177 (BPL7 AND BPL10) -> BPL7 178 (BPL9 AND BPL10) -> BPL9 179 180 Show => BPL3, BPL6, BPL10, BPL11, BPL12 181 Copy Image -> Viewer 1 >> ALL BPL 182 Copy Viewer 1 -> Image >> ALL BPL 183 Show => All 184 185 Save Image '< file name based on source >' with Bitplanes File: < file name based on source >.cxi Path: I:\CISP IaFiles\Images\NASA Microgravity\84G\84G set 1 Overwrite Protection: No 186 Hide => All 187 Show => BPL3, BPL6, BPL9 188 189 Comment ' Begin measurements: pores on BP3, cut grains on BP6, uncut ' grains on BP4, RoI on BP10, edge grid on BP11, original liquid on ' BP12. 190 191 ' Area fractions (note numbers are relative to process frame size) 192 Comment ' FLDM1 is W fraction, FLDM2 is Porosity, FLDM6 is Sample Area, ' FLDM7 is liquid fraction. 193 Field Measures (BPL7) -> FLDM1

� Area � Average Area � Perimeter � Mean Horizontal Chord � Mean Vertical Chord � Circular Diameter � Spherical Diameter � Area Percent � Count � Horizontal Intercept � Vertical Intercept � Hor Intercept Density � Ver Intercept Density � Density

Regenerate sample area from manually edited bitplanes: more accurate due to color variations found at physical edges of sample. Recorrect any remaining porosity and solid/liquid phase areas Store img & all BP in Vwr 1: debug storage. Save image: overwrite original, as only thing changes are bitplanes. MUST be .CXI format. Must edit this line for proper file locations. Directions on what’s where, and how field measurements are set up. Measure unsegmented tungsten phase field measures

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� Anisotropy � Intensity � X Centroid � Y Centroid � ASTM E112-96 � Mean Curvature for micrograph

194 Field Measures (BPL3) -> FLDM2 � Area � Area Percent � Count

195 Field Measures (BPL10) -> FLDM6 � Area � Area Percent

196 Field Measures (BPL9) -> FLDM7 � Area � Average Area � Perimeter � Mean Horizontal Chord � Mean Vertical Chord � Circular Diameter � Spherical Diameter � Area Percent � Count � Horizontal Intercept � Vertical Intercept � Hor Intercept Density � Ver Intercept Density � Density � Anisotropy � Intensity � X Centroid � Y Centroid � ASTM E112-96 � Mean Curvature for micrograph

197 Hide => All 198 199 ' Contiguity Measurements 200 ' Find S-S and S-L Boundaries 201 Square Grid 1x1 -> BPL11 Overall Grid Dimensions 1280 x 1024 pixels 905 x 724 µm 202 Dilate CROSS x5 => BPL11 203 Hide => BPL11 204 Copy BPL6 -> BPL1 205 Chord Size BPL1 -> None Diameter = 6 206 Closing CIRC x2 => BPL1 207 Invert BPL1 -> BPL4 208 (BPL1 DIFF BPL6) -> BPL1 209 (BPL1 DIFF BPL4) -> BPL1 210 Trap BPL1 -> None 4x4 211 Copy BPL4 -> BPL2 212 Dilate CIRC x2 => BPL2 213 (BPL6 AND BPL2) -> BPL2 214 Trap BPL2 -> None 3x3 215 (BPL2 DIFF BPL11) -> BPL2 216 ' Straighten/clean S-S, S-L boundaries 217 Copy BPL1 -> BPL8 218 Dilate CIRC x1 => BPL8

Measure porosity field measures Measure sample area (may be less than field area) Measure liquid phase field measures End field measurements For contiguity, must find S-L and S-S boundaries through bitplane manipulation. Measure fields first avoid loss of information. Redraw edge grid and expand Generate solid-solid boundaries Copy seg grains, close segments, invert original, subtract. Difference is solid-solid boundaries. Generate solid-liquid boundaries Dilate liquid phase, find overlap with segmented solid grains. Remove SL boundaries on edges Clean up SS, SL boundaries by applying convex hull to

