Microstructural investigation of the volume beneath nanoindentations in copper

The deformed volume below nanoindentations in copper single crystals with a 〈11¯0〉{111} orientation is investigated. Using a focused ion beam workstation, cross-sections through nanoindentations were fabricated and examined using the electron backscatter diffraction technique. Additionally a transmission electron microscopic foil through the middle of an imprint was prepared and analysed. Due to changes in the crystal orientation around and beneath the indentations the plastically deformed zone can be visualized and compared with the measured hardness values. Furthermore, the hardness data were analysed in terms of geometrically necessary dislocations using the Nix–Gao model, where a linear relationship was found for H2 vs. 1/hc, but with different slopes for large and shallow indentations. The orientation “micrographs” indicate that this behaviour is associated with a change in the deformation mechanism. Consequently, two models based on possible dislocation arrangements are presented and compared with the experimental findings. For large indentations a dislocation pile-up model similar to those used to explain the Hall–Petch effect is suggested, while the model for shallow imprints uses far-reaching dislocation loops to accommodate the shape change of the indenter.