X-Ray Microdiffraction Analysis of Diamond-shaped NiTi for Enhanced Biomedical Applications

1. X-Ray Microdiffraction on Diamond-shaped NiTi for Biomedical Applications Apurva Mehta SSRL/ SLAC, Stanford University Valentina Imbeni SEM X International Congress

2. New Boss SEM X International Congress

3. Collaborators • Valentina Imbeni – SRI • Brad Boyce – Sandia Labs • Nobumichi Tamura – LBL • Xiao-Yan Gong, Alan Pelton, & Tom Duerig – NDC • Rob Ritchie’s Group (Scott Robertson, Monica Barney) – LBL/ UC Berkeley SEM X International Congress

4. fractures In vivo loading Motivation:Macroscopic --- 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. SEM X International Congress

5. MotivationE.g., understanding Fatigue Tests Location of Fracture Increase of Fatigue Life Above 1.5% Strain !! A. Pelton et. al. - NDC SEM X International Congress

6. Talk Outline • What did we do? • Methodology • What did we find? • Diamond in Compression • Diamond in Compression Cycling • Diamond in Tension • Five “New” Insights SEM X International Congress

7. compression Tension MethodologyLoad Cell X-ray Beam • Nitinol Tube 4.67mm OD with 0.38mm wall • Laser machined • Fully Annealed – Grains ~ 20-100 microns FEA Simulations SEM X International Congress

8. MethodologyX-ray Microdiffraction Bend Magnet Source (250x40mm) CCD camera 4 Crystal Si(111) Monochromator 1:1 Toroidal mirror 1:1 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 SEM X International Congress

9. 10 mm MethodologyX-ray Microdiffraction-1 micron spot • Ni & Ti Fluorescence • Austenite Diff. Pattern Grain Map Elastic Strain Plastic Strain NiTi Diffraction Patterns SEM X International Congress

10. Deviatioric Dilational Strain Tensor From Laue Patterns deviations (broad bandpass (White) X-rays) From energy scan (Variably Monochromated X-rays) Strain Tensors In crystal reference frame In Sample reference frame + Crystal Orientation From Laue Patterns SEM X International Congress

11. Displacement  Strain SEM X International Congress

12. Findings SEM X International Congress

13. CompressionD = 0 mm : F = 0 N eyy exx SEM X International Congress

14. CompressionD = 0.5 mm : F = -0.393 N eyy exx SEM X International Congress

15. CompressionD = 1.0 mm : F = -0.747 N eyy exx SEM X International Congress

16. CompressionD = 1.5 mm : F = -1.080 N eyy exx SEM X International Congress

17. CompressionD = 2.5 mm : F = -1.465 N eyy exx SEM X International Congress

18. CompressionD = 3.7 mm : F = -1.543 N eyy exx SEM X International Congress

19. CompressionD = 3.7 mm : F = -1.543 N Austenite Martensite eyy Phase Map SEM X International Congress

20. Insight #1 Finite Elem. Analysis Microdiffraction X. –Y. Gong et al. 3.7 mm compression Qualitative agreement with FEA But – Granular and Speckled SEM X International Congress

21. Martensite Austenite Aust + Mart 2 phase region Transformation strain @ const. Stress s 1.5% Molar vol ~ strain 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 SEM X International Congress

22. Austenite 1.5% Molar vol ~ strain Insight #3 • Strain relief on transformation • Strain reversal Nucleation energy SEM X International Congress

23. CompressionD = 2.5 mm unload : F = -1.037 N eyy exx SEM X International Congress

24. CompressionD = 0.0 mm unload : F = +0.282 N eyy exx SEM X International Congress

25. Load Cycling @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 SEM X International Congress

26. 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 > 1.5 % SEM X International Congress

27. Tension : eyy SEM X International Congress

28. 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. SEM X International Congress

29. Summary • Insights: • Strain map granular, martensite evolution speckled. • In the superelstic region max stress doesn’t 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? SEM X International Congress

30. Crystallographic Relationships SEM X International Congress

31. Thanks ! SEM X International Congress