Size dependence of micro-scale mechanical properties on heavy-ion irradiated tempered-martensitic steel evaluated through nanoindentation and micropillar compression tests

The size dependence of microscale mechanical properties is crucial for evaluating irradiation hardening using nanoindentation and micropillar compression tests. To predict bulk hardness without the indentation size effect in heavy-ion irradiated materials, this study designed a modified Nix-Gao model incorporating the effect of irradiation hardening. The model was used to determine the bulk hardness of irradiated film and unirradiated substrate in tempered F82H martensitic steel irradiated with 10.5 MeV Fe-ion. The bulk hardness of unirradiated F82H predicted from the nanoindentation hardness was consistent with the traditional Vickers hardness obtained using a multiscale hardness relationship. The effects of irradiation conditions and the irradiation damage gradient were also determined by the modified model. In a micropillar compression test performed using micropillars with sizes of 1-8 mu m, the flow stress at 8 % plastic strain of the micropillars showed a stable and comparable value with that of a bulk tensile specimen, while the yield stress at 0.2 % strain offset decreased with decreasing pillar sizes. The irradiation hardening calculated using the bulk hardness obtained in the nanoindentation tests and the flow stress determined from the micropillar compression tests at 8 % plastic strain were consistent, as predicted by Tabor's law. These findings prove that the proposed method is effective for assessing the mechanical characteristics of a size-limited region, even on the subsurface of metallic materials exposed to heavy-ion irradiation.