Ultrastable large elastocaloric effect in densely nanoprecipitated NiTi alloy for low-temperature refrigeration

Elastocaloric cooling using shape memory alloys (SMAs) offers a green alternative to conventional vapor compression-based technology. However, extending this novel technology to subzero Celsius temperatures is challenging due to the weak caloric effect (typical adiabatic temperature change Δ T <10 K) and severe functional degradation of existing low-temperature superelastic SMAs. Here, we achieved an ultrastable large elastocaloric effect at low temperatures in Ni_51.5Ti_48.5 SMA by introducing hyperdense Ti₃Ni₄ nanoprecipitates with average spacing of only 8.2 nm into the phase-transition (PT) matrix. The formation of hyperdense nanoprecipitates reduces the Ni/Ti ratio of the PT matrix to improve the stress-induced PT latent heat and simultaneously creates a strong coherent strain field to significantly suppress the thermally induced PT. These dual benefits enable a considerable caloric effect with Δ T of 9.4−23.7 K in the temperature window of 213−295 K. Furthermore, these nanoprecipitates and the associated strain field significantly strengthen the PT matrix and improve austenite-martensite compatibility. This effectively suppresses dislocation accumulation and residual martensite formation during cyclic deformation, enabling near-zero functional degradation over 10⁵ PT cycles in the entire temperature window mentioned above. Our alloy overcomes the key limitations of existing low-temperature superelastic SMAs, paving the way for subzero Celsius elastocaloric refrigeration.