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219 Convex Hull CIRC x5 => BPL8 220 Thin CIRC to End => BPL8 221 Copy BPL8 -> BPL1 222 Convex Hull CIRC x2 => BPL2 223 Thin CIRC to End => BPL2 224 225 ' Find Pore contiguity 226 Copy BPL7 -> BPL8 227 Dilate CIRC x2 => BPL8 228 (BPL8 AND BPL3) -> BPL4 229 Copy BPL9 -> BPL8 230 Dilate CIRC x2 => BPL8 231 (BPL8 AND BPL3) -> BPL12 232 Copy BPL12 -> BPL8 233 Dilate CIRC x1 => BPL8 234 (BPL4 DIFF BPL8) -> BPL4 235 (BPL12 DIFF BPL11) -> BPL12 236 Trap BPL12 -> None 3x3 237 Convex Hull CIRC x2 => BPL12 238 Thin CIRC to End => BPL12 239 (BPL4 DIFF BPL11) -> BPL4 240 Trap BPL4 -> None 3x3 241 Convex Hull CIRC x2 => BPL4 242 Thin CIRC to End => BPL4 243 244 ' Measure pore boundary lengths 245 Object Measures (BPL4, 12) -> OBJM9

� Perimeter � Length � String Length � X � Y

246 Object Measures (BPL1) -> OBJM3 � Perimeter � Length � String Length � X � Y

247 Object Measures (BPL2) -> OBJM5 � Perimeter � Length � String Length � X � Y

248 Show => All 249 250 ' Connectivity/Grain Size Measurements 251 Set Guard Frame to 127,1 650x470 µm Set Guard Frame to 180,1 920x664 pixels 252 Copy BPL1 -> BPL8 253 Triple points B =>BPL8 254 Dilate CIRC x1 => BPL8 255 (BPL1 DIFF BPL8) -> BPL1 256 Trap BPL1 -> None 3x3 257 Copy BPL7 -> BPL8 258 Closing CIRC x1 => BPL8 259 Erode CIRC x1 => BPL8 Extend 260 (BPL1 AND BPL8) -> BPL1 261 Copy BPL1 -> BPL8

regularize geometry. Thin features to 1 pixel width. Find SV, LV boundaries (req by Guneet Sethi) to determine pore contiguity: similar to SL and SS above. Measure boundary lengths. Use string length, perimeter based measure rather than length (linear, inaccurate for curved lines). String length avoids stereological factor of 2 in traditional contiguity by line-intercept. Reset guard frame per sample max grain size: modify this per sample Find triple points in solid-solid boundaries. Separate SS boundaries into individual boundaries between two grains only by dilating triple points (1 px) and subtracting from T or Y shaped boundaries.

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262 Zone CIRC x4 => BPL1 263 Disconnect HEX => BPL1 264 Trap BPL1 -> None 1x1 265 Hide => All 266 Show => BPL1, BPL2, BPL6 267 Transfer (BPL1 SEL BPL11) -> None 268 Transfer (BPL6 SEL BPL11) -> None 269 Object Transfer BPL1 -> None Child Count.BPL6 less than 0.9n 270 271 ' Pore solid contact-V-S boundary modifications for child count 272 Copy BPL6 -> BPL8 273 Zone CIRC x3 => BPL8 274 (BPL4 AND BPL8) -> BPL4 275 Copy BPL4 -> BPL8 276 Triple points B =>BPL8 277 Dilate CIRC x1 => BPL8 278 (BPL4 DIFF BPL8) -> BPL4 279 Trap BPL4 -> None 3x3 280 Zone CIRC x4 => BPL4 281 Disconnect HEX => BPL4 282 Trap BPL4 -> None 1x1 283 284 Object Measures (BPL6) -> OBJM4

