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The migration of a (100) 9 = 43.6° (E29) twist grain boundary is observed during the course of a molecular-dynamics simulation. The atomic-level details of the migration are investigated by determining the time dependence of the planar structure factor, a function of the planar interparticle bond angles, and the location of the center of a mass of planes near the grain boundary. It is found that a migration step consists of local bond rearrangements which, when the simulation cell is made large enough, produce domain-like structures in the migrating plane. Although no overall sliding is observed during migration, a local sliding of the planes near the migrating grain boundary accompanies the migration process. It is suggested that a three-dimensional cloud of thermally produced Frenkel-like point defects near the boundary accompanies, and facilitates, its migration.

I. INTRODUCTION Grain-boundary (GB) migration, defined as the motion of an interface between two crystals in the direction of the interface normal, is a widely observed process of which the atomic-level details are not well known. Over the years there have been several theories advanced which attempt to describe the mechanism for migration. Mott, for example, proposed that GB migration is a thermally activated process that is accompanied by the transfer of groups of atoms across the boundary.1 This proposed multi-atom process was used to explain the rather high activation energies associated with migration. Later Turnbull, noting that a boundary can move under the influence of a driving force, derived a Nernst-Einstein relation for the mobility of a grain boundary based on the assumption that single atoms are transferred across the boundary.2 He proposed that the activation energy for GB migration should be similar to that for diffusion at the boundary. Unfortunately, most experiments to date have produced little information about the details of the migration mechanism. There are at least three current views on the mechanism of GB migration, each having some experimental support. The first view holds that migration can occur by means of the climb and glide of secondary grainboundary dislocations (SGBDs); it is supported primarily by TEM structural observations.3'4 The second view is that migration can occur by the aforementioned mechanisms of single- or multi-atom transfer across a grain boundary. The third view is that at low temperatures, in a system with impurities, the driven migration of

^Permanent address: Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.

a GB together with its absorbed impurity atoms is an activated process.5'6 By contrast, at higher temperatures the impurity atoms may be left behind by the moving grain boundary, resulting in nonactivated grain-boundary migration.5'6 While none of these views precludes the others, they involve different assumptions; for example, migration of SGBDs involves the motion of line defects as opposed to the transfer of point defects across a gra