Nanoporous gold (npg), prepared by the process of dealloying, acts as a model material for studying size effects on nanoscale metal networks. In this prominent material Au ligaments act together as a three-dimensional (3D) open porous network of interconnected struts which can be tailored in size within the nm-µm range. With decreasing ligament size stresses close to the theoretical shear strength of Au have been reported in former compression and nanoindentation studies. In many of these studies, the standard Gibson-Ashby scaling equation for macroscopic foam plasticity has been applied to determine mechanical properties. However recent 3D analyses on npg reveal that the Gibson-Ashby relation might not be directly applicable to npg due to it's distinct network architecture. The objective of this study is to understand the interplay between the size of single struts, the nanoporous network topology and the defect structure.
The aim is realized by combining in situ micromechanical testing with non-destructive tomographic techniques and experimentally-informed simulations. Micropillar compression is performed in scanning electron microscopy (SEM) and transmission electron microscopy (TEM). By advanced tomographic imaging, a 3D characterization of the npg network before and after deformation is achieved. For small strut sizes, 360° electron tomography (ET) is applied enabling high quality reconstructions of the 3D morphology of npg without missing-wedge artefacts. The experimentally derived 3D data are used as input for ultra large-scale atomistic simulations and compared with corresponding simulations on geometrically constructed structures. This approach allows the correlation of experimental and simulated flow stress, the explanation of defect mechanisms observed in the experiment and the study of different topology and boundary conditions.
For larger strut sizes mechanical testing and 3D structure analyses of npg pillars are carried out by in situ SEM and high-resolution X-ray tomography (XRT, Nano-CT), respectively. Image correlation analysis applied to in situ SEM image series reveals the evolution of local strain gradients during deformation and, in particular, local yielding in the very early stages of deformation. By correlation with experimentally-informed finite element simulations the overall stress-strain response and the observed local deformation fields are evaluated.
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|Presentation||Figure 1||Figure 1 – Advanced characterization of the deformation process of npg nanopillars exhibiting small strut sizes by combining 360° ET and in situ TEM nanomechanics followed by subsequent defect analysis in the TEM.||82 KB||Download|
|Presentation||Figure 2||Figure 2 – 3D analysis of the deformation process of npg micropillars exhibiting large struts sizes by means of in situ SEM micromechanics and X-ray microscopy (XRM), specifically 360° XT, revealing the distribution of plastic strain after deformation.||83 KB||Download|