Low-fatigue large elastocaloric effect in NiTi shape memory alloy enabled by two-step transition

NiTi shape memory alloys are promising elastocaloric materials owing to their substantial adiabatic temperature change (ATad). However, the simultaneous attainment of large ATad and low fatigue poses challenges due to the significant hysteresis and severe functional fatigue associated with the autocatalytic avalanche-like martensitic transformation. This study demonstrates a continuous two-step transition in Ni50.8Ti49.2 (at.%), showcasing cyclic-stable superelasticity with large recoverable strain (5.9 %), substantial ATad (19.1 K) and high coefficient of performance. In-situ loading analysis indicates a stress-induced continuous transition from the B2 to R and subsequently to B19 '. Nanoscale lattice analysis exposes heterogeneous strain network, harboring metastable R phase preceding the B19 ' phase. By exploiting differences in critical stress and transformation potential between R and B19 ' phases, this study demonstrates the possibilities to synergistically integrate functional performances of different martensitic phases in NiTi into a sequential and continuous two-step transition to provide controlled strain release with unprecedented properties.