When 9:00 AM - 11:00 AM Sep 03, 2015
Where 1013 H.H. Dow
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Quantitative Studies of Microstructural Phase Transformation: Critical Features of Nickel Titanium Polycrystals Undergoing Superelastic Deformation


Michael Kimiecik
Thesis Defense

Samantha Daly and J. Wayne Jones, advisors

 

Shape memory alloys are a class of materials able to recover from large strains via a solid-to-solid, diffusionless phase transformation. Despite their adoption in a number of industries, ranging from biomedical devices to actuators in automotive and aerospace applications, there is still a poor understanding of how their microstructure influences the martensitic transformation that gives these alloys their desirable properties. 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 discussed and found to be inaccurate. 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 related to the resolved shear stress on twin planes as identified using logistic regression on the configuration fractions and the underlying features 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 a foray into understanding the complex interactions taking place when cycling superelastic material.