Emergence of enhanced strengths and Bauschinger effect in conformally passivated copper nanopillars as revealed by dislocation dynamics

The ability to precisely control the surface state of a nanostructure may offer a pathway towards tuning the mechanical properties of small-scale metallic components. In our previous work [Jennings et al., Acta Mater. 60 (2012) 3444-3455], single-crystalline Cu nanopillars were conformally coated with a 5-25 nm thick layer of TiO2/Al2O3. Uniaxial compression tests revealed two key findings associated with these passivated samples: (i) similar to 80% higher strengths as compared with the uncoated samples of the same diameter, 200 nm; and (ii) Bauschinger effect-like hysteresis during unloading-reloading segments. Dislocation dynamics simulations of uniaxially compressed 200 nm diameter Cu nanopillars with coated surfaces revealed the contribution of dislocation multiplication, pinning, and pile-up processes to the experimentally observed enhancement in pillar strength. They further helped explain the transition of plasticity mechanisms from dislocation multiplication via the operation of single-arm dislocation sources to dislocation nucleation from the crystal-coating interface. Hysteresis in stress strain data is discussed in the framework of dislocation structure evolution during unloading reloading cycles in experiments and simulations. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.