Grain-boundary structures in polycrystalline metals at the nanoscale

We present a detailed analysis of grain-boundary structures in computer-generated Cu and Ni three-dimensional nanocrystalline samples. The study includes both totally random and textured microstructures with grain sizes in the range of 5-12 nm. A detailed direct visualization technique is used at the atomic scale for studying the grain-boundary structural features. The study focuses on determining the presence of regions in the boundary exhibiting order and structural units normally expected for high-angle boundaries. For low-angle boundaries we investigate the presence of dislocation networks accommodating the misfit between the grains. A significant degree of crystalline order is found for all the boundaries studied. The highest degree of structural order was identified for boundaries with misfits within about 10 degrees deviation from the perfect twin. These grain boundaries contain a repeated building structure consisting of structural units typical of a Sigma = 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 regular pattern. The cases studied include grain boundaries with a high-index common axis and show structural coherency that is independent of the grain size. Similar results are obtained for textured samples containing only low-angle grain boundaries, where regular dislocation arrays that are typical of larger grain materials are observed. These results provide evidence against the view of grain boundaries in nanocrystals as highly disordered, amorphous, or liquidlike interfaces;The results suggest that the grain-boundary structure in nanocrystalline materials is actually similar to that found in larger grain polycrystals.