Microstructure evolution, mechanical properties and diffusion behaviour of Mg-6Zn-2Gd-0.5Zr alloy during homogenization

The microstructure evolution and mechanical properties of Mg-6Zn-2Gd-0.5Zr alloy during homogenization treatment were investigated. The as-cast alloy was found to be composed of dendritic primary α-Mg matrix, α-Mg + W (Mg₃Zn₃Gd₂) eutectic along grain boundaries, and icosahedral quasicrystalline I (Mg₃Zn₆Gd) phase within α-Mg matrix. During homogenization process, α-Mg + W (Mg₃Zn₃Gd₂) eutectic and I phase gradually dissolved into α-Mg matrix, while some rod-like rare earth hydrides (GdH₂) formed within α-Mg matrix. Both the tensile yield strength and the elongation showed a similar tendency as a function of homogenization temperature and holding time. The optimized homogenization parameter was determined to be 505 °C for 16 h according to the microstructure evolution. Furthermore, the diffusion kinetics equation of the solute elements derived from the Gauss model was established to predict the segregation ratio of Gd element as a function of holding time, which was proved to be effective to evaluate the homogenization effect of the experimental alloy.