The role of zinc in the eutectic solidification of magnesium-aluminium-zinc alloys

Previous experimental work has suggested that the addition of zinc to hypoeutectic magnesium-aluminium alloys promotes the formation of a divorced eutectic. However, the amount of zinc required to produce this effect has not been quantified and the mechanism by which it occurs has not been determined. This paper reports the results of a study of the effect of zinc on eutectic morphology in permanent mould cast hypoeutectic magnesium-aluminium alloys, including the commercial alloy AZ91. The results show that the strength of this effect varies with aluminium content and cooling rate. The addition of 1.6 wt% Zn to a Mg-9Al alloy cooled at approximately 80 Ws was sufficient to cause a fully divorced eutectic to form when a partially divorced eutectic formed in the binary alloy solidified at the same rate. An addition of 1.6 wt% Zn to a Mg-15Al alloy cooled at approximately 20 K/s was sufficient to cause a partially divorced eutectic to form when a granular eutectic was observed in the binary alloy solidified at the same cooling rate. Directionally solidified samples of AZ91 were used to investigate the segregating behaviour of zinc during solidification. The effect of zinc on the solidification behaviour of hypoeutectic Mg-Al-Zn alloys, and its relation to and dependence on cooling rate, is discussed.