A model to explain extensive superplasticity in polycrystalline materials
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Superplastic behavior has been observed in a variety of metallic systems during high temperature deformation. The possibility of superplastic flow in nanocrystalline materials at low temperatures is reported in molecular-dynamics simulations [1, 2] and in experimental evidences [3, 4, 5, 6]. Karch et al.  observed that conventional brittle ceramics became ductile at low temperature if a polycrystalline ceramics was generated with a crystal size of a few nm. Lu et al.  showed recently that bulk, highly pure nanocrystalline copper could be strained at room temperature by up to 5,000% without strain hardening and changing grain size. These experimental results indicate that conventional dislocation mechanisms are not responsible for the large strain and that the extensive superplasticity seems to originate from grain boundary (GB) diffusion (Coble creep). This novel behavior, however, could not be explained by present models and theories.
KeywordsGrain Boundary Polycrystalline Material Nanocrystalline Material Diffusion Creep Average Strain Rate
This work was supported by the National Science Foundation of China under Grant No.10572088 and the National Basic Research Program of China through Grant No. 2004CB619303.