Effect of aspect ratio on the elastocaloric effect and its cyclic stability of nanocrystalline NiTi shape memory alloy

Abstract

NiTi shape memory alloy (SMA) has been suggested as an environmentally friendly solid-state refrigerant that is expected to replace the traditional vapor compression technology. However, its various refrigeration indicators still need to be further explored and enhanced. Nanocrystalline NiTi SMA has significant advantages in terms of elastocaloric effect (eCE) and cyclic stability compared with traditional coarse-grained NiTi SMA. In this work, the influence of aspect ratio (L/W, where L and W are the gauge length and width of the samples, respectively) on the eCE and its cyclic stability of nanocrystalline NiTi SMA was investigated experimentally. The in-situ cyclic tensile tests were performed to observe the axial strain field and temperature field on the surface of the specimens with different aspect ratios. Some microscopic observations were also carried out to clarify the internal mechanism. The experimental results show that as the aspect ratio decreases, the adiabatic temperature change ΔTad (elasocaloric cooling capacity) of the nanocrystalline NiTi SMA gradually decreases, while the coefficient of the performance of material (COPmat) increases, and the cyclic stability of eCE was enhanced. However, the dependence of eCE and its cyclic stability on the aspect ratio is gradually weakened when L/W > 4. The eCE and its cyclic stability of nanocrystalline NiTi SMA are not related to the dislocation slipping but rather attributed to the cyclically accumulated residual martensite. This work can provide valuable reference for designing efficient and reliable nanocrystalline NiTi SMA based solid-state refrigeration devices.

Keywords

• Nanocrystalline NiTi shape memory alloy
• Elastocaloric effect
• Cyclic stability
• Aspect ratio