Abstract
Alumina‐matrix composites with and without SiC particulates have been produced by directed melt oxidation of a multicomponent Al alloy. In the more general case, the microstructure consists of three interpenetrating phases: the SiC preform, a continuous α‐Al2O3 matrix, and a network of unoxidized metal. The volume fraction of metal within the oxidation product decreases with increasing processing temperature, and its distribution is less uniform when a preform is present. The preform does not show evidence of degradation by the molten alloy, but the growth front tends to climb up the particles, increasing the oxidation area and enhancing the rate of composite formation. The total porosity of the composite was found to increase with increasing Mg content, processing temperature, and/or SiC particle size. Porosity within the channels is associated primarily with insufficient metal flow to feed the solidification shrinkage. Larger pores are common in the SiC composites and seem to evolve by encroachment of interparticle spaces by the convoluted growth front. Coarse chemical segregation of the heavily alloyed residual metal is pervasive regardless of the presence or characteristics of the preform, but can be significantly refined by increasing the cooling rate of the composite after growth.
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