Growth Stage Kinetics in the Synthesis of Al₂O₃/Al Composites by Directed Oxidation of Al-Mg and Al-Mg-Si Alloys

Although synthesis of ceramic matrix composites by the directed oxidation process offers significant advantages over traditional composite processing routes, the scientific basis for the process is not fully understood. This paper is addressed to understanding the mechanism of composite growth from Al-Mg and Al-Mg-Si alloys theoretically and experimentally. Analysis of the oxidation kinetics of Al-Mg and Al-Mg-Si alloys for various oxygen pressures, temperatures and durations of oxidation, obtained in this study and reported in the literature, demonstrates that the growth kinetics can be tailored by the control of alloy composition. For the Al-Mg alloys, transport of oxygen through a thin alloy layer near the surface controls the growth rate. When Si is added to the alloy, the oxidation mechanism is completely changed. The rate of oxidation of Al-Mg-Si alloys depends on the transport of electronic species through a thin MgO layer at the top surface of the composite. Apart from contributing to a more complete understanding of the growth stage, the mechanism of composite growth will serve as a basis for improving growth rates.