Superelastic nanocrystalline NiTi tubes are promising candidates for eco-friendly elastocaloric cooling, but their cyclic stability suffers severely from functional degradation and the limited fatigue life of conventional coarse-grained NiTi remains a crucial bottleneck. First, the functional degradation and its effect on elastocaloric cooling performance were investigated. The results show that the functional degradation accompanies with progressive accumulation of residual strain and significant reduction in both hysteresis loop area (D) and forward transformation stress (σ_f^tr). Such functional degradation arises from phase transition-induced dislocations and dislocation-pinned residual martensite. The dislocations partition the original austenite grains into much smaller nanodomains,...[ Read more ]

Superelastic nanocrystalline NiTi tubes are promising candidates for eco-friendly elastocaloric cooling, but their cyclic stability suffers severely from functional degradation and the limited fatigue life of conventional coarse-grained NiTi remains a crucial bottleneck. First, the functional degradation and its effect on elastocaloric cooling performance were investigated. The results show that the functional degradation accompanies with progressive accumulation of residual strain and significant reduction in both hysteresis loop area (D) and forward transformation stress (σ_f^tr). Such functional degradation arises from phase transition-induced dislocations and dislocation-pinned residual martensite. The dislocations partition the original austenite grains into much smaller nanodomains, leading to the macroscopic residual strain and reduced D. The nanosized residual martensite can directly grow without overcoming martensite nucleation barrier and induce compressive residual stress in the austenite phase, contributing to the decrease in σ_f^tr . As a result of functional degradation, the material coefficient of performance was doubled for full phase transition and enhanced by 40% for partial phase transition compared with the first cycle, mainly due to the cyclically-decreased D. The study shows that the cyclic stability and elastocaloric cooling performance of NiTi can be improved via training at a suitable stress.

Then, the ultrahigh fatigue life of nanocrystalline NiTi tubes was achieved via high frequency fatigue test after training, exceeding 120 million cycles under 800 MPa. The NiTi tubes demonstrate stable cyclic stress-strain responses and a stable adiabatic temperature drop of 6.6 °C in the lifespan. The material coefficient of performance increases from the initial 8.8 to 11.6 of the 10⁸th compressive cycle. The high resistance to nucleation and growth of compression-parallel cracks results in the ultrahigh fatigue life of the tubes. The research shows the great potentials of nanocrystalline NiT tubes with both stable thermomechanical properties and long compressive fatigue life for reliable elastocaloric cooling.

Keywords: Elastocaloric cooling; Cyclic response; NiTi; Shape memory alloy (SMA); Martensitic phase transformation; Ultrahigh fatigue life.