Achieving broad-range cryogenic elastocaloric effect in a TiNiHfCo nanocrystalline alloy with R-phase nanodomains

Achieving large, reversible elastocaloric effects at low temperatures remains a key challenge for solid–state cooling. Here, we report a nanocrystalline Ti_46.2Ni_45.8Hf₃Co₅ shape memory alloy that delivers robust cryogenic elastocaloric performance, featuring a fully recoverable superelastic strain of 6%, a large adiabatic temperature change (ΔT_ad) of 10.0 K at −100 °C, and a broad working window spanning ∼100 °C. These properties originate from a hierarchical microstructure composed of nanograins, dense dislocations, and uniformly dispersed R-phase nanodomains. The defect-rich nanocrystalline structure reinforces the matrix, suppresses plastic slip and stabilizes the austenite phase. Crucially, the R-phase nanodomains may provide correlated precursor regions that reduce the reliance on fresh martensite nucleation during stress-induced transformation, thereby weakening the temperature dependence of the transformation stress. These findings offer a microstructural design principle for high-performance elastocaloric materials capable of efficient and reversible operation in cryogenic environments.