M. Rester

Yield stress influenced by the ratio of wire diameter to grain size – a competition between the effects of specimen microstructure and dimension in micro-sized polycrystalline copper wires

Polycrystalline copper wires with diameters of 25, 30 and 50 µm were annealed at temperatures between 200°C and 900°C, resulting in different microstructures with ratios of wire diameter to grain size between 1.1 and 15.6. The microstructure evolution and tensile behavior were studied systematically. In comparison with experimental data available in the literature, the results revealed that the tensile yield stresses of these micro-sized wires are influenced not only by the grain size but also by the ratio of wire diameter to grain size. This is clearly seen when comparing identical grain sizes but different wire diameters where thinner wires reveal smaller flow stress values. A model is proposed to explain the ‘smaller is softer’ phenomenon, taking into account the higher strengthening effect of grain boundaries compared to the free surface.

Influence of external and internal length scale on the flow stress of copper

Abstract The flow stress of bulk specimens is known to depend on the microstructure. With a reduction of specimen dimensions into the micrometer and nanometer regime, specimen size-effects also influence the mechanical properties. We characterized the size-dependent flow stress of copper over more than three orders of magnitude, starting from several tens of micrometers down to a few tens of nanometers. For this purpose nanoindentation, micro-compression, and tensile testing experiments were performed. Additionally, different grain sizes were generated by severe plastic deformation. The observed increase in flow stress with reduced critical dimension is discussed with respect to the different stress states and microstructures present in the reported experiments. The mechanism controlling deformation changes from dislocation pile-up for critical dimensions > 1 μm via a transition regime (1 μm > critical dimensions > 100 nm) to dislocation nucleation for critical dimensions < 100 nm.

The deformation-induced zone below large and shallow nanoindentations: A comparative study using EBSD and TEM

A comparison is made between the deformation-induced zone beneath nanoindentations obtained by Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). Since there are resolutional limitations associated with EBSD, especially at very small scan sizes, it is not known how accurately the deformed volume beneath the imprints can be characterized. To aid in answering this question, cross-sectional EBSD and TEM samples of nanoindentations were fabricated by means of a Focused Ion Beam (FIB) workstation, analyzed, and subsequently compared with each other. For large indentations as well as for shallow ones, agreement of the determined zones was found. The results of the EBSD and TEM experiments were also used to characterize the deformed volumes. In the EBSD maps of large indentations, strongly confined deformation patterns were found, while for the shallow indentations the observed patterns are more diffuse. The TEM micrographs and the Selected-Area Electron Diffraction (SAED) support these facts and give insight into the dislocation structure of the deformation zone.

Microstructural Investigation of the Deformation Zone below Nano-Indents in Copper

The deformation zone below nanoindents in copper single crystals with an 1 0>{111} orientation is investigated. Using a focused ion beam (FIB) system, cross-sections through the center of the indents were fabricated and subsequently analyzed by means of electron backscatter diffraction (EBSD) technique. Additionally, cross-sectional TEM foils were prepared and examined. Due to changes in the crystal orientation around and beneath the indentations, the plastically deformed zone can be visualized and related to the measured hardness values. Furthermore, the hardness data were analyzed using the Nix-Gao model where a linear relationship was found for H 2 vs. 1/h c , but with different slopes for large and shallow indentations. The measured orientation maps indicate that this behavior is presumably caused by a change in the deformation mechanism. On the basis of possible dislocation arrangements, two models are suggested and compared to the experimental findings. The model presented for large imprints is based on dislocation pile-ups similar to the Hall-Petch effect, while the model for shallow indentations uses far-reaching dislocation loops to accommodate the shape change of the imprint.

Conventional TEM Investigation Of The FIB Damage In Copper

Several years ago, the focussed ion beam (FIB) technique has found a broad variety of applications in material science [1]. Recently, the FIB became popular as a tool for making TEM samples as well as machining miniaturized mechanical test samples [2-7] to investigate the influence of sample dimensions on mechanical properties. As the surface to volume ratio is large for submicron sized test structures, any surface modifications by ion bombardment and implantation may critically alter the mechanical properties.