Effect of composition on the microstructure and mechanical properties of Mg–Zn–Al alloys

Magnesium is receiving great attention for transport applications, particularly its cast alloys. This investigation focuses on the as-cast microstructure and mechanical properties of permanent-mould cast Mg–Zn–Al alloys with typical compositions within the high zinc castable domain. Three types of alloys were identified and characterized by Mg32(Al, Zn)49, also known as the τ phase; MgZn phase, also known as the ɛ phase; and a ternary icosahedral quasi-crystalline phase, denoted as the Q phase, respectively. A schematic phase diagram is proposed to show the change of microstructral constituents with element content and the Zn/Al ratio. The diagram reveals that the microstructral constituent is dominated by both the content of Zn or Al and the Zn/Al mass ratio; alloys with a high Zn/Al ratio and a low Al content fall into the ɛ-type; alloys with an intermediate Zn/Al ratio and an intermediate Al content favour the τ-type; and those with a low Zn/Al ratio and a high Al are dominated by the icosahedral quasi-crystalline phase. No Mg17Al12 (γ) phase was found in those ZA series alloys. The solidification process and its effects on the phase constituents were discussed. Preliminary mechanical property testing showed that all the experimental alloys possess comparable ultimate strength and yield strength with the AZ91 alloy at ambient temperature, but show far superior creep resistance at elevated temperatures. Moreover, while ambient-temperature properties solely depend on the total element contents, the τ- and the Q-type alloys show greater potential than the ɛ-type alloys on the improvement of elevated temperature properties.