Radiation-induced grain boundary segregation and sensitization of a neutron-irradiated austenitic stainless steel

Analytical electron microscopy and electrochemical potentiokinetic reactivation (EPR) testing were applied to the radiation-induced segregation (RIS) and sensitization of a titanium-modified austenitic stainless steel neutron-irradiated to 9 dpa. The EPR testing of both solution annealed (SA) and 25% cold-worked (CW) materials indicated a significant increase in the reactivation charge ( P a ). Both optical and scanning electron microscopy of the specimen surface after EPR testing showed preferential attack at grain boundaries, indicative of sensitization. In addition, localized attack of the matrix was observed. Though precipitates were occasionally present on grain boundaries, they were not chromium-rich M 23C 6, but nickel- and silicon-enriched G phase. Faulted dislocation loops, small γ' precipitates, and isolated cavities were observed in the matrix. X-ray microanalysis showed significant RIS at high-angle boundaries in both materials. Depletion of chromium to apparent levels of 10 at% was observed in the irradiated SA material. The boundaries were enriched in nickel, silicon, and titanium to 28, 6, and 1 at%, respectively) and depleted of iron and molybdenum (as low as 54 and 0.7 at%, respectively). The width of the segregation zone was very narrow (< 6 nm). Similar grain boundary RIS was observed in the cold-worked material. There was also evidence for boundary migration in the cold-worked material (boundary facetting and asymmetric composition profiles). Voids and faulted dislocation loops in the SA material also exhibited similar RIS.