Improved elastocaloric effect of NiTi shape memory alloys strengthened by Ni4Ti3 nanoprecipitates

Abstract

NiTi shape memory alloys (SMAs) exhibit extraordinary potential in the field of solid-state cooling due to their exceptional elastocaloric effect (eCE). However, the inherent trade-off between cooling efficiency and cooling capacity often makes it difficult to enhance both simultaneously, and the relatively poor cyclic stability poses a significant challenge to the application of such material as a solid-state refrigerant. A NiTi elastocaloric material containing uniformly distributed high-density Ni4Ti3 nanoprecipitates is obtained in this work, with a remarkable coefficient of performance of material (COPmat) as high as 52 and a satisfactory adiabatic temperature change (ΔTad) of 11 K, breaking through the limits of the eCE reported in existing SMAs. The precipitate-strengthened NiTi SMA fabricated in this work exhibits a low hysteresis and excellent cyclic stability, with the former attribute stemming from the strain-glass-like phase transformation and its reverse induced by the high-density Ni4Ti3 nanoprecipitates, and the latter resulting from the effective suppression of dislocation slip by the uniformly distributed high-density Ni4Ti3 nanoprecipitates. The precipitate-strengthened NiTi SMAs with a <001>{111} texture exhibit great eCE and simultaneously possess good strength and superelasticity controllability, which can maintain excellent cyclic stability even at a high stress level of 800 MPa. The precipitate-strengthened NiTi SMAs developed in this work not only hold significant advantages for solid-state refrigeration applications but also demonstrate extraordinary potential in high-precision control devices.