OPTIMIZATION OF CHEMICAL-REACTIONS BETWEEN ALUMINA SILICA FIBERS AND ALUMINUM-MAGNESIUM ALLOYS DURING COMPOSITE PROCESSING/

An Al-Si-Cu-Mg alloy reinforced with alumina/silica fibres (Fiberfrax( (R)), alumina/silica ratio = 45/55) has been extensively characterized in terms of microstructure, interfacial chemical reactions and mechan ical properties. The composite was fabricated by squeeze casting. The above characteristics were measured as a function of (a) calcination t emperature of the fibre preform before infiltration, and (b) subsequen t composite heat treatment. The main reaction that occurs during the p rocessing of aluminium alloy matrix composites is the reduction of sil ica in the binder and fibres by magnesium from the matrix. When calcin ed below 1000 degrees C, the fibres remain amorphous with a coating of porous silica binder. In this condition, the reinforcement reacts str ongly with the matrix during heat treatment of the composite. In contr ast, at high calcination temperatures (1200 degrees C), the fibres tra nsform partially into mullite and the silica binder densities; these f ibres are somewhat less reactive with the matrix. In both cases, the m atrix/reinforcement reactions are very strong during high-temperature heat treatment, leading to a complete reduction of silica in some case s. The degradation caused by chemical reactions adversely affects the mechanical properties of these composites. Therefore, in order to opti mize the mechanical properties of this composite, the fibre preform fi rst must be calcined at high temperature, then the composite heat trea tment limited to low temperature.