Dynamics of grain boundary motion at the atomic level
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Dynamics of grain boundary motion at the atomic level
K. L. Merkle, L. J. Thompson and F. Phillipp* Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A. *Max-Planck-Institut für Metallforschung, Heisenbergstr. 3, D-70569 Stuttgart, Germany.
ABSTRACT
Grain boundaries (GBs) in polycrystalline materials play a pivotal role in controlling their mechanical and physical behavior. High-resolution electron microscopy (HREM) was used to study thermally activated GB migration in thin films of Al and Au at elevated temperatures (T > 0.5 Tm). Grain boundary engineering via epitaxial templating allowed the manufacture of well-defined grain and interfacial geometries. These techniques enabled the observation of tilt, but also twist and general GBs at atomic resolution in-situ at high temperatures. Surface-energy driven GB migration occurred in general GBs, whereas tilt GB motion was curvature driven. Digital analysis of HREM video recordings have given considerable insight in the dynamics of GB motion at elevated temperatures. It is not surprising that the complex and diverse migration mechanisms depend on GB geometry as well as on interatomic interactions. The results provide, among others, direct evidence for collective effects by concerted atomic shuffles, ledge propagation in (113) symmetric tilt GBs, and motions of triple junctions at elevated temperatures.
INTRODUCTION
Past research on GB migration has largely focused on macroscopic observations in poly or specially designed bicrystalline specimen. It was found that GB mobilities of high-angle GBs in metals depend strongly on GB geometry and the presence of impurities. However, it was clear that information on atomic-scale processes could not be directly obtained by such experiments. The present experimental investigations utilize direct observations of the atomic-scale dynamics of GB structures in thin films at elevated temperature. Although real-space, real-time HREM is necessarily limited to observation of thin films we shall show that GB migration mechanisms can be elucidated, including their dependence on high-angle GB crystallographic parameters. The considerable complexity (involving five macroscopic degrees of freedom) of possible geometric arrangements between two adjoining crystals presents a special challenge for GB structure - property correlations. In addition, atomic-scale relaxations in GBs also depend on the detailed form of interatomic interaction. Macroscopic investigations indicate that GB mobilities are linearly related to the driving force for GB migration [1]. Capillary forces have most often been used in bicrystal GB migration experiments. Such macroscopic curvature-driven migration
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studies are complicated by the fact that GB energy depends on the GB plane. On a microscopic scale many GB facets could be involved, each of which may have different energetic and kinetic properties. Nevertheless, it is well established that GB mobilities may range over orders of magnitude, depending on misorientat
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