Microstructural characterisation and properties of quaternary Ni ₅₀Mn₂₉Ga₂₀Gd₁ ferromagnetic shape memory alloy

A Heusler Ni₅₀Mn₂₉Ga₂₀Gd₁ ferromagnetic shape memory alloy is obtained by substituting 1 at- %Gd for Ga in a ternary Ni₅₀Mn₂₉Ga2_1-xGd_x alloy. The microstructure, phase transformation, magnetic and mechanical properties of Ni₅₀Mn₂₉Ga₂₀Gd₁ alloy are investigated. It is shown that the Ni₅₀Mn₂₉Ga ₂₀Gd₁ alloy has the best overall mechanical properties among the Ni₅₀Mn₂₉Ga2_1-xGd_x (x50, 0·1, 0·5, 1, 2, 5) alloys. Compression tests show that a compressive strength of 1124·42 MPa with a compressive strain up to 16·25% can be achieved in this alloy. The mechanism of the improved mechanical properties is also discussed. The results demonstrate that the grains are refined obviously by the addition of 1 at-% Gd. The microstructure of the Ni₅₀Mn₂₉Ga₂₀Gd₁ alloy consists of the matrix and the hexagonal Gd(Ni, Mn)₄Ga phase distributing mainly along the grain boundaries. Martensitic structure changes from five-layered martensite with 0 at-%Gd to seven-layered modulated martensite with 1 at-%Gd. One-step thermoelastic martensitic transformation occurs in this quaternary alloy. The martensitic transformation temperatures of the Ni₅₀Mn ₂₉Ga₂₀Gd₁ alloy increase and its Curie temperature keeps unchanged compared with that of ternary Ni₅₀Mn ₂₉Ga2_1-xGd_x alloy. Coupling of the magnetic and structural transitions is observed in the Ni₅₀Mn₂₉Ga ₂₀Gd₁ alloy.