POSTIRRADIATION ANNEALING EFFECTS ON MICROSTRUCTURE AND HELIUM BUBBLES IN NEUTRON-IRRADIATED TYPE-304 STAINLESS-STEEL

Helium bubble nucleation and growth processes were studied at elevated temperatures on type 304 stainless steel which had been neutron irrad iated to 1.4 x 10(26) n/m(2) (E > 1 MeV) and annealed at 400, 550, 650 and 900 degrees C for 1 h. After annealing, specimens for microstruct ural observation were prepared and observed (by transmission electron microscope (TER?).) Radiation defects were present at high density in the as-irradiated specimens and annealed out with increasing annealing temperature. Helium bubbles became visible where defects were anneale d out (above 650 degrees C). The bubbles; grew preferentially at dislo cations and grain boundaries. At the grain boundaries, most of the hel ium bubbles formed on grain boundary dislocations. It was concluded th at during post-irradiation annealing. the dislocations which remained after annealing played an important role in helium bubble growth. even in the grain boundaries.