Influence of Co Content on the High-Temperature Oxidation Performance of (Ni,Co)-25Cr-0.4C-6Ta Cast Alloys

Tantalum carbide (TaC) is a classical strengthening precipitate phase, like chromium carbides, in equi-axed cast Cr-rich cobalt-based alloys. When the former carbides are of a eutectic nature and in quantity high enough, they may be of great interest for applications at elevated temperature, due to their favorable script-like morphology and their high stability at high temperature. Unfortunately, they cannot be so easily obtained in Cr-rich nickel-based alloys of similar composition. This can be resolved by adding cobalt in substitution to nickel, but such chemical modification may change the high temperature oxidation behavior in the wrong direction. In this work, several alloys, all containing 25% Cr, 0.4% C, and 6% Ta (wt%), were fabricated and studied at 1127 A degrees C. It appears that the 27 wt% Co content is high enough to allow obtaining primary carbides, which are essentially TaC. The alloys rich enough in Co containing only tantalum carbides are clearly disadvantaged in terms of oxidation resistance at high temperature by the high Co content and by the absence of interdendritic chromium carbides known to act as useful Cr reservoirs. Among the six studied alloys, these are the three ones for which Co/Ni > 1 that demonstrated starts of catastrophic oxidation before 24 h of exposure to air.