Nanoindentation applied to ion-irradiated and neutron-irradiated Fe-9Cr and Fe-9Cr-NiSiP model alloys

Abstract

Nanoindentation of ion-irradiated materials has attracted much interest as a tool envisaged to derive the dose dependence of bulk-equivalent hardness from small samples. A major challenge arises from the steep damage gradient in the thin ion-irradiated layer and its unavoidable interplay with the indentation size effect. The present study relies on a number of choices aimed at simplifying the interpretation of the results and strengthening the conclusions. The studied alloys are two ferritic Fe-9Cr model alloys differing in controlled amounts of Ni, Si, and P known to enhance irradiation hardening. Both ion-irradiated (5 MeV Fe2+ ions) and neutron-irradiated samples along with the unirradiated references were investigated using Berkovich tips. According to the collaborative nature of the study, tests were conducted in two different laboratories using different equipment. A generalized Nix-Gao approach was applied to derive the bulk-equivalent hardness and characteristic length scale parameters for the homogeneous unirradiated and neutron-irradiated samples. Comparison with Vickers hardness indicates a 6% overestimation of the bulk-equivalent hardness as compared to the ideal correlation. For the case of ion irradiation, a first model assumes a homogeneous irradiated layer on a homogeneous substrate, while a second model explicitly takes into account the damage gradient. The first model was combined with both the original and the generalized Nix-Gao relation. We have found that the results revealed for Fe-9Cr vs Fe-9Cr-NiSiP are compatible with expectations based upon known irradiation-induced microstructures. The bulk-equivalent hardness derived for ion-irradiated samples reasonably agrees with the observation for neutron-irradiated samples.