Multi-scale Modeling of Dislocation-driven Plasticity in Sub-micron Scale Metals

Metals are the cornerstone for industrial and structural applications due to their strength and ductility. To meet the constantly increasing requirements of advancing technology, a superior combination of strength and ductility is necessary. With this motivation, materials research has discovered novel methods to manipulate the internal microstructure leading to changes in the mechanical behavior. Since the plastic deformation in metals is commonly mediated by the motion and interaction of crystalline defects, it is vital to attain an in-depth understanding of how these defect microstructures could affect the macroscopic properties and how they can be manipulated for optimal performance. Interestingly, as the sample size approaches microstructural dimensions, crystalline metals start to show size dependence in their mechanical properties, deviating from the bulk behavior where mechanical response is size independent. At the micron and sub-micron length scales, plastic deformation is mainly driven by individual dislocations rather than the stochastic interaction between dislocations, so that it is imperative to delve into the detailed mechanics of individual dislocations for a fundamental understanding of microstructure-property relations. Dislocation dynamics (DD) simulations can be a useful tool due to their capability to keep track of the detailed dislocation microstructure evolution and to predict the corresponding macroscopic material response. In this regard, the DD framework along with theoretical approaches are utilized to study the correlation between the size dependent mechanical response and the dislocation microstructure at the micron and sub-micron length scales. In the first part of this dissertation, the plasticity in body-centered cubic (BCC) metals is examined with particular focus on the strong temperature dependence, which stems from the thermally activated motion of screw dislocations. In Chapter 2, we develop a DD model based on the atomistic characterization of dislocation mobility and potential source mechanisms to investigate the temperature dependent plasticity in BCC micropillars. Our models of molybdenum (Mo) and niobium (Nb) show the dislocation source mechanism changes with respect to temperature due to the change in mobility of screw dislocations, and these results are compared with experimental results and theoretical models. In addition, the size dependence increases with temperature, which agrees with recent experimental observations. To understand the underlying mechanisms that control the mechanical properties of nanostructured metals, an insight into the role of the grain boundary in dislocation-driven plastic deformation is vital. The grain boundary has been observed as a dislocation source, sink, or having no effect, which in turn, gives rise to different macroscopic mechanical responses. With this motivation, the second part of this dissertation (Chapter 3) describes the atomistic simulations and threedimensional DD simulations that were performed to investigate dislocation interactions at various grain boundaries and their role in the plastic deformation of face-centered cubic (FCC) bicrystalline micropillars. The atomistically-informed DD simulations show that bicrystalline samples containing a high angle grain boundary (HAGB) display hardening and higher flow stresses compared to single crystals, while micropillars with a coherent twin boundary (CTB) show similar flow stresses to the reference single crystalline samples. This is due to the transparency of the grain boundary to slip transmission, which is observed in the atomistic simulations. Interestingly, allowing dislocation glide on the grain boundary exhibits a decrease in flow stress as slip transmission becomes easier. To further investigate deformation mechanisms, it is necessary to delve into the detailed motion of individual defects under complex loading conditions. Current dislocation dynamics modeling techniques are limited to fixed geometries and small deformation. To overcome these limitations, a newly developed multi-scale model called the defect dynamics element method (DDEM) could be used, as it couples dislocation dynamics with a finite element model. In the third part of this dissertation, Chapter 4, this new coupled model is applied to model the Taylor impact test of BCC tantalum (Ta) single crystals, and investigates the anisotropic mechanical behavior observed in recent experimental findings. Finally, a summary of the completed work is provided in Chapter 5.