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

The transforming behavior of Fe doped Ni₅₅Mn20Ga25 ferromagnetic shape memory alloys is investigated in this study. The Ni₅₅Mn ₂₀Ga₂₅ 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 Ni₅₅Mn₂₀Ga ₂₅ alloy induces an intermartensitic transition and the transition sequence of Ni₅₃Fe2Mn₂₀Ga₂₅ alloy changes into P (parent phase) →7 M (seven-layer modulated martensite) →non-modulated T. It was also found that the intermartensitic transition of Ni₅₃Fe2Mn₂₀- Ga₂₅ 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.