Internal corundum growth in alumina-silica refractory during exposure to molten Al-5Mg alloy

The present work is aimed at clarifying the internal degradation processes of an alumina-silica refractory through microstructural observations and analysis after exposure to a molten Al-5Mg alloy for an extended period. An alumina-silica crucible was pretreated by a molten Al-5Mg alloy to cause the initial stage of degradations at the inner crucible surface. Fresh alloy was melted in the pretreated crucible and held at 1150˚C for 96 hrs to cause further degradations into the crucible wall. Results indicate that Mg reacts with aggregate (Al6Si2O13, SiO2) and matrix materials to form MgAl2O4, Si, Al, and Al-Si-Ca intermetallic compounds that were crystallized around MgAl2O4 to form network-like structures. In the absence of Mg, Al reacts with the aggregate to produce Si and αAl2O3 and reacts with calcium-aluminum silicate in the matrix to form Si and CaO-Al2O3 compounds. These indicate that MgAl2O4 spinel forms preferentially before the corundum (αAl2O3) formation. Mg and Ca, segregated at the interface of " αAl2O3 / Al6Si2O13" or "αAl2O3 / SiO2", may promote corundum formation. The lower concentration of O and Si in the degraded areas in comparison with those in un-degraded areas suggests that gaseous SiO might have been generated during the degradation processes.