Fe substitution induced intermartensitic transition and its internal stress dependent transforming behavior in Ni-Mn-Ga based alloy

摘要

The transforming behavior of Fe doped Ni_(55)Mn_(20)Ga_(25) ferromagnetic shape memory alloys is investigated in this study. The Ni_(55)Mn_(20)Ga_(25) alloy only undergoes P (parent phase) -> non-modulated T (non-modulated tetragonal martensite) martensitic transition. It does not exhibit intermartensitic transition, since the non-modulated T structure is the most stable martensitic state in Ni-Mn-Ga system. However, 2% (at.) Fe substitution for Ni of the Ni_(55)Mn_(20)Ga_(25) alloy induces an intermartensitic transition and the transition sequence of Ni_(53)Fe_2Mn_(20)Ga_(25) alloy changes into P (parent phase) ->7M (seven-layer modulated martensite) -> non-modulated T. It was also found that the intermartensitic transition of Ni_(53)Fe_2Mn_(20)Ga_(25) alloy can be suppressed by the internal stress produced by water quenching and sample crushing. This is considered to stem from the inhomogeneous and non-directional internal stress states generated during water quenching and sample crushing process.