Controlling dislocation nucleation-mediated plasticity in nanostructures via surface modification

A straightforward strategy for reaching the ultimate strength of crystalline metals is to synthesize materials with a scarcity of defects. Bottom-up synthesis of nanostructures is an ideal means of approaching this limit, where nucleation of dislocations is a requirement in otherwise perfect crystals to facilitate plastic flow and mitigate brittle fracture. In this deformation mechanism regime where thermal fluctuations assist in the nucleation process, surface self-diffusion emerges as the rate-limiting step needed to promote displacive activity. We conducted tensile experiments in situ on pristine Au nanowires in addition to nanowires with thin Al2O3 coatings produced by atomic layer deposition. Distributions of yield strengths of nanowires with fully conformal coatings show a pronounced reduction in scatter and a measurable shift toward higher values. The presence of coatings did not curtail the ability of the nanowires to sustain significant amounts of plastic strain. In all conditions, nucleation of partial dislocations governed incipient plasticity. Analyses of the data using a thermal activation model in tandem with Bragg coherent X-ray diffraction point to the potent effect of control over the near surface zone in mediating diffusion, and thereby dislocation nucleation and the ensuing plastic response of the nanostructures. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.