SEM/TEM FRACTOGRAPHY HANDBOOK

McDonnell Douglas Astronautics Company Huntington Beach, California

Sponsored by

Air Force Materials Laboratory Air Force Wright Aeronautical Laboratories

Air Force Systems Command Wright-Patterson Air Force Base, Ohio

AFML-TR-75-159

DECEMBER 1975

METALS AND CERAMICS INFORMATION CENTER A Department of Defense Information Analysis Center

Approved for public release; distribution unlimited.

MCIC·HB-06

ACKNOWLEDGEMENT

This document was prepared by the Metals and Ceramics Information Center (MCIC), Battelle's

Columbus Laboratories, 505 King Avenue, Columbus, Ohio 43201. MCIC's objective is to provide

a comprehensive current resource of technical information on the development and utilization

of advanced metal- or ceramic-base materials.

The Center is operated by Battelle-Columbus under Contract Number DSA900-75-C-1803 for the

U.S. Defense Supply Agency; technical aspects of MCIC operations are monitored by the Army

Materials and Mechanics Research Center. The support of these sponsor organizations is gratefully

acknowledged.

This document was prepared under the sponsorship of the Department of Defense. Neither the United States Government nor any person acting on behalf of the United States Government assumes any liability resulting from the use or publication of the information contained in this document or warrants that such use or publication will be free from privately owned rights.

Approved for public release; distribution unlimited.

All rights reserved. This document, or parts thereof, may not be reproduced in any form without written permission of the Metals and Ceramics Information Center.

ii

FOREWORD

This final Technical Report was prepared by the McDonnell Douglas Astro­nautics Company, Huntington Beach, California under Contract No. F336l5-74-C-5004. The time period covered by the contract was from 15 October 1973 to 15 June 1975. The work was conducted under the direction of the Air Force Materials Laboratory, with Mr. R. Henderson (AFML/MXA) as Project Engineer. This contract was initiated under Task No. 738103.

The program at McDonnell Douglas was under the direction of Mr. G. F. Pittinato, Principal investigator, with Mr. V. Kerlins, Mr. A. Phillips, and Mr. M. A. Russo as coinvestigators. Appreciation is expressed to Mr. H. Taketani and Mr. R. A. Rawe for their assistance in this study. The metal­lographic work was accomplished by Mr. J. L. Evans and Mr. L. Hodde. The SEM fractographs of the fatigue samples were taken by Mr. R. R. Wilcox.

The authors would like to thank the following for their contribution of either material or test specimens.

Dr. C. P. Sullivan Pratt & Whitney Aircraft East Hartford, Connecticut

Mr. H. A. Williams General Electric Company Cincinnati, Ohio

Mr. R. A. Lauchner Northrop Corporation Hawthorne, California

Mr. D. E. Lane Martin Marietta Aluminum, Inc. Torrance, California

iii

Mr. J. Moore Pratt & Whitney Aircraft West Palm Beach, Florida

Mr. L. J. Barker Kaiser Aluminum & Chemical Corp. Pleasanton, California

Mr. P. W. Kroger ALCOA Vernon, California

Mr. R. V. Turley Douglas Aircraft Company Long Beach, California

NOTICES

When Government drawings, specifications, or other data are used for any purpose other than in connection with a definitely related Government procurement operation, the United States Government thereby incurs no responsibility nor any obligation whatsoever; and the fact that the Government may have formulated, furnished, or in any way supplied the said drawings, specifications, or other data, is not to be regarded by implication or otherwise as in any manner licensing the holder or any other person or corporation, or conveying any rights or per­mission to manufacture, use or sell any patented invention that may in any way be related thereto.

This technical report has been reviewed and is approved for publication by the Public Information Office (PIO) and is releasable to the National Technical Information Service (NTIS).

RUSSELL L. HENDERSON Project Engineer Corrosion Control & Failure Analysis

FOR THE COMMANDER

T.€~PE~

Copies of this report should not be returned unless return is required by security considerations, contractual obligations, or notice on a specific docu­ment.

iv

TABLE OF CONTENTS

INTRODUCTION • • • • . • . • . • • • • • • • • • • • • • • • • • .. 1

TECHNIQUES - SECTION I • • • • . • • • • • • • • • • • . . • • . .• 3

Introduction • • . . . . • . .

Care and Handling of Fractures

Cleaning Fractures

Visual Examination .

SEM Sample Preparation

TEM Sample Preparation

Special Techniques • • •

FRACTURE MODES - SECTION II

Introduction .

