Interpreting anomalous low-strength and low-stiffness of nanoporous gold: Quantification of network connectivity

The strength and elastic modulus of macroscopic nanoporous Au samples are much lower than that predicted by using Gibson-Ashby scaling laws. Here we attribute this discrepancy to a lowered network connectivity in nanoporous structure, and propose to modify the scaling equations (for both strength and stiffness) by introducing a concept of effective relative density. It is presented that the effective relative density can be determined by measuring the elasticity of nanoporous Au; under this scenario, the corrected strengths of Au nano-ligaments agree (on the order of magnitude) well with the previous data for similar-sized Au nanowires or nanopillars, which confirms our hypotheses. This study also revealed a low power-law exponent of size-dependent strength for Au nano-ligaments ( beta = -0.34) compared with that of Au submicron-pillars machined by focused ion beam (FIB) (beta = -0.61), which may be related to the critical roles of surface defects played in deformation. A ratio between effective relative density and relative density is suggested to quantify the network connectivity in nanoporous structure, which decreases rapidly during structure coarsening - it explains why most previous nanoporous Au samples, which more or less experienced coarsening, were unexpectedly soft. Current study also leads to some general strategies for syntheses of strong and stiff nanoporous metals. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.