Evaluation of Helium effects on swelling behavior of oxide dispersion strengthened ferritic steels under ion irradiation

Effects of ion-irradiation on the swelling behavior were investigated for a high-chromium oxide dispersion strengthened (ODS) ferritic steel and a 9Cr-2W reduced-activation ferritic (RAF) steel up to a high dose (nominal 60 dpa) with and without helium (900 appm) by using single and dual-ion irradiation techniques at 773 K. Microstructure observations revealed that the average size and number density of cavities formed in the ODS steel are half the size and twice the density of those in the RAF steel, indicating that the ODS steel has superior resistance to swelling. Trapping of helium atoms and vacancies at the boundaries between the nano-sized oxide particles and the matrix probably accelerated nucleation of bubbles and prevented the voids from growing. It is concluded that small oxide particles in high density are effective traps for helium atoms and/or vacancies by offering a high number density of trapping sites.