Micro-Mechanical Testing For Nuclear ApplicationsStress –Strain Responses and Size Effects...

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Micro-Mechanical Testing For Nuclear Applications David E.J. Armstrong, CCFE Junior Research Fellow, Department of Materials, University of Oxford St Edmund Hall, University of Oxford PFMC/FEMaS Conference May 2011

Transcript of Micro-Mechanical Testing For Nuclear ApplicationsStress –Strain Responses and Size Effects...

Page 1: Micro-Mechanical Testing For Nuclear ApplicationsStress –Strain Responses and Size Effects Measuring Properties of Grain Boundaries Conclusions Introduction 28/06/2011 D.E.J Armstrong

Micro-Mechanical Testing For Nuclear Applications

David E.J. Armstrong,

CCFE Junior Research Fellow,

Department of Materials, University of Oxford

St Edmund Hall, University of Oxford

PFMC/FEMaS Conference

May 2011

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IntroductionWhy Micro-mechanical Testing?

BasicsFIB MachiningNanoindentation

Micro-mechanical Testing Case StudiesStress – Strain Responses and Size EffectsMeasuring Properties of Grain Boundaries

Conclusions

Introduction

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The People

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Steve RobertsAngus WilkinsonDavide Di MaioJicheng GongBen Britton

Fiona HalidayMike RogersWill Herbert

Lawrence WhyattJames Robinson

Ele GrievesonJames Gibson

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Steve RobertsAngus WilkinsonDavide Di MaioJicheng GongBen Britton

Fiona HalidayMike RogersWill Herbert

Lawrence WhyattJames Robinson

Ele GrievesonJames Gibson

The People

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Micro-mechanical testing

• Recently developed testing techniques

• Utilizes Focused Ion Beam (FIB) machining and nanoindentation

• Allows manufacture of samples with well-defined stress states

• Allows fracture properties, yield strengths and elastic properties to be measured

• Temperature variation now available

+

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Why use micro-mechanical testing?

• Useful where only small samples are available

– Cost

– Processing

• Need for a sample design that can be machined in surface of bulk samples

• Suitable for measuring individual microstructuralfeatures

• Samples that can be manufactured quickly and reproducibly

1um

3 m 10 m

4 m

3 m

3 m

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Types of micro-mechanical testing?

• Compression

• Tension

• Three Point Bend

• Cantilever bending

1um

3 m 10 m

4 m

3 m

3 m

• Electro-deposition

• Selectively etched

• FIB machined

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Nanoindentation

• Nanoindentation mechanical probe which allows local hardness and modulus to be measured

• A sharp diamond is driven into the surface with a known force

• Displacement is measured using a capacitance gauge

• Sharp tip can also be used as a surface profilometertool

• Also very useful to deform and test specimens

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Nanoindentation

• By knowing the contact area between sample and indenter hardness and modulus from unload can be calculated

• A small ac sinusoidal can also be placed on the load

• This Continuous Stiffness Measurement (CSM) allows the modulus and hardness to be continually measured as a function of depth

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Nanoindentation

Ion Beam

8mm

See talk on Wednesday for selected results on nanoindentation of ion implanted surfaces in

tungsten and tungsten alloys

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Focused Ion Beam Machining• FIB uses gallium ion

(Ga+) to “knock” atoms out of the sample being machined

• Ions focused on surface of sample using electromagnetic lens (similar to SEM)

• Beam currents from 1pA to 45nA allow features as small as 5nm to be machined

• Also allows deposition of Pt/W/C

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• FIB uses gallium ion (Ga+) to “knock” atoms out of the sample being machined

• Ions focused on surface of sample using electromagnetic lens (similar to SEM)

• Beam currents from 1pA to 45nA allow features as small as 5nm to be machined

• Also allows deposition of Pt/W/C

Focused Ion Beam Machining

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Microcantilever Manufacture

10 m

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2 m

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When FIB goes wrongMy First Cantilever-

November 2004

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When FIB goes wrong

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When FIB goes wrong

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When FIB goes wrong

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Case Study One:Elastic

Anisotropy

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Measuring Elastic Anisotropy

