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The drag on a grain boundary produced by an impurity atmosphere is examined in detail, and is found to depend on the velocity of the grain boundary relative to the diffusivity of the impurity and its interaction with the grain boundary. At high velocities the faster diffusing impurities have the greater drag; whereas at low velocities the reverse is true. With increasing impurity concentration or with decreasing temperature a boundary may experience a transition due to a changing interaction with its impurity atmosphere. The nature of the transition depends on driving force and may give rise to a large apparent activation energy, as well as jerky boundary motion due to the existence of a range of conditions where two boundary velocities are possible. Special orientation effects that may result in textures are expected to occur more easily at high velocities.

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