ABSTRACT- Nanoscale metals have become increasingly important as advanced materials for energy-related applications, while fundamentally pushing the scientific frontiers of mechanics, materials science and energy at the nanoscale. Examples vary from silver nanowire films for transparent conductive electrodes in next-generation solar cells, touch-screen displays and stretchable electronics, to conductive metals strengthened by nanoscale twin boundaries resisting extreme environments. This seminar presents our latest research progress in understanding the small-scale mechanics of strength and superplasticity in nanoscale metals, by using state-of-the-art combined in-situ nanomechanical experiments and molecular dynamics simulations. First, the ability of twin boundaries in strengthening and maintaining ductility has been well documented; yet most understanding of the origin of these properties relies on perfect-interface assumptions. We will show that growth twins in nanotwinned Cu and Ag metals are inherently defective with kink-like steps and curvature, and that these imperfections play a key role in plastic deformation mechanisms and the Hall-Petch strengthening limit. Second, we will show a new type of size effect in the fracture of ultra-thin Au nanowires containing angstrom-scale twins that results in tensile strengths near the maximum theoretical limit. Third, following the “smaller is stronger” trend, we will show unusual room-temperature super-elongation without softening in Ag nanowires over a sample diameter range between 15 nm and 50 nm, which extends far beyond the maximum size for pure surface-diffusion-mediated deformation (e.g. Coble-type creep), and where it is shown experimentally and theoretically that crystal slip could serve as a stimulus to surface diffusional creep.

BIO- Frederic Sansoz is a professor of mechanical engineering and materials science at the University of Vermont. He earned his BS degree in mechanical and aerospace engineering and MS degree in materials science and engineering from the Ecole Nationale Supérieure de Mécanique et Aérotechnique in Poitiers, France in 1996, and a PhD degree with honors in materials science and engineering from Ecole des Mines de Paris in 2000. He was a post-doctoral fellow in mechanical engineering at the Johns Hopkins University until 2003. Sansoz received the National Science Foundation CAREER award in 2008. His research focuses on understanding the mechanics and physics of interface-dominated nanomaterials using materials theory, atomistic computer simulations and nanoscale experiments.