EFFECTS OF ALLOYING ELEMENTS ON THE POSTIRRADIATION MICROSTRUCTURE OF 9-PERCENT-CR-2-PERCENT-W LOW ACTIVATION MARTENSITIC STEELS

Post-irradiation microstructure observations were carried out for five 9%Cr-2%W low activation martensitic steels of which the chemical compositions of elements such as B, Mn, Zr and W were altered. Neutron irradiations were performed in FFTF/MOTA cycle 10 irradiations (10 dpa at 638 K, 30 dpa at 693, 793 and 873 K). Transmission electron microscope observations revealed that void swelling as high as 0.2% occurred in boron (30 wppm) doped 9%Cr-2%W steel irradiated to 40 dpa at 693 K with a void density of 4.7 X 10(20) m-2. Some voids were observed in each steel irradiated at 638 K, but the swelling was negligible. A void size distribution diagram indicated that small bubbles less than 5 nm in diameter were observed in boron doped steels, while no such small bubbles were observed in a steel without an addition of boron. No voids were observed after irradiations at 793 and 873 K in any steels. An addition of 2% Mn caused a significant increase in the void density accompanied by a decrease in the void size. No chi-phase was observe in any steels after the irradiations, while Laves-phase was observed in specimens irradiated at 873 K to 33 dpa. Void shapes in these steels were determined to be dodecahedron and cube type.