Multiscale modeling and uncertainty analysis of mechanical behavior of nanostructural metals

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Zeng, Zhi
Zhu, Ting
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Metals with heterogeneous nanostructures hold great promise for achieving a synergy of ultra-high strength and ductility, thus overcoming the conventional strength-ductility tradeoff in nanostructured materials. To provide a fundamental understanding of the mechanics and physical mechanisms governing the strength and ductility in heterogeneous nanostructured metals, we conduct both atomistic and crystal plasticity modeling studies of heterogeneous nanostructured metals in this thesis. The heterogeneous nanostructured metals studied include gradient nano-grained copper, transmodal grained aluminum and additively manufactured stainless steel. A general modeling framework is developed for heterogeneous nanostructures. Specifically, we develop a Voronoi tessellation-based geometrical method to build the heterogeneous nano-grained structures. The distribution of grain size and the spatial arrangement of nonuniform grains are fully controllable. We also develop a crystal plasticity finite element model that accounts for grain-size-dependent yield strength and strain hardening. The associated finite element simulations reveal both the gradient stress and gradient plastic strain. To gain mechanistic insights into the controlling deformation mechanisms in heterogeneous nanostructured metals, we perform large scale molecular dynamics simulations to reveal grain boundary-dominated plastic deformation. Moreover, we also perform atomistic studies of unit processes of plastic deformation, including dislocation slip, deformation twinning and grain boundary sliding, through direct coupling with in situ transmission electron microscopy experiment. The uncertainties arising from the heterogeneous grains with a variety of size and spatial distributions are quantified. Heterogeneous microstructures turn microstructure uncertainties into valuable features of material properties. This thesis work provides the fundamental understanding of strength and ductility as well as unit deformation mechanisms of nanostructured metals. Furthermore, our uncertainty study has important implications for the design and fabrication of high-performance nanostructured materials.
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