• Elastic properties can control deformation processes and important for engineering design

• Most materials display elastic anisotropy

• Difficult to measure experimentally unless large single crystal available

• Traditional techniques – static or dynamic require large (mm to cm) samples

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Elastic Anisotropy In Copper• Copper: highly

anisotropic well characterised material

• Should be an “easy” starting material

• Cantilevers manufactured in single crystal sample at 15o

intervals between [100] and [110] directions

• Cantilevers scanned using “nano-vision” stage to produce topographical image

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Multiple loading method

• Longer, thinner cantilevers

• Cantilever loaded using nanoindenter close to free end

• Each loading to 200nm (no yield)

• Indenter moved 700nm towards fixed end and cantilever loaded

• Repeated between 5 and 13 times

• Use unload data

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Elastic Anisotropy In Copper

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Analysis of Elastic Properties 1

• From simple beam theory:

• But due to non-fixed end

there is extra deflection at

the fixed end:

• Ignoring lower order terms

beam compliance can be

written as:

EI

LS

3

3

00

3

3L

EI

PLL

2

0

3

3

1LL

EIS m

• Thus for longer beams the effect of the fixed end

is less important28/06/2011 D.E.J Armstrong 2011

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• Plot of S versus L3

shows linear relationship at larger values of L

• The gradient of this linear region can be used to find Young’s modulus

• Analysis carried out on cantilevers at 15o

intervals between [100] and [110]

• Found to give good results for aspect ratio greater than 6

Analysis of Elastic Properties 2

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Analysis of Elastic Properties 3

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Elastic Anisotropy in Copper

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[ 15 12 1]

Measured range

117-131GPa

B

( 7 8 9 ):[ 15 12 1]

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Size Effects on Yield Stress

• Well known that as specimen size decreased yield stress increases

• Exact form of this relationship in triangular microcantilevers unknown

• Cantilevers machined in single crystal copper with long axis in [110] direction

• Range in size from 1 m thick and 10 m long to 18 m thick and 100 m long

• Tested at constant displacement rate of 5nm/s

• Only smallest cantilevers can be used to study ion implanted layers - difficult

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Size Effects on Yield Stress

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• Pillars machined into the ion implanted layers, using multi stage approach

• Width approx 500nm

• Height 3 m

• Flat Punch type nanoindenter tip used to compress the pillars

FeCr Micro Pillars

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Yield Properties in FeCr Alloys

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FeCr Micro Pillars

Unimplanted Implanted

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FeCr Micro Pillars

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Case Study Two:Fracture of Grain

Boundries

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Measuring grain boundary fracture toughness

• Polycrystalline material properties often controlled by grain boundaries

• Measurement of single boundaries difficult/expensive

• Bi-crystals may only contain “special” boundaries

• Need to be able to compare local chemistry with mechanical properties –especially after irradiation 1 m

10 m

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Page 35: Micro-Mechanical Testing For Nuclear ApplicationsStress –Strain Responses and Size Effects Measuring Properties of Grain Boundaries Conclusions Introduction 28/06/2011 D.E.J Armstrong

Sample manufacture• Copper bismuth well known

for GB fracture at room

temperature

• Mechanism and anisotropy of

embrittlement not well

understood

• Sample contains 0.02wt%Bi

(60ppm)

• Cast in vacuum inside quartz

tubes @1374K - slow cooled

• Samples sectioned into bars

and discs for testing

• Large grains with no visible

precipitates 300 m

5mm

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• Only grain boundaries

running normal to

surface tested

• Cantilevers have

pentagonal cross-

section

• Sharp notch milled at

grain boundary to act

as fracture initiation

site

Cantilever manufacture

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D.E.J Armstrong 201139

Cantilever manufacture

• Only grain boundaries

running normal to

surface tested

• Cantilevers have

pentagonal cross-

section

• Sharp notch milled at

grain boundary to act

as fracture initiation

site

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D.E.J Armstrong 2011D.E.J.Armstrong 2008 40

