Effects of Helium and Displacement Damage on Microstructural Evolution in Helium-Implanted Martensitic Steel HCM12A Examined by TEM and Positron Annihilation Lifetime Measurement

The effects of helium concentration and displacement damage on microstructural evolution were mainly investigated in a high chromium martensitic steel (HCM12A) implanted uniformly with helium at 823 K to 1, 10, and 30 appm-He by 50 MeV cyclotron accelerator using an energy degrader under 2x10 dpa/appm-He. After the helium implantation, the microstructures were examined by a transmission electron microscopy (TEM) and positron annihilation lifetime (PAL) measurement. Bubbles (He-V clusters) with about 10 nm to a few 10 nm in diameter were formed on grain boundaries, sub-boundaries, and dislocation lines, and bubbles with a few nm were also observed in matrix. The long lifetime component of positron tended to increase with helium implantation concentration, especially, in the regime more than 10 appm-He implantation. Preset results of the long lifetime in the HCM12A implanted with 10 appm-He and 30 appm-He are 345 ps in τ3 and 308 ps in τ2, respectively, and these values were found to close to the value of 360 ps which was obtained by the other study in F82H steel irradiated in the SINQ irradiation program.