Chemical and structural analysis of grain boundaries in Inconel 690 for corrosion resistance

[摘要] (cont.) Low E grain boundaries exhibited a thin band of Cr-carbide on the boundary that was approximately 50 nm thick. On low angle grain boundaries, coarsened Cr-carbides were observed in semi-continuous form; with an average size of 230 nm. On high angle grain boundaries, further coarsening of the carbides resulted in a discontinuous distribution with an average precipitate size of 430 nm. Cr depletion occurred in the vicinity of the carbides; depletion was the most severe on high angle grain boundaries, down to 20wt-%. The suspected cause of the varying degree of coarsening of the Cr-carbides was the differences in diffusivity that control the kinetics of precipitation at the grain boundary. The ;;mean field;; model for the coarsening of a distribution of carbides was used for quantitatively comparing the diffusivity of Cr at the high and low angle grain boundaries. The result indicated that diffusivity of Cr at high angle grain boundaries was an order of magnitude higher than at low angle grain boundaries, at the temperature of Cr-carbide formation 600-950;; C. High angle grain boundaries have been shown to be the most susceptible to corrosion and SCC previously. The results of this work suggest that the higher diffusivity of Cr at the high angle boundaries of alloy 690 could contribute to SCC susceptibility through two mechanisms: 1) The coarser carbides, formed because of higher diffusivity, can more easily initiate microcracks if they are present. 2) The higher diffusivity leads to greater Cr redistribution, which could leave the boundary in a chemical state more prone to corrosion.

[发布机构] Massachusetts Institute of Technology