Hysteresis effect on austenite-martensite interface in Ni-Mn-Ga single crystal

The experiments on Ni-Mn-Ga single crystal shape memory alloy under a heating-cooling cycle demonstrate that the automatically generated twin laminate structure of the compatible austenite-martensite interface in the forward martensitic phase transition is significantly different from that in the reverse phase transition, even though the temperature hysteresis is small (As - Ms around 4 ◦C). Moreover, after the cooling-induced austenite → martensite transition, the remaining twin laminate is so fine (layer width around 1 μm) that the neighbouring twin boundaries merge with each other and disappear, making the fine twin laminate evolve into a single martensite variant (i.e., spontaneous detwinning). These observations provide insights into the relation between
the hysteresis (phase-transition driving forces), austenite-martensite interface, and the basic material parameters such as twin boundary energy and softening modulus during the phase transition.