� Area � Filled Area � Perimeter � Convex Perimeter � Length � Width � Breadth � Feret Average � String Length � Outer Diameter � Circular Diameter � Spherical Diameter � ASTM E112-96 � Sphericity � Roundness � Aspect Ratio � Roughness � Fractal Dimension � Child Count.BPL1 � Child Perimeter.BPL1 � Hole Area � Hole Fraction � Hole Count � Hole Average Area � Hole Perimeter � X Centroid � Y Centroid � X � Y � X1 Bounding Rectangle � Y1 Bounding Rectangle � X2 Bounding Rectangle � Y2 Bounding Rectangle � Area Percent � Contiguity for Grain.BP1

Expand SS boundaries without overlapping Remove edge SS boundaries after separation Remove all SS boundaries not touching a segmented grain (happens after rem edge grains & boundaries) Pore-solid boundaries for pore connectivity: similar to SS bondary treatment above. Measure tungsten grain object measures including connectivity: connectivity is count of solid-solid boundaries touching each grain object. Large number of measures selected for possible later analysis, including 2D shape. X and Y give position to master relative zero, upper left position on sample usually. Gravitational samples oriented with bottom side at minimum Y. Microgravity samples oriented to arbitrary edge. Also includes individual contiguity per grain, for Junwu

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� Pixel Count-1280x1024 Image � Mean curvature for each grain

285 Object Measures (BPL3) -> OBJM8 � Area � Filled Area � Perimeter � Convex Perimeter � Length � Width � Breadth � Feret Average � String Length � Outer Diameter � Circular Diameter � Spherical Diameter � ASTM E112-96 � Sphericity � Roundness � Aspect Ratio � Roughness � Fractal Dimension � Child Count.BPL4 � Hole Area � Hole Fraction � Hole Count � Hole Average Area � Hole Perimeter � X Centroid � Y Centroid � X � Y � X1 Bounding Rectangle � Y1 Bounding Rectangle � X2 Bounding Rectangle � Y2 Bounding Rectangle � Area Percent � Contiguity for Grain.BP1 � Pixel Count-1280x1024 Image � Mean curvature for each grain

286 Show => All 287 Save Image '< file name based on source >' with Bitplanes File: < file name based on source >.cxi Path: I:\CISP IaFiles\Images\NASA Microgravity\84G\84G set 1\84G set 1 connect-grain Overwrite Protection: Yes

Shen’s work Measure pore object measures Must show all or bitplanes are not saved Save final image: MUST be saved separately from initial image since bitplanes are now different.

EPILOG No epilog code required.

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B.3 Hardness Analysis Script

PROLOG: None FIELD 001 Load Image '*' File: *.tif Path: C:\IaFiles\Images 002 Gray Threshold BPL1 range 0..104 003 Copy BPL1 -> BPL2 004 Reconstruct HEX x19 => BPL2 005 Erode CROSS x1 => BPL2 Extend 006 Chord Size BPL2 -> None Diameter = 37 007 Square Grid 1x1 -> BPL12 Overall Grid Dimensions 1280 x 1024 pixels 905 x 724 µm 008 Transfer (BPL2 SEL BPL12) -> None 009 Dilate CROSS x1 => BPL2 010 Object Measures (BPL2) -> OBJM1

� Area � Length � Breadth � Orientation � X Centroid � Y Centroid � Vickers Hardness 10kgf

EPILOG: None

“Vickers Hardness 10kgf” is custom measure defined as

1854.4×10000/((Breadth+Length)/2)^2.

Length is longest feret; Breadth is feret perpendicular to longest feret.

Angles (orientation) were confirmed to be 90° apart and align with diagonals observed in

image.

