Eutectic solidification in Mg-9Al-0.7Zn: From divorced to coupled growth

In hypoeutectic Mg-Al-based alloys, Mg17Al12 forms in the last stages of solidification by a eutectic reaction that is usually fully or partially divorced. Here we investigate the nucleation, growth and morphology of the Mg17Al12-containing eutectic in a multicomponent Mg-9Al-0.7Zn-0.2Mn alloy. Under all solidification conditions studied, Mg17Al12 formed an interconnected skeleton in 3D and Mg17Al12 orientation domains spanned multiple α-Mg grains, indicating that Mg17Al12 nucleation events are relatively infrequent. While most regions of α-Mg+Mg17Al12 had no preferred orientation relationship (OR), occasional eutectic regions had the Burgers OR that may be related to the nucleation of Mg17Al12 on α-Mg during solidification. It is shown that the degree of partially divorced growth depends on a simple geometrical measure for the space available for eutectic growth involving the dendrite arm spacing, the eutectic fraction and the eutectic spacing. We then demonstrate that fully coupled eutectic microstructures can be generated in this alloy by independently manipulating the magnesium dendrite solidification time and the eutectic cooling rate.