Large low-field magnetocaloric effect in directionally solidified Ni55Mn18+xGa27−x (x=0, 1, 2) alloys

Highlights: • Textured polycrystalline Ni-Mn-Ga alloys were prepared by directional solidification. • Large ΔT_ad up to 1.57 K were achieved under the low field change of 1.5 T. • Enhanced magnetostructural coupling and strong texture result in large MCE. The magnetostructural transformation and magnetocaloric effect of directionally solidified polycrystalline Ni₅₅Mn_18+xGa_27−x (x = 0, 1, 2) alloys were studied. It is shown the directionally solidified alloys form coarse columnar-shaped grains with strong 0 0 1_A (the subscript A refers to austenite) preferred orientation along the solidification direction. Through Mn substitution for Ga, a coupled magnetostructural transformation was realized in Ni₅₅Mn₁₉Ga₂₆ and Ni₅₅Mn₂₀Ga₂₅ alloys. Large adiabatic temperature variation (ΔT_ad) of 1.47 K and 1.57 K under the low field change of 1.5 T were achieved in these two alloys, respectively. Such ΔT_ad values are obviously higher than that obtained from a single martensitic transformation and magnetic transition. The present results demonstrate that proper composition tuning to achieve magnetostructural transformation as well as increasing the grain size and preferential orientation degree through directional solidification could be an economical processing route to optimize magnetocaloric properties in polycrystalline Ni-Mn-Ga based alloys.