Grain-boundary structures in polycrystalline metals at the nanoscale

We present a detailed analysis of grain-boundary structures in computer-gen erated Cu and Ni three-dimensional nanocrystalline samples. The study inclu des both totally random and textured microstructures with grain sizes in th e range of 5-12 nm. A detailed direct visualization technique is used at th e atomic scale for studying the grain-boundary structural features. The stu dy focuses on determining the presence of regions in the boundary exhibitin g order and structural units normally expected for high-angle boundaries. F or low-angle boundaries we investigate the presence of dislocation networks accommodating the misfit between the grains. A significant degree of cryst alline order is found for all the boundaries studied. The highest degree of structural order was identified for boundaries with misfits within about 1 0 degrees deviation from the perfect twin. These grain boundaries contain a repeated building structure consisting of structural units typical of a Si gma = 3 symmetrical tilt twin boundary and highly disordered steps between those structural units. For all other types of misfit, we also observe some degree of structural coherence, and misfit accommodation occurs in a regul ar pattern. The cases studied include grain boundaries with a high-index co mmon axis and show structural coherency that is independent of the grain si ze. Similar results are obtained for textured samples containing only low-a ngle grain boundaries, where regular dislocation arrays that are typical of larger grain materials are observed. These results provide evidence agains t the view of grain boundaries in nanocrystals as highly disordered, amorph ous, or liquidlike interfaces;The results suggest that the grain-boundary s tructure in nanocrystalline materials is actually similar to that found in larger grain polycrystals.