Large low-field magnetocaloric effect in directionally solidified Ni_(55)Mn_(18+x)Ga_(27-x) (x = 0, 1, 2) alloys

摘要

The magnetostructural transformation and magnetocaloric effect of directionally solidified polycrys-talline Ni_(55)Mn_(18+x)Ga_(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 Ni_(55)Mn_(19)Ga_(26) and Ni_(55)Mn_(20)Ga_(25) 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.