The elastocaloric cooling performance of NiTi-based shape memory alloys is enhanced by grain refinement (and partial amorphization), dislocation strengthening and alloying (combined with multiaxial forging and aging). Firstly, the effects of grain refinement and partial amorphization on temperature drop (ΔT), coefficient of performance (COP)and cyclic stability of NiTi are investigated. It is found that grain refinement and partial amorphization decrease ΔT by reducing the amount of B2 phase, increase COP by altering phase transition mode and enhance cyclic stability by grain boundary and amorphous phase strengthening. A good combination of large ΔT, enhanced cyclic stability and high COP is achieved in a 35 nm-grain-size NiTi. Secondly, to further enhance cyclic stability, nanocryst...[ Read more ]

The elastocaloric cooling performance of NiTi-based shape memory alloys is enhanced by grain refinement (and partial amorphization), dislocation strengthening and alloying (combined with multiaxial forging and aging). Firstly, the effects of grain refinement and partial amorphization on temperature drop (ΔT), coefficient of performance (COP)and cyclic stability of NiTi are investigated. It is found that grain refinement and partial amorphization decrease ΔT by reducing the amount of B2 phase, increase COP by altering phase transition mode and enhance cyclic stability by grain boundary and amorphous phase strengthening. A good combination of large ΔT, enhanced cyclic stability and high COP is achieved in a 35 nm-grain-size NiTi. Secondly, to further enhance cyclic stability, nanocrystalline high-density-dislocation (HDD) NiTi is fabricated via cold rolling and annealing. The HDD NiTi exhibits large and stable elastocaloric effect with a ΔT of 16-18 ℃ and a residual strain of only 1.25% over 10⁶ cycles under 1400 MPa. The nano-grains and the high-density dislocations suppress new dislocations and residual martensite, leading to the stable elastocaloric effect. Thirdly, to largely reduce transition stress, Cu and Co are added in NiTi to obtain quaternary NiTiCuCo. Precipitated NiTiCuCo is further successfully fabricated via multiaxial forging followed by aging. With a low transition stress of around 400 MPa at room temperature, the precipitated NiTiCuCo under the working stress of 550 MPa shows a large and stable ΔT of about 17 ℃, low residual strain of 0.19% and high COP of over 40 in 10⁷ cyclic compressions. The excellent cyclic stability, on one hand, originates from the reduced sweep distance of the transformation interfaces due to introduction of dense precipitates, and on the other hand, from precipitation strengthening.