Synergistic effect of hydrogen, helium and displacement damage on cavity evolution in pure Fe and CLF-1 steel under multiple beams irradiation

14 MeV fusion neutrons induce substantial displacement damage in materials and generate transmutation gases, such as hydrogen (H) and helium (He) through (n, p) and (n, α) nuclear reactions. Previous studies have shown that the synergistic effects of H, He and displacement damage can accelerate microstructural degradation in materials. To investigate their influence on cavity nucleation, growth and swelling, systematic experiments were conducted on China Low-activation Ferritic/Martensitic Steel (CLF-1) and pure iron at 450 °C using single-, dual- and triple-beam irradiations at 15 dpa. Additional Fe + He dual-beam irradiations were performed at both 15 and 30 dpa. By comparing of CLF-1 steel and pure iron under the same irradiation conditions, our experimental results show that the effects of hydrogen and helium were similar in stabilizing small clusters and promoting nucleation in both materials. Specifically, the results of CLF-1 steel revealed that the reduced defect mobility arising from the complex elemental composition, together with the enhanced recombination and the effective He trapping at precipitate–matrix interfaces, enhances its tolerance to irradiation-induced swelling and delays the dose-dependent transition from the nucleation-dominant stage to the growth-dominant stage to doses beyond 30 dpa, whereas pure iron undergoes this transition between 15 and 30 dpa. At 15 dpa, although both materials are in nucleation stage, and the co-implanted gas atoms promote nucleation and shorten the incubation period in CLF-1 steel, since no detectable cavities were observed under single-beam irradiation.