Grain Boundary Dynamics: A Novel Tool for Microstructure Control
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Grain Boundary Dynamics: A Novel Tool for Microstructure Control G. Gottstein1, D.A. Molodov1, L. S. Shvindlerman1,2, M. Winning1 1 Institut für Metallkunde und Metallphysik, RWTH Aachen, D-52056 Aachen, Germany 2 Institute of Solid State Physics, Russian Academy of Science, Chernogolovka, Moscow, distr., 142432 Russia ABSTRACT The reaction of grain boundaries to a wide spectrum of forces is reviewed. Curvature, volume energy and mechanical forces are considered. The boundary mobility is strongly dependent on misorientation, which is attributed to both grain boundary structure and segregation. In magnetically anisotropic materials grain boundaries can be moved by magnetic forces. For the first time a directionality of boundary mobility is reported. Flat boundaries can also be moved by mechanical forces, which sheds new light on microstructure evolution during elevated temperature deformation. Curvature driven and mechanically moved boundaries can behave differently. A sharp transition between the small and large angle boundary regime is observed. It is shown that grain boundary triple junctions have a finite mobility and thus, may have a serious impact on grain growth in fine grained materials. The various dependencies can be utilized to influence grain boundary motion and thus, microstructure evolution during recrystallization and grain growth.
INTRODUCTION Defects play an essential role in microstructure evolution, in particular 2D defects, i.e. internal surfaces between similar and dissimilar phases. Interfaces between equal phases but different crystallographic orientations are referred to as grain boundaries. Grain boundaries have the unique property that they react to exerted forces by a change of position. Grain boundary dynamics, i.e. the motion of grain boundaries under the action of forces is the main subject of this paper. A more detailed account of this and related topics can be found in a recent review by the current authors [0]. Commercial materials are polycrystalline, i.e. they consist of a large number of crystallites (grains) which are separated by grain boundaries. Each crystallite is surrounded by more than one grain which implies that grain boundaries form a spatial boundary network comprising boundaries, triple lines and quadruple junctions. If all boundaries and their junctions had equal properties, respectively, the temporal change of grain structure evolution would be sufficient to characterize the kinetic properties of grain boundaries. Fortunately, nature has chosen to make boundaries different, which gives us a powerful tool for microstructure control. Therefore, by definition, there is no average representative grain boundary, and grain boundary properties are most appropriately measured on specific individual grain boundaries, i.e. in bicrystals. Equivalently, measurement of junction properties requires tricrystal experiments, etc.
MEASUREMENT OF GRAIN BOUNDARY AND TRIPLE JUNCTION VELOCITY The measurement of grain boundary velocity seems to be a trivial problem, since only the displac
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