The effect of tantalum additions on the microstructure and oxidation resistance of a cobalt-based superalloy

• Ta addition formed MC carbides in all alloys and Laves + σ phases in 3.1 at.% alloy. • At 800°C and 1000°C, Ta improved oxidation by MC carbides disrupting Cr-rich phase. • At 1200°C, Ta worsened oxidation by disrupting silica subscale with CrTaO 4 . • Intermediate temperature exposure formed intermetallics in the Ta alloys. Refractory metal additions are known to enhance the mechanical properties of cast superalloys. This study examines the effect of Ta additions (0, 0.9, 1.4, and 3.1 at.%) on the microstructure and oxidation behaviour of a carbide-reinforced Co-based alloy (Co-101a, Co-34.4Cr-9.5Ni-2.9Mo-2.0Si-0.5Mn-2.3C, at.%). Ta promoted the formation of Ta-rich MC carbides, and the 3.1 at.% Ta alloy also formed Laves and σ phases in place of Cr-rich carbides. Solution heat treatment at 1200°C for 24 h decomposed the interdendritic lamellar intergrowths, producing M 23 C 6 carbides in all but the 3.1 at.% Ta alloy, with notable MC carbide spheroidisation in the lower-Ta compositions. Isothermal oxidation at 800°C, 1000°C, and 1200°C for up to 100 h revealed two distinct mechanisms. At 800°C and 1000°C, Ta additions up to 1.4 at.% reduced mass gain by promoting MC carbide formation, which disrupted the Cr-rich carbide network and was accompanied by CrTaO 4 formation. However, at 3.1 at.% Ta, the presence of an extensive σ-rich interdendritic network was associated with deeper internal attack and higher mass gain. At 1200°C, mass gains increased by up to 40% with higher Ta content. CrTaO 4 formation coincided with a less continuous silica-rich subscale, reducing oxidation resistance. Microstructurally, Ta promoted intragranular needle-like Laves phase formation at 800°C and formation of Laves-like phases adjacent to MC carbides at 1200°C. Although moderate Ta additions improved oxidation resistance at intermediate temperatures, intermetallic formation, and poor performance at 1200°C suggest Ta may not be advantageous in this system.