It was demonstrated that strain-rate sensitivity emerging in single-crystalline Cu nanopillars with diameters ranging from 75 to 500nm through uniaxial deformation experiments performed at different constant strain rates. In the range of pillar diameters and strain rates tested, it was found 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. This transition diameter, Dt, was found to be a function of strain rate, where faster strain rates shift the transition diameter to smaller pillar diameters: Dt of about 150nm at 10−3/s and Dt of up to 75nm at 10−1/s. The activation volumes, Ω, were computed as a function of the pillar diameter at each strain rate and it was found that, for pillar diameters below Dt, the activation volumes were relatively small, Ω < 10b3. This range agreed favorably with atomistic simulations for dislocation nucleation from a free surface. 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 Dt, to dislocation nucleation from the surface in the smaller samples, was postulated.

Emergence of Strain-Rate Sensitivity in Cu Nanopillars: Transition from Dislocation Multiplication to Dislocation Nucleation. A.T.Jennings, J.Li, J.R.Greer: Acta Materialia, 2011, 59[14], 5627-37