Effect of Irradiation with Varying H/He Ratio on Defect Clusters and Hardening in CLAM Steel

High energy neutron irradiation in fusion reactors compared to fission reactors necessitates more attention to the effects of hydrogen (H) and helium (He) generated by transmutation on structural materials. Due to the intricate synergistic effect between H and He, the experimental results have not exhibited satisfactory consistency and, sometimes were contradictory. Via summarizing the previous results, it is suggested that the mechanism of the HHe synergistic effect is highly correlated with the H/He ratios. Based on this perspective, multi-beam ion irradiation experiments with four distinct H/He ratios were implemented on China Low Activation Martensitic (CLAM) steel, a candidate structural material for fusion reactors. The results showed that a low H/He ratio contributed to the dispersion of cluster nucleation, resulting in high-density small dislocation loops, whereas a high H/He ratio promoted the growth of defect clusters. Irradiation-induced hardening was observed to increase at low H concentrations and decrease at high H concentrations. The present study proposed that the H-He synergistic effect can be viewed from the perspective of the formation of H2 molecules. The disparate responses of materials to the H-He synergistic effect may originate from the different H critical concentrations for the formation of H2 molecules. This critical concentration should be determined by the factors such as material composition and irradiation conditions.