Quasi-linear superelasticity and associated elastocaloric effect in boron-doped polycrystalline Ni-Mn-Ti alloys

Abstract

Ni-Mn-Ti shape memory alloys show great potential in solid-state elastocaloric cooling owing to very prominent elastocaloric effect along with first-order stress-induced martensitic transformation. However, large stress hysteresis inherent to martensitic transformation greatly restricts the energy efficiency and cyclic stability of elastocaloric response. Here, we demonstrate the effective manipulation of stress hysteresis as well as the resulting elastocaloric effect through doping boron to Ni-Mn-Ti alloys. With the incremental boron content in (Ni₅₀Mn₃₁Ti₁₉)_100–xB_x (x = 0, 0.2, 0.5, 1, 1.5) alloys, a plateau-type superelastic behavior with large stress hysteresis gradually evolves into a quasi-linear one with slim hysteresis, giving rise to significant improvement in the energy conversion efficiency of elastocaloric response. In a (Ni₅₀Mn₃₁Ti₁₉)₉₉B₁ alloy, the coefficient of performance of material (COP_mat) can be as high as 24 ∼ 33. Moreover, under a compressive strain of 4%, large cooling |ΔT_ad| values higher than 6.5 K in the (Ni₅₀Mn₃₁Ti₁₉)₉₉B₁ alloy are maintained for over 8000 superelastic cycles, showing an enhancement of one order of magnitude in the cyclability with respect to that of boron-free Ni₅₀Mn₃₁Ti₁₉ alloy. We attribute the enhanced elasocaloric response to the significantly improved mechanical properties but reduced stress hysteresis endowed by relatively high content of boron doping.