Microstructure of helium-implanted and proton-irradiated T91 ferritic/martensitic steel

In the effort to understand the effect of helium on the irradiated microstructure, the ferritic/martensitic steel T91 was implanted to three helium concentrations of 720 appm, 1260 appm and 1800 appm. The alloy with and without helium pre-implantation was irradiated with 2.0 MeV protons to doses of 2.2 dpa, 7 dpa and 9.2 dpa at 450 °C. The irradiated microstructure, consisting of dislocation loops and bubbles, was characterized using transmission electron microscopy (TEM). Small bubbles were found in helium pre-implanted samples. No cavities were observed in irradiated samples without helium pre-implantation. Helium was found to promote swelling in T91. Irradiation may not be sufficient for cavity nucleation. However, when combined with helium pre-implantation, irradiation plays a role by assisting bubble growth and promoting swelling.