Inter-martensite strain evolution in NiMnGa single crystals

Stress-induced martensitic transformations are clarified in classes of NiMnGa alloys which undergo the stress-free, thermal-induced inter-martensite transformation austenite (A) ⇌ pre-martensite (PM) ⇌ martensite. This study implements a comprehensive experimental approach, including analysis of the strain–temperature and stress–strain response, which discloses stress-induced inter-martensite transitions. The evolution of the transitions is elucidated using in situ digital image correlation (DIC) measurements of meso-scale strain fields. Under stress, this body of work unequivocally demonstrates that the transformation path becomes A ⇌ PM ⇌ I ⇌ 10M. The I-phase is an intermediate stress-induced martensite with a modulation period between three and five-layers. Owing to the intermediate transition, the thermal hysteresis in the strain–temperature response is tiny (<10 °C) compared with the hysteresis (32 °C) for A ⇌ 10M. The differential hysteresis levels are rationalized based on a thermo-mechanical formulation. Meso-scale DIC measurements quantify inter-martensite strain levels, which are indistinguishable from macro-scale stress–strain and strain–temperature responses.