CAREER: Understanding Nanoscale Deformation by Characterizing the Mechanical Behavior of Nanoporus Noble Metals

A persistent problem exists in studies of nanocrystalline metals, thin films and nanostructured materials: what role do dislocations and other defects play in nanoscale deformation 7 It is understood that constraints on dislocation nucleation and motion arise as the available deformation volume decreases, but it is unclear whether dislocations are able to mediate plasticity in metal volumes that are several to tens of nm in size. Even if dislocations are involved in the deformation process, their behavior is likely to be heavily influenced by the presence of free surfaces and interfaces. Additional mechanisms such as diffusion may also occur. In order to correctly interpret and model the deformation behavior of nanocrystalline metals, we must understand the actual mechanisms that dominate deformation. The objectives of the proposed CAREER research and education plan are to: (1) investigate nanoscale deformation behavior in nanoporous gold, palladium and iridium, using in situ transmission electron microscopy; (2) systematically study the mechanical properties of thin tilm and bulk nanoporous noble metals, and determine the appropriate scaling laws that describe these properties; (3) evaluate the damping behavior of nanoporous metals, which are expected to exhibit significantly higher damping and anelasticity versus dense or flm-scale porous metals; (4) integrate international research opportunities into the educational experience of undergraduate materials engineering students at the University of Kentucky. Combining these areas of research and education will facilitate a wide-reaching, mechanism-based understanding of the mechanical behavior of nanoporous metals. The results will be relevant to nanostructured materials such as nanowires, since in both cases the volume available for deformation is highly constrained. This study has a strong fundamental scientific basis, but will benefit the application of nanoporous metals by enabling improvements in their mechanical stability. The intellectual merit of this proposal lies in its aim to uncover the fundamental mechanisms governing the mechanical behavior of nanoporous structures. The results from this project, which will focus on face-centered cubic noble metals, should be applicable to other nanoporous metals and relevant to the study of nanoscale materials subjected to deformation. Additionally, by attaining a better understanding of the mechanical behavior of nanoporous structures, fellow scientists will be able to predict and tailor properties for a given application. Preliminary results have already provided promising insight into the mechanical behavior of nanoporous noble metals. The first ever nanoindentation experiments on thin film nanoporous gold in the TEM, performed by the PI and collaborators in Berkeley, have suggested that unique dislocation activity occurs in nanoporous gold. By testing palladium and iridium, the homologous temperature range will be probed in a manner that would otherwise not be possible. The broader impact of this research proposal will enhance the undergraduate education experience for materials engineers at the University of Kentucky, by providing them with a unique opportunity to study abroad _an_dperform research in a world-leading materials laboratory in Germany. Both graduate and undergraduate students will be directly involved in this research. The engineering student exchange program between Kentucky and Karlsruhe is a continuing focus of the PI, who will recruit UK undergraduate students to work with graduate students and with visiting German students in his laboratory. This experience will provide UK undergraduates with international exposure and help them learn how to live and work in a global society. The results of this project will be presented at conferences and disseminated in the scientific literature, with joint authorship by each team of student researchers.