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Browsing by Author "Van Swygenhoven, H."

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    Grain-boundary structures in polycrystalline metals at the nanoscale
    Van Swygenhoven, H.; Farkas, Diana; Caro, Alfredo (American Physical Society, 2000-07-01)
    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.
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    Intergranular fracture in nanocrystalline metals
    Farkas, Diana; Van Swygenhoven, H.; Derlet, P. M. (American Physical Society, 2002-08-01)
    Crack propagation studies in nanocrystalline Ni samples with mean grain sizes ranging from 5 to 12 nm are reported using atomistic simulations. For all grain sizes pure intergranular fracture is observed. Intergranular fracture is shown to proceed by the coalescence of microvoids formed at the grain boundaries ahead of the crack. The energy released during propagation is higher than the Griffith value, indicating an additional grain-boundary accommodation mechanism.