MICROSTRUCTURAL ANALYSIS OF NEUTRON-IRRADIATED MARTENSITIC STEELS

Four martensitic steels for fusion applications were examined by trans mission electron microscopy after irradiation in the Fast Flux Test Fa cility (FFTF) at 420 degrees C to 7.8 x 10(26) n/m(2) (E > 0.1 MeV), a bout 35 dpa. There were two commercial steels, 9Cr-1MoVNb and 12Cr-1Mo VW, and two experimental reduced-activation steels, 9Cr-2WV and 9Cr-2W VTa. Before irradiation, the tempered martensite microstructures of th e four steels contained a high dislocation density, and the major prec ipitate was M(23)C(6) carbide, with few MC carbides. irradiation cause d minor changes in these precipitates. Voids were found in all irradia ted specimens, but swelling remained below 1%, with the 9Cr-1MoVNb hav ing the highest void density. Although the 12Cr-1MoVW steel showed the best swelling resistance, it also contained the highest density of ra diation-induced new phases, which were identified as chi-phase and pos sibly alpha'. Radiation-induced chi-phase was also observed in the 9Cr -1MoVPNb steel. The two reduced-activation steels showed very stable b ehavior under irradiation: a high density of dislocation loops replace d the original high dislocation density; moderate void swelling occurr ed, and no new phase formed. The differences in microstructural evolut ion of the steels can explain some of the mechanical properties observ ations made in these steels.