EBSD

Used to characterise

misorientation at g.bs being

tested

Allows g.bs of specific

misorientation to be selected

for testing

70 m

1-SEM image

2- Grain

orientations

(normal IPF map)

3-Grain

boundaries of

greater than 5o

misorientation

1

2 3

10 m

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General boundary

3 - twin

Testing of micro-cantilevers

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Tested - Fracture

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Tested - No Fracture

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D.E.J.Armstrong 2008 44

For a pentagonal beam assuming small deflections:

I

Lypc 4)(

6288)

2(

12

2242

3 wyb

bwbw

by

wbI

)(1b

aFaK cc

432 )(22)(57.35)(5.14)(53.21)(b

a

b

a

b

a

b

a

b

aF

This allows the fracture toughness for pentagonal

beams to be calculated from the load displacement data

and beam dimensions.

Calculation of GB fracture toughness

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p=load at fracture

w=width

b=beam depth

a=crack depth

L=length

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Do we have all dimensions?

Load – Easy. From NanoindenterWidth – Easy. From SEM images pre testDepth – Medium. From SEM image, more difficult than W as sample must be tilted and only end can be measuredCrack depth- Hard. Can estimate before testing but MUST be measured post testing as reproducibility is poorLength – Hard. Can’t be directly measured on fractured specimens. Can be measured using AFM scan

p=load at fracture

w=width

b=beam depth

a=crack depth

L=length

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Results

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Results

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4848

EDX spectrum from bulkEDX spectrum from grain boundary

Bi

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TEM EDX – FIB lift-out

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Applied to real systems

• Copper-bismuth is not of engineering use

• Many important nuclear materials are brittle– steels, under the right

conditions– tungsten– ceramics

• Investigation into GB fracture in temper-embrittled steels

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Temper-embrittled Steel

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Load-displacement data

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Load-displacement data

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Cantilever after testing

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High strain rate testing

MML nanotest platform

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Failure

• It was not possible to achieve brittle fracture in temper embrittled S80 steel

• Although it is brittle the macro-fracture toughness is estimated to be 20 MPam0.5

• The plastic zone around the crack tip is large

• For a micro-scale specimen to be fractured would need to be ≈10mm (Not very micro!!!)

• But James did write up a very good thesis!!

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Page 54: Micro-Mechanical Testing For Nuclear ApplicationsStress –Strain Responses and Size Effects Measuring Properties of Grain Boundaries Conclusions Introduction 28/06/2011 D.E.J Armstrong

Applications to Tungsten

• Tungsten is brittle (5MPam0.5 )

• Important for nuclear fusion applications

• Need to understand how to control brittle behaviour

• Tests now being used to characterize brittle boundaries (James Gibson)

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Grain Boundaries in Tungsten

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Grain Boundaries in Tungsten

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Grain Boundaries in Tungsten

Cantilever Fracture Toughness (KJC) (MPapm)

1 4.9

5 17.9

9 0.53

11 21.2

12 6.52

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60

• Micro-cantilever tests allow us to measure a range of material properties

• Effect of single grain boundaries can be measured

• Small volumes of materials needed for many results

• Allows results which are not obtainable using conventional tests

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Summary and Future Questions

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Summary and Future Questions

• Problems in working in such small specimens?

• Are the results representative of bulk samples?

• How do Ga+ ions damage the specimens?

• Can modelling explain size effects?

• Can tests be performed at high temperature?

• Micro-cantilever tests allow us to measure a range of material properties

• Effect of single grain boundaries can be measured

• Small volumes of materials needed for many results

• Allows results which are not obtainable using conventional tests

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Page 60: Micro-Mechanical Testing For Nuclear ApplicationsStress –Strain Responses and Size Effects Measuring Properties of Grain Boundaries Conclusions Introduction 28/06/2011 D.E.J Armstrong

Thanks To

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• Steve Roberts

• Angus Wilkinson

• Michael Rieth

• Ben Britton

• CCFE

• St Edmund Hall, Oxford


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