Multiscale Modeling, Simulation and Visualization and Their Potential for Future Aerospace Systems

We have combined molecular-dynamics simulations with mesoscopic simulations to study the mechanism and kinetics of grain growth in a thin film of nanocrystalline palladium with a columnar grain structure. The conventional picture is that grain growth results solely from the migration of grain boundaries in response to the driving force associated with the reduction in the grain-boundary area. However, our moleculardynamics simulations suggest that, at least in a nanocrystalline microstructure, grain rotations play an equally important role. Based on this insight we have developed a kinetic model for grain-boundary-diffusion accommodated grain rotation, extending the formalism of Raj and Ashby (1971) for grain-boundary sliding. We have incorporated this model into a mesoscopic-simulation code based on the Needleman-Rice (1980) variational formalism for the dissipated power. 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, if the average grain size is greater than Rc, grain growth is dominated by grain-boundary migration. Our study reveals that the growth exponents characterizing the power-law time dependence of the average grain size are different for the two growth regimes. We discuss how this methodology can be extended to allow the study of deformation process. This combination of atomic-level with mesoscopic simulations then enables the investigation, in a physically realistic manner, of grain growth in systems containing a large number of grains and over long times. *Work supported by the U.S. Department of Energy, Office of Science under Contract W-31-109-Eng-38.

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