The isothermal sections at 1250 and 1650 °С, liquidus surface and solidus surface for the Al₂O₃–HfO₂–Y₂O₃ phase diagram were constructed for first time. Phase equilibria were established at these temperatures. No ternary compounds and appreciable solid solution regions on the base of components or binary compounds were found in the ternary system. As far as HfO₂ interacts with every other phases eutectically, this fact allows to combine in materials the unique properties of HfO₂-based T- and F-phases with the properties of other phases of the Al₂O₃–HfO₂–Y₂O₃ system. The liquidus surface is completed by eight primary crystallization fields. Four four-phase nonvariant eutectic equilibria, one four-phase nonvariant transformation equilibrium and three three-phase nonvariant eutectic equilibria were found in the ternary system. It solidus surface consists of five isothermal fields which correspond to four invariant eutectic equilibria and one invariant transformation equilibrium. The solidus surface includes also six linear surfaces of binary eutectics crystallization end. The highest solidus temperature in the system is 2810 ^oC – the HfO₂ melting point, the lowest – 1755 °C – the ternary eutectic AL+F+Y₃A₅ melting temperature. The diagram of phase equilibria and the equilibrium alloys crystallization scheme for the Al₂O₃–HfO₂–Y₂O₃ system were constructed using data on bounding binary systems, liquidus and solidus surfaces.

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1. Phase diagrams of the systems Al₂O₃–ZrO₂–Ln(Y)₂O₃ as a scientific base for creating new high-temperature structural ceramics