Microstructure and mechanical properties of sand mold cast Mg-4.58Zn-2.6Gd-0.18Zr magnesium alloy after different heat treatments

Microstructure and mechanical properties of sand mold cast Mg-4.58Zn-2.6Gd-0.18Zr magnesium alloy after different heat treatments were investigated. As-cast alloy was composed of α(Mg) matrix, interdendritic α(Mg) + W(Mg₃Zn₃Gd₂) eutectic, icosahedral quasicrystalline I(Mg₃Zn₆Gd) phase and Mg₃Gd particles within α(Mg) matrix. After solution treatment at 505 °C for 16 h, most of α(Mg) + W(Mg₃Zn₃Gd₂) eutectic dissolved into α(Mg) matrix, but a new phase, Zn₂Zr₃, precipitated within α(Mg) matrix. The appropriate aging treatment was done at 220 °C for 16 h, after which some dispersive fine eutectics re-precipitated along the grain boundary. Furthermore, rod-like β₁′ and plate-like β₂′ formed within α(Mg) matrix. The ultimate tensile strength, yield strength and elongation of the experimental alloy in the T6 state at room temperature were 280 MPa, 175 MPa and 7.5%, respectively. According to strengthening contribution calculation, it is revealed that both the second phase strengthening and the solid solution strengthening played important roles in improving the mechanical properties after T6 treatment.