Observations on the role of Mg and Si in the directed oxidation of Al–Mg–Si alloys

The initiation and growth of α–Al₂O₃/metal composites by the directed oxidation of molten Al–Mg, Al–Si, and Al–Mg–Si alloys at 1373 and 1523 K were investigated. Spontaneous initiation and growth occurred only on the ternary Al–Mg–Si alloy. Growth also occurred on the Al-Mg binary alloy, but in this case initiation required the mechanical disruption of the protective oxide formed during initial heating to the growth temperature. In addition, mechanical disruption of the protective oxide scale on the Al–Mg–Si alloy enhanced growth initiation. Growth on the Al–Si alloy could not be induced under any conditions. From these observations it is concluded that, in the directed oxidation of Al–Mg–Si alloys, Mg is essential to the accelerated oxidation reaction, while Si appears to play a role in promoting the breakdown of the protective oxide layers. The most uniform initiation and growth results were obtained by providing a thin layer of SiO₂ particles at the initial growth surface of the alloy.