Radiation-induced segregation in FFTF-irradiated austentic stainless steels

Analytical electron microscopy has been applied to radiation-induced segregation (RIS) to defect sinks in two austenitic stainless steels irradiated in the Fast Flux Test Facility to ≅ 15 dpa at 520deg C. RIS to faulted dislocation loops has been shown to be strongly dependent on loop size. Several mechanisms for this effect are discussed, including increased vacancy-driven RIS for large dislocation loop sizes. The spatial distribution of solutes near large dislocation loops is consistent with dominance of the inverse Kirkendall effect and not with an interstitial mechanism of RIS. In a nickel-stabilized austenite (USPCA), RIS to grain boundaries results in nickel enrichment and chromium depletion. In a manganese-stabilized austenite (EP838), manganese is the species strongly depleted at boundaries, resulting in little chromium depletion relative to that in USPCA. Therefore, EP838 should exhibit greater resistance to radiation-induced sensitization. Manganese depletion at grain boundaries in the EP838 may induce a loss of austenite stability, but that effect will be offset in part by an enrichment of nickel. (orig.)