Synchrotron-based multiscale study on phase transformation in a cold-rolled NiTi shape memory alloy: Effects of preexisting defects

Abstract

Deformation and phase transformation of a cold-rolled, textured, shape memory alloy sheet Ni50.6Ti49.4 are investigated via real-time, in situ synchrotron X-ray diffraction and digital imaging correlation. The specimens are subjected to tensile loading along two orthogonal directions. Bulk stress–strain curves (macroscale), strain fields (mesoscale) and X-ray diffraction patterns (microscale) are obtained simultaneously. Martensite nucleation is highly correlated with the preexisting defects. Different from the widely observed Lüders bands within defect-free NiTi, uniform martensite phase transformation occurs in the cold-rolled alloy as a result of the relatively widely distributed preexisting defects, giving rise to progressive growth of martensite, relatively uniform strain fields, the sigmoidal stress–strain curves and increased strain hardening rate. Different favorably oriented martensite variants are induced due to the initial texture, and the martensite phase transformation is more active for loading along the less favorable orientation, giving rise to the anisotropy in the phase transformation.