Microstructure investigation and first-principle analysis of die-cast AZ91 alloy with calcium addition

In order to get improved mechanical properties of die-cast AZ91 alloy under elevated temperatures, Ca element was added as a cost-effective alloying constituent. It appeared that minor Ca addition less than 0.5 wt% would result in no apparent change in microstructure, but the tensile strength at elevated temperatures was improved considerably. When increasing Ca addition to more than 1.0 wt%, Al₂Ca phase will precipitate during solidification, no Mg₂Ca phase was discovered. Homogeneous microstructure and high temperature stability in tensile strength of die-cast AZ91 alloy with Ca addition was mainly attributed to the precipitation of Al₂Ca phase, which considerably refined the bulky β-Mg₁₇Al₁₂ phase distributed originally at the grain boundaries of die-cast AZ91 alloy with no Ca addition. The priority of Al₂Ca phase compared to Mg₂Ca phase in precipitation sequence was verified by first-principle calculation of their cohesive energy and formation enthalpy, and can also be associated with more bounding electrons between Al and Ca atoms.