Anomalous compliance and early yielding of nanoporous gold

Abstract

We present a study of the elastic and plastic behavior of nanoporous gold in compression, focusing on molecular dynamics simulation and inspecting experimental data for verification. Both approaches agree on an anomalously high elastic compliance in the early stages of deformation, along with a quasi immediate onset of plastic yielding even at the smallest load. Already before the first loading, the material undergoes spontaneous plastic deformation under the action of the capillary forces, requiring no external load. Plastic deformation under compressive load is accompanied by dislocation storage and dislocation interaction, along with strong strain hardening. Dislocation-starvation scenarios are not supported by our results. The stiffness increases during deformation, but never approaches the prediction by the relevant Gibson-Ashby scaling law. Microstructural disorder affects the plastic deformation behavior and surface excess elasticity might modify elastic response, yet we relate the anomalous compliance and the immediate yield onset to an atomistic origin: the large surface-induced prestress induces elastic shear that brings some regions in the material close to the shear instability of the generalized stacking fault energy curve. These regions are elastically highly compliant and plastically weak.

Nanoporous Gold—Testing Macro-scale Samples to Probe Small-scale Mechanical Behavior

“…Independent confirmation comes from molecular dynamics simulation [25] that uses an embedded-atom potential for gold. Studying NPG with 3.2 nm ligament size, the simulation finds the graph of Y eff (ϕ) and specifically the absolute value at low strain to practically coincide with that of the 40 nm sample in the experiments of Figure 3(a).…”

“…It is thus conceivable that surface-induced shear stresses may bring some of the ligaments locally close to a shear-unstable state. Ngô et al [25] suggests that these configurations may be responsible for the anomalously compliant behavior of NPG.…”

“…Ngô et al [25] have pointed out that the most obvious phenomenon in the latter context is the shear instability at the points of inflection of the generalized stacking-fault energy function. In fact, experiment [6] as well as numerical simulation [25,43] show that the surface-induced prestress in the bulk of the ligaments, which on theoretical grounds must have a large shear component, [44] is extremely high and can induce spontaneous plastic yielding even when there is no external load. It is thus conceivable that surface-induced shear stresses may bring some of the ligaments locally close to a shear-unstable state.…”

“…The corrosion behavior of grain boundaries is equally relevant, as illustrated in Figure 2. NPG samples are readily cleaved by applying slight pressure with a scalpel, and Figure 2(a) shows the resulting cleavage surface of a sample made by dealloying carefully annealed Cu 75 Au 25 . The intergranular nature of the fracture is apparent, as are crack branches that propagate along grain boundaries.…”

“…The large strain that is reached in compression affords testing protocols comprising, for example, strainrate jumps [7] or load-unload segments. [25] Besides enabling model studies of small-scale mechanical behavior, macroscopic NPG samples also demonstrate the implementation of the favorable mechanical behavior that is suggested by academic studies of nanoscale objects (nanowires, nanopillars) into a 'material', that is, into a substance from which things-such as engineering components-can be formed.…”