The Al2O3-HfO2-Gd2O3 phase diagram. I. Isothermal sections at 1250 AND 1650A°C

The isothermal sections of the Al2O3-HfO2-Gd2O3 phase diagram at 1250 and 1650A degrees C are constructed for the first time and phase equilibria at these temperatures are established. No ternary compounds or appreciable solid solution regions based on components or binary compounds are found in the ternary system. Interaction in the system is determined by the most thermodynamically stable compound, HfO2, which equilibrates with all phases in the system. In the region with Gd2O3 content up to similar to 65%, the sections are similar, only width of the regions changes. This is connected with changes in the extension of M and F solid solutions in the HfO2-Gd2O3 binary bounding system. The presence of AL + F, GA + GH(2), and G(2)A + F two-phase regions at the isothermal sections suggests that there are triangulating sections of the Al2O3-HfO2-Gd2O3 system in them. Since the F and GH2 phases are of variable composition, these sections can be qualified as conditionally quasibinary. In wide three-phase regions, like in the Al2O3-ZrO2-Gd2O3 system, ternary eutectic points are expected to exist.