Cavity formation and hardness change in He implanted EUROFER97 and EU-ODS EUROFER

Abstract

RAFM steels EUROFER97 and EU-ODS EUROFER samples have been implanted with He ions at 40 keV (ion penetration depth ~300 nm) at a dose of ~ 1 × 1015 ion/cm2 and different temperatures up to 550 °C. Post-irradiation examination of the samples has been performed using nanoindentation along with conventional and scanning transmission electron microscopy (CTEM/STEM in Annular Dark Field mode). The specimens were indented up to 500 nm by CSM method (Continuous Stiffness Measurement), in order to assess the changes in nanoindentation hardness values due to the irradiation. After indentation tests, lamella was extracted from each implanted sample which included at least one nanoindentation cross-section. The changes in hardness were correlated with any microstructure modification detected by TEM. A clear trend can be observed; the hardness values increased with irradiation temperature, as well as the He bubble nucleation and population density. A remarkable change in bubble distribution in EUROFER97 was found at the two highest irradiation temperatures (450 °C and 550 °C). In contrast, an increase in the cavities size was observed in EU-ODS EUROFER, but no new nucleation was observed for the same temperatures. These observations suggest that for these particular conditions, the cavity growth is enhanced, rather than new nucleation, which depends strongly on the material microstructure.