Quantitative Studies of Microstructural Phase Transformation: Critical Features of Nickel Titanium Polycrystals Undergoing Superelastic Deformation.

Shape Memory Alloys;Martensite;Fatigue;Digital Image Correlation;Superelasticity;Materials Science and Engineering;Engineering;Materials Science and Engineering

Kimiecik, Michael GregoryMarquis, Emmanuelle ;

University of Michigan

关键词: Shape Memory Alloys;    Martensite;    Fatigue;    Digital Image Correlation;    Superelasticity;    Materials Science and Engineering;    Engineering;    Materials Science and Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/116620/mkimieci_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This dissertation addresses the stress-induced phase transformation that occurs upon the loading of Nickel-Titanium alloys, leading to their superelastic properties. Superelasticity is widely utilized in biomedical applications including stents and valves, but there is still much that is unknown regarding the interaction between the superelastic martensitic transformation and microstructure of these materials. To help fill this knowledge gap, an experimental technique combining high resolution Scanning Electron Microscopy and Digital Image Correlation was applied to the analysis of superelastic nickel titanium shape memory alloy. The displacement and subsequent strain maps generated from this technique, combined with crystallographic orientation gathered via Electron Backscatter Diffraction, allowed the tracking of subgrain martensite transformation in polycrystalline specimens. Additional analytical tools were developed to determine the configuration of martensite variants in each transformed grain. These observations provide new details on how the macroscopic martensite band progresses through polycrystals and how that martensite is configured in the transformed martensite band. The previously held assumption that in a superelastic polycrystal similarly oriented grains transform similarly is found to be inaccurate based on experimental observations. Previously unobserved lattice correspondence variant configurations of martensite were found to readily develop in the polycrystalline specimens in addition to the previously observed habit plane variants. The appearance of these correspondence variant configurations was due to the large resolved shear stress on twin planes as identified using logistic regression on the configuration fractions and the underlying grain orientations of the polycrystal. Those areas which primarily transformed to correspondence variant configurations accumulated an increased amount of residual strain, which served as a blueprint for subsequent transformation. While the interaction between martensite, plasticity, and polycrystalline microstructures requires more work to fully characterize, this dissertation represents an attempt to understand the complex interactions taking place in superelastic material undergoing cyclic loading.