On the phase relations in the ZrO2–YO1.5–AlO1.5 system

Experimental studies of samples with the composition 23.5 mol% ZrO2, 59.2 mol% YO1.5 and 17.3 mol% AlO1.5 indicated a change of tie-lines in the temperature range 1523–1613 K due to the invariant reaction YAM + F = YAP + δ as predicted by Calphad method. The thermodynamic description of the ZrO2–YO1.5–AlO1.5 system based on the modelling of fluorite (F) and bixbyite (C) structures as different phases is derived. The isothermal sections at 1523, 1573 and 1623 K are calculated and two invariant solid state reactions are indicated: YAM + F = YAP + δ at 1614 K and F = δ + YAM + C at 1602 K. Ternary interaction parameter in liquid phase is assessed to get an improved fit of experimental data on liquidus surface. The calculated liquidus surface is in a good agreement with experimental data. Reasonable agreement with experimental data on isoplethal sections is obtained. The T0-lines for fluorite ⇔ tetragonal and tetragonal ⇔ monoclinic transformations are calculated in the ZrO2–YO1.5–AlO1.5 system.