Martensitic transformation behaviour and mechanical property of dual-phase Ni-Co-Mn-Sn-Fe ferromagnetic shape memory alloys

Abstract

Ni-Co-Mn-Sn Heusler alloys possess attractive multifunctional properties, owing to the unique metamagnetic shape memory effect. However, the intrinsic high brittleness limits their applications. In this work, we studied the effects of Fe doping and heat treatment on the microstructures, mechanical properties, martensitic transformation and magnetic behaviours of Ni-Co-Mn-Sn-Fe Heusler alloys. A eutectic dual-phase was successfully introduced in Ni48Co4Mn39-xSn9Fex (x = 3, 4, 5, 6, 7 and 8) alloys by doping Fe for Mn. The engineering strength and ductility increased almost by twofold in a eutectic dual-phase alloy compared to the single-phase alloy. Such microstructure also shows a large quasilinear pseudoelasticity of 4.3% with a very small stress hysteresis and good cyclic stability. After the heat treatment, the γ phase lamellae was much coarsened which resulted in decrease of strength and ductility by ~20%. The magnitude of pseudoelasticity decreased to 2.6% and worse cyclic stability occurred because of the grain coarsening. The metamagnetic transformation was obtained when Fe content is between 2.5 at% and 6.5 at%, with Fe3 alloy showing a A(ferro) ⇔ M(para) type and Fe4-Fe6 alloys showing a A(ferro) ⇔ M(ferro) one.