Phase stability of an HT-9 duct irradiated in FFTF

A fuel test assembly known as the ACO-3 duct made out of a normalized and fully tempered ferritic/martensitic steel (HT-9) was previously irradiated in the Fast Flux Test Reactor Facility (FFTF) up to 155 dpa at a temperature range of 380–504 °C. The microstructures of the samples from five different zones along a face of the duct were analyzed using a combination of transmission electron microscopy (TEM) based techniques, small angle neutron scattering (SANS) and atom probe tomography (APT). A high density of Cr rich α′ precipitates together with a moderate density of G-phase precipitates with average sizes of ∼4 and 11 nm respectively were found in the 20 dpa, 380 °C zone. It was found that precipitation of the second phases is more sensitive to the temperature history then to the exposed neutron dose. In general, the density of both precipitates decreases with increasing irradiation temperature. No significant change is observed in average size of α′ while the average size of G-phase precipitates increases up to 27 nm at 440 °C. Voids are observed after irradiation at 100 dpa (irradiation temperature of 410 °C) and 155 dpa (irradiation temperature of 440 °C) but no voids were detected after irradiation to 96 dpa (irradiation temperature of 466 °C). In contrast to what is previously reported in the literature, no Laves, M6C or Chi phases were found in any of the zones.