Negative temperature-dependence of stress-induced R→B19′ transformation in nanocrystalline NiTi alloy

The 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. IMPACT STATEMENT Negative temperature-dependence of stress-induced R -> B19' transformation was discovered for the first time in NiTi alloys, being in sharp contrast to previous reports. This calls for a revisit of such transformation.

Automated summary

The temperature dependence of the critical stress of the R -> B19' transformation in NiTi shape memory alloys has been found to be negative in this study, in contrast to previous reports. This discovery calls for a reevaluation of this transformation.