Refractory metal (molybdenum, niobium, and tantalum) matrix composite materials in combination with refractory ceramics (alumina, zirconia, and mullite) are of particular interest for high-temperature applications, e.g., in refractory linings in the casting industry. [1] However, besides the high-temperature applicability, other aspects need consideration as well for the choice of material combination, such as: 1) the reaction between the metal and ceramic particles, 2) the chemical interaction with the environment, and 3) the thermal mismatch between the ceramic and the metallic phases. [2] The majority of the past publications dealt with so-called fine-grained refractory composites. [3][4][5][6][7] However, their disadvantages are high shrinkage on sintering, thus resulting in limited thermal shock ability. The refractory metals Nb and Ta are promising candidates for application with Al 2 O 3 due to their similar thermal expansion behavior. Wang et al. [8] and White et al. [9] showed for temperatures between 1000 and 2000 K a similarly linear increase of the linear thermal expansion coefficient for the three elements, with 8.8 to 11.2 Â 10 À6 K À1 for α-Al 2 O 3 , [8] 8.3 to 10.4 Â 10 À6 K À1 for Nb, and 7.1 to 8.4 Â 10 À6 K À1 for Ta. [9] Recently, Zienert et al. [10] demonstrated the fabrication of coarse-grained refractory composites based on alumina with niobium or tantalum, while Weidner et al. [11] reported first mechanical properties at high temperatures (1300-1500 °C) under compressive load. In these coarse-grained refractory composites, alumina with different particle classifications from 0-20 μm up to 2-5 mm (with different volume

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The binary tantalum–oxygen system is assessed using the CALculation of PHase Diagrams (CALPHAD) method with experimental data from the literature. The oxygen solubility in the Ta solid‐solution phase is discussed and modeled. The low‐ and high‐temperature modifications of Ta2O5 are described as stoichiometric compounds. This dataset is extended into the ternary Al–Ta–O system by complementing it with binary datasets for Al–O and Al–Ta from the literature and adding mixed‐oxide AlTaO4. The dataset for the ternary system Al–Nb–O is created by combining the three corresponding binary datasets from the literature and by assessing the quasibinary section Al2O3–Nb2O5. The ternary aluminum niobates are described as stoichiometric compounds. Phase equilibria between refractory metals and alumina at high temperature are discussed.