Characterization of in situ Deformation Texture in Superelastic Nitinol

This dissertation is motivated by the increasing usage of Nitinol in biomedical implant devices as well as the disturbing numbers of device failures as reported in the literature. Given the recent awareness of the complex in vivo loading conditions experience by the devices, combined with the lack of commercial finite element analysis program capable of accurate lifetime prediction for such devices, it is imperative to understand the fundamental mechanism of the underlying phase transformation behind the superelastic mechanical properties of Nitinol. Macroscopic multi-mode fatigue is examined through classical continuum mechanics. It is found that normalization of the dissimilar mechanical behaviors of superelastic Nitinol under multi-modes loading is possible through Coffin-Manson approach. The influence of austenitic texture under in situ multi-mode monotonic and fatigue loading is explored with micro-X-ray Diffraction. It appears that the martensitic transformation propagation occurs in competing pathways between the grain boundary and grain with favorable transformation orientations. It is found that initial austenitic textures as well as loading modes favors different propagation paths. The resulting evolution of austenite texture may explain the fatigue degradation of Nitinol. Austenite texture sharpness as well as grain boundary density is found to have negative influence on the fatigue lifetime of Nitinol. The results determined through this research can be used to derive a better constitutive relation as well as methods for better fatigue lifetime prediction.