A detailed analysis was made of grain-boundary structures in computer-generated 3-dimensional nanocrystalline samples. It included both totally random and textured microstructures having grain sizes of 5 to 12nm. A detailed direct visualization technique was used at the atomic scale for the study of grain-boundary structural features. The study focussed upon determining the presence of regions in the boundary which exhibited order, and the structural units which were normally expected in high-angle boundaries. The presence of dislocation networks which accommodated misfit between the grains was investigated for the case of low-angle boundaries. A significant degree of crystalline order was found in all of the boundaries which were studied. The highest degree of structural order was found for boundaries with misfits which were within about 10º of a perfect twin. These grain boundaries contained a repeated structure which consisted of structural units that were typical of a Σ = 3 symmetrical tilt twin boundary. There were also highly disordered steps between the structural units. For all other types of misfit, some degree of structural coherence was observed, and misfit accommodation occurred in a regular pattern. The cases which were studied included grain boundaries which had a high-index common axis, and exhibited a structural coherency that was independent of the grain size. Similar results were obtained for textured samples which contained only low-angle grain boundaries; where regular dislocation arrays that were typical of larger-grained materials were observed. The results provided evidence that was contrary to the view that grain boundaries in nanocrystals were highly-disordered, amorphous or liquid-like interfaces. The results suggested instead that the grain-boundary structure in nanocrystalline materials was similar to that found in larger-grained polycrystals.

Grain-Boundary Structures in Polycrystalline Metals at the Nanoscale. H.Van Swygenhoven, D.Farkas, A.Caro: Physical Review B, 2000, 62[2], 831-8