A new physical mechanism or mode of plastic deformation in nanocrystalline metals and ceramics is suggested and theoretically described. The mode represents the cooperative grain boundary (GB) sliding and stress-driven GB migration process. It is theoretically revealed that the new deformation mode is more energetically favorable than "pure" GB sliding and enhances the ductility of nanocrystalline solids in wide ranges of their structural parameters.
A theoretical model is suggested that describes the behavioral features and energetic characteristics of dipoles of grain boundary dislocations in nanocrystalline films. Such dislocation dipoles in nanocrystalline films are shown to play the role of misfit defect configurations that compensate, in part, for misfit stresses that occur due to a mismatch between crystal lattice parameters of films and substrates. Ranges of parameters (misfit parameter, grain size, etc.) are revealed at which the formation of dislocation dipoles is energetically favorable in nanocrystalline films. It is demonstrated that dislocation dipoles are typical structural elements of nanocrystalline films fabricated at highly nonequilibrium conditions.
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