Phase equilibrium investigations of the Al2O3–Ta2O5 system: New experiments and thermodynamic modeling

Abstract

AbstractPhase equilibria in the Al2O3–Ta2O5 system have been studied by means of CO2 laser heater, high‐speed pyrometer, X‐ray powder diffraction, and scanning electron microscopy. The liquidus and invariant reaction temperatures in the whole composition region were measured by cooling trace tests. The monotectic reaction of Liquid 1→Liquid 2 + Al2O3 was determined to be at about 1547°C, but the typical microstructures for the miscibility gap were not directly observed in our experiments. The tetragonal AlTaO4 phases formed from as‐cast samples were found to congruently melt at 1669°C, and they could maintain stability during the cooling process but would gradually transform into orthorhombic and monoclinic polymorphs after heat treatment. Two eutectic reactions of Liquid → Al2O3 + T‐AlTaO4 and Liquid → T‐AlTaO4 + β‐Ta2O5 were measured to be at 1447°C and 36.7 mol%Ta2O5 and 1513°C and 63.5 mol% Ta2O5, respectively. Due to the undercooling effect, the invariant reaction temperatures determined by this work might be preliminary and need to be verified by more accurate methods. Our experiments confirmed that doping Al2O3 in Ta2O5 phases could improve phase stability of β‐Ta2O5. Finally, thermodynamic optimization of the Al2O3–Ta2O5 system was performed based on the obtained experimental data by means of the CALPHAD method. The self‐consistent thermodynamic parameters are helpful in further efforts for construction of Al2O3–Ta2O5‐based multicomponent thermodynamic database and the development of alumina–tantala‐based materials.