Deformation-induced martensite stabilisation in [1 0 0] single-crystalline Ni–Ti

Abstract

It is known that deformation causes stabilisation of the thermoelastic martensite in shape memory alloys. Several hypotheses have been proposed in the literature to explain this phenomenon. One such explanation suggests that grain boundaries in polycrystalline materials contribute to the stabilisation effect by acting as a plastic region to resist the shape recovery during the reverse transformation. This study was carried out to verify this hypothesis in single crystal NiTi. It was found that tensile deformation along the low index directions of the austenite caused stabilisation of the martensite. Transmission electron microscopic examination showed that numerous dislocations were generated by deformation, even at low strain levels. However, the stabilisation effect, as measured by the magnitude of the increase of the critical temperature for the reverse transformation, was much weaker than that observed in similar, but polycrystalline NiTi, supporting the hypothesis that grain boundaries contribute to martensite stabilisation in polycrystalline materials.