Dimple Rupture •

Cleavage .

Fatigue

Decohesive Rupture

Miscellaneous Fracture Surface Features

ATLAS OF FRACTOGRAPHS - SECTION III

Introduction • • •

Mechanical Testing .

Fractographic EXamination

Index of Atlas Fractographs

5

5

5

7

7

9

17

23

25

25

13

39

49

54

59

61

61

64

67

REFERENCES • • • • • • • • • • • • • • • • • • • • • • • • • • • • . 687

v

INTRODUCTION

The use of electron fractography has become a standard practice in service

failure analysis. As an aid to interpreting electron fractographs, a trans­

mission electron microscope (TEM) Fractography Handbook (ML-TDR-64-4l6) was

prepared in 1965. This handbook, which characterizes the fracture appearance

of various aircraft materials fractured under known conditions, has proven to

be an invaluable aid in the identification of fracture modes in service fail­

ures. However, since the publication of the TEM Fractography Handbook, new

alloys for aircraft and engine applications have been developed and there has

been increasing use of the scanning electron microscope (SEM) for examining

fracture surfaces.

The SEM provides a rapid means for the direct examination of fracture surfaces,

thus permitting fracture modes to be determined more rapidly than they could

be with the TEM. However, at equivalent magnifications, the TEM fractographs

exhibit sharper detail. It has also been found that identical fracture features

appear somewhat different when viewed in the SEM as compared to the TEM. Con­

sequently, there is a need for SEM characterization of fractures obtained

under known conditions as well as a pictorial comparison of fracture modes

obtained using both the SEM and TEM.

The present handbook contains both SEM and TEM fractographs and can be used

as an effective reference handbook to aid investigators in fracture surface

analyses. Basic specimen preparation techniques and the interpretation of

electron fractographs are discussed in detail. However, it is assumed that the

reader is a materials engineer, is familiar with the operation of an electron

microscope, and has had experience in failure analysis, since no effort is

made to expound on these subjects. The use of the electron microscope for

fracture analysis adds one more tool to assist the failure investigator in

his analysis, and augments, rather than replaces, well established failure

analysis techniques.

TECHNIQUES

SECTION 1

3

INTRODUCTION

The diversity of problems associated with fracture surface analysis precludes

the use of fixed rules or techniques for examining a fracture. Instead, the

investigator must decide what specific information is required from the frac­

ture and what techniques are available for obtaining this information. This

section of the handbook covers the basic procedures and techniques used in

electron fractography. Some of these procedures and techniques apply

regardless of the type of electron microscope used in the examination, while

others are specifically designed for scanning (SEM) or transmission (TEM)

electron fractography.

CARE AND HANDLING OF FRACTURES

When a fracture requires laboratory examination, both mating surfaces should

be preserved either by the application of a protective coating*, by placing in

a desiccator, or by sealing in a plastic bag containing a desiccant to prevent

any accumulation of undue moisture until the examination can be made. The

coating used should be soluble in an organic or other mild solvent so that it

can be completely removed prior to examination. Touching the fracture surface

with fingers, rubbing, or fitting the fractures together can cause serious

damage. Picking at the fracture with a sharp instrument should also be

avoided. Rough treatment or the formation of corrosion products on the fracture

can obscure vital information. Education in the proper handling of specimens

prior to any fractographic examination is strongly recommended for anyone deal­

ing in fractures either in the field or in the laboratory.

CLEANING FRACTURES

The fracture appearance should be documented by photographing or the taking of

notes before any cleaning is attempted. Also, it must be ascertained whether

identification of foreign products on the fracture will aid in the failure

analysis. Identification of these products can be quite useful in pinpointing

adverse environmental conditions that contributed to the fracture. Hasty clean­

ing can remove important evidence. The problem of cleaning the fracture

surface should be approached with caution and common sense.

* Krylon Crystal Clear Spray No. 1302, Borden, Inc. Department CP, N.Y.,N.Y.

5

It is difficult to present detailed cleaning procedures which would apply to

all fracture surfaces, since different metals are involved, and varying degrees

of surface contamination are encountered. As a general rule, the mildest,

least damaging cleaning procedure should be used. In most cases, repeated

stripping of a plastic replica is sufficient to clean a fracture. If a

cleaning solution is required, one should be chosen that will not attack the

fracture surface, but yet remove the undesirable contamination. In the case

of oil or grease, organic cleaning solutions such as acetone or trichloroethy­

lene may be used. If an immersion treatment is not sufficient, vapor degreasing

or ultrasonic procedures may be employed. It is not recommended that a metal

brush or other harsh mechanical means be used to remove contaminants; however,

light cleaning with a soft organic fiber brush is permissible.

