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

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_49.2Ni_44.8Cu₆ alloy exhibits a large adiabatic temperature change (ΔT_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 ΔT_ad after 200 cycles. Superelastic testing on micropillars confirms stable recoverable strain of 4.3% after 10⁶ 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.