Refractory metal-alumina composites are investigated for a new generation of coarse-grained composites.^[1] The knowledge about phase transformations and reactions during the sintering process and the long-term heat treatments are a fundamental basis in developing new materials. The quaternary systems Al-Nb-Si-O and Al-Ta-Si-O are not yet thermodynamically assessed in the literature. For the CALPHAD-type assessment key experiments such as X-ray powder diffraction (XRD) and thermal analysis (e.g. differential thermal analysis (DTA) and differential scanning calorimetry (DSC)) are used to determine phase compositions, homogeneity ranges, thermochemical and thermophysical properties.

The samples are prepared by dry-mixing powders, which are then cold isostatically pressed, heat treated (only solid-state reactions) and annealed. If thermodynamic equilibrium is not reached, samples are regrinded and the previous steps are repeated. The experiments are used to create an experimental dataset as input for the CALPHAD optimization.

According to the planned conditions of use of the composites, the samples were placed in the composition ranges M-Al₂O₃-SiO₂-M₂O₅ (M=Nb, Ta) for the key experiments. XRD experiments show that AlTaO₄ is practically a stoichiometric phase and the only ternary oxide in the quasi-binary section Al₂O₃-Ta₂O₅. XRD patterns of quenched samples are used to determine the solubility of Al₂O₃ in α-Ta₂O₅ and β-Ta₂O₅. The same procedure is applied to determine the dissolution of SiO₂ in Nb₂O₅ and Ta₂O₅. No ternary oxide is formed in the quasibinary sections SiO₂-M₂O₅ (M=Nb, Ta). Within the Al₂O₃-Nb₂O₅ section three stoichiometric ternary oxides can be identified: AlNbO₄, AlNb₁₁O₂₉ and AlNb₄₉O₁₂₄. All four ternary compounds show no solid-solid phase transition. Based on the present experimental results and evaluations of binary systems from the literature, thermodynamic datasets will be evaluated which describe both quaternary systems, Al-M-Si-O (M=Nb, Ta), from the melt down to room temperature. For Al-M-O (M=Nb, Ta) datasets have already been published.^{[2, 3]}

References
[1] T. Zienert; Ceram. Int., 2018, 44, 16809–16818.
[2] J. Gebauer; Adv. Eng. Mater., 2022, 24, 2200162.
[3] M. Eusterholz; Adv. Eng. Mater., 2022, 24, 2200161.