Grain boundary segregation during heat treatment at 600°C in a model Alloy 600

The carbon distribution in the vicinity of grain boundaries, in solution in the matrix, and in carbides has been studied by the atom probe at various stages of the intergranular carbide precipitation process in an austenitic nickel-base model alloy similar to commercial Alloy 600. The grain boundary precipitation evolved from small discrete carbides after 4 h at 600°C to a semi-continuous carbide film after 16 h, and finally to large discrete carbides after 213 h. Enrichments of carbon, boron, nitrogen and phosphorus were found at the grain boundaries in all ageing conditions. Large variations were observed between different grain boundaries and at different positions of an individual grain boundary in the same ageing condition. The highest carbon concentration was accompanied by chromium enrichment, indicating the nucleation of chromium-carbides. After ageing for 213 h, the amount of carbon segregation to the boundary was very low, which suggests that the driving force for carbon segregation is less than that for carbide precipitation. Phosphorus was only detected at the grain boundaries when the carbon concentration was low.