Fe-ion irradiation induced changes in precipitates of 11Cr ferritic/martensitic steel

• Study focusing on the precipitates change of 11Cr martensitic steel under irradiation is presented. • Irradiation induced the precipitation of rod- and parallelogram-like M 23 C 6 within laths and matrix. • Irradiation induced the precipitation of a great many dispersed Fe-Cr-rich M 3 X 2 (Cr 3 C 2 type) within δ-ferrite. • Irradiation induced the formation of NbC (fcc, a = 1.115 nm), Nb 2 C and Nb 6 C 5 precipitate phases. • Irradiation induced the formation of intermetallic compounds σ-FeCrW and Fe-Cr within δ-ferrite. The precipitate phases of an 11Cr F/M steel in normalized-and-tempered state and after irradiation with 3.5 MeV Fe 13+ ions at 700 °C to 0.84 dpa were studied using transmission electron microscopy. Cr-rich precipitate phase underwent great changes under irradiation. While the existing Cr-rich M 23 C 6 precipitates with a Cr/Fe ratio of about 1.8 remained, Cr-rich M 23 C 6 precipitates with a Cr/Fe ratio of about 1.0 were precipitated during irradiation. Irradiation induced the precipitation of rod-like Cr-rich M 23 C 6 precipitates within martensite laths, and Cr-rich M 23 C 6 precipitates with a parallelogram morphology in the matrix. Irradiation-induced Cr-rich M 2 X (Cr 2 N type) carbonitride and Cr-rich M 7 C 3 carbide were identified. Irradiation significantly affected δ-ferrite in the steel, resulting in the precipitation of dense and small Cr-rich M 3 X 2 carbonitrides based on Cr 3 C 2 within δ-ferrite grains. Irradiation caused significant changes in the nature of Nb-rich precipitate phases. While the existing Nb-Ta-rich MX carbonitrides based on NbC (fcc lattice, a = 0.4469 nm) remained under irradiation, irradiation induced three types of Nb-rich phases, including Nb-Ta-rich MX carbonitride based on NbC (fcc lattice, a = 1.115 nm), Nb 2 C (simple orthorhombic lattice) carbide, and Nb-Ta-rich M 6 X 5 carbonitride based on Nb 6 C 5 (base-centered monoclinic lattice). Irradiation also induced the formation of two types of intermetallic compound phases, σ-FeCrW (base-centered tetragonal lattice) and Fe-Cr (bcc lattice) which are absent in the normalized-and-tempered steel, within the δ-ferrite adjacent to δ-ferrite-martensite boundaries. The formation of the irradiation-induced precipitate phases is also discussed.