Design of a Superelastic Alloy Uniting Large Functional Response and Exceptional Cyclic Stability

Abstract The development of a Ni‐rich TiNiCu alloy that achieves both large functional output and exceptional cyclic stability, addressing a challenging trade‐off in shape memory alloy design, is reported. By tuning the Ni/Ti ratio, the B19 martensite phase is stabilized at low Cu concentrations, enabling a direct B2–B19 transformation with low thermal hysteresis and preserved processability. The optimized Ti _49.2 Ni _44.8 Cu ₆ alloy exhibits a large adiabatic temperature change ( Δ T _ad ) of 20.3 K, and high recoverable strain above 5% with low stress hysteresis. Thermal cycling shows negligible degradation in transformation temperatures and latent heat after 5000 cycles, while elastocaloric test reveals only a 2% drop in Δ T _ad after 200 cycles. Superelastic testing on micropillars confirms stable recoverable strain of 4.3% after 10 ⁶ cycles. These robust properties arise from two key design features: a compositionally tuned transformation pathway that promotes twinless B19 martensite formation with large spontaneous strain, and a nanocrystalline microstructure strengthened by residual dislocations and nanoprecipitates that suppress plasticity. This integrated design strategy offers a scalable route toward high‐performance TiNi‐based alloys, with promising potential for solid‐state cooling and superelastic medical applications.