Oxidation, corrosion, or other chemical reaction products are usually more

difficult to remove. In these instances, mild acid or alkaline solutions such

as acetic acid, orthophosphoric acid, or sodium hydroxide, heated if required,

may be employed. Commercial ultrasonic cleaning equipment manufacturers

supply special inhibited cleaning solutions which remove oxides from metal

surfaces. It should be remembered that chemical reactions such as oxidation

and corrosion consume the base metal. Therefore, part of the fracture is

inherently destroyed and removing this oxide layer will not restore the frac­

ture to its original condition.

The following are cleaning solutions which are used for specific applications:

Removal of oxide from aluminum alloys:

70 cc orthophosphoric acid (85%)

32 g chromic acid

130 cc water

Solution may be warmed.

Removal of rust from steel:

1. Orthophosphoric acid. Use concentrated or diluted up to 50% with water.

Solution may be warmed.

2. 100 cc 6N HC1 inhibited with 0.2 g hexamethylene-tetramine.

Use at ambient temperature.

6

Removal of residual salt (NaCl) deposits:

Immerse specimen in glycerin.

After cleaning by any of the above solutions, the specimen must be thoroughly

rinsed separately in water and alcohol and then dried.

VISUAL EXAMINATION

A fracture must be considered in its entirety because the examination of only

one small area may lead to an inaccurate interpretation of the fracture mode.

It is essential, therefore, to examine the fracture origin as well as adjacent

areas.

The initial step in the examination of a fracture is to determine the location

of the fracture origin, and subsequently, the exact areas for examination in

the electron microscope. Visually or by using a stereoscopic light microscope,

it is usually possible to determine the origin of a fracture by such features

as chevron marks, hackle marks, fLip-flops, texture changes, radial fracture

traces, or by the absence of shear lips along the edges. These methods are

discussed in detail in Reference 1. A fractographic method for locating the

fracture origin is discussed later in this section under Special Techniques.

SEM SA}fPLE PREPARATION

Viewing a fracture in the SEM requires that the sample be cut and subsequently

mounted on a relatively small sample holder. In mounting the sample, it is

absolutely essential that a conductive path (ground) exists between the point

where the electron beam strikes the sample and the holder. For metallic

specimens which have a clean surface and are electrically conductive, the

sample is simply grounded to the holder by using a commercially available

conductive paint such as Television Tube Koat.* For optimum grounding, the

area where the conductive paint contacts the sample and the holder should be

clean and free of oxide coatings. This can be accomplished by lightly sanding

the contact surface of the sample, as well as the holder, and wiping the sanded

*Television Tube Koat is manufactured by G. C. Electronics, Rockford, Illinois.

7

areas with a solvent. If a specimen is cut, the clean, cut surface serves as

a good contact area for grounding. Sample holders which are repeatedly used

generally accumulate fingerprints or debris and are normally lightly oxidized.

Since oily deposits and oxides (especially aluminum oxide) are insulators,

the cleanliness of the sample holder is essential, and often overlooked, in

obtaining a proper ground.

Nonconductive surfaces on the sample must be coated with a thin conductive

material to prevent them from accumulating an electrical charge from the elec­

tron beam, Figure 1. In practice, this is accomplished by grounding the sample

to the holder and then vacuum vapor depositing or sputtering a thin conductive

coating such as gold, gold-palladium, or carbon on its surfaces. Rotating the

sample during vapor deposition ensures a uniform conductive coating and pre­

vents the formation of shadows, Figure 2. For most applications, a 1.5 inch

(3.8 cm) length of 0.008 inch (0.020 cm) diameter gold wire evaporated on a

rotating sample placed approximately 2.5 inches (6.4 cm) from the gold source

(basket) will provide a satisfactory coating. These coatings can also be

S14685 sm 2000X

Figure 1 Charged particle (arrow) on a fracture surface.

8

S14586 8m 1600X

Figure 2 Dark area (arrow) resulting from an uneven distribution of vapor­deposited gold.

S14686 SEM (A) l800X S14687 SEM (B) 2200X

Figure 3 SEM fractographs of a lightly oxidized fracture showing the effect of a poorly conductive surface. (A) as oxidized, (B) gold coated.

applied to metal surfaces to improve their image quality, Figure 3. Some con­

ductive spray coatings are available, however, these are inferior to vapor­

deposited metals and are generally unsatisfactory for fracture analysis.

