Thermal cycle degradation of alumina-silica refractories exposed to molten Al-5Mg alloy

In this study, degradation by so-called “Obake”, an advanced stage of an external corundum growth on alumina-silica refractory surfaces, was investigated. The Obake was successfully recreated by melting Al-5%Mg in the alumina-silica crucible under thermal cycle conditions. The thermal test temperatures were chosen at 1000°C or 1150°C. It was confirmed that internal degradation occurred by formation of spinel (MgAl2O4) where the refractory was in touch with molten Al-5Mg and by corundum growth at completely separate location that is at or in the vicinity of the melt line of the refractory surface where the following reaction would take place. (4/3) Al(l)＋O2(g)→(2/3) Al2O3(s) As the degradation of refractory progress, the concentration of Si, Fe, Ti, and Ca in the melt increased. Cracks observed at the "MgAl2O4 / refractory" interface, exposed to thermal cycles, are most likely due to the difference in the linear expansion coefficient between MgAl2O4 and the refractory. αAl2O3, which is a constituent phase of Obake, exists as an aggregate of fine particles including Al metal in between. All αAl2O3 is found in the eutectic regions (Al-Si), suggesting that the eutectic reaction (577°C) may be accompanied by the formation of αAl2O3 © 2022 The Japan Institute of Light Metals.