SiO₂-Al₂O₃ metastable phase equilibrium diagram without mullite

Abstract

A metastable binary phase diagram between SiO₂ (cristobalite) andα-Al₂O₃ (corundum) in the absence of any mullite phase is presented. A eutectic is indicated at a temperature of ≈ 1260° C and a composition of ≈ 18 wt% (≈ 12 mol%) Al₂O₃. The liquidi of the proposed metastable system were positioned on the basis of the thermodynamic data calculated from the stable equilibrium diagram of Aksay and Pask [2]. Experimental evidence is also presented. A SiO₂-Al₂O₃ melt containing 80 wt% Al₂O₃ cooled at a slow rate in sealed molybdenum crucibles shows crystalline Al₂O₃ plus a glass phase whose composition followed the calculated extension of the stable Al₂O₃ liquidus to lower temperatures. Compacts of cristobalite—corundum mixtures were fired at subsolidus temperatures to estimate the eutectic temperature experimentally. The proposed metastable phase diagram effectively explains the formation of non-crystalline phases in subsolidus reactions, and microstructure obtained on solidification of high alumina melts.