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

摘要

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.

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