Structural Investigation of Size Effects in Plasticity using Indentation Techniques

Abstract

It was found, that contrary to the predictions of classic continuum plasticity theory, the plastically deformed zone below nano-, micro- and macroindentations is not selfsimilar. Rather, different stages of deformation associated with varying sizes of the deformed regions were detected. Examining cross-sections through nanoindentations in copper by means of electron backscatter diffraction (EBSD) technique, show that different characteristic deformation patterns occur. For large nanoindentations (2.5 mN–10 mN) a plastically deformed zone, which consists of three characteristic regions is found, while for shallow nanoindentations (? 1 mN) only two characteristic sections appear. Due to these findings it can be assumed, that a change in the deformation mechanism between large and shallow nanoindentations takes place. Analysis of the corresponding hardness data in terms of geometrically necessary dislocations (GNDs) using the Nix-Gao model, supports the assumption of a “mechanism change??. To explain the observed behavior, two models based on possible dislocation arrangements are suggested and compared to the experimental findings. The model presented for large imprints is similar to the dislocation pile-up model explaining the Hall-Petch effect, while the model for shallow nanoindentations uses far-reaching dislocation loops to accommodate the shape change caused by the indenter. Further evidence for a change of the deformation mechanism were delivered by additionally performed transmission electron microscopy (TEM) experiments. As the TEM experiments show, the plastically deformed zone of large nanoindents consists of high density dislocation networks, intermitted by almost dislocation free regions. The deformation zone found for small nanoindentations, however, looks somewhat different. Instead of dense networks of dislocations, the plastically deformed zone is built up by single dislocation loops surrounding the imprint. The plastic deformation zone below microindentations (> 10 mN–300 mN) can as well be divided into three characteristic regions. Noticeable is, that the dimension of the zone where significant changes of the orientation occur, is proportional to the size of the imprint. For macroindentations (> 300 mN–100 N) the plastically deformed zone consists of only two characteristic regions. The identified regions exhibit a structure, which is typical for low and medium deformed face-centered cubic single crystals of pure metals. With increasing load, dislocation substructures which exhibit orientation fluctuations in the micron regime, occur. Summarizing the microstructural results of all examined indentations it becomes apparent, that the size of the indentations cover a wide range of the different scales of structural evolution, appearing during the deformation of a single crystal. It seems that the hardness of a material varies with the size of indentation, as the flow stress of a single crystal with the evolving substructure.