Helium-induced hardening effect in ferritic/martensitic steels F82H and Optimax-A irradiated in a mixed spectrum of high energy protons and spallation neutrons

Ferritic/martensitic (FM) steels, F82H and Optimax-A, were irradiated up to 11.3 dpa/1175 appm He in Target-3 of the Swiss Spallation Neutron Source (SINQ). To investigate helium-induced hardening effect, micro-hardness and microstructure of the irradiated specimens were investigated in as-irradiated condition and after annealing at 600 °C. The results show that the hardening in the as-irradiated specimens increased linearly with √ dpa at low doses and more rapidly at higher doses, which is contributed by both defect clusters and helium bubbles. After annealing, although the hardening decreased greatly, a significant amount remained, which is proportional to √ C He (CHe is helium concentration) and essentially contributed by helium bubbles. From the results of the annealed specimens, it is deduced that the helium bubbles with sizes 1–1.5 nm are weak obstacles with barrier strength about 0.1. The significant hardening in the annealed specimens can be attributed to the high density of bubbles.