Effect of gadolinium on microstructure and the mechanical properties of Mg-Zn-Zr cast magnesium alloys

Mg-5Zn-0.6Zr and Mg-5Zn-3Gd-0.6Zr were prepared by sand casting. The effects of Gd addition on the microstructure and properties of Mg-5Zn-0.6Zr magnesium alloy were studied by XRD, OM, SEM, EDS and mechanical property tests. The results show that: as-cast microstructure in Mg-5Zn-0.6Zr alloy consists of α-Mg grains with MgZn₂ phase on the grain boundary, while microstructure in Mg-5Zn-3Gd-0.6Zr alloy also is a ternary Mg-Zn-Gd phase on the α-Mg grain boundary with finer structure. A large amount of dispersive particles of Mg-Zn-Gd phase are formed nearby the grain boundary. After solution heat-treatment, eutectic in Mg-5Zn-0.6Zr alloy is fully dissolved into the matrix, in which the microstructure in Mg-5Zn-3Gd-0.6Zr is fined obviously. However, there is still Mg-Zn-Gd phase on the grain boundary of Mg-5Zn-3Gd-0.6Zr alloy. Moreover, the morphology of Mg-Zn-Gd phase was changed from net into insular. The as-cast mechanical property of Mg-5Zn-0.6Zr alloy is higher than that of Mg-5Zn-3Gd-0.6Zr alloy. After T4 and T6 heat-treatment, the yield strength and the elongation of Mg-5Zn-3Gd-0.6Zr alloy becomes higher than that of Mg-5Zn-0.6Zr alloy. With the increase of temperature, the ultimate strength and yield strength of Mg-5Zn-0.6Zr alloy at elevated temperature decrease sharply. At elevated temperature the mechanical property of Mg-5Zn-3Gd-0.6Zr alloy is better than that of Mg-5Zn-0.6Zr alloy.