Grain orientation dependence of phase transformation in the shape memory alloy Nickel-Titanium

The dependence of grain orientation on stress-induced martensitic transformation in superelastic, polycrystalline Nickel-Titanium sheet was examined at the microstructural length scale. Full-field strains, indicative of transformation extent, were characterized in fields of view of nominally 100 μm × 100 μm using a custom combination of scanning electron microscopy with distortion-corrected digital image correlation. It was found that similarly oriented grains do not necessarily transform similarly, in contrast to a common assumption in mean-field theories. Specifically, grains with similar orientation (as determined by the misorientation of the grain and specimen axes) showed variation in both the mean strain of the grain as well as the range (heterogeneity) of strain across the grain, as determined from surface measurements. Additionally, neither grain size nor degree of misorientation (of common crystal axes from the loading axis) affected the mean strain and strain range.