| Control of microstructural evolution is the goal of much of materials processing. Properties of grain boundaries and associated higher order defects determine fundamental microstructural parameters such as grain size/shape and texture, which in turn control an amalgam of material properties and applications. Microstructural evolution theories are based on certain assumptions, and attempts to experimentally validate them have not been promising, predominantly due to the presence of impurities.; In this thesis, classical molecular dynamics simulation techniques are used to investigate boundary kinetics. Validity of the assumptions inherent in the theory of grain boundary migration is first ascertained. The U-shaped half-loop geometry is employed in a two-dimensional (triangular lattice) Lennard-Jones system to observe steady-state, curvature driven boundary migration. The classical linear relation between the migration rate and the driving force is recovered at low driving forces. Three-dimensional, highly parallelized simulations of ⟨111⟩ tilt grain boundaries in aluminum (EAM potentials) also confirm this result. The boundary mobility is found to have an Arrhenius dependence on temperature. However, the extracted activation energies of migration are significantly lower than those extracted in experiments, confirming the presence of impurities in the latter. Structurally similar boundaries are found to exhibit the compensation effect. Both boundary mobility and energy vary non-monotonically with the boundary misorientation, exhibiting maxima and minima for high symmetry (low Σ) special misorientations, respectively. Using these anisotropic boundary properties in a Potts model reveals that the evolution of two-dimensional random textures is mostly controlled by boundary energy anisotropy, not the mobility anisotropy.; Atomistic migration mechanism studies suggest that while single hops across the boundary are frequent, migration occurs primarily due to correlated shuffling of groups of atoms. The atomic population and frequency of the shuffles depend on the boundary structure and temperature. The atomic-level details also reveal the origin of frequent non-steady-state effects observed during boundary migration, i.e. the phenomenon of vacancy generation and emission from the boundaries. The excess boundary volume ejected into the bulk during the reduction in area of the grain boundaries manifests itself as vacancy/void generation events, which introduce transients in the otherwise steady-state migration process.; Simulations of migration of individual triple junctions show that significant deviations occur between the dynamic and static dihedral angles, in the case of special, (low Σ) tri-crystallographies. This implies that triple junctions have finite mobilities, and they impose a significant drag on the grain boundaries. Triple junction drag is also found to have a strong directional dependence. Finally, the simulations of simple circular grains prove that boundaries rotate to low energy misorientations. Grain rotation can potentially serve as an additional dynamic effect during microstructural evolution. Both triple junction drag and grain rotation are observed to have strong size dependence, implying their importance during nano-structural evolution of ultra-fine-grain-sized materials. |
