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

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.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116620/1/mkimieci_1.pd

Microstructural strain memory and associated plasticity in superelastic niti under low cycle fatigue

When cyclically loaded in tension, superelastic Nickel Titanium (NiTi) undergoes a characteristic shakedown behavior which dramatically changes its hysteretic stress–strain response. As many uses of superelastic NiTi involve cyclic loading, a detailed understanding of the interaction between phase transformation and associated plasticity is necessary to predict the lifetime behavior of NiTi devices. Earlier macroscopic studies have dealt with this phenomenon on a bulk material level, but its microstructural origin and small scale analogues remain largely uninvestigated. To that end, low cycle, low strain-rate fatigue tests were performed on superelastic NiTi sheet to examine the local damage and accumulation of plastic deformation that contribute to the evolution of its stress strain response. Local strain measured in situ with Scanning Electron Microscopy Digital Image Correlation was matched with individual microstructural features – such as individual parent grains and grain neighborhoods – measured with Electron Backscatter Diffraction. Martensitic transformation associated with superelasticity was inferred from the full-field strain maps captured each load cycle. Special attention was paid to the particular martensite variants and twinning modes that nucleate in the first cycle and their similitude to subsequent martensite transformation. In addition, cyclic behavior such as martensite retention and ratcheting, strain memory of both martensite and austenite configurations, and damage accumulation are also considered