Several attempts had been made in recent years to understand the indentation size effect. The theoretical models, based essentially on strain gradient plasticity theories, such as the Nix–Gao model, predicted that the indentation size effect was caused by an increase in the density of dislocations as the indentation size decreased. Molecular dynamics simulation results tended to confirm this fact, but the truth was that very few experimental studies exist on the direct observation of how dislocations were generated and accommodated in the vicinity of nano-indentations. Using a Ni transmission electron microscopy thin foil as model material, it was shown here that, when the material was submitted to atomic force microscopy-based nano-indentation, a high dislocation density zone was generated at the centre of the indented region and that prismatic loop and helical dislocations were emitted sideways from the central region of the nano-indentation along the <110> directions. It was also shown that the dislocation array formed during the nano-indentation process was far from the ideal model proposed by Nix and Gao; based upon load axi-centered dislocation loops.

Dislocation Structures in Nanoindented Ductile Metals - a Transmission Electron Microscopy Direct Observation. S.Graça, P.A.Carvalho, R.Colaço: Journal of Physics D, 2011, 44[33], 335402