Indentation size effect, geometrically necessary dislocations and pile-up effects in hardness testing of irradiated nickel

Abstract The indentation size effect response, based on dislocation density, of commercially pure Ni irradiated at 120°C with 6MeV protons to 0.1dpa, was compared with non-irradiated Ni. Irradiation-induced defects were characterised by TEM. Nano-scale and micro-scale indentation tests were carried out. The Nix-Gao (NG) model was applied to determine the increase in the bulk yield strength arising from irradiation induced defects, which agreed well with the predicted increase in yield strength from the Bacon-Kocks-Scattergood (BKS) obstacle hardening model, using a superposition of SFT, glissile perfect loops and sessile Frank loops. A bi-linear trend was observed in the NG model in both the non-irradiated and irradiated material, indicating a transition in deformation mechanisms for indents in the ‘nano-scale’ regime. The extent of the bi-linear trend was minimized if indentation pile-up was measured and accounted for. It is shown that, in this material, care must be taken when interpreting measurements of the increase in hardness due to irradiation induced defects when measurements are made from depths shallower than 500nm. It is experimentally shown using both strain gradient plasticity modelling and independent SEM/EBSD analysis that there is a higher density of geometrically necessary dislocations (GNDs) present in the deformation volume under an indent in the irradiated material when compared to the analogous indent in the non-irradiated material; this results from a more confined plastic deformation volume under the indent in the irradiated material. It is proposed that sessile Frank loops inhibit plastic deformation by acting as obstacles to dislocation motion which results in deformation under the indenter tip being more localised. The result is a larger difference in hardness between the irradiated and non-irradiated material when measured using nanoindentation compared to bulk hardness testing. The increase in GND density in nanoindentation of irradiated material can be accounted for by correcting for indentation pile-up.