Microstructural Evolution of Ferritic--Martensitic Steels Irradiated in the Fast Breeder Reactor Phenix

The microstructural evolution of a 17Cr ferritic steel (F17) and five stabilized (EM12, FV448, HT9, and 1.4914) or unstabilized (EM10) 9-12Cr ferritic--martensitic steels irradiated in Phenix is discussed. The TEM examinations performed on these steels show that their microstructures are only modified by irradiation at low temperature (T < 500 deg C). Few voids are generally observed, leading to swelling values close to zero, except in duplex EM12 and ferritic F17 steels. On the other hand, fine and uniform precipitation of alpha ', M sub 6 C and chi -phases is induced by irradiation in alloys that contain either > 13% Cr ( alpha '), or > 0.3% Ni (M sub 6 C) or 2% Mo ( chi ). This precipitation coupled with the radiation-induced dislocation microstructure is responsible for radiation-induced embrittlement which depends both on number density and nature of particles, alpha ' being more detrimental than M sub 6 C or chi .