  Superelastic NiTi tubes are promising candidates for eco-friendly
elastocaloric cooling, but their cyclic stability suffers severely from
functional degradation. Herein, we investigate the functional degradation of
nanocrystalline NiTi tubes via in-depth analysis and find out that it is
beneficial to elastocaloric cooling performance. 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. The accumulation of residual strain arises from
phase-transition-induced dislocations and dislocation-pinned residual
martensite. The former separates the original austenite grains to much smaller
nanodomains (equivalent to grain size effect) and contributes to the strain
hardening during phase transition, leading to the significant reduction of D.
The latter induces compressive residual stress in the austenite phase and thus
gives rise to the evolutive reduction of forward transformation stress.
Consequently, the material coefficient of performance (COPmater) was
self-enhanced for 40~104 %. The beneficial effect is mainly because of the
cyclically-decreased D. The study might provide a processing route to tailor
COPmater and stabilize the mechanical response of NiTi by cyclic compression.
