Helium irradiation hardening in Fe, F82H-IEA and F82H-ODS steel

Helium implantation at room temperature was performed to investigate irradiation hardening of ferritic/martensitic steels. Bulk hardness was extracted from Nix-Gao model using a Berkovich nano-indentor. The correlation between irradiation hardening and helium concentration ranging 500 2000 appm was examined. Nano-hardness increases as a function of helium concentration. The hardening of Fe is saturated when the concentration of helium exceeds 1000 appm. No saturation tendency is observed for F82H-IEA and F82H-ODS. F82H-ODS, having a higher hardness, provides a lower irradiation hardening than F82H-IEA. Transmission electron microscopy (TEM) reveals that cavities with a uniform distribution are formed after helium implantation to 2000 appm, showing a mean size of 1.4 nm with an average number density of 5.8 × 1023 m-3 in Fe, 1.1 nm with 4.9 × 1023 m-3 in F82H-IEA and 1.3 nm with 7.4 × 1023 m-3 in F82H-ODS. The results reveal that the formation of defects by helium irradiation at room temperature is independent from the as-received microstructure, while the change of mechanical property is dependent on the microstructure of the as-received materials. This suggests the same type of irradiation induced defect may have a quite different hardening effect on materials with different microstructure.