Large-scale Atomistic Simulations for Deformation and Fracture Mechanisms of Nanotwinned Metals Li, Xiaoyan creator 2012 xxvi, 161 p. born digital Thesis (Ph.D. -- Brown University (2012) Gao, Huajian Director Kumar, Sharvan Reader Bower, Allan Reader Brown University. ENGINEERING: Solid Mechanics sponsor theses nanotwinned metals atomistic simulation deformation fracture twin boundary dislocation RPB 20121023 engEnglishNanotwinned metals have exhibited enhanced mechanical properties, including ultra-high strength, good ductility and remarkable fracture toughness. In this dissertation, we used large-scale atomistic simulations to investigate deformation and fracture behaviors of nanotwinned metals, in forms of bulk, thin films and nanopillars. The work is organized in three parts.<br/> In the first part (Chapter 4), we performed large-scale atomistic simulations to study deformation mechanisms in equiaxed- and columnar-grained polycrystalline nanotwinned bulk Cu. Three dislocation-based mechanisms are revealed. In the equiaxed-grained nanotwinned Cu, it was found that dislocation cutting through or pileup on twin boundary (TB) is a controlling mechanism, leading to strength hardening. However, when TB spacing falls below a critical size, the strength softening governed by nucleation and motion of partial dislocations parallel to TBs was observed in our simulations. To characterize such twin size-mediated softening, we have proposed a scaling law (including two microstructural dimensions, i.e. mean grain size and TB spacing) based on kinetic theory of dislocation nucleation. For columnar-grained nanotwinned Cu, when the tensile axis is parallel to TBs, the movement of threading dislocations confined inside twin lamellae dominates plastic deformation.<br/> The second part (Chapter 5) is dedicated to interpret a novel toughening mechanism (crack bridging) observed in in-situ experiments on nanotwinned Cu thin films. Atomistic simulations showed that during plastic deformation, initially clean TB can transform to a dislocation wall which can block further dislocation activity and resist crack propagation, leading to experimentally observed crack-bridging behavior, i.e. nanoscale twins serve as uncracked ligaments. The formation of dislocation wall is attributed to interaction between dislocations emitted from crack tip and TBs.<br/> The third part (Chapter 6) is devoted to investigate mechanical responses of nanotwinned Cu nanopillars with different TB orientations. Atomistic simulations showed that the strain localization (such as necking or shear banding) is a governing deformation mechanism in nanotwinned nanopillars with orthogonal TBs, while the detwinning dominates plastic deformation of nanopillars with slanted TBs. The difference in deformation mechanisms leads to substantially different strengths in corresponding nanopillars. Atomistic simulations also demonstrated that TB-surface intersection is a preferential dislocation nucleation site in nanopillars with different TB orientations.10.7301/Z0N014TRIn CopyrightCollection is open for research.dissertations