The Zirconia-Hafnia System: DTA Measurements and Thermodynamic Calculations

The equilibrium temperature (T0) at which the tetragonal and monoclinic phases of either ZrO2 or HfO2 coexist is generally defined by the middle temperature of As (the onset transformation temperature on heating) and Ms (the onset transformation temperature on cooling). It cannot be directly determined due to the athermal nature of the martensitic transformation. Practically, the determination of T0 is important for the prediction of As and Ms in ZrO2 or HfO2‐based materials. In this work, the ZrO2–HfO2 system was studied experimentally by differential thermal analysis (DTA) to obtain the martensitic tetragonal ⇔ monoclinic transformation temperatures in the temperature range of 1273–1973 K. The T0 temperatures obtained for ZrO2 and HfO2 are 1367±5 and 2052±5 K, respectively. They are adopted for the assessments of the Gibbs energy parameters of these two oxides. A reasonably calculated ZrO2−HfO2 phase diagram is presented.