Effect of silica surface dopants on the formation of alumina/aluminum composites by the directed metal oxidation of an aluminum alloy

This study focused on clarifying the effect of SiO2 surface dopants on the formation of Al2O3/aluminium composites' especially on oxidation phenomena during the incubation period. The present results showed that a surface dopant decreased the incubation period of an Al-Mg-Si alloy, as well as that of an AI-Mg alloy, and that addition of an external surface dopant decreased the incubation period more effectively than did an internal alloying of silicon. A two-step oxidation process was also conducted. In the first step of the process, an aluminum alloy was oxidized without a surface dopant and cooled to room temperature during the incubation stage. In the second step, the same specimen was surface-doped with SiO2 powder and reoxidized. The incubation time for the specimen subjected to the two-step oxidation process was the same as that for the single-step specimen oxidized with a surface dopant. The substantial decrease in the incubation period, especially for the Al-Mg alloy, is ascribed to interaction between the SiO2 surface dopant and the MgO layer. This interaction made the MgO layer thinner and increased the number of magnesium vacancies in the MgO layer, thus providing an appropriate microstructure in the MgO layer for bulk-growth initiation.