Investigation of deformation processes in shape memory alloys by 3D x-ray diffraction and x-ray microscopy

Abstract

In the case of shape memory alloys (SMAs), fundamental micromechanical theory has been an active area of research for more than 70 years. However, experiments to validate these theories on the microstructural scale are relatively new, challenging, and often limited to two-dimensional surface measurements. As a result, there persists a general lack of understanding regarding the relationships that link microstructure evolution to macroscopic behavior. To address this open area, we utilize cutting-edge in situ synchrotron X-ray techniques such as near-field and far-field 3D X-Ray Diffraction (3DXRD) and Dark-Field X-Ray Microscopy (DFXM). Using these techniques, I present results from three particular experiments on NiTi SMAs: (1) The micromechanics behind localized deformation bands in martensite reorientation are reported using 3DXRD, revealing a geometric softening effect across the band interfaces; (2) A forward model algorithmic approach to indexing martensite in two-phase 3DXRD data is presented and used to illustrate the shortcomings of the maximum work criterion; (3) The topology, misorientation, and elastic strains inside an austenite single crystal during thermally-induced transformation are shown using DFXM with a spatial resolution of 100 nm.

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