Microstructural characterization of high pressure die cast Mg-Zn-Al-RE alloys

Mg-(1-10)Zn-(4-6)Al-4RE alloys were prepared by high pressure die casting and their microstructures were characterized. Optical observations revealed the presence of externally solidified grains (ESGs) in the microstructure. The amount of ESGs initially decreased with increasing Zn content, but further additions of Zn led to a steady increase in the amount of ESGs. The Mg-4Zn-4Al-4RE alloy was found to have the least ESGs and the most uniform grain structure. The alloys with higher Zn and Al contents showed a more dendritic microstructure with coarse RE-containing intermetallic phases and network of a Mg-Zn-Al ternary phase. Compared with the commercial AE44 (Mg-4Al-4RE) alloy, additions of Zn reduced the amount of Al-RE phases and resulted in the formation of Al2REZn2 phase.