The Al₂O₃–HfO₂–Y₂O₃ Phase Diagram.
III. Solidus Surface and Phase Equilibria in Alloy Crystallization

Abstract

The solidus surface for the Al₂O₃–HfO₂–Y₂O₃ phase diagram is constructed for the first time. It consists of five isothermal fields, which correspond to four invariant eutectic equilibria and one invariant transformation equilibrium. The solidus surface includes also six ruled surfaces of binary eutectic crystallization end and T + F co-crystallization end. The highest solidus temperature in the system is 2810 °C (HfO₂ melting point) and the lowest is 1755 °C (ternary eutectic AL + F + Y₃A₅ melting temperature). No ternary compounds or appreciable solubility of the third component in binary compounds and components are found in the ternary system. The phase equilibria diagram and reaction scheme for equilibrium crystallization of Al₂O₃–HfO₂–Y₂O₃ alloys are constructed using data on the bounding binary systems and liquidus and solidus surfaces.