Micro-Mechanical Testing For Nuclear ApplicationsStress –Strain Responses and Size Effects...
Transcript of 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
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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
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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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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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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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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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• 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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Thanks To
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• Steve Roberts
• Angus Wilkinson
• Michael Rieth
• Ben Britton
• CCFE
• St Edmund Hall, Oxford