Microstructural evolution of silicon carbide aluminum oxide composites processed by melt oxidation

The nucleation and growth mechanisms during high temperature oxidation of l iquid Al-3 wt% Mg and Al-3 wt % Mg-7 wt % Si alloys were studied to provide a better understanding of the composite fabrication process, especially in the presence of SiC reinforcement. Al2O3-matrix composites with and withou t SiC particulates have been produced by directed oxidation of aluminum all oys. The microstructure consists of three interpenetrating phases: the SiC preform, a continuous alpha-Al2O3 matrix, and a network of unoxidized metal . The volume fraction of metal within the oxidation product decreases with increasing processing temperature. The preform does not show any evidence o f degradation by the molten alloy but the growth front tends to climb up th e particles, increasing the oxidation area and therefore enhancing the rate of composite growth. The amount of porosity was found to increase with the Mg content in the alloy, from 2.0 vol % for 0.5 wt% Mg to 5.8 vol % for 3 wt % Mg. The role of magnesium and silicon in the growth process are discus sed. (C) 1998 Kluwer Academic Publishers.