Temperature-dependent mechanical properties and elastocaloric effects of multiphase nanocrystalline NiTi alloys

We investigate the temperature-dependent mechanical properties and elastocaloric effects of different multiphase nanocrystalline NiTi alloys with grain sizes of 10 nm, 15 nm, 20 nm, 27 nm, 34 nm and 55 nm. These multiphase nanocrystalline NiTi alloys mainly consist of B2, B19′ and R phases and can undergo the thermally induced phase transition over a wide temperature range from 208 K to 353 K. Their superelastic deformation (loading to a strain of 4%) is contributed by combinations of the elastic deformation of B2 phase, the R → B2 phase transition, the reversible martensite reorientation and the B2 → R and B2/R → B19' phase transitions. Such complex deformation mechanism together with the microstructure evolution with temperature brings strong temperature-dependences of the mechanical properties and elastocaloric effects. As results, at the grain sizes of 15 nm ∼ 20 nm, the stress under a constant superelastic strain can show a significant V-shaped variation with temperature. At the grain size of 55 nm, the adiabatic temperature drop can even change from −8.5 K to −29 K ∼ −35.8 K as the ambient temperature rises from 293 K to 333 K. By comparison of the mechanical properties and elastocaloric effects of the different multiphase nanocrystalline NiTi alloys, it is found that the one with the grain size of 20 nm exhibits the best comprehensive cooling performances, which has a soft near-linear superelasticity (a moderate driving force), a very wide refrigeration temperature window (> 245 K) with considerable adiabatic temperature drops (∆Tmax= −17.1 K), and a good functional stability. This work provides an experimental basis for using the multiphase nanocrystalline NiTi alloys as solid-state refrigerants.