Preferential Oxidation of Mg in Mechanically Alloyed Al–Mg–O Based Systems

Abstract

For the purpose of attesting preferential oxidation of solute Mg in Al and obtaining fine dispersion of MgO in the Al matrix, a powder of Al-10 at%Mg alloy was mechanically alloyed with or without addition of metal oxide. Metal oxides added are CuO, Fe2O3, SiO2, V2O5, Nb2O5 and Ta2O5. MA was carried out under an argon atmosphere by using an Attritor ball mill with addition of methanol as the process control agent. After ball-milling for 30 h, the MAed powders were degassed and consolidated to P/M materials by hot extrusion. The P/M materials were heated at 773 K for 24 h. Constituent phases were studied for the MAed powders, as-extruded and heated P/M materials by X-ray diffraction. Metallography and hardness measurement were also done for the P/M materials.In Al–Mg alloy MAed with methanol, MgO is formed by preferential oxidation. In all the Al–Mg-Metal oxide systems studied, reduction of the added metal oxide and preferential oxidation of Mg were observed at different stages of mechanical and thermal processes. No diffraction lines of Al2O3 were observed throughout. In the Al–Mg–CuO system, decomposition of CuO occurred during ball-milling. In the systems other than Al–Mg–CuO, decomposition of the added oxide and formation of MgO were observed in the as-extruded P/M material. Such a solid state reaction proceeded by further heating at 773 K. After heating for 24 h, decomposition of the added oxides was completed except Ta2O5. The rate of decomposition of metal oxides is in correlation with their values of standard free energy of formation. Thus, in MAed Al–Mg based systems, dispersion of MgO particles in the Al alloy matrix can be attained by preferential oxidation of solute Mg. In case of addition of excess oxygen over MgO equivalent, formation of spinel (MgAl2O4) was observed.