Atomistic simulations of defect nucleation and free volume in nanocrystalline materials
Tucker, Garritt J.
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Atomistic simulations are employed in this thesis to investigate defect nucleation and free volume of grain boundaries and nanocrystalline materials. Nanocrystalline materials are of particular interest due to their improved mechanical properties and alternative strain accommodation processes at the nanoscale. These processes, or deformation mechanisms, within nanocrystalline materials are strongly dictated by the larger volume fraction of grain boundaries and interfaces due to smaller average grain sizes. The behavior of grain boundaries within nanocrystalline materials is still largely unknown. One reason is that experimental investigation at this scale is often difficult, time consuming, expensive, or impossible with current resources. Atomistic simulations have shown the potential to probe fundamental behavior at these length scales and provide vital insight into material mechanisms. Therefore, work conducted in this thesis will utilize atomistic simulations to explore structure-property relationships of face-centered-cubic grain boundaries, and investigate the deformation of nanocrystalline copper as a function of average grain size. Volume-averaged kinematic metrics are formulated from continuum mechanics theory to estimate nonlocal deformation fields and probe the nanoscale features unique to strain accommodation mechanisms in nanocrystalline metals. The kinematic metrics are also leveraged to explore the tensile deformation of nanocrystalline copper at 10K. The distribution of different deformation mechanisms is calculated and we are able to partition the role of competing mechanisms in the overall strain of the nanocrystalline structure as a function of grain size. Grain boundaries are observed to be influential in smaller grained structures, while dislocation glide is more influential as grain size increases. Under compression, however, the resolved compressive normal stress on interfaces hinders grain boundary plasticity, leading to a tension-compression asymmetry in the strength of nanocrystalline copper. The mechanisms responsible for the asymmetry are probed with atomistic simulations and the volume-averaged metrics. Finally, the utility of the metrics in capturing nonlocal nanoscale deformation behavior and their potential to inform higher-scaled models is discussed.