Oxidation processes and phase changes in metastable Al-Mg alloys

Oxidation behavior of metastable mechanical alloys in the Al-Mg binary system has been examined in the context of high energy density materials and combustion applications. Mechanical alloy powders with compositions ranging from Al_0.95Mg_0.05 to Al_0.5Mg_0.5, as well as the component metals were heated at 20 K/min in oxygen. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) showed that oxidation proceeds in two separate steps. During the first step occurring over the range of 550 - 600 °C, Mg is oxidized and thereby quantitatively removed from the metallic phase. The selective removal of Mg from the alloy was identified by correlation of weight gain with the Mg concentration of the alloy, by and x-ray diffraction (XRD) and scanning electron microscopy (SEM) of intermediate products. The second step during which the remainder of the metallic phase is oxidized occurs over a wider range of temperatures (900 - 1200 °C). The temperatures of both effects decrease slightly with increasing Mg content in the alloy. Oxidation is increasingly incomplete as the Mg concentration of the alloy decreases below 30 mol-%. It was concluded that the low-temperature selective oxidation of Mg is controlled by the volatilization of Mg from the alloy. No correlation could be established between the oxidation reactions and subsolidus phase transitions, which occur over the temperature range of 100 - 400 °C and are associated with the relaxation of the metastable state of the mechanical alloys.