Near-ideal strength in gold nanowires achieved through microstructural design

The ideal strength of crystalline solids refers to the stress at elastic instability of a hypothetical defect-free crystal with infinite dimensions subjected to an increasing load. Experimentally observed metallic wires of a few tens of nanometers in diameter usually yield far before the ideal strength, because different types of surface or structural defects, such as surface inhomogeneities or grain boundaries, act to decrease the stress required for dislocation nucleation and irreversible deformation. In this study, however, we report on atomistic simulations of near-ideal strength in pure Au nanowires with complex faceted structures related to realistic nanowires. The microstructure dependence of tensile strength in face-centered cubic Au nanowires with either cylindrical or faceted surface morphologies was studied by classical molecular dynamics simulations. We demonstrate that maximum strength and steep size effects from the twin boundary spacing are best achieved in zigzag Au nanowires made of a parallel arrangement of coherent twin boundaries along the axis, and {111} surface facets. Surface faceting in Au NWs gives rise to a novel yielding mechanism associated with the nucleation and propagation of full dislocations along {001}110 slip systems, instead of the common {111}112 partial slip observed in face-centered cubic metals. Furthermore, a shift from surface dislocation nucleation to homogeneous dislocation nucleation arises as the twin boundary spacing is decreased below a critical limit in faceted nanowires. It is thus discovered that special defects can be utilized to approach the ideal strength of gold in nanowires by microstructural design.