| In this work, a 3D Dislocation Cellular Automaton (CA) model is developed and calibrated first; then the model is applied to study the mechanical properties of nanoscale and finescale multilayer thin films. In the 3D dislocation model, the study object has a FCC structure. To calibrate this model, three examples involving operation of a Frank-Read source are chosen. These examples also highlight the benefits and drawbacks of the method. A benefit to discretization is that dislocation evolution may be analyzed via spatial averaging over collections of patches, so that the discrete versus continuum nature of the results may be studied. Further, dislocation reactions and cross slip are accommodated easily and, in principle, Monte-Carlo schemes can be integrated into the evolution formalism. Overall, the discrete nature of the method is attractive for incorporating the kinetics of thermally activated states and for simplifying the range of geometries and threshold criteria associated with dislocation reactions.; This 3D Dislocation Cellular Automaton model is employed to simulate yield and hardening in nanostructured metallic multilayer thin films. Threading and interfacial dislocation sources are studied. The films are composed of 2 types of alternating single crystalline FCC layers with a (001) epitaxy, a mismatch in stress-free lattice parameter, but no elastic modulus mismatch. Interfaces are assigned no additional strength except that from lattice parameter mismatch and interfacial dislocation arrays. Three regimes of tensile plastic response are identified based on the evolution of interfaces during tensile deformation. For smaller individual layer thickness, interfaces are coherent initially and remain so up to bulk yield (Regime I). For intermediate layer thickness, interfaces are coherent initially but become semi-coherent prior to bulk yield (Regime IIa). For larger layer thickness, interfaces are semi-coherent initially and acquire additional dislocation content prior to bulk yield (Regime IIb). The evolution of interfacial structure during deformation in Regimes IIa and IIb occurs due to deposition of dislocation content along interfaces by confined layer slip (CLS). The overall outcome is that the plastic strength of multilayer thin films increases with decreasing layer thickness until Regime I is encountered. Strength in Regime I may increase, reach a plateau, or even decrease. The results are consistent with experimental measurements of hardness, including the Ag/Al system in particular. (Abstract shortened by UMI.)... |
