Fe substitution induced intermartensitic transition and its internal stress dependent transforming behavior in NiaMnaGa based alloy

The transforming behavior of Fe doped Ni55Mn20Ga25 ferromagnetic shape memory alloys is investigated in this study. The Ni55Mn20Ga25 alloy only undergoes P (parent phase) a 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 NiaMnaGa system. However, 2% (at.) Fe substitution for Ni of the Ni55Mn20Ga25 alloy induces an intermartensitic transition and the transition sequence of Ni53Fe2Mn20Ga25 alloy changes into P (parent phase) a 7 M (seven-layer modulated martensite) a non-modulated T. It was also found that the intermartensitic transition of Ni53Fe2Mn20Ga25 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.