Invited
SMALL-SCALE PLASTICITY AND ELASTICITY: VERIFYING THE ROLE OF CAPILLARITY BY EXPERIMENT

Microstructure design at the nanoscale affords tailoring materials behavior by mixing interfacial contributions into the otherwise bulk-dominated behavior. The strengthening at small scale has been linked to interfacial effects, as have observations of stiffening or enhanced compliance in nanoscale objects. Yet, the underlying mechanisms remain poorly understood. While atomistic simulation studies provide a wealth of suggestions, meaningful experimental verification is more elusive. This talk discusses experiments designed to verify the suggested contribution of capillary forces to the plastic and elastic response of small bodies. Dealloying-made nanoporous metals provide model systems, homogeneous metal networks with struts or ligaments at the nanoscale. The mm-size nanoporous bodies are highly deformable in compression and can be tested with reliable macroscopic methods. In-situ tests in electrolyte afford control over their surface state tension and surface stress; the response of the mechanical behavior provides robust conclusions on the role of capillary forces in small-scale mechanics. Observations of an anomalous (effective, macroscopic) compliance of nanoporous gold also raise the issue of a possible contribution of surface excess elasticity. Density functional theory studies have recently produced the first numerical data for the excess elastic parameters of a metal surface. When discussed based on a careful selection of the reference frame, they indeed suggest a more compliant elastic response at small scale.