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.