Effect of Convection Solidification on the Halo Formation in Zn-Al Alloys

The formation of halos was investigated for hypoeutectic, eutectic and hypereutectic Zn-Al alloys under conventional and under convection solidification. It was found that the Zn-rich halos occurred in the surroundings of the Al-rich primary phase for the hypereutectic Zn-Al alloys at Al>5 wt.% and no halos occurred for the hypoeutectic and the eutectic Zn-Al alloys at Al < 5 wt.% under conventional solidification. However, the Zn-rich halos were completely vanished from the Al-rich phase under intensive convection solidification. The interaction among the solid phases and the liquid phase was found to be responsible for the halo formation.