KINETICS OF DIRECTED OXIDATION OF AL-MG ALLOYS IN THE INITIAL AND FINAL STAGES OF SYNTHESIS OF AL2O3 AL COMPOSITES/

In the directed oxidation of AL-Mg alloys, MgO forms in the initial st age. The mechanism of formation of MgO from the Al-Mg alloy in the ini tial stage of oxidation was studied. The variables studied were the to tal pressure ill the reaction chamber and partial pressure of oxygen. The oxidation rate in the initial stage was proportional to both the o xygen partial pressure and oxygen diffusivity. These results suggest t hat MgO forms by reaction-enhanced vaporization of Mg from the alloy f ollowed by oxidation of the Mg vapour in the gas phase. The end of the initial stage corresponds to the arrival of the oxygen front close to the melt surface, when spinel formation occurs. The kinetics of forma tion of Al2O3 in the growth stage of directed oxidation of the Al-5wt% Mg alloy was also investigated as a function of time, temperature and oxygen partial pressure. The growth rate decreased as a function of t ime, was practically independent of oxygen pressure and exhibited an a ctivation energy of 361 kJ mol(-1). In the growth stage, the kinetics of oxidation is controlled by the rate of transport of oxygen through the alloy layer near the surface to the alumina-alloy interface.