Emergence of strain-rate sensitivity in Cu nanopillars: Transition from dislocation multiplication to dislocation nucleation

We demonstrate strain-rate sensitivity emerging in single-crystalline Cu nanopillars with diameters ranging from 75 up to 500 nm through uniaxial deformation experiments performed at different constant strain rates. In the range of pillar diameters and strain rates tested, we find that the size dependence of the pillar strength deviates from the ubiquitously observed power law to a relatively size-independent flow strength, markedly below the predicted theoretical strength for strain rates slower than 10(-1) s(-1). We find this transition diameter, D(t) to be a function of strain rate, where faster strain rates shift the transition diameter to smaller pillar diameters: D(t) similar to 150 nm at 10(-3) s(-1) and D(t) similar to <= 75 nm at 10(-1) s(-1). We compute the activation volumes, Omega, as a function of pillar diameter at each strain rate and find that for pillar diameters below D(t) the activation volumes are relatively small, Omega < 10b(3). This range agrees favorably with atomistic simulations for dislocation nucleation from a free surface. We postulate a plasticity mechanism transition from dislocation multiplication via the operation of truncated dislocation sources, also referred to as single-arm sources, in pillars with diameters greater than D, to dislocation nucleation from the surface in the smaller samples. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.