Grain growth and deformation in nanocrystalline materials; A combined atomistic and mesoscopic simulation study
Dr. Dorel Moldovan
rDepartment of Mechanical Engineering
Louisiana State University, Baton Rouge, LA 70803
Date: Tuesday, April 6, 2004
Time: 3:30 p.m.
Place: 105 Othmer Hall
Combined molecular-dynamics and mesoscopic simulations are used to elucidate the mechanism of grain growth and deformation in nanocrystalline materials. Our studies suggest that in nanocrystalline microstructures grain rotations play a complementary, equally important role to grain growth by grain boundary migration. The presence of both grain-boundary migration and grain rotation introduces a physical length scale, Rc, into the system. The growth process is characterized by two regimes: if the average grain size is smaller than Rc, grain growth is grain-rotation dominated; by contrast, growth is dominated by grain-boundary migration for grain sizes greater than Rc. This study reveals that the growth exponents are different for the two growth regimes. The combination of atomic-level with mesoscopic simulations based on the Needleman-Rice (1980) variational formalism for dissipative processes, enables the investigation of grain growth in systems containing a large number of grains and over long times. By extending the variational functional approach, the deformation mechanism by grain-boundary diffusional creep (Coble creep) in polycrystalline materials is investigated. The stress distribution along the grain-boundaries in a polycrystalline solid under externally applied stress is determined and the mechanism of how topological inhomogenieties (distributions in grain size and shape) introduces stress concentrations into a microstructure beyond those already present in perfectly homogeneous microstructure (equiaxed hexagonal grains) is investigated. Moreover, the diffusional creep response and the additional stress concentrations at the grain-boundaries in an idealized microstructure with variable grain-boundary diffusivities are explored.
