Premartensitic tweed engineering for high-efficiency elastocaloric cooling and exceptional superelasticity in a Co-V-Ti-Fe-Cu bulk alloy

Shape memory alloys are promising elastocaloric refrigerants due to the substantial adiabatic temperature change (Δ T ad ) associated with stress-induced martensitic transformation. However, their cooling efficiency and functional durability are severely hampered by the intrinsic stress hysteresis of phase transformation. Here, we design a superelastic alloy that overcomes the inherent trade-off between Δ T ad and hysteresis by introducing a premartensitic structure in the Co-V-Ti system. The resulting alloy simultaneously achieves a fully recoverable superelastic strain of 3.3% with an ultra-low stress hysteresis of 25 MPa and a large Δ T ad of −16.2 K upon unloading. This combination of low energy dissipation and large caloric effect yields a strikingly high coefficient of performance (COP mat ) of 29.5, alongside excellent functional stability maintained over 20,000 cycles. We demonstrate that the cross‑hatched tweed structure acts as pre‑existing embryos for martensite nucleation, drastically reducing the transformation energy barrier. In-situ high-energy X-ray diffraction further confirms that this tweed structure enables improved lattice compatibility (λ 2 = 0.9708) between austenite and martensite phases. This work establishes a novel design strategy for developing highly efficient and stable elastocaloric materials by engineering premartensitic structures that preserve the strong first-order transition (to ensure a large Δ T ad ) while reducing the nucleation barrier (to achieve low hysteresis).