Phase equilibrium investigations and thermodynamic modelling of the ZrO2-HfO2-Ta2O5 system

The phase equilibria in the ZrO₂-HfO₂-Ta₂O₅ system were studied at 1673 K and 1873 K by equilibration technique and in the temperature range up to 2473 K by differential thermal analysis (DTA). The phase and chemical composition of the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX). Melting relations were studied by DTA followed by microstructural investigation by SEM/EDX. The composition dependence of the eutectic and peritectic temperatures was shown. The heat capacity of the (Zr,Hf)₆Ta₂O₁₇ compound with different compositions was measured by differential scanning calorimetry (DSC) in three temperature ranges from 100 K to 230 K, from 220 K to 570 K, and from 330 K to 1295 K. Based on the obtained results and our previous thermodynamic descriptions, a database for the ternary ZrO₂-HfO₂-Ta₂O₅ system using the CALPHAD approach was created.