FORMATION OF AL2O3/METAL COMPOSITES BY THE DIRECTED OXIDATION OF MOLTEN ALUMINUM MAGNESIUM SILICON ALLOYS .1. MICROSTRUCTURAL DEVELOPMENT

The growth of alpha-Al2O3/metal composites by the directed oxidation of molten Al-Mg-Si alloys proceeds through four distinct stages. The first stage encompasses the early heating of the alloy ingot, melting, and continued heating to between 1123 and 1173 K. In this latter temperature range, the molten alloy surface rapidly oxidizes to form a MgO-covered MgAl2O4 layer. During further heating and initial soak at the composite growth temperature (1373 to 1573 K), the duplex layer slowly thickens (second stage). The start of the third stage, growth initiation, is marked by the spread of a metal-rich zone over the duplex layer; this metal-rich zone is believed to be connected to the molten alloy through microcracks in the thickened MgO/MgAl2O4 layer. Small nodules of the oxide/metal composite nucleate from the metal-rich layer. During the final rapid growth stage, the small composite nodules grow and coalesce to form a macroscopically planar growth front, which persists until growth is complete. Throughout the growth process, the external surface of the alpha-Al2O3/metal composite is covered by a thin MgO layer. Immediately under this external layer and separating it from the alpha-Al2O3 is a thin layer of molten metal.