Optimization of chemical reactions between alumina/silica fibres and aluminium-magnesium alloys during composite processing

Abstract

An Al-Si-;Cu-Mg alloy reinforced with alumina/silica fibres (Fiberfrax®, alumina/silica ratio=45/55) has been extensively characterized in terms of microstructure, interfacial chemical reactions and mechanical properties. The composite was fabricated by squeeze casting. The above characteristics were measured as a function of (a) calcination temperature of the fibre preform before infiltration, and (b) subsequent composite heat treatment. The main reaction that occurs during the processing of aluminium alloy matrix composites is the reduction of silica in the binder and fibres by magnesium from the matrix. When calcined below 1000°C, the fibres remain amorphous with a coating of porous silica binder. In this condition, the reinforcement reacts strongly with the matrix during heat treatment of the composite. In contrast, at high calcination temperatures (1200°C), the fibres transform partially into mullite and the silica binder densifies; these fibres are somewhat less reactive with the matrix. In both cases, the matrix/reinforcement reactions are very strong during high-temperature heat treatment, leading to a complete reduction of silica in some cases. The degradation caused by chemical reactions adversely affects the mechanical properties of these composites. Therefore, in order to optimize the mechanical properties of this composite, the fibre preform first must be calcined at high temperature, then the composite heat treatment limited to low temperature.