Quantitative Studies of microstructural phase transformation in Nickel–Titanium

A new experimental approach is used to quantify the full field heterogeneous strain accommodation associated with martensitic phase transformation in Nickel–Titanium at the microstructural length scale. Local deformation measures were matched with underlying microstructural characteristics obtained by EBSD, providing point-by-point correlation between transformation extent and microstructure. It was found that a single grain can accommodate both a heavily deformed martensite phase and an austenite phase, and that the transformation does not progress in an ordered fashion but rather can skip over grains or portions of grains in an extremely heterogeneous manner. The martensite front was diffuse at the microstructural length scale, and formed a cross-hatch structure in advance of the stable band. Strain accommodation was strongly heterogeneous at the grain level; while on average the martensite band accommodated approximately 10% strain, some individual regions accommodated up to 20% strain while regions of retained austenite remained at 1% strain.

► A new approach to characterize phase transformation in SMAs at the microstructural length scale is proposed. ► In superelastic Nitinol, martensite formed heterogeneously across and within grains as it progressed from grain to grain. ► Microscale Martensite formed a diffuse network in advance of the stable band; transformation and recovery took place only at the phase front. ► Approach enables new insights into microscale mechanisms of transformation and its relation to underlying crystallography.