Modeling the effects of damage rate and He/dpa ratio on microstructural evolution

The effects of displacement damage rate, helium generation rate and other irradiation conditions on irradiation-induced microstructural changes and swelling in an austenitic stainless steel were investigated by numerical calculations. The computer model developed in this study dynamically integrates rate equations which describe the eveolution of point-defects, small point defect clusters including cascade vacancy clusters, cavity nucleation and the evolution of the cavity size distribution. The model was calibrated using data obtained from a dual-beam ion irradiation experiment. In this experiment, solution annealed type 316 stainless steel was irradiated by 4 MeV nickel ions with simultaneous implantation of helium ions up to 25 dpa at 873 K. The He/dpa ratios and the displacement damage rates were in the range of 1 to 50 appm He/dpa and 2x10-3 to 1x10-2 dpa/sec, respectively. These experiments showed that helium effects on dislocation evolution significantly influence the nucleation and growth of cavities. The predicted effects of helium over a broad range of damage rates were calculated using the calibrated model. (orig.)