Nanocrystalline Metals and Thin Films: Quantized Plasticity, Internal Stress, and Grain Boundary Strength

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TECHNICAL SUMMARY:The proposed research aims to understand unusual characteristics of nanocrystalline metals through study of the underlying physics of deformation. This is accomplished by developing a new continuum description called quantized crystal plasticity and informing it with results from atomistic simulations and mechanical testing. The atomistic studies achieve this by measuring distributions of critical strength to trigger slip events. They also explore fundamental descriptions of the local features of grain boundaries that control dislocation absorption, repulsion, and nucleation. In this sense, two scales of simulation?continuum and atomistic?are coupled. The informed quantized crystal plasticity model can address experimentally relevant sample sizes and strain rates that are not possible with atomistic simulations. Specifically, the model is coupled to tensile experiments to study stress-assisted grain growth and the effect of bimodal grain size distributions on nc ductility. There are two primary expected technical outcomes of this work. The first is a fundamental description of the distribution of critical strengths and distribution of internal stress state in nanocrystalline metals. This includes an understanding of how these distributions evolve with macroscopic deformation. The second is a finite element based model that captures these features, and permits the prediction of the mechanical response of nanocrystalline metals to various stress or deformation paths. These outcomes provide a basis for structure-mechanical property relations for nanocrystalline metals.NON-TECHNICAL SUMMARY:Nanocrystalline metals embody an extreme strategy to make a high strength metal?namely to shrink the size of individual crystals to only a few hundred atoms across. This can reduce or eliminate defects that traditionally weaken metals and also introduce barriers that strengthen the metal. Indeed, nanocrystalline metals show promise in terms of extraordinary resistance to yielding, large fracture energy at low temperature, and improved resistance to failure under oscillating loads (fatigue). The proposed work will team researchers from US and international institutions to understand the fundamental strengthening processes. A key outcome is a computational tool to predict the mechanical properties of nanocrystalline metals based on their underlying structure. This enables optimization of the performance of such materials. Two graduate students will be trained in the use of advanced computational and experimental techniques. They will also assist in the development, implementation, and refinement of project-based learning modules for a new Science, Technology, Engineering, and Mathematics (STEM) secondary school. A spin off of this effort is novel, web-based interfaces to improve comprehension of materials engineering concepts, and the anticipated involvement of students from under-represented groups.