Stability of propagating Austenite-Martensite interface in Ni-Mn-Ga single crystal

The transformation between the austenite and martensite phases in Shape Memory Alloys (SMAs) occurs via the nucleation and propagation of an Austenite-Martensite interface (A-M interface), whose morphology/configuration determines the driving force (energy dissipation) of the phase transformation. In this paper, we develop an experimental setup to control the temperatures at the two ends of the Ni-Mn-Ga single crystal to create an A-M interface and observe its cyclic propagation during the forward and reverse martensitic phase transformations. It is found that, during the heating-induced M→A transformation, the A-M interface is not always the planar habit-plane, but can be a non-planar interface consisting of numerous wedges/needles that contain different martensite twins forming compatible internal structures (like self-accommodation configuration). By contrast, during the cooling-induced A→M transformation, the interface is always planar with simple martensite twin laminates in the interfacial transition zone. These observations imply that the interface propagation and the associated kinetics (driving forces and energy dissipation) are not the same during the forward and reverse martensitic phase transformations. These results not only provide insights into the physical mechanism of the phase transformation, but also encourage the improvement of the theoretical modeling to better predict the material behaviors.