Elastocaloric Effect at Stress-Induced Martensitic Transformation in NiFeGa(B) Alloys Produced by Directional Solidification Method

The influence of microalloying with 0.3 at % boron and γ-phase particles on the martensitic transformation, the cyclic stability of superelasticity and elastocaloric effect in textured Ni₅₄Fe₁₉Ga₂₇ polycrystals, produced by directional solidification method, was investigated. There is a preferential orientation of grains along the [012]- and [013]-directions along the crystallization direction in studied polycrystals. Ultimate compressive strength increases from 1.1 to 1.3 GPa with boron microalloying. As-cast polycrystals Ni₅₄Fe₁₉Ga₂₇ and (Ni₅₄Fe₁₉Ga₂₇)_99.7B_0.3 demonstrate elastocaloric effect with a stable adiabatic temperature change ∆T_ad = 6.4 K in 100 cycles, despite the formation of macrocracks. Aging at 1273 K for 0.5 h results in the precipitation of γ-phase particles at grain boundaries and within the grain body, which prevents the formation of macrocracks in cyclic tests up to 500 loading/unloading cycles. The combination of boron microalloying, texture formation and precipitation of the ductile γ-phase leads to increased strength and reduced grain boundary stress gradient during the martensitic transformations. As a result, the aged polycrystalline (Ni₅₄Fe₁₉Ga₂₇)_99.7B_0.3 alloy demonstrate high cyclic stability of superelasticity and elastocaloric effect at room temperature with ∆T_ad value up to 5.4 K and the coefficient of performance up to 31.