Heterogeneous dislocation nucleation from surfaces and interfaces as governing plasticity mechanism in nanoscale metals

We report the results of constant strain rate experiments on electroplated, single crystalline copper pillars with diameters between 75 and 525 nm. At slow strain rates, 10−3s−1, pillar diameters with 150 nm and above show a size-dependent strength similar to previous reports. Below 150 nm, we find that the size effect vanishes as the strength transitions to a relatively size-independent regime. Strain rate sensitivity and activation volume are determined from uniaxial compression tests at different strain rates and corroborate a deformation mechanism change. These results are discussed in the framework of recent in situ transmission electron microscopy experiments observing two distinct deformation mechanisms in pillars and thin films on flexible substrates: partial dislocation nucleation from stress concentrations in smaller structures and single arm source operation in larger samples. Models attempting to explain these different size-dependent regimes are discussed in relation to these experiments and existing literature revealing further insights into the likely small-scale deformation mechanisms.