Phase equilibria in the ZrO2-Ta2O5-Nb2O5 system: Experimental studies and thermodynamic modeling

The phase relations in the ZrO2-Ta2O5-Nb2O5 system were experimentally investigated, and thermodynamic modeling was consequently performed in the present work. Samples with the selected compositions were synthesized using coprecipitation method, and then separately equilibrated at 1573 and 1673 K. The microstructural characterizations and phase analyses were carried out with the support of the X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDX). The phase transformations were measured through differential thermal analysis (DTA). No ternary compounds exist, while Ta2O5 and Nb2O5 phases substantially extend into the ternary system. Moreover, binary Ta2Zr6O17 and Nb2Zr6O17 phases form a continuous solid solution (A(2)Zr(6)O(17)) in the ternary system. A three-phase equilibrium region beta-Ta2O5 + A(2)Zr(6)O(17) + Nb2O5 was speculated. Based on the available literature, the ZrO2-Nb2O5, ZrO2-Ta2O5, and Nb2O5-Ta2O5 binary systems were critically evaluated and optimized by means of the CALPHAD (CALculation of PHAse Diagram) method. Combining with thermodynamic parameters of the binary systems, phase relations in the ternary system were extrapolated. However, the extrapolation cannot reproduce our experimental results, especially for the ternary solubilities of ZrO2 in the Ta2O5 and Nb2O5 phases and the melting behavior of selected samples. Finally, the ternary mixing parameters were introduced, and our experimental data were well reproduced by the improved thermodynamic descriptions.