Lattice Correspondence Analyses of Grain Boundary Formation and Migration in Hexagonal Close-Packed Metals

NON-TECHNICAL SUMMARY Grain boundaries in crystalline metals are very important planar defects and play a crucial role in physical and mechanical properties of metals and alloys. During thermal/mechanical processing or deformation, new grains nucleate from the matrix and coarsen as a result of grain boundary migration. But how a new grain chooses the boundary plane with the matrix and how the grain boundary migrates has remained a long-standing fundamental problem in materials science. When a grain boundary migrates, the lattice of one grain is transformed into the lattice of the neighboring grain. Numerous grain boundary models have been proposed to describe grain boundary structures and their migration mechanisms. However, the current grain boundary theories do not consider lattice transformation and are unable to account for many phenomena obtained from experimental and simulation studies. To resolve the complex mechanisms for grain boundary formation and migration in important engineering metals such as lightweight magnesium and titanium with hexagonal close-packed structures, a novel strategy that fundamentally differs from the existing framework is proposed. Computational studies on the atomic scale and advanced materials characterization are being combined to reveal the underlying physics of grain boundary formation and migration. The project is resolving the grain boundary physics with unprecedented clarity. Because grain boundaries significantly affect material mechanical behavior, the project is also impacting the processing of lightweight metals in terms of microstructure control, which is critical for improved energy efficiency. The project is also engaging students of various ages through educational outreach and curriculum development. TECHNICAL SUMMARY The existing theories such as the coincidence site lattice and the disconnection model can be described as a point-to-point matching scheme. However, experimental and simulation studies show that during grain boundary migration, an atomic plane of one grain is transformed into a corresponding plane in the neighboring grain. Thus, the grain boundary migration is achieved by a plane-to-plane lattice transformation, and a unique lattice correspondence can be established, similar to deformation twinning which linearly maps the matrix lattice into the twin lattice. The proposed research is applying the principle of lattice correspondence to resolving the fundamental physics of grain boundary formation and migration. First, the orientation distribution and structure of grain boundaries in wrought magnesium and titanium are being characterized. Electron backscatter diffraction and high-resolution transmission electron microscopy are being performed to investigate the grain boundary structure. Then the experimental results are being used as input for atomic-scale simulations in which lattice correspondence analyses are conducted. Invariant planes in lattice transformation are being identified. Grain boundary mobility and migration kinetics are being obtained from the simulation results. The outcome of the proposed research is establishing an inherent connection between deformation twinning, phase transformation and grain boundary migration, which is transformative and profoundly advancing the knowledge of grain boundary properties in low symmetry crystal structures. The proposed fundamental research well aligns with the mission of National Science Foundation, and the outcome is expected to significantly impact the materials science and engineering community. In addition, educational engagement and outreach to students of various ages are being carried out in partnership with established university programming. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.