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B.4 Operator Dependence

The amount of manual corrections required for the quantitative image analysis

caused some concern over operator dependence of the measurement. To address this

issue, multiple operators analyzed a single set of 10 fields (images) for sample 141R,

78WNiFe 600 min µG. These analyses were done in two sessions, once during initial

verification of the analysis procedure and once after improvements were made in the

procedure. Due to the learning effect of working on similar images, replicates by

operators could not be made easily, so a true gage R&R could not be performed.

However, operator-to-operator variance can and was analyzed using ANOVA single-

factor analysis. Results of this analysis are summarized briefly in Table 25.

Operator A was the author of the script, who performed one analysis of the test set

before and once after the procedure was modified: the time between measurements 1 and

2 was over 45 days, reducing the learning effect. The data analyzed in this study was

done using operators A and C only: operators B and D were trained in this analysis as

part of general education on image analysis and their data was only used for the purpose

of the repeatability study.

Significant differences were only noted in contiguity, and then only between

sessions 1 and 2. The upgrade to the procedure was primarily aimed at improving

contiguity analysis. Session 2 techniques were used for all data presented in this study.

The results of the ANOVA testing indicated negligible operator dependence of

the analysis technique used to generate quantitative microstructural data on grain size,

contiguity, and phase fraction.

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Table 25: Summary of operator dependence ANOVA testing results on test image set. Operators are labeled A through D, numbers indicate initial test session (1) and after procedure improvement (2, final procedure). Similarity confidences were in excess of 99%.

Operator Contiguity Grain Size Phase Fraction

A1

B1 SIMILAR

A2

C2

D2

SIMILAR

SIMILAR SIMILAR

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Appendix C Ultrasonic Signal Analysis

MATLAB Script for Time of Flight Analysis

Written for MATLAB v 7.1.0.246 (R14) Service Pack 3, no extra toolboxes required. %% utrimplotenv.m Matlab script file %Opens user selected oscilloscope .csv output and plots intensity vs. % time. Prompts user for ranges of interest for first peak ranges. %Calculates Hilbert transforms for signals in selected ranges, finds peaks % within ranges, and calculates peak position (time) and time difference % between the peaks. %Input data should be comma or space delimited ASCII files, first column % time in seconds, second column intensity in arbitrary units. %Output data is prompted to save to comma-delimited ASCII text file, % with first entry filename, second entry peak 1 position, third entry peak % 2 position, third entry time difference between peaks. %LCampbell Dec2005, based on original core calculation code by MPetrick (ESM at PSU) outfile = {'File;P1 (\mus);P2 (\mus);Env ToF (\musec)\n'}; n=1; cnt = 1; running = 0; h=gcf; hold off while cnt %Auto loop, multi file handling. Comment this and last END statement for single run. %Data import-user select, single-file select. [fn pn] = uigetfile('*.csv', 'Select oscilliscope output file for analysis:'); if fn==0 | isempty(fn) disp('Action cancelled.'); if running cnt = 0; continue else return end end running = 1; %Add relative path and open file for reading path(path, pn); fid = fopen(fn); if fid == -1 disp(['Error opening file ', pathname, '/',filename]); return; end %Get data from file raw=textscan(fid,'%s','delimiter','\n','whitespace','','bufsize',8190); raw1=raw{1}; %eliminate one cell nest m = size(raw1, 1); j=0; time = []; signal = []; for i=1:m, rawstr=raw1{i}; if ~isempty(rawstr) && ~isempty(sscanf(raw1{i},'%f')) %ignore empties and non-numeric entries in file j=j+1; %using loop to handle blank lines-faster if vectorize. numout = sscanf(rawstr, '%f,%f'); time(j) = numout(1); signal(j) = numout(2); end end fclose(fid); %close file-important to avoid errors

