| new mechanism/mode of plastic deformation in ultrafine- and coarse-grained polycrystalline materials is theoretically described which occurs through nanograin formation induced by stress fields and transformations of dislocation pile-ups. It is demonstrated that lattice dislocations of a pile-up stopped at a grain boundary (GB) are trapped at the GB and transform into GB dislocations. The GB dislocations climb, and this leads to formation of a locally non-equilibrium GB fragment with tilt misorientation different from that of its neighboring (equilibrium) grain boundary fragments. The locally non-equilibrium GB fragment under shear stress splits into an immobile GB segment and mobile GB segment that moves within an adjacent ultrafine grain, in which case a new nanoscopic grain within the ultrafine grain is formed. Our theory is consistent with the corresponding experimental data reported in the literature. |
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