Microstructural characterisation and properties of quaternary Ni 50 Mn 29 Ga 20 Gd 1 ferromagnetic shape memory alloy

Abstract

A Heusler Ni₅₀Mn₂₉Ga₂₀Gd₁ ferromagnetic shape memory alloy is obtained by substituting 1 at-Gd for Ga in a ternary Ni₅₀Mn₂₉Ga₂₁ alloy. The microstructure, phase transformation, magnetic and mechanical properties of Ni₅₀Mn₂₉Ga₂₀Gd₁ alloy are investigated. It is shown that the Ni₅₀Mn₂₉Ga₂₀Gd₁ alloy has the best overall mechanical properties among the Ni₅₀Mn₂₉Ga_21-xGd_x (x = 0, 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 Ni₅₀Mn₂₉Ga₂₀Gd₁ alloy increase and its Curie temperature keeps unchanged compared with that of ternary Ni₅₀Mn₂₉Ga₂₁ alloy. Coupling of the magnetic and structural transitions is observed in the Ni₅₀Mn₂₉Ga₂₀Gd₁ alloy.