On the role of surfaces and interfaces in dislocation nucleation-controlled plasticity of nano-objects: comparison of in-situ experiments with atomistic simulations
Nanoscale metallic objects like thin films, nanoparticles, nanowires, or nanoporous metals receive sustained attention due to their size-dependent mechanical properties, which can include changes in deformation mechanisms, pseudoelastic behavior and increased yield strength compared to the bulk material. Many of these nano-objects are initially dislocation-free, such that plastic deformation is controlled by the nucleation of dislocations. Surfaces as nucleation sites and surface morphology therefore play an important role in small-scale plasticity. However, surfaces can also facilitate dislocation reactions or cross-slip at the intersection of dislocations with free surfaces. Furthermore, many nano-objects contain coherent twin boundaries, which strongly influence dislocation and twinning-mediated plasticity.
Here we present recent results on in situ experiments on single crystalline and nanotwinned nanowires as well nanoporous gold and compare them with atomistic simulations. These allow for controlled variation of the size and morphology as well as surface quality. The simulations highlight the need for realistic sample sizes and morphologies to capture and understand essential deformation mechanisms. Furthermore, twin boundaries are shown to not only act as obstacles to dislocation motion leading to hardening, but to fundamentally change the deformation mechanisms and failure mode. Here, the intersection of twin boundaries with free surfaces is shown to be of critical importance, highlighting fundamental differences between dislocation processes in nanotwinned polycrystals and nanotwinned nano-objects.