Indentation response of nanoporous gold from atomistic simulations
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Michael J. Demkowicz Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, USA (Received 25 January 2018; accepted 15 March 2018)
We present classical potential molecular dynamics simulations of nanoporous gold (np-Au) impacted by a spherical indenter. The atomic structure was generated using a phase field model as a template. In agreement with previous experiments, we observe densification in the region under the indenter. The hardness values obtained from our simulations exhibit a transition from an initially perfect-plastic plateau to hardening behavior in the later stages of indentation. This transition occurs when the relative density beneath the indenter exceeds ;0.9. Hardness values obtained from the nanoindentation simulations reach 0.6 GPa, due to the densification of the material under the indenter. Elevated dislocation densities are observed in the densified region. The mechanism of pore collapse in the densified layer under the indenter is seen to switch from uniaxial to triaxial, consistent with a change in deformation mechanism from one based on shearing of individual ligaments in np-Au to one involving dislocation-mediated plasticity around voids in a Au single crystal undergoing uniaxial compression.
I. INTRODUCTION
Nanoporous gold (np-Au) is a widely studied model nanostructured material.1–3 This material exhibits several unique behaviors due to the high surface-to-volume ratio and bicontinuous structure of its interconnected Au ligaments and intervening void space as well as the distinctive properties arising from the nanoscale dimensions of the ligaments. In particular, significant experimental effort has been devoted to understanding its mechanical properties,4–7 which include ductility in compression,8 a “smaller is stronger” size effect for nanoscale ligament diameters,9,10 and pronounced strain hardening in compression due to densification.11 We present atomistic simulations of the response of np-Au to nanoscale spherical indentation, providing a point of comparison to previous experimental studies as well as insights into the fundamental mechanisms governing the mechanical properties of this material. The mechanical behavior of np-Au has been studied using a variety of approaches, including nanoindentation as well and other techniques.4,12–22 These investigations demonstrate that the standard Gibson–Ashby (G–A) scaling relation23,24 describes the yield stress, r, of np-Au remarkably well, provided that the yield stress of the ligaments, rl, is written as a function of ligament diameter, d, to account the size effect in ligament plasticity.25–30 Indeed, the yield strength of Au nanopillars may reach a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.72
several GPa, as opposed to the 200–300 MPa yield stresses found in fully dense Au.31 This relation may therefore be written as r 3=2 0:3qrel rl ðd Þ
;
where the relative density, qrel 5 q/ql, compares the average density, q, of np-Au to the density ql of indi
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