A thermogravimetric study of the oxidative growth of Al₂O₃/Al alloy composites

Abstract

The oxidation of liquid Al-Mg-Si alloys at 900-1400 °C was studied by thermogravimetric analysis (TGA). The development of a semi-protective surface layer of MgO/MgAl₂O₄ allows the continuous formation of an Al₂O₃-matrix composite containing an interpenetrating network of metal microchannels at 1000-1350 °C. An initial incubation period precedes bulk oxidation, wherein Al₂O₃ grows from a near-surface alloy layer by reaction of oxygen supplied by the dissolution of the surface oxides and Al supplied from a bulk alloy reservoir through the microchannel network. The typical oxidation rate during bulk growth displays an initial acceleration followed by a parabolic deceleration in a regime apparently limited by Al transport to the near-surface layer. Both regimes may be influenced by the Si content in this layer, which rises due to preferential Al and Mg oxidation. The growth rates increase with temperature to a maximum at ~1300 °C, with a nominal activation energy of 270 kJ/mole for an Al-2.85 wt. % Mg-5.4 wt. % Si alloy in O₂ at furnace temperatures of 1000-1300 °C. An oscillatory rate regime observed at 1000-1075 °C resulted in a banded structure of varying Al₂O₃-to-metal volume fraction.