In situ formation of Al2O3–SiC–mullite from Al-matrix composites

Five composite batches (M1–M5) containing Al2O3–SiC–mullite–Al were designed and prepared in situ from different proportion mixtures of alumina, silicon carbide and aluminum alloy. The phase composition and microstructure were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Sinterability in terms of bulk density, apparent porosity and compressive strength were also measured. All samples exhibit XRD reflections characteristic of alumina, SiC, and mullite. The amount of these phases depended on starting batch compositions, oxidation of Al-alloy, oxidation of SiC, and reaction of starting and oxidized materials together to form mullite. In the composites-containing lower mullite content, new silica-rich phase (aluminum silicate phase, i.e. Al2Si4O10) and/or cristobalite appeared. The formed mullite or silica rich phase prevents further oxidation of SiC or Al–metal. These phase compositions reflect the higher density, lower porosity and good mechanical properties of such composites. The presence of low melting silicate phases was found to increase the bulk density and consequently to decrease the apparent porosity.