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

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 mu m via a transition regime (1 mu m > critical dimensions > 100 nm) to dislocation nucleation for critical dimensions < 100 nm.