Microstructure in martensitic steels T91 and F82H after irradiation in SINQ Target-3

T91 and F82H martensitic steels were irradiated in the Swiss spallation neutron source Target-3 to a maximum dose of 11.8 dpa in a temperature range of 90–360 °C. Transmission electron microscope observations were performed to study radiation effects on the microstructure at different irradiation doses and temperatures. The results for the unirradiated and irradiated conditions indicate that there is no significant difference between the microstructures of the two steels. High-density helium bubbles of size about 1 nm are observed in samples irradiated at temperatures above about 200 °C to about 10 dpa with 550 appm He. The helium bubbles observed in T91 steel have slightly higher density but smaller size as compared to those in F82H steel at the same irradiation conditions. The density and size of defect clusters in a T91 sample are almost the same as those in a F82H sample at the same dose. In both steels, the size increases while the density drops rapidly with irradiation temperature above about 250 °C. The amorphization of M23C6 precipitates was observed again in T91 and F82 samples irradiated at 220 °C and below.