Swelling at high radiation damage levels of 120 and 240 dpa in 3.5 MeV self-ion irradiated ferritic/martensitic steels

The swelling behavior of ferritic/martensitic steels (FC92–B/-N, HT9, and Gr.92) was investigated following 3.5 MeV Fe++ ion irradiation. Tested alloys were helium-pre-implanted up to the peak contents of 120 and 240 appm with He/dpa ratio of 1 appm/dpa at room temperature and then exposed to self-ion beam to the peak damage conditions of 120 and 240 dpa at 475 °C. Field-emission transmission electron microscopy was used to characterize the cavity evolution. FC92–B exhibited the highest resistance to swelling among the irradiated alloys. The final volumetric swelling of FC92–B reached 1.3 % at 70 dpa and 2.9 % at 140 dpa. On the other hand, HT9 exhibited the highest swelling, reaching 7.4 % at 140 dpa. Comparing the present swelling results at 140 dpa/140 appm He with swelling data at 280 dpa/280 appm He from our previous study, it was observed that Gr.92 and FC92–N swelled more at 140 dpa/140 appm He than at 280 dpa/280 appm He. This negative correlation between swelling and dose in Gr.92 and FC92–N is primarily attributed to the helium-associated swelling suppression at higher helium concentration of 280 appm. A bimodal cavity size distribution appeared only in Gr.92 and FC92–N at 280 dpa/280 appm. This result demonstrates that the excess amount of helium over 200 appm promoted early-stabilization of new-born cavities, resulting in preferentially enhanced cavity nucleation, while impeding the growth of nucleated cavities. An inhibition in cavity growth possibly led to an extended duration of nucleation-dominant stages, finally suppressing swelling in ion-irradiated Gr.92 and FC92–N alloys.