Cavity evolution and hardness changes in a 15Cr-ODS ferritic steel by post He-implantation annealing

15Cr-ODS ferritic steel was implanted with 3500 appm He (0.2 dpa) at 300, 550 and 700 °C. The post-implantation annealing (PIA) at 800 °C/100 h was conducted on the specimen implanted at 300 °C. Nanoindentation (NI) tests were carried out to investigate the temperature dependent hardening, and transmission electron microscopy (TEM) observation was performed to characterize the cavity evolution. A limited hardening was found in the as-implanted ODS specimens at all implantation temperatures, which tended to reduce with increasing the temperature. The PIA caused no further hardening, while TEM images revealed that there was an evident change in the cavity distribution morphology. The cavity diameter increased from 2.5 ± 0.4 nm to 5.0 ± 1.7 nm and the corresponding number density decreased from (22.2 ± 1.6) × 1022 m−3 to (6.7 ± 0.5) × 1022 m−3 before and after the PIA. The negligible hardness changes induced by the PIA were interpreted in terms of the Orowan-type dislocation barrier model with the corresponding barrier strength factor of cavities to be less than 0.1.