Hypereutectic Zn–Al Alloys: Microstructural Development under Unsteady-State Solidification Conditions, Eutectic Coupled Zone and Hardness

The present study investigates the effects of Al content and solidification thermal parameters on the microstructural development under transient heat flow conditions for two hypereutectic Zn–Al alloys: Zn-6wt.%Al and Zn-11wt.%Al. The alloys were directionally solidified and had experimental cooling profiles monitored permitting cooling rates and growth rates to be determined along the length of the directionally solidified (DS) castings. The microstructure of the Zn-6wt.%Al alloy is shown to be formed by eutectic colonies, constituted by a eutectic mixture of (Zn) and (Al′) phases in the form of lamellae and the Zn-11wt.% Al alloy by the pro-eutectic (Al′) dendrites and the eutectic mixture in the interdendritic regions. Growth laws are experimentally determined relating eutectic and dendritic spacings to the growth rate and cooling rate. A diagram exhibiting the coupled zone of Zn–Al alloys as a function of cooling rate is proposed, which shows different microstructural morphologies influenced by composition and thermal parameters, that is, growth rate and the temperature gradient, synthesized by the cooling rate (Ṫ = G.V). The microhardness of both Zn-6wt.%Al and Zn-11wt.%Al alloys were shown not to depend on the length scale of the resulting microstructure.