Size effects and strength fluctuation in nanoscale plasticity

Stochastic, discontinuous flow is ubiquitous in the plastic deformation of small-volume metallic materials. We have identified a size-strengthening effect on the stress to initiate the jerky plastic yielding in nanoscale volumes of copper single crystals, subjected to nanoindentation in different orientations. Such a nanoscale size effect arises due to the stochastic nature of dislocation sources, in contrast to the microscale size effect often attributed to plastic strain gradients. The jerky response can result from the activation of either surface or bulk heterogeneous dislocation sources, as governed by the distribution and resistance of dislocation locks. Implications concerning the deformation mechanism in materials with flow defect-limited characteristics are discussed.