Kinetics and Connectivity of Grain Boundaries in Three-dimensional Metallic Systems
Creators: name: Kang, Jiwoong
Publisher: name: My University
Year: 2022
Subjects: subject: microstructure
subject: grain growth
subject: x-ray tomography
subject: time resolved 3D imaging
subject: Chemical Engineering
subject: FOS: Chemical engineering
subject: Engineering

description: As the intrinsic microstructure of polycrystalline materials governs their properties, it is of fundamental interest to understand the underlying microstructures and their evolution mechanisms. Despite decades of research on this topic, many open questions remain unanswered (see Chapters 1-4). This is mainly because conventional experimental approaches provide limited information on microstructures either spatially (two-dimensional) or temporally (destructive). Thus, in this dissertation, nondestructive three-dimensional modalities (Chapter 5), namely absorption contrast tomography (ACT) and laboratory-based diffraction contrast tomography (LabDCT), are employed to capture the evolution of three-dimensional microstructures in polycrystalline materials. Armed with these techniques, this dissertation examines two scientific phenomena: (1) abnormal grain growth and (2) percolation behavior of grain boundary networks. Firstly, Chapter 6 of this dissertation describes the newly developed in-house 3D x-ray diffraction (3DXRD) data processing framework, PolyProc. As LabDCT becomes more and more accessible, needs are placed on developing data processing frameworks that are as efficient as possible. PolyProc fulfills the demand as it can intake a range of 3DXRD datasets and output analysis-ready datasets with further functionalities, such as a visualization and grain statistics. The framework is heavily utilized throughout the following chapters. The following Chapter 7 and 8 focus on identifying the mechanism of abnormal grain growth (AGG) in a particle-containing alloy. By integrating ACT and LabDCT, we capture occurrence of AGG together with the spatial distributions of second phase particles. The holistic view of microstructure enables us to conclude that distribution of articles determines the trajectories of grain boundaries. That is, the particle distribution is highly correlated to the occurrence of AGG. We further investigate the origin of non-random particle distribution, which manifests during the isothermal anneal close to the solvus temperature. We find that it stems from residual segregation of the solute phase following solidification. Finally, Chapter 9 covers the percolation behavior of 3D polycrystalline materials. By collecting a large-scale 3D grain structure by LabDCT, we determine a percolation threshold of the high angle grain boundaries. We harness finite-size scaling analysis from bond percolation theory. We further confirm good agreement of the threshold with the theoretical result, validating the applicability of percolation theory on grain boundary networks. We further investigate the percolation behavior of triple junction (TJ) lines, which we confirm to show a lower percolation threshold than GB. We also observe vastly different percolation behaviors between TJ networks and theoretical diamond lattice, despite their topological similarities. We attribute the discrepancy to hyper-coordination of nodes and a spatial clustering of TJs in the microstructure. Overall, the findings in the dissertation expand our knowledge on kinetics and connectivity of grain boundaries in 3D metallic systems, which can contribute for designing efficient metallurgical processing methods and tailored microstructures for industrial applications.