Nitinol is a Ni-Ti shape-memory alloy which is important for biomedical implant applications due to its mechanical property of superelastically. It derives this property (of up to -10% reversible deformation) from an in situ stress-induced martensitic transformation between its austenite and martensite phases. This transformation is traditionally assumed to be the major source of strain below ~8%, and occurs when local strain within the austenite phase exceeds ~1.2%. In this work, we used synchrotron x-ray micro-diffraction to examine the grain-by-grain source of the deformation during the in situ phase transformation of NiTi tensile ("dogbone") samples loaded in the x-ray beam. Although most individual austenite grains were observed to transform to martensite at local strains exceeding 1.2%, we found that numerous austenite grains remained untransformed at strains above this level. Although this effect can be due to grain orientation, we believe that it is also associated with specific austenite grains becoming locally plastically deformed instead of transforming, i.e., that the martensite transformation is not the sole contribution to the strain below ~8%. We consider this notion of local austenite plasticity to be a feasible explanation for the limits to the reversibility of transformation during the cyclic loading of NiTi.
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