A fundamental study of the formation of aluminum oxide/aluminum composites from oxidation of liquid aluminum alloy

Thermogravimetric and microstructural analyses coupled with updated thermodynamic data for the Al-Mg-O system, were used to study the oxidation behavior and underlying mechanisms during formation of Al$sb2$O$sb3$/Al composites by direct oxidation of molten Al-Mg alloys between 1000 and 1300$spcirc$C.;Oxidation starts with the formation of surface MgO followed by rapid growth of a mixed MgAl$sb2$O$sb4$ (spinel) + metal layer during heating to composite growth temperatures. The temperature at which the transformation occurs depends on the Mg content in the alloy and the heating rate. Rapid spinel growth typically leads to surface passivation and an abrupt drop in the oxidation rate. A long incubation period of slow and negligible oxidation ensues wherein the initial spinel further develops into a multilayer structure consisting of MgO/dense spinel/spinel + metal between the surface and the bulk alloy. Incubation ends with the nucleation of composite nodules when continuous metal paths breach the dense spinel to reach the external MgO. The role of SiO$sb2$ in substantially reducing the incubation period is discussed.;Composite nucleation starts with the formation of nodules consisting of additional spinel + metal which is later replaced by Al$sb2$O$sb3$ + metal. The extent to which the spinel grows before Al$sb2$O$sb3$ forms was examined as a function of temperature and Mg content in the alloy.;Further growth and coalescence of the Al$sb2$O$sb3$+Al nodules leads to bulk Al$sb2$O$sb3$/Al composite formation. In the present alloys, composite growth occurs primarily in an oscillatory rate regime (1000-1200$spcirc$C). The rate oscillations manifest themselves in the composite microstructure as bands with alternating Al$sb2$O$sb3$ to metal volume fraction ratios. The formation of bands was associated to periodic variations of the microstructure at the growth front. The evolution of these bands was discussed in light of the current understanding of the growth mechanism.