Numerical simulations were performed on the mesoscale and nanoscale, in order to investigate how the localized nature of a deformation field induces nucleation and affects the motion of dislocations. The applications of different simulation techniques to provide information that can be correlated with, or can shed light on, experimentally observed phenomena were investigated.; An atomistic analysis of nanoindentation using molecular statics was undertaken. The simulation was within the two-dimensional framework involving pair molecular interactions in single-crystal films having a close-packed lattice structure. Attention was devoted to characterizing the nucleation of dislocations in response to indentation loading. The location of subsurface nucleation was found to coincide with the maximum resolved shear strain along the slip directions. A unique critical strain value appears to exist for initiating nano-scale plasticity in a perfect crystal under indentation. The effects of residual strain and existing material defects on the nucleation process was also investigated. It was found that residual tensile strain enhances the nucleation process and residual compressive strain delays it. Vacancies in the crystal act as stress concentration points and nucleation sites.; Dislocation dynamics simulations were carried out to study the cyclic stress-strain response of crystals containing misfit particles. The strength differential, manifested by the difference in the magnitudes of tensile and compressive flow strength during continuous loading, is examined. The computational model consists of a spherical particle and a single Frank-Read source in a specified slip plane inside a face-centered-cubic crystal. Attention is devoted to the dislocation glide behavior affected by the misfit elastic field, even when the slip plane does not intersect the particle. The multiplication of dislocation from the single source, the formation of pile-up loops as well as the unraveling of the loops upon reversed loading were all captured by the simulation. It was observed that the existence of a misfit particle gives rise to strength differential, a phenomenon of fundamentally different nature with regard to the widely recognized Bauschinger effect. The "back stress" concept was employed to analyze the simulation result. The effects of particle size and applied strain rate on the overall strength differential were also examined.
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