Significant elastocaloric effect over a wide temperature range in a polycrystalline (Ni51Fe18Ga27Co4)99.7B0.3 alloy

The effect of microstructure on stress-induced martensitic transformation and elastocaloric properties was investigated in as-cast and aged at 873 K for 1 h samples of polycrystalline (Ni51Fe18Ga27Co4)99.7B0.3 alloy. This alloy has a columnar grain structure with preferential grain orientation along the [001]A-direction. Microalloying with 0.3% boron leads to the formation of a multiphase structure consisting of austenite with a B2/L21-structure, as well as borides and γ-phase particles primarily located along grain boundaries. After aging at 873 K for 1 h, austenite with an ordered L21-structure forms, and γ′-phase particles (up to 400 nm in size) precipitate inside the grains. The preferential orientation of grains along the [001]A-direction and the multiphase structure formed by boron microalloying result in high strength with significant plasticity. The as-cast and aged alloys demonstrate the elastocaloric effect with stable adiabatic temperature change, ΔTad, over a wide temperature range: ΔTEC = 125 K and ΔTEC = 90 K, respectively. The ΔTad reaches (9.4 ± 0.5) K in the as-cast alloy and (8.3 ± 0.5) K in the aged alloy and corresponds to the experimental values of ΔTad = 9–11 K for [001]A-oriented NiFeGa(Co) single crystals. This enhancement of the elastocaloric properties by creating a multiphase microstructure with defined crystallographic texture of polycrystalline NiFeGa(Co, B) alloys provides important insights into the design of energy-efficient solid-state refrigeration materials.