Cyclic stable superelasticity and elastocaloric effect via the R→B19′ transformation in NiTi

NiTi-based shape memory alloys are promising candidates for solid-state refrigeration owing to the latent heat associated with stress-induced martensitic transformations. However, the conventional B2→B19′ pathway is constrained by a fundamental trade-off between elastocaloric performance and cyclic stability. In this work, we demonstrate that activating the R→B19′ transformation pathway effectively circumvents this limitation. Differential scanning calorimetry confirms stable and reversible R→B19′ transformations in binary NiTi alloys. The reduced energy barrier between the R-phase and B19′ martensite facilitates a more continuous and efficient transformation, thereby suppressing the accumulation of irreversible defects. Through integrated thermomechanical processing and microstructural characterization, we show that NiTi alloys undergoing reversible R↔B19′ transformations exhibit a large adiabatic temperature change (18.59 K), high recoverable strain (4.86%), and exceptional cycling stability, retaining over 99% of performance after 200 tensile cycles. These findings establish a robust design strategy for high-performance shape memory alloys.