Reversible elastocaloric effects with small hysteresis in nanocrystalline Ni-Ti microwires

The elastocaloric effect (eCE) with high reversibility and small hysteresis associated with the B2 <-> R transformation was demonstrated in nanocrystalline Ni50.5Ti49.5 microwires. The microwire, with diameter of 100 mu m and nanoscale grains 15-75 nm, was perpared by multi-step cold-drawing followed by low-temperature annealing. R-B2 transition occurs in the annealed microwire at ambient temperatures. Large isothermal entropy change (Delta S-iso) 20.5 J/kg.K during cooling and 21.4 J/kg.K during heating under a stress of 335 MPa associated with the R-B2 transformation was confirmed. As a result, a refrigeration capacity (RC) 257.3 J/kg during cooling and 249.5 J/kg during heating over a working temperature window (Delta T-FWHM) 298.5-313.6 K and 304.5-313.6 K, respectively, was achieved. Furthermore, the annealed microwire exhibited a small temperature hysteresis 5-8.5 K and very equivalent Delta S-iso during forward and backward R phase transitions, giving rise to a highly reversible eCE with small hysteresis. (C) 2018 Author(s).