Aside from a poorly conductive surface, any sample which is even slightly

magnetic will yield poor image quality due to a defocusing effect.(l) There­

fore, it is a good practice to demagnetize (degauss) samples of materials

that can be magnetized because such operations as magnetic particle inspection

or cutting can result in residual magnetism. Small inexpensive demagnetizing

coils are commercially available.

TEM SAMPLE PREPARATION

The actual fracture surface can not be viewed in the TEM because the image is

formed by an electron beam which must pass through the sample. For this

reason, various methods have been developed for replicating the fracture

9

detail on very thin, shadowed carbon films that are transparent to the electron

beam. There are a number of techniques that can be used to replicate the (2 to 5)

fracture surface ; however, only the plastic-carbon method will be

discussed in detail. This technique is convenient to use, does not destroy

the fracture surface, has good resolution, and provid~s satisfactory results

for all routine fractographic examinations.

The plastic-carbon replication technique is shown schematically in Figure 4.

This technique involves replicating the fracture surface with plastic, deposit­

ing a metal (shadowing) and carbon on the plastic, and finally dissolving the

plastic away from the shadowed carbon replica. Each of these steps is

examined in detail in the following discussion.

Plastic Replication

Cellulose acetate tapes ranging in thickness from 0.001 to 0.005 inch (0.003

to 0.013 cm) are used to replicate fracture surfaces. A general rule for

selecting a tape thickness is the more jagged the fracture, the thicker the

tape. However, a thin tape should be used in preference to a thicker one

whenever possible. In many instances, rather than USing a single thickness

of the 0.005 inch (0.013 cm) tape, a double thickness of the 0.001 inch

(0.003 cm) tape may be preferred. The plastic used should be dipped in a

cleaning solution, such as DuPont Freon PCA, to remove any surface contamina­

tion. The plastic can also be cleaned by wiping with lens tissue.

Two different methods can be used to obtain a plastic replica of a fracture

surface. The thickness of the tape that is being used establishes the method

of application. When using cellulose acetate tapes that are less than approx­

imately 0.003 inch (0.008 cm) thick, an acetone solution is used to partially

soften the plastic prior to placing it on the fracture surface. The tape is

softened only to the extent that it will follow the contours of the

fracture. Too soft a plastic film or excessive use of acetone may result in

the formation of vapor bubbles at the plastic-metal interface. The applied

tape is then permitted to dry in place at least 10 minutes without any

application of pressure or heat. When the tape has thoroughly dried and

hardened, tweezers are used to remove it from the fracture. This "negative

replica" of the fracture surface, trinnned to the desired size, is now ready

10

y-- CHROMIUM COATING

............ --CHROMIUM......... ......... - - -VAPORIZED .................. - -_. IN TUNGSTEN SPIRAL

SIDE

SPHERE

- - - -

METAL SURFACE WITH SCRATCH

PLASTIC F ILIvi

PLASTIC :FILM STRIPPED, IhvERTED, AND SHADOWED WITH CHROMIUM.

-

CARBON ARC DEPOSITION OF CARBON

ELECTRON BEAM

v o Figure 4 Plastic-carbon replication method.

11

CELLULOSE ACETATE REMOVED IN ACETONE FILM,INVERTED,AND EXAMINED IN MICROSCOPE.

for shadowing and carbon deposition. Arrow points may be cut on the plastic

replica to orient it with respect to the fracture surface.

If the fracture surface is rough or jagged and the use of a double-thickness

thin tape is not satisfactory, a 0.005 inch (0.013 cm) thick cellulose acetate

tape should be employed. Because of the thickness of the tape, even if it is

softened in acetone, it has little tendency to follow the rough surface

contours. For this reason, some liquid replicating solution (cellulose ace­

tate dissolved in acetone) is applied to one surface of the tape. The

replicating solution is allowed to partially soften the tape (usually 2 to 3

minutes). Just prior to replication, a coating of the replicating solution is

applied to the fracture. The solution-covered side of the tape is then

pressed on to the wetted fracture surface. A firm pressure is exerted on

the tape for about 1 to 2 minutes. When the acetone has evaporated (usually

30-45 minutes), the cellulose acetate from the solution and the tape form a

continuous replica which is then removed from the fracture surface as one unit.

