Effects of Al and Zn contents on the microstructure and mechanical properties of Mg-Al-Zn-Ca magnesium alloys

The microstructure and tensile properties of Mg-xAl-yZn-2Ca (x + y = 8 wt%) alloys with various Al and Zn contents have been investigated. The results showed that all the alloys exhibited dendritic microstructure with α-Mg matrix and eutectic compounds distributed at dendritic boundaries. The category and amount of eutectic compounds were influenced obviously by Al and Zn contents. Two Laves phases, i.e. C36 and C15, Q phase and/or Ca₂Mg₆Zn₃ isomorphs phases were observed in as-cast alloys. The majority of second phase was C36 in high Al-containing alloys. By decreasing Al content, the C36 was transformed to C15 Laves phase and Ca₂Mg₆Zn₃ isomorphs phase. Both C36 and C15 Laves phases contained a certain amount of Zn. The tensile strength and ductility of Mg-Al-Zn-Ca alloys were greatly dependent on the category and the morphology of second phases. The C36 Laves phase exhibited better strengthening effect, while the Q phase easily led to the formation of cavities at high temperature, thus decreasing heat resistance of the alloy. The alloy containing the majority of networked C36 phase shows optimal mechanical properties at both room temperature (RT) and elevated temperature.