Effect of loading stress on cyclic stability of elastocaloric cooling performance of NiTi tubes

Superelastic NiTi tubes are promising candidates for eco-friendly elastocaloric cooling devices due to their large transformation entropy change and favorable tubular geometry for heat transfer; however, their functional performance often degrades under cyclic compression. In this work, the cyclic elastocaloric behavior of nanocrystalline NiTi tubes under different compressive stresses is systematically investigated, revealing a strong stress dependence of cyclic stability. Under a partial transformation stress of 800 MPa, the adiabatic temperature change increases with cycling, whereas it progressively decreases under a full transformation stress of 1200 MPa. In contrast, at an intermediate stress of 1000 MPa, the adiabatic temperature change stabilizes with cycling, exhibiting the highest elastocaloric stability. Microstructural analysis indicates that the stress-dependent evolution of residual martensite and residual stress governs the cyclic elastocaloric response. Residual strain accumulation arises from transformation-induced dislocations and dislocation-pinned residual martensite: the former subdivides austenite grains into nanodomains and reduces transformation hysteresis through strain hardening, while the latter introduces compressive residual stress in austenite, leading to a gradual reduction in critical transformation stress. This study provides a mechanical route to stabilize the elastocaloric performance of NiTi alloys.