If difficulty is experienced in stripping, it may be found advantageous to

heat the replica and fracture in an oven at 200°F (93°C) for approximately

15 minutes and cool in air to room temperature. The heating process thoroughly

dries the plastic permitting easier stripping with a minimum of replica distor­

tion. The replicas are then placed on a glass slide with the impression side

up in preparation for shadowing and carbon deposition.

Shadowing Techniques and Carbon Deposition

In order to increase the contrast and give a replica a three-dimensional

effect, a process known as shadowing is used. Shadowing is an operation

whereby a heavy metal is deposited at an oblique angle to the surface by

evaporating it from an incandescent filament or an arc in a vacuum chamber,

Figure 4. The shadowing material is deposited at an angle of approximately

45° (smooth surfaces require lower angles) in such a way as to relate with

a known direction such as the macroscopic fracture direction. The vaporized

metal atoms travel in essentially straight lines from the filament and strike

the replica at an oblique angle. Upon contact, the metal condenses where

it strikes. Certain favorably oriented surface features receive a thicker

metal deposit than others, and in fact, some areas adjacent to surface

12

protrusions or depressions receive no metal deposit at all. The difference

in deposited metal thickness between the front and back side of a surface

feature produces a difference in contrast due to an increase in electron

scattering by the metal coated areas. Since electron scattering is a function

of the atomic number and mass density of the element, it would require less

deposition of a heavy metal to produce a desired contrast. Commonly used

shadowing materials are chromium, germanium, palladium, palladium-platinum,

platinum-carbon, and gold-palladium. The minimum amount of shadowing material

necessary to produce contrast should be employed when maximum resolution is

required.

There are several methods used to judge the thickness or amount of shadowing

material deposited. One method is to use a drop of silicone oil on a glass

slide which is placed over a white piece of paper. The difference in contrast

between the paper under the oil drop and the surrounding area gives an indication

of the amount of metal deposited. A finger print on a white piece of paper

or a small solid object placed on a slide may be used to observe the development

of an actual shadow and thus provide a way for estimating the amount of metal

deposited. A good indication of a sufficient amount of deposited material is

the inception of a change in contrast between the shadowed and unshadowed regions.

An insufficient amount of shadow material will result in poor contrast, while

too much material may obscure surface detail and give accentuated shadows.

To avoid granulation of the shadow material, it is necessary to maintain a

high vacuum condition (about 10-4 to 10-5 torr) during the shadowing process.

After shadowing, granulation can also result from exposure to too high an intensity

electron beam. Gold shadowing is particularly sensitive to this phenomenon.

After the shadowing operation, carbon is deposited on the replica. It is this

carbon film with the shadowed metal that is ultimately examined in the TEM.

Carbon is deposited either normal to or at a slight angle to the surface of the

replica. In order to ensure a uniform deposition of carbon, a rotating stage

should be used. The complete evaporation of a 0.040 inch (0.1 em) diameter

0.40 inch (1 cm) long carbon rod located about 5 inches (12.7 cm) above the

13

replica is usually sufficient to yield a sound replica. For optimum results,

the vacuum should be less than 5 x 10-4 torr while depositing the carbon.

Dissolution of Plastic

The replica is immersed in acetone to dissolve the cellulose acetate. A

gentle periodic agitation of the acetone is recommended to facilitate the

dissolving of the plastic. Due to the expansion of the cellulose acetate

during dissolution in acetone, the carbon film may distort and fragment into

pieces so small as to render the replica useless. To prevent this undesirable

condition, the following procedures may be employed:

(1) Use a thinner strip of plastic.

(Z) Use a solution of acetone diluted to 50% by ethyl alcohol or distilled

water. After soaking the replica in this diluted acetone solution until

most of the plastic has dissolved, place it in pure acetone to dissolve

the remaining cellulose acetate.

(3) Use a warm solution of acetone or vapors of acetone to dissolve the

plastic.

(4) Place the trimmed replicas, carbon side up, on a piece of filter paper

in a Petri dish. Using an eyedropper, add a 60 to 70% solution of

acetone in water to the dish by allowing the drops of solution to fall

near the edge of the dish well away from the replicas. The objective is

to float the replicas without getting the carbon surface wet. Allow

the replicas to float for several hours and then add straight acetone to

remove the remaining plastic.

(5) Use paraffin to strengthen the carbon film during the dissolution of

the plastic. The paraffin is removed by soaking the replica in benzene.

After the plastic film is dissolved, the carbon replica is picked up on a 75

to ZOO mesh screen. Screens of various grid configurations are available. The

screen holding the replica is then placed on filter paper in a covered container

until it is inserted in the TEM.

14