Enhanced elastocaloric stability in NiTi alloys under shear stress

Significant elastocaloric effects (eCE) generated from stress-induced B2↔B19’ transition under axial load in NiTi shape memory alloys have already been proved. However, the huge hysteresis of B2↔B19’ transition directly leads to severe functional fatigue, which restricts its practical application. Here, we prepared linear NiTi spring using Ni_50.8Ti_49.2 wire with diameter of 300 μm aiming at converting normal stress into shear stress considering the fact that martensitic transition is achieved by shear deformation. An isothermal entropy change of ∼41 J/kg·K as well as an adiabatic temperature decrease of −16.8 K have been obtained in present spring. Furthermore, no significant degradation of elastocaloric strength emerged after 254 cycles, while only ∼2.5 K temperature degradation after 1505 cycles. Such superior stability of elastocaloric effect was mainly attributed to pure shear stress condition and gradient martensitic transition (MT) on the wire cross-section during a superelasticity cycle, which effectively reduced transition actuation load and avoided adverse factors under normal stress condition. Therefore, our spring allows for not only stable and huge caloric effect but also fast cycle frequency, thus, being attractive for micro-cooling devices or heating pumps.