Grain boundary segregation during heat treatment at 600 degrees 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 degree s C to a semi-continuous carbide film after 16 h, and finally to large disc rete carbides after 213 h. Enrichments of carbon, boron, nitrogen and phosp horus were found at the grain boundaries in all ageing conditions. Large va riations 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, indic ating the nucleation of chromium-carbides. After ageing for 213 h, the amou nt of carbon segregation to the boundary was very low, which suggests that the driving force for carbon segregation is less than that for carbide prec ipitation. Phosphorus was only detected at the grain boundaries when the ca rbon concentration was low. (C) 1999 Elsevier Science S.A. All rights reser ved.