Evaluation of irradiation hardening and microstructure evolution under the synergistic interaction of He and subsequent Fe ions irradiation in CLAM steel

• Sequential dual-ion irradiation was used to study the synergistic effects of helium accumulation and cascade damage.
• The critical helium dose for notable growth of bubbles is around 1.0 × 1017 He+/cm2.
• Bubbles and dislocation loops grew greatly after subsequent Fe-ions irradiation.
• The tested and calculated hardness increments are all in proportion to the helium dose.

Sequential dual-ion irradiation is a useful technique for experimental exploration on the synergistic effects of ion accumulation and cascade damage. In this research, the helium-ion accumulation concomitant with displacement damage induced by helium and iron ions irradiation in China low active martensitic (CLAM) steel was studied by ion-irradiation, transmission electron microscopy and nano-indentation technique. The helium bubbles formed under continuous implantation of helium ions at room temperature and the helium dose forming observed bubbles resolved by TEM clearly in CLAM steel is around 0.7 × 1017–1.0 × 1017 He+/cm2 under single 100 keV He+ irradiation. Significant irradiation hardening was observed in the samples with various ion- and dose-irradiations. Positive correlation between the hardening increment and the helium-ion dose has been established in single helium-ion irradiated samples. On the other hand, the subsequent Fe-ion irradiation greatly promoted the formation and growth of helium bubbles as well as dislocation loops in sequential dual-ion irradiated samples. No significant contribution of the subsequent Fe-ion irradiation on the hardness increment was found for the sequential dual-ion irradiated samples. It is suggested that the defects recombination, the combination effects of size and density of defects contribute to the degree of irradiation hardening. The calculated hardness increment based on dispersion strengthening model and the experimental microstructure analysis followed the same trend as the experimental nano-indentation data.