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%Data pretreatment time = 1e6 .* time; %convert to µs % signal = signal; %changes to signal here if necessary-smoothing? % u = [time; signal]'; % figure; hold on; plot(time, signal); xlabel('Time (µs)'); ylabel('Intensity'); title(['Signal plot for ', fn]); hold off; l1 = input('Enter peak 1 lower limit in µs: '); %Original 4-value user input l2 = input('Enter peak 1 upper limit in µs: '); if l1 == 0 & l2 == 0 %Skip provision disp(['Skipped file ', fn]); continue; end u1 = input('Enter peak 2 lower limit in µs: '); u2 = input('Enter peak 2 upper limit in µs: '); TOL=1e-3; %Tolerance for finding indices: typical timestep is 1e-3 microseconds. l1i = find(abs(time-l1)<TOL); l1i = l1i(1); l2i = find(abs(time-l2)<TOL); %Error tolerant index finding. l2i = l2i(end); u1i = find(abs(time-u1)<TOL); u1i = u1i(1); u2i = find(abs(time-u2)<TOL); if isempty(u2i) u2i=size(time,2); else u2i = u2i(end); end signal_o = signal; signal(1:l1i)=0; % Second zero-fill: faster, avoids find() calls. signal(l2i:u1i)=0; signal(u2i:end)=0; %Begin envelope calculation u = [time; signal]'; % Original Code-MPetrick t=u(:,1); a=u(:,2); A=fft(a); A=2*A; % A(fix(length(A)/2):end)=0; %Original MPetrick code, arbitrary 1/2 split. A(fix((u1i-l2i)/2):end) = 0; %LC 7Dec revision, center of space between peak windows. b=ifft(A); F=sqrt(real(b).^2+imag(b).^2); % End original code MPetrick [ax splot aplot] = plotyy(t,signal_o,t,F); title(fn); set(splot, 'linewidth', 1, 'color', 'b'); set(aplot, 'linewidth', 1.5, 'color', 'r'); axes(ax(1)); ylabel('Intensity', 'color', 'b'); axes(ax(2)); ylabel('Amplitude', 'color', 'r'); set(ax(2), 'ycolor', 'r'); xlabel('Time (µs)');

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[Y max1] = max(F(l1i:l2i)); max1 = max1 + l1i; %Correct offset from narrow range max() call. [Y max2] = max(F(u1i:u2i)); max2 = max2 + u1i; %OUTPUT T1 = t(max1); T2 = t(max2); if isempty(T2) & ~isempty(T1) T2=2*T1; elseif isempty(T1) & ~isempty(T2) T1=0.5*T2; end DelT = abs(T1-T2); hold on; plot(t(max1), F(max1), 'rx', 'markersize', 7); plot(t(max2), F(max2), 'gx', 'markersize', 7); %Can also add pretty lines on chart, not done here. text(T1+0.05*T2, F(max1)*0.95, ['T1 = ', num2str(T1, '%2.2f'), ' µs']); text(T2+0.05*T2, F(max2), ['T2 = ', num2str(T2, '%2.2f'), ' µs']); text(T2, F(max1)*0.95, ['\DeltaT = ', num2str(DelT, '%2.2f'), ' µs']); %End envelope stuff. hold off com = input('Enter comment: ', 's'); outfile{n} = [fn ',' num2str(T1, '%11.8f') ',' num2str(T2, '%11.8f') ',' num2str(DelT, '%11.8f') ',' com ]; n=n+1; %Figure saving [ps fname ext] = fileparts([pn filesep fn]); saveas(gcf, fname, 'fig'); saveas(gcf, fname, 'bmp'); disp(['T1=' num2str(T1) ' T2=' num2str(T2) ' DelT=' num2str(DelT)]); R = upper(input('Continue? Y/N [Y]:', 's')); cnt = (isempty(R) || R(1)=='Y'); % if cnt delete(gca); %Get rid of current figure, avoid massive overlays delete(gca); % end end %end master while continue loop %File data output-prompt user for filename & location [pfn ppn] = uiputfile; if isequal(pfn,0) || isequal(ppn,0) disp('Data save cancelled') else dlmwrite([ppn pfn], char(outfile), ''); disp(['Data written successfully to:']); disp([ppn pfn]); end %End of line

