Surface-controlled mechanical properties of nanoporous metals

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Surfaces play a significant role in plastic and elastic deformation, especially at small crystal size when the specific surface area is large. This is well documented by observations on the size-dependent strength, for instance in recent work on nanowire deformation. The proposed research aims to contribute towards identifying the processes that underlie the role of the surface in mechanical the behaviour. We propose to exploit a new and unconventional approach: rather than changing the size or the specific surface area of the sample, we shall instigate cyclic changes of state of the surface. This will be done in-situ during mechanical tests, and the consequences for the mechanical behavior will be recorded. Our main focus is on the constitutive plastic behaviour, as parametrized by yield- and flow stress, work hardening and strain rate sensitivity. Information on the elastic response will be sought as a closely related supplementary issue. As suitable model materials with large specific surface area we will study nanoporous noble metals and their alloys, using recent synthesis routes that yield millimetersized nanoporous metal samples which exhibit large (>50% strain) ductile deformation in compression. The surface modifications will use i) electrochemical experiments and ii) reversible gas adsorption studies to cycle selectively the surface chemistry (for instance, superficial oxidation/reduction or hydrogen adsorption/desorption cycles) or the superficial electric charge density. Through these variables, our approach affords control over the two relevant capillary forces, the surface tension and the surface stress, and over the surface diffusivity. We aim at i) establishing a first experimental data base for the response of plastic flow and of excess elastic behavior to the surface state, and ii) pinpointing the microscopic phenomena via which the surface affects the mechanics, for instance dislocation nucleation or egression, dislocation endpoint drag, step edge energetics, surface diffusion, or surface (excess-) elasticity.