Microstructural Examination of 12% Cr Martensitic Stainless Steel After Irradiation at Elevated Temperatures in FFTF

Abstract

A remelted 12% Cr martensitic stainless steel (HT-9) has been examined by transmission electron microscopy before and after irradiation in the Materials Open Test Assembly (MOTA) of the Fast Flux Test Facility (FFTF). The irradiation temperatures were 365, 420, 520, and 600°C with the fluences as high as 7.3 × 1022 n/cm2 (E > 0.01 MeV) or 34 dpa. The extracted precipitates from each specimen were identified using X-ray microanalysis and selected area diffraction. The precipitates in the unirradiated condition were primarily M23C6 carbides, which formed at martensite lath and prior austenite grain boundaries. During irradiation at elevated temperatures, small amounts of other phases formed, which were tentatively identified as the chromium-rich α′, the nickel-silicon rich G-phase, and the intermetallic Chi phase. Irradiation-induced voids were observed only in specimens irradiated at 420°C to a dose of 34 dpa; no voids were found for specimens irradiated at 365, 520, and 600°C (∼11, ∼34, and ∼34 dpa). These results are not in agreement with previous experiments in that voids have not been reported in this alloy at relatively high fluence level (∼67 dpa) following irradiation in another fast-spectrum reactor (EBR.II). This is, however, the first observation following FFTF irradiation. The present results indicate that cavities can form in HT-9 at modest fluence levels even without significant generation of helium. Hence, the cavity formation in this class of ferritic alloys is not simply caused by helium generation but rather more complex mechanisms.