Title: ATOMISTIC SIMULATIONS OF DEFECT NUCLEATION AND FREE VOLUME IN NANOCRYSTALLINE MATERIALS

Summary: In this research, atomistic simulations are employed 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, we utilize atomistic simulations to explore structure-property relationships of face-centered-cubic (fcc) grain boundaries, and investigate the deformation of nanocrystalline copper as a function of average grain size.

Molecular statics employing an embedded atom method potential are utilized in this research to construct fcc bicrystalline grain boundary structures.

The boundaries are then deformed at 10K under uniaxial tension and simple shear at a constant strain rate to elucidate the influence of interfacial structure on inelastic deformation. An algorithm is also presented to compute interfacial free volume in the bicrystalline structures and quantitatively track its evolution with imposed strain. Representative non-equilibrium grain boundaries are instantiated using excess free volume as a measure of the degree of non-equilibrium state, and then deformed to explore the influence of structure on deformation response. It is shown that excess free volume alters interfacial atomic processes critical for dislocation nucleation and grain boundary sliding, resulting in lower grain boundary strength.

Volume-averaged kinematic metrics are formulated from continuum mechanics theory and applied to the results of atomistic simulations to provide new insight into atomic deformation and rotation fields. Inelastic deformation mechanisms common to nanocrystalline metals, such as heterogeneous dislocation nucleation, grain boundary sliding, and grain boundary migration are analyzed with the proposed metrics using bicrystalline grain boundaries. The results indicate that unique deformation fields are associated with each mechanism and a sense of the deformation history of the atomic fields are provided through the utilization of neighbor lists from the reference configuration. Other metrics use current configuration quantities to display the fronts of propagating dislocation networks.

The kinematic metrics are also leveraged to explore the tensile deformation of nanocrystalline copper at 10K. The distribution of different strain accommodation mechanisms is estimated and we are able to partition the role of competing mechanisms in the 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 non-local nanoscale deformation behavior and their potential to inform higher-scaled models is discussed.