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Appendix D Statistical Testing

D.1 Distribution Similarity

Statistical testing on the similarity of grain size distributions was performed using

a chi-squared test [78]. Data was prepared by examining 1G and µG grain size data and

preparing frequency histograms of both distributions using identical bins. Bins were

determined separately for each time or alloy condition, to minimize the number of bins

with counts <5 as recommended for the testing. The test is nearly identical to the

traditional chi-squared test for independence, where the chi-squared statistic is calculated

by:

( )∑

−=

sr

ji ji

jiij

nnnnnnn,

, ..

2..2χ

Equation 15

where ni. is notation for marginal line totals and i and j index from 1 to r and s

respectively. In this case, s is two as only two distributions are being compared, and r is

the number of bins selected. For the test, the χ2 value is compared to the critical value,

with significance α and degrees of freedom ν=(r-1)(s-1). If χ2> χ2(0.05,#Bins) then the grain

size distributions are statistically similar. The limitation of this test is that it assumes

large samples, and does not function well with many empty bins or bin frequencies under

5. With the large number of grains examined in this study and careful selection of bin

sizes, the test is robust in this case. Statistical calculations were performed using

MINITAB to calculate a P-value, or significance level at which χ2≥ χ2(0.05,#Bins). A low P-

value indicates that the distributions are dissimilar with (1-P) certainty. A significance

level of α=0.05 was chosen for this and all other statistical testing in this work.

D.2 Sample Similarity

The similarity of contiguity and other replicated data available for different

samples was determined using a student’s t-test assuming unequal variances [78]. For

contiguity, data was pulled as field data, with one contiguity value measured per field,

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calculated from the sums of boundary lengths in that field. This gave one data point per

field, in most samples not enough to perform distributional testing. For contiguity, the

testing was simply to detect whether a significant difference existed between contiguity

based on gravitational condition during sintering.

The test was an equal means test with unknown, assumed unequal variances.

Calculation of the t-statistic proceeds using:

2

121 n

nxxu iii −= (i=1,2,…,ni)

∑=

=in

ii

i

un

u1

1

( )2

11

21

1

21 ⎟⎟

⎠

⎞⎜⎜⎝

⎛−=−= ∑∑∑

===

iii n

ii

n

ii

n

ii uunuunQ

( )1121

21

−

−=

nnQ

xxt

Equation 16

Equation 17

Equation 18

Equation 19

The values are dissimilar if |t| > tα/2, n1-1. Statistical calculations were performed using

MINITAB to calculate P-values: a P-value below the significance level indicates that the

values come from statistically different samples with (1-P) certainty. A significance level

of α=0.05 was chosen for this and all other statistical testing in this work.

160

Appendix E Grain Size Distributions

Full plots of the measured two-dimensional grain size distributions are given here

in Figure 73 through Figure 78 for 78WNiFe, Figure 79 for 35WNiFe, and Figure 80for

93WNiFe. All distributions are number (population) based, and grain size is represented

as circular equivalent diameter. Grain size plots are given on linear scale, with both

frequency histogram and cumulative undersize percentage curve. Number frequencies

were calculated with a bin size of 2 µm for all plots.

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78WNiFe Grain Size Distributions

Grain Size Histogram: 1G 1m

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Figure 73: Grain size distribution (2D) for 78WNiFe 1 min sintering time. Top:

1G. Bottom: microgravity.

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Grain Size Histogram: 1G 15m

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Grain Size (2D) Histogram: 1G 45m

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Grain Size (2D) Histogram: µG 45m

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Gravitational Effects Mechanical Micro Structural Properties Tungsten Heavy Alloys 2006

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Transcript of Gravitational Effects Mechanical Micro Structural Properties Tungsten Heavy Alloys 2006