The Effects of Solidification Cooling and Growth Rates on Microstructure and Hardness of Supersaturated Al-7%Si-x%Zn Alloys

There is a lack of information in the literature about the effects of the solidification cooling (Ṫ) and growth (v) rates on both microstructure and mechanical properties of Al-Si-Zn alloys. Moreover, the relation between microstructural parameters and mechanical behavior is valuable information to evaluate the potential of Al-Si-Zn alloys. New applications can be attained when Zn is added to Al-Si alloys, due to the reduction in the liquidus temperature, T_L, and improvement in mechanical properties. The present study investigates the evolution of microstructure and the corresponding effects on microhardness of Al-7wt.%Si-(10, 15, 20) wt.%Zn alloys, directionally solidified under unsteady-state conditions. The microstructure of the Al-7%Si-x%Zn alloys is shown to be characterized by a dendritic α-Al matrix, with the interdendritic region composed of α-Al, Si, and an AlFeSi-type intermetallic. The decrease in Ṫ and v resulted in coarsening of the dendrites. Dendritic scaling laws are experimentally determined relating primary, secondary, and tertiary dendritic spacings to Ṫ and v. The main mechanism of reinforcement is shown to be related to the supersaturated solid solution. The hardness of the Al-7%Si-10%Zn alloy is not influenced by refinement of the dendritic spacings, however, each further increment of about 5% Zn to this alloy resulted in a hardness increase of about 30 HV.