Influence of Heat Treatments on Martensitic Transformations and Elastocaloric Effect in Two-Phase (β + γ) NiFeGa Alloys

This study reveals the impact of the formation mechanism of a two-phase (β + γ) structure during heat treatment on thermoelastic L2₁(B2)-10M/14M-L1₀ martensitic transformations and elastocaloric parameters of polycrystalline Ni₅₄Fe₁₉Ga₂₇ alloy. It is experimentally shown that annealing of the as-cast Ni₅₄Fe₁₉Ga₂₇ alloy in the temperature range 1173–1463 K for 0.5 h followed by water quenching leads to the precipitation of the γ phase at grain boundaries and inside grains. As the annealing temperature increases from 1173 to 1463 K, the thickness of the γ-phase layer at the grain boundaries doubles, particles inside the grains coarsen, and their volume distribution becomes nonuniform. Simultaneously, the martensitic transformation temperatures increase by 31–69 K. The nonuniform distribution of the γ-phase particles and the morphological features of martensite (refinement of its twinned structure) lead to a 5–6-fold widening of the martensitic transformation intervals in crystals annealed at 1448 K compared to the as-cast alloy. After cyclic superelastic tests with 20 to 100 loading/unloading cycles, two-phase (β + γ) polycrystals demonstrate the stable adiabatic cooling temperature ∆T_ad (2.7–3.0 K) and do not crack along grain boundaries, unlike those in the as-cast state. Significant fatigue strength and a high coefficient of performance (up to 18.3) make (β + γ) Ni₅₄Fe₁₉Ga₂₇ polycrystals promising for practical use in solid-state cooling.