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Understanding the Role of Microstructural Stability and Variability on Plastic Deformation Pathways in Polycrystalline Materials: A Multi-scale Modeling Approach
AdvisorMushongera, Leslie L.T.M.
Materials Science and Engineering
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The focus of this work is to understand the role of microstructural stability and variability on the plastic deformation behavior of polycrystalline materials. Emphasis is put on: developing a unifying understanding of the governing deformation mechanisms and identifying the critical driving force metrics for damage in nano- and coarse-grained materials. Synthetic microstructures are systematically generated using microstructural statistics obtained from experimental images and used as inputs in the simulations. A hybrid Molecular Dynamics and Monte Carlo framework is used to understand the question of whether local grain boundary state as modified by dopant segregation affects the stability and plastic deformation pathways in nano-grained materials. The simulation studies revealed that grain boundary dopants affect plasticity of nano-grained materials in two ways: the dopants reduce the excess free volume within the grain boundaries, thus bringing them to a lower, and stable energy state, which prohibits the nucleation and growth of intergranular cracks; and the formation of large, disordered grain boundaries in doped nano-grained materials under applied load allows them to accommodate the deformation and prohibit crack growth. A mesoscale crystal plasticity framework implemented using the finite element approach is then used to analyze the influence of variabilities in the microstructure on fatigue of coarse-grained materials under strain-controlled conditions. The crystal plasticity studies are done using synthetic columnar and equiaxed microstructures; which are two extreme microstructural characteristics typically observed in fusion-based additive manufacturing techniques. It is shown that columnar microstructures exhibit poor resistance to fatigue damage than equiaxed microstructures. The poor resistance to fatigue damage by columnar microstructures is attributed to the large anisotropy associated with this type of microstructure. The stored plastic strain energy density is used as a critical metric to predict fatigue life. It is demonstrated that stored plastic strain energy density is necessary and sufficient to predict the scatter in fatigue life.