Effects of minor Gd addition on microstructures and mechanical properties of the high strain-rate rolled Mg-Zn-Zr alloys

Abstract Effects of minor Gd addition (0, 0.2, 0.5 and 0.8 mass%) on microstructure and mechanical properties of the high strain-rate rolled (HSRR) Mg–5.5Zn–0.6Zr-based alloys are investigated by OM, XRD, SEM, TEM and mechanical testing. The Mg–5.5Zn–0.6Zr alloy consists of the α-Mg matrix and the Mg 7 Zn 3 phase. Minor Gd addition can effectively refine grains and change phase compositions. The quasicrystal I-phases (Mg–Zn–Gd ternary phase) are formed and mainly aggregated along the grain boundaries in the Mg–5.5Zn–0.6Zr-based alloys with minor Gd addition. The amount of the I-phase increases with the increase of Gd addition, but the amount of the Mg 7 Zn 3 phase decreases. With the Gd addition up to 0.8 mass%, the main phases of the alloy are the α-Mg matrix and I-phases. The Gd addition can reduce the stacking fault energy of the α-Mg matrix and thus promote the dynamic recrystallization (DRX) during the HSRR process. The broken quasicrystal I-phase particles resulted from the HSRR process play a role in promoting the nucleation of DRX, inhibiting the growth of the DRX grains and refining grains. Meanwhile, the dispersion strengthening and precipitation strengthening of the I-phase particles contribute to the enhanced strength. The as-rolled Mg–5.5Zn–0.6Zr–0.8Gd alloy exhibits an optimal strength-ductility balance, with the ultimate tensile strength of 327 MPa, the yield strength of 242 MPa and the elongation to rupture of 22%, respectively. The addition of Gd can effectively weaken the recrystallization texture and modify the tensile fracture mechanism from quasi-cleavage to ductile.