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 (ΔT_ad). However, the simultaneous attainment of large ΔT_ad 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 Ni_50.8Ti_49.2 (at.%), showcasing cyclic-stable superelasticity with large recoverable strain (5.9 %), substantial ΔT_ad (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.