Indentation size effects of ferritic/martensitic steels: a comparative experimental and modelling study

Abstract

The paper presents a comparative study of different nanoindentation methods as applied to the ferritic/martensitic steels T91 and Eurofer97, here investigated in the non-irradiated reference state, but envisaged as structural materials for nuclear fission and fusion applications, respectively. Depth-controlled single cycle measurements at various indentation depths, force-controlled single cycle, force-controlled progressive multi-cycle measurements, and continuous stiffness measurements (CSM) using a Berkovich tip at room temperature have been combined to determine the indentation hardness and the elastic modulus, and to assess the robustness of the different methods in capturing the indentation size effects (ISE) of those steels. The Nix−Gao model is found inappropriate because it does not account for the breakdown of the scaling regime at small indentation depths that is linked to the extremely high density of dislocations associated with martensitic lath boundary misorientation. A generalization of the Nix–Gao model is therefore developed which allows the prediction of the dislocation densities in the lath structure in accordance with neutron diffraction results. Amplitude and frequency of the CSM oscillations influence the ISE observed. Differences of the microstructure-based parameters describing the ISE of quasi-static and dynamic measurements on T91 and Eurofer97 may reflect differences in the associated deformation mechanisms and histories.