Microstructural characterisation of irradiated CT material

The characterisation of these materials using electron-optical techniques provides quantitative data concerning the hardening of the grain interiors by dislocation loops and the changes in grain boundary chemistry caused by irradiation. The microstructure and microchemistry has been studied in three irradiated and one archive samples of material used in the Halden IASCC programme. In the archive condition, the 316NG steel exhibits slight thermal non-equilibrium segregation of chromium and molybdenum to the grain boundaries. On irradiation these effects are reduced by irradiation-induced segregation and the enrichments of nickel and silicon are similar to those found in a 304 steel irradiated under the same conditions. The 347 steel was found to have similar grain boundary microchemistry to the 348G steel irradiated in the first phase of the Halden IASCC programme. The high fluence 304 steel showed significant radiation-induced segregation with pronounced depletion of chromium and enrichment of silicon and nickel at the grain boundaries. High densities of small dislocation loops were found in the irradiated steels. The size and the density of the loops depended at least as much on the composition of the steel as on the irradiation level. A significant finding in the 304 steel was the presence of a high density of nano metre-scale cavities which give rise to a calculated swelling of ∼0.1%. This is the most significant swelling data yet published for a BWR-irradiated steel. (Author)