In-situ Observation of the Reverse Martensitic Transformation in a Ti–Pd Shape Memory Alloy

Thermoelastic martensitic transformation governs the shape memory effect in shape memory alloys. In this study, in-situ digital holographic microscopy was employed to clarify the mechanism of thermoelastic martensitic transformation in a Ti–Pd high temperature shape memory alloy during reverse transformation. At the initial stage of the reverse transformation, a habit plane variant (HPV) pair satisfying favorable self-accommodation and kinematic compatibility (KC) conditions transformed simultaneously from within a grain interior. Subsequently, secondary reverse transformation occurred in HPVs located adjacent to the first-transformed HPV pair, selected by their KC compatibility with the initially transformed HPV. These results demonstrate that, at elevated temperature, the reverse transformation in Ti–Pd is governed by strain-reduction and interfacial compatibility, rather than by the release of stored elastic energy, as typically observed in low-temperature SMAs such as Ti–Ni.