Achieving large room-temperature elastocaloric effect and ultrahigh cyclic stability by grain size engineering

The trade-off between a large room-temperature adiabatic temperature drop (Δ T _ad) and high cyclic stability has long been an obstacle for developing high-performance elastocaloric cooling materials and devices. To overcome this challenge, we tailored the grain size (GS) of a NiTiCuCo shape memory alloy through high-pressure torsion followed by annealing. It is found that the NiTiCuCo with an average GS of 70 nm shows a large room-temperature Δ T _ad of 21.2 ± 0.3 K which does not degrade over 10⁷ phase-transformation cycles. This unique combination of properties makes this alloy highly competitive among existing elastocaloric cooling materials. The large Δ T _ad is due to improved phase-transformation reversibility with reduced dislocation-pinned and thermomechanical-coupling-induced residual martensite during unloading. The high cyclic stability stems from inhibited dislocation motion which is due to enhanced lattice compatibility and a significantly lower work stress ( σ _w) compared to the material’s yield stress ( σ _y). Our work provides not only a high-performance elastocaloric material but also an effective strategy to break the performance bottleneck of shape memory alloys by GS engineering.