Papers by Author: Elizabeth A. Holm

Abstract: In this paper we discuss the principles of a combined approach to solve the problem of solute drag as it occurs in microstructure evolution processes such as grain growth, recrystallization and phase transformation. A recently developed irregular grid cellular automaton is used to simulate normal grain growth, in which the energy of the grain boundaries is the driving force. A new, discrete diffusion model is used to simulate solute segregation to the grain boundaries. The local concentration of the solute is then taken into account in the calculation of the local grain boundary mobility and/or grain boundary energy, thereby constituting a drag force. The relation between solute concentration and grain boundary mobility/energy is derived from molecular dynamics simulations.

Abstract: With the development of new, fully three-dimensional metallographic techniques, there is considerable interest in characterizing three-dimensional microstructures in ways that go beyond twodimensional stereology. One characteristic of grain structures is the surface of lowest energy across the microstructure, termed the critical manifold (CM). When the grain boundaries are sufficiently weak, the CM lies entirely on grain boundaries, while when the grain boundaries are strong, cleavage occurs. A scaling theory for the cleavage to intergranular transition of CMs is developed. We find that a critical length scale exists, so that on short length scales cleavage is observed, while at long length scales the CM is rough. CMs for realistic polycrystalline grain structures, determined by a network optimization algorithm, are used to verify the analysis.

Abstract: We compare the ability of three different types of microstructural model to simulate particle pinning. The microstructural models are the Phase Field model, the Front Tracking model and the Monte Carlo Potts model. The same 3D test geometry is simulated using each method. This is an hexagonal network with spherical particles located at the centre of each hexagonal grain. The hexagonal grain network provides a constant driving force for a moving boundary and includes triple line and quadruple point motion. This geometry allows detailed investigation of the boundary/particle interaction. The pinning force acting on the migrating curved grain boundary is calculated and compared with theoretical predictions for each model.

Abstract: We describe a general approach to obtaining 3D microstructures as input to computer simulations of materials properties. We introduce a program called MicroConstructor, that takes 2D micrographs and generates 3D discrete computer microstructures which are statistically equivalent in terms of the microstructural variables of interest. The basis of the code is a genetic algorithm that evolves the 3D microstructure so that its stereological parameters match the 2D data. Since this approach is not limited by scale it can be used to generate 3D initial multiscale microstructures. This algorithm will enable microstructural modellers to use as their starting point, experimentally based microstructures without having to acquire 3D information experimentally, a very time consuming and expensive process.

Abstract: Because both nucleation and growth are local phenomena, recrystallization depends on the spatial distribution of strains in a plastically deformed polycrystal. Using a polycrystal plasticity finite element model, we calculated these distributions for equiaxed polycrystals of copper with a random texture. We incorporated the results into a mesoscale recrystallization simulation with a nucleation model based on subgrain evolution. The coupled simulation results indicate that differences in local structure cause significant differences in recrystallization kinetics and grain size distribution. Furthermore, recrystallization in non-uniformly deformed polycrystals, even those with a random texture, is quantitatively and qualitatively different than predicted by continuum models that assume a uniform distribution of strains. This work highlights the need to examine all length scales relevant to the recrystallization process.

Abstract: The origin of the strain-free crystallites that nucleate the recrystallization process has been debated for decades. Realistic, three-dimensional computer simulations indicate that the nucleation event is the mobility-driven abnormal growth of certain subgrains. Based on these observations, we derive a model that incorporates subgrain topology, texture, boundary distribution and boundary properties to predict the frequency of the abnormal growth events that lead to nucleation. The qualitative and quantitative agreement between theory, simulation, and experiments is excellent.