Probing the initial stage of synthesis of Al₂O₃/Al composites by directed oxidation of Al-Mg alloys

In the directed oxidation of Al-Mg alloys, the amount of MgO that forms in the initial stage prior to the incubation period affects the rate of oxidation of Al to Al₂O₃ in the composite growth stage. The mechanism of formation of MgO and the duration of the initial stage were investigated experimentally and theoretically. The variables studied were total pressure in the reaction chamber, partial pressure of oxygen, and the nature of the diluent gas which affects the diffusion coefficients of magnesium vapor and oxygen in the gas phase. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and the diffusivity of oxygen. The duration of the initial stage decreased with the increase in oxygen pressure. To understand the role of magnesium evaporation in the oxidation behavior of the alloy, the velocity, temperature, and concentration fields in the gas phase were simulated numerically. The calculated concentration profiles of magnesium vapor and oxygen as a function of time were consistent with the experimentally measured oxidation rates and confirm reaction-enhanced gaseous diffusion-limited vaporization of magnesium in the initial stage of oxidation of Al-Mg alloys. The region where the magnesium vapor is oxidized in the gas phase moved progressively closer to the alloy surface during the initial stage of oxidation. The end of the initial stage and the start of the incubation period corresponded to the arrival of the oxygen front close to the surface when the spinel formation occurred.