The influence of helium on cavity evolution in ion-irradiated T91

The effect of helium pre-implanted in concentrations spanning 0 to 1000 appm on the swelling evolution in ferritic-martensitic alloy T91 was explored. Irradiation with 5.0 and 4.4 MeV Fe2+ ions at 460 °C up to damage levels of 350 dpa were performed with pre-implanted helium levels of 0, 1, 10 and 1000 appm. Low concentrations of helium were found to enhance nucleation at low damage levels. However, at high damage levels, differences in nucleation became insignificant, and nearly identical cavity distributions were created. At low helium levels, the cavity evolution was facilitated by similar cavity sink strengths. High helium concentrations reduced the critical cavity radius and resulted in the formation of a high-density bubble population. The bubbles were <4 nm and remained stable even up to high damage levels, while the growth of larger cavities was suppressed. The suppression of swelling was found to be a result of helium trapping or alteration of cavity and dislocation bias. Dislocation and precipitate evolution was also studied and exhibited relative insensitivity to helium concentration, and did not significantly influence the cavity evolution.