Solidification microstructural constituent and its crystallographic morphology of permanent-mould-cast Mg-Zn-Al alloys

The microstructural constituents and the crystallographic morphology of the primary intermetallic phases in permanent-mould-cast Mg-Zn-Al alloys with typical compositions within high zinc castable domain were investigated. Three kinds of primary compounds with distinct morphology were identified as Mg32(Al,Zn)49(τ), MgZn(ɛ), and a ternary icosahedral quasi-crystalline compound (denoted with Q). The constituent is found to change with Zn and Al content and their concentration ratio. Alloys with middle mass ratio of Zn to Al and Al content, consist of α-Mg and τ phase, while alloys with high mass ratio of Zn to Al and low Al are composed of α-Mg, ɛ and a small amount of τ, those with low ratio of Zn to Al and high Al consist of α-Mg with Q. Solidification characteristics and process were proposed. The solidification ranges and liquidus temperature decrease with increasing Zn and Al content for τ- and Q-type alloys, whereas ɛ-type alloy shows reverse tendency. The second phase transformation moves to higher temperature range when Al content increases and ratio of Zn to Al decreases.