Fundamentals of the directed oxidation process for synthesis of alumina/aluminum composites

Synthesis of oxide matrix composites by the directed metal oxidation process offers significant advantages over traditional composite processing routes. Much of the previous work on directed oxidation has been focused on understanding the microstructural evolution during the process. In this work, the kinetics of formation of Al{dollar}rmsb2Osb3{dollar}/Al composites from Al-Mg alloys has been examined. In addition, growth kinetics of Al{dollar}rmsb2Osb3{dollar}/Al composites through Al{dollar}rmsb2Osb3{dollar} preforms has been studied. Furthermore, the nature of metal distribution in Al{dollar}rmsb2Osb3{dollar}/Al composites, and its dependence on growth temperature, have been investigated.; The amount of MgO which forms in the initial stage and the mechanism of initial stage oxidation are important. It is shown that reaction enhanced gaseous diffusion limited vaporization of magnesium occurs in the initial stage. It is also shown that the interface, where magnesium vapor reacts with oxygen to form MgO, progressively moves towards the alloy surface during the reaction.; Analysis of the oxidation kinetics of Al-Mg and Al-Mg-Si alloys demonstrates that the growth kinetics can be tailored by the control of alloy composition. For Al-Mg alloys, transport of oxygen through a thin alloy layer near the surface controls the growth rate. When Si is added to the alloy, the oxidation mechanism is completely changed. The rate of oxidation of Al-Mg-Si alloys depends on the transport of electronic species through a thin MgO layer at the top surface of the composite.; The metal distribution in Al{dollar}rmsb2Osb3{dollar}/Al composites and its dependence on growth temperature were examined. Both the amount of metal in the composite and the extent of interconnection of the metal channels decrease with increasing growth temperature. The observed changes in microstructure with temperature can be explained by considering temperature variations of grain boundary energies in alumina and the alumina/aluminum interfacial energy.; The mechanism of oxidation of Al-Mg alloys into Al{dollar}rmsb2Osb3{dollar} preforms has been investigated. The study demonstrates that the growth kinetics of Al alloys into Al{dollar}rmsb2Osb3{dollar} preforms can be tailored by the control of preform particle size.