Modeling of the microstructural effects on the mechanical response of polycrystals

The modeling and hence exploitation of the connection between the microstructure and the mechanical response of polycrystals is and continues to be at the forefront of the longstanding challenges in the materials science and metallurgical engineering. The macroscopic mechanical response of polycrystalline materials is intricately governed by the propensity of the micro-mechanisms of crystal plasticity, which are controlled by the instantaneous hierarchical microstructure and its evolution. Therefore, the microstructure almost exclusively controls the macroscopically observable mechanical response of polycrystalline aggregates in terms of the stress response and its variation (the stress rate or strain hardening). In this thesis, the microstructural effects on the mechanical response/properties of polycrystals are classified into four groups: the polarity, size, composite, and porosity effects. The historical background as well as the research on the modeling of the microstructural effects, which has so far lasted almost a century, are concisely reviewed. The primary microstructural effects, the size and polarity effects, are modeled for different polycrystalline metallic materials at various length scales. First, the size effect was modeled at the macro-scale using a nonlocal (physics-based) microstructural model for polycrystal plasticity to simulate the behavior of a ferritic-pearlitic steel during large deformation in the cold and warm regimes. Then, the model was applied to simulate industrial cold and warm forging processes of a bevel gear shaft and predict its final microstructure and properties (process-microstructure-properties linkage). Second, the polarity effect was modeled at the mesoscale using a physics-based crystal plasticity model to simulate the (macroscopic) anisotropic mechanical response of an additively manufactured austenitic high-Mn steel (microstructure-properties linkage). It was, then, demonstrated that the mesoscale model can be applied for the optimal computational design of an additively manufactured lattice structure