Experimental and thermodynamic study of nickel-based alloys containing chromium carbides, Part II: Study of the sub-surface characteristics of Ni–30 wt% Cr–xxC alloys oxidized at high temperature using thermodynamic calculations

Six ternary Ni–30 wt% Cr–xxC alloys (x=0.2–2.0x=0.2–2.0) were oxidized at 1000, 1100 and 1200 ∘C. For several of them, the sub-surface was studied with accuracy by division into several successive parts for local measurements of carbide fractions by image analysis. Additionally thermodynamic calculations were performed for local analysis of the samples. By these means, the outer carbide-free zone and the inner zone enriched in carbides were characterized, with special attention to both local carbon contents and local solidus temperatures. Profiles of carbon contents (and thereafter of local solidus temperatures) across the zone affected by oxidation were obtained by thermodynamic calculations. The content of carbon still present in the carbide-free zone is necessarily very low. The carbon content in the zone enriched in carbides is higher than in the bulk. The loss of carbon leads to a higher refractoriness for the carbide-free zone, which could protect the bulk against fast oxidation if it becomes partially molten because of a sudden increase in temperature in service.