Microstructures and Magnetic Properties of Fe–Ga and Fe–Ga–V Ferromagnetic Shape Memory Alloys

The microstructures and magnetic properties of bulk Fe₇₃Ga₂₇ and Fe₇₃Ga₁₈V₉ (at.%) ferromagnetic shape memory (FSM) alloys were investigated by transmission electron microscopy (TEM), a magnetostrictive-meter setup, and a superconducting quantum interference device magnetometry. Results showed that in the Fe₇₃Ga₂₇ FSM alloy solution treated (ST) at 1100°C for 4 h and quenched in ice brine, the antiphase boundary segments (APBs) of the D0₃ domain were in the A2 (disordered) matrix and the Fe₇₃Ga₂₇ FSM alloy had an optimal magnetostriction and a higher magnetostrictive susceptibility (Δλ^s_||/ΔH). In Fe₇₃Ga₂₇ FSM alloy ST, aged at 700°C for 24 h and furnace cooled (7 HTFC), D03 nanoclusters underwent phase transformation to an intermediate tetragonal phase (i.e., L1₀-like tetragonal martensite) via Bain distortion, and finally L12 (Fe3Ga) structures precipitated, as observed by TEM. The L1₀-like tetragonal martensite and L1₂ phases in the aged Fe₇₃Ga₂₇ FSM alloy drastically decreased the magnetostriction from positive to negative. However, by adding V into the Fe₇₃Ga₂₇ alloy system to produce Fe₇₃Ga₁₈V₉ FSM alloy ST and ageing at 7 HTFC, the phase transformation of D0₃ to an intermediate tetragonal martensite phase and precipitation of L1₂ structures were not found. The results indicate that the aged Fe₇₃Ga₁₈V₉ FSM alloy maintained stable magnetostriction. However, the addition of V into the Fe₇₃Ga₂₇ alloy system to produce Fe₇₃Ga₁₈V₉ does not result in a higher saturation magnetization (Ms) or magnetocrystalline anisotropy energy constant (Ku) in the Fe₇₃G- ₁₈V₉ FSM alloy.