Refractory metal-alumina composites are investigated for a new generation of coarse-grained materials. The knowledge about phase transformations and reactions during the sintering process and the long-term heat treatments are a fundamental basis in developing the new materials. The ternary systems Al-Nb-O and Al-Ta-O are not thermodynamically assessed in the literature – binary parameter sets are available. For the CALPHAD-type assessment key experiments such as X-ray powder diffraction (XRD) and thermal analysis (e.g. differential thermal analysis (DTA), differential scanning calorimetry (DSC) and dilatometry (DIL)) 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 optimization.

The main technological focus for the composites is on the thermodynamic stabilities of both refractory metals in contact with alumina. Due to oxygen impurities (such as the native oxide layer of metal particles) phase equilibria in the quasi-binary sections Al₂O₃-Nb₂O₅ and Al₂O₃-Ta₂O₅ are mainly investigated. XRD experiments show that AlTaO₄ is a stoichiometric phase and the only ternary oxide in the quasi-binary section Al₂O₃-Ta₂O₅. XRD patterns of samples consisting of AlTaO₄ and a second phase Al₂O₃ or Ta₂O₅, respectively, have the same AlTaO₄ peak positions as the pattern of a AlTaO₄ single-phase sample. 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 in DTA and DIL experiments.

The thermodynamic parameters and coefficients of the Gibbs energy functions of ternary Al_xM_yO_z (M = Nb, Ta) oxides are assessed for the first time and the descriptions of the ionic liquids are extended with ternary parameters.