Helium effects on EUROFER97 martensitic steel irradiated by dual-beam from 1 to 50 dpa at 250 and 300 °C with 10 He appm/dpa

Reduced activation martensitic steel specimens were irradiated by dual-beam from 0.1 to 50 dpa at 250 and 300 °C with 10 He appm/dpa. The in-situ microstructure observation shows that the bubbles were present after 1.7 dpa for 250 °C irradiation and after 0.1 dpa for 300 °C irradiation. The bubble size distribution is a Maxwell distribution, which evolves to a bimodal distribution with increasing damage dose. The transition occurs after 10 dpa at 300 °C irradiation and after 30 dpa at 250 °C irradiation. The bubble concentration and peak bubble size are similar for various doses at 250 °C, and are 3.8×10²¹ m^-3 and 2.5 nm, respectively. The size of cavities at the second peak of the bimodal distribution is around 6 nm. The cavity size distribution increases and spreads out with damage dose increase. The dislocation loops are typical interstitial dislocation loops that evolve from black-white spots to visible interstitial loops. The size range of interstitial loops is between 2.5 and 11 nm and the dislocation loop density increases monotonically with the damage dose.