Effects of phase transformation on the microstructures and magnetostriction of Fe-Ga and Fe-Ga-Zn ferromagnetic shape memory alloys

The phase transformation and magnetostriction of bulk Fe₇₃Ga₂₇ and Fe₇₃Ga₁₈Zn₉ (at. %) ferromagnetic shape memory alloys (FSMs) were investigated by transmission electron microscopy (TEM), x-ray diffraction (XRD), and a magnetostrictive-meter setup. For the Fe₇₃Ga₂₇ FSM alloy solution treated at 1100 °C for 4 h and quenched in ice brine, the antiphase boundary segments of the D0₃ domain were observed in the A2 (disordered) matrix, and the Fe₇₃Ga₂₇ FSM alloy had an optimal magnetostriction (λ_‖^s= 71 × 10⁻⁶ and λ_⊥^s= −31 × 10⁻⁶). In Fe₇₃Ga₂₇ FSM alloy as-quenched, aged at 700 °C for 24 h, and furnace cooled, D0₃ nanoclusters underwent phase transformation to an intermediate tetragonal phase (i.e., L1₀-like martensite) via Bain distortion, and finally L1₂ (Fe₃Ga) structures precipitated, as observed by TEM and XRD. The L1₀-like martensite and L1₂ phases in the aged Fe₇₃Ga₂₇ FSM alloy drastically decreased the magnetostriction from positive to negative (λ_‖^s= −20 × 10⁻⁶ and λ_⊥^s= −8 × 10⁻⁶). However, in Fe₇₃Ga₁₈Zn₉ FSM alloy as-quenched and aged, 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 Fe₇₃Ga₁₈Zn₉ FSM alloy maintained stable magnetostriction (λ_‖^s= 36 × 10⁻⁶ and λ_⊥^s= −31 × 10⁻⁶). Adding Zn can improve the ferromagnetic shape memory effect of aged Fe₇₃Ga₁₈Zn₉ alloy, which may be useful in application of the alloy in high temperature environments