Addition of Co in Ni(Cr)-based cast superalloys for tantalum carbide stabilisation: consequences on the behaviour in oxidation at elevated temperatures

Abstract

ABSTRACT Strengthening by TaC is a possible way of improving Cr-rich Ni-based alloys at high temperatures. Unfortunately, in a Ni–Cr environment, TaC co-exist with chromium carbides and induces a significant decrease in refractoriness of the alloys. Adding cobalt may correct this problem, but it shows oxidation behaviour. Here, 14 and 27 wt-% were added in a Ni–25Cr–0.4C–6Ta reference alloy. The evolution of the microstructures and properties of these alloys were examined during a 170 h-long exposure at 1200°C in laboratory air. Cobalt limits the high-temperature detrimental morphologic evolution of the TaC carbides, but it effectively degrades the alloy towards a forthcoming loss of its chromia-forming behaviour, as suggested by the chromium contents and concentration gradients in the subsurface. However, even for the 27 wt-%Co-added alloy this did not occur, and no local catastrophic oxidation was noticed.