BSTRACTThe temperature dependence of the critical stress of R→B19’ transformation in NiTi shape memory alloys remains unclear due to the difficulties in precisely identifying the phase constitution and acquiring sole R phase. Here we investigate the thermally- and stress-induced martensitic transformation in a near-equiatomic nanocrystalline NiTi alloy.

In situ

synchrotron X-ray diffraction is used to identify the phase evolution such that the temperature window for sole R phase is firmly determined. We find that the temperature dependence of R→B19’ is negative (−0.2 MPa/K) in our alloy

in sharp contrast to 3–5 MPa/K reported. Possible reasons for the discrepancies are discussed.