Transition from Homogeneous to Localized Deformation in Nanoporous Gold.

We report a transition from homogeneous deformation to localized densification for nanoporous gold (NPG) under compression, with its solid fraction (φ) increasing to above ∼1/3. Results obtained herein suggest that this transition is inverted compared to that of conventional porous materials. Consequently, under compression, the low-density NPGs with φ<1/3 showed evident strain hardening, whereas a stress plateau was observed for high-density NPGs with φ>1/3, which is contrary to the established notions for conventional porous materials. The ligament pinch-offs and bending-dominated structures are responsible for the homogeneous deformation of low-density NPGs. For high-density NPGs, the compression- or tension-dominated structure enables the collective strain bursts in nanoligaments, resulting in localized densification and stress plateau in their compression curves. In addition to the nanosize effect, the surface-diffusion-mediated topology evolution and the large-scale crystal-lattice coherency arising from the large grain size are also decisive to the mechanical response of dealloyed NPGs, which might be universal for self-organized nanonetwork materials.

Surface Triple Junctions Govern the Strength of a Nanoscale Solid.

Surface triple junctions (STJs), i.e., the termination lines of grain boundaries at solid surface, are the common line defects in polycrystalline materials. Compared with planar defects such as grain boundaries and surfaces, STJ lines are usually overlooked in a material's strengthening although abundant atoms may reside at STJs in many nanomaterials. In this study, by in situ compression of coarse-grained and nanocrystalline nanoporous gold samples in an electrochemical environment, the effect of STJs on the strength of nanoporous gold was successfully decoupled from grain-boundary and surface effects. We found that the strength of nanoporous gold became sensitive to STJ modification when ligament size was decreased to below ∼100 nm, indicating that STJs started to influence ligament strength at sub-100 nm scale. This STJ effect was associated with the emission of dislocations from STJs during plastic deformation. Our findings strongly suggest that the structure and chemistry at STJs should be considered in understanding the mechanical response of sub-100 nm scale materials.