X-Ray Microdiffraction on Diamond-shaped NiTi for Biomedical Applications

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X-Ray Microdiffraction on Diamond-shaped NiTi for
Biomedical Applications

• Apurva Mehta
• SSRL/ SLAC, Stanford University

Valentina Imbeni
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New Boss
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Collaborators

• Valentina Imbeni SRI
• Brad Boyce Sandia Labs
• Nobumichi Tamura LBL
• Xiao-Yan Gong, Alan Pelton, Tom Duerig NDC
• Rob Ritchies Group (Scott Robertson, Monica
  Barney) LBL/ UC Berkeley

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MotivationMacroscopic ?---? Microscopic

• Understanding of Deformation and Failure of NiTi
  components at Local Level under Multiaxial
  Loading.
• Validation of Design Models.
• Towards Improved Models that include
• Austenite to Martensitic Phase Transition
• Mechanics Beyond Continuum Mechanics.

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MotivationE.g., understanding Fatigue Tests

• Location of Fracture
• Increase of Fatigue Life Above 1.5 Strain !!

A. Pelton et. al. - NDC
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Talk Outline

• What did we do?
• Methodology
• What did we find?
• Diamond in Compression
• Diamond in Compression Cycling
• Diamond in Tension
• Five New Insights

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MethodologyLoad Cell
X-ray Beam

• Nitinol Tube 4.67mm OD with 0.38mm wall
• Laser machined
• Fully Annealed Grains 20-100 microns

FEA Simulations
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MethodologyX-ray Microdiffraction
Bend Magnet Source (250x40mm)
CCD camera
4 Crystal Si(111) Monochromator
11 Toroidal mirror
11 image at slits
Elevation view
Sample on scanning XY stage
Plan view
Horizontal focusing K-B mirror
Vertical focusing K-B mirror
Schematic layout of the X-ray Microdiffraction
Beamline (7.3.3.) at the ALS
Beam size on sample 0.8x0.8 mm2 Photon energy
range 5-14 keV
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MethodologyX-ray Microdiffraction-1 micron spot

• Ni Ti Fluorescence
• Austenite Diff. Pattern

Grain Map
Elastic Strain
Plastic Strain
NiTi Diffraction Patterns
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Strain Tensors
In crystal reference frame
In Sample reference frame
exx exy exz
exy eyy eyz
exz eyz ezz
Crystal Orientation From Laue Patterns
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Displacement ?? Strain
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Findings
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CompressionD 0 mm F 0 N
eyy
exx
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CompressionD 0.5 mm F -0.393 N
eyy
exx
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CompressionD 1.0 mm F -0.747 N
eyy
exx
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CompressionD 1.5 mm F -1.080 N
eyy
exx
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CompressionD 2.5 mm F -1.465 N
eyy
exx
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CompressionD 3.7 mm F -1.543 N
eyy
exx
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CompressionD 3.7 mm F -1.543 N
Austenite Martensite
eyy
Phase Map
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Insight 1
Finite Elem. Analysis
Microdiffraction
X. Y. Gong et al.
3.7 mm compression
Qualitative agreement with FEA But Granular and
Speckled
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Insight 2

• Local Strain Never exceeds 1.5
• NiTi Superelastic because the Aust. And Mart.
  Elastic region separated by a large region of
  Transformation Strain

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Insight 3

• Strain relief on transformation
• Strain reversal

Nucleation energy
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CompressionD 2.5 mm unload F -1.037 N
eyy
exx
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CompressionD 0.0 mm unload F 0.282 N
eyy
exx
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Load Cycling _at_3.7 mm
One Cycles 3.7- 0- 3.7 mm
Eleven Cycles 4.9 2.5 - 3.7 mm
Zero Cycles 0 3.7 mm
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Insight 4

• On cycling Martensitic region grows.
• Growth Pattern unpredictable from FEA
• Strain relief as Martensite grows
• Explanation for increased Fatigue Life for
  macroscopic strains gt 1.5

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Tension eyy
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Insight 5

• Transformation front and hence stress hotspot
  changes direction, and traverses down the stem of
  the diamond.
• Failure occurs when the hotspot encounters a
  defect or weakness in the material. Location of
  failure maybe different from FEA prediction.

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Summary

• Insights
• Strain map granular, martensite evolution
  speckled.
• In the superelstic region max stress doesnt
  exceed stress corresponding to 1.5 Austenite
  strain.
• Strain relief and strain reversal at the
  transformation front.
• On load cycling, the martensite region grows.
  Overall stress drops.
• Transformation and max stress front changes
  directions.
• Further Questions
• What is the crystallographic relationship between
  the Martenite and the Austenite phase?
• What happens around a crack tip?

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Crystallographic Relationships
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Thanks !