Microstructural analysis of neutron-irradiated martensitic steels

Four martensitic steels for fusion applications were examined by transmission electron microscopy after irradiation in the Fast Flux Test Facility (FFTF) at 420°C to 7.8 × 10²⁶ n/m² (E > 0.1 MeV), about 35 dpa. There were two commercial steels, 9Cr-1MoVNb and 12Cr-1MoVW, and two experimental reduced-activation steels, 9Cr-2WV and 9Cr-2WVTa. Before irradiation, the tempered martensite microstructures of the four steels contained a high dislocation density, and the major precipitate was M₂₃C₆ carbide, with few MC carbides. Irradiation caused minor changes in these precipitates. Voids were found in all irradiated specimens, but swelling remained below 1%, with the 9Cr-1MoVNb having the highest void density. Although the 12Cr-1MoVW steel showed the best swelling resistance, it also contained the highest density of radiation-induced new phases, which were identified as chi-phase and possibly α′. Radiation-induced chi-phase was also observed in the 9Cr-1MoVNb steel. The two reduced-activation steels showed very stable behavior under irradiation: a high density of dislocation loops replaced the original high dislocation density; moderate void swelling occurred, and no new phase formed. The differences in microstructural evolution of the steels can explain some of the mechanical properties observations made in these steels.