Tailorable elastocaloric cooling performance of wire-arc directed energy deposition NiTi alloy through concentration gradient design

Abstract

The inherent hysteresis of NiTi alloy samples is one of the key factors limiting their elastocaloric cooling performance. However, reducing hysteresis often leads to a decrease in adiabatic temperature change (ΔT_ad), thereby hindering the application of NiTi alloys in the refrigeration field. Here, NiTi alloys with alternating high-Ni and low-Ni content were fabricated by tailoring heat input during the wire-arc directed energy deposition (DED) process, which modifies the Ni concentration gradient and enables the modulation of the elastocaloric cooling performance of NiTi alloys. The coefficient of performance of material (COP_mat) of the high-Ni NiTi alloy samples is relatively high, but their ΔT_ad during deformation is lower. On the other hand, the low-Ni NiTi alloy samples, while exhibiting higher ΔT_ad, show poorer stability during cycling. Due to the synergistic effect of the microstructures in the high-Ni and low-Ni region, a favorable combination of low cyclic hysteresis and high ΔT_ad were achieved in the composite NiTi samples. Additionally, the composite NiTi samples also demonstrate excellent cyclic stability, with a degradation rate of only 4% during the cycling process under a 2% strain condition. This study proposes a feasible approach for regulating the elastocaloric effect of NiTi alloys, paving the way for additive manufacturing to prepare elastocaloric cooling materials.