On the yielding and densification of nanoporous Au nanopillars in molecular dynamics simulations

We report on the plastic deformation characteristics of nanoporous gold nanopillars under compression in molecular dynamics simulations. The stress-strain curves demonstrate an initial non-linear regime up to compressive true strains of about 5%, followed by a stress plateau and a strong hardening stage. The plateau stress is increasing with the ligament size. In addition, a power-law is fitted to the stress variation during the plateau and hardening stages. In order to relate the mechanical response to the dislocation activity, an in-house post-processing technique was developed to skeletonize the structure and to identify the yielded ligaments. We found that the elastic-plastic transition starts at a strain substantially lower than that of the plateau. The load-bearing capacity is further retained by the unyielded ligaments within the three-dimensionally connected network, allowing further increase in the stress and at the onset of the plateau stress 40% of the ligaments have already yielded plastically. Calculation of the genus of the subnetwork of unyielded ligaments shows that it drops to zero at the onset of the plateau stage. Additionally, the rate of dislocation nucleation is maximum at the onset of the plateau, allowing to continue the plastic flow without increasing the stress. The stress starts increasing again as the ligaments coalesce and the nanoporous structure densifies. Based on the analysis of the genus of the whole structure during compression, we propose a correlation between the hardening and the topology of the structure, suggesting that hardening is dominated by the coalescence of ligaments. During coalescence, grain boundaries were formed between coalesced ligaments, some are either removed or retained. As a result, at very large compressive strains the single crystal nanoporous nanopillar turns into a polycrystalline specimen with some disconnected pores.