Browsing by Author "Caro, Alfredo"

Now showing 1 - 4 of 4
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    Annealing twins in nanocrystalline fcc metals: A molecular dynamics simulation
    Farkas, Diana; Bringa, Eduardo M.; Caro, Alfredo (American Physical Society, 2007-05-23)
    We report fully three-dimensional atomistic molecular dynamics studies of grain growth kinetics in nanocrystalline Cu of 5 nm average grain size. We observe the formation of annealing twins as part of the grain growth process. The grain size and energy evolution was monitored as a function of time for various temperatures, yielding an activation energy for the process. The atomistic mechanism of annealing twin formation from the moving boundaries is described.
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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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    Implications of ab initio energetics on the thermodynamics of Fe-Cr alloys
    Caro, Alfredo; Caro, M.; Lopasso, E. M.; Crowson, D. A. (AIP Publishing, 2006-09-01)
    The authors analyze the implications of the recently reported results of ab initio calculations of formation energies of the Fe-Cr alloy. The formation energies show a change in sign from negative to positive as Cr composition increases above similar to 10%. By developing a classic potential to evaluate the thermodynamic properties, they determine the location of the solubility limit and compare it with earlier results. A significant difference appears in a region of temperature and composition that is relevant for the nuclear applications of this alloy. Experimental results seem to confirm the validity of the location of the new solvus line. (c) 2006 American Institute of Physics.
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    Sputtering from a porous material by penetrating ions
    Rodriguez-Nieva, J. F.; Bringa, Eduardo M.; Cassidy, T. A.; Johnson, R. E.; Caro, Alfredo; Fama, M.; Loeffler, M. J.; Baragiola, R. A.; Farkas, Diana (IOP Publishing Ltd., 2011-12-01)
    Porous materials are ubiquitous in the universe and weathering of porous surfaces plays an important role in the evolution of planetary and interstellar materials. Sputtering of porous solids in particular can influence atmosphere formation, surface reflectivity, and the production of the ambient gas around materials in space. Several previous studies and models have shown a large reduction in the sputtering of a porous solid compared to the sputtering of the non-porous solid. Using molecular dynamics simulations we study the sputtering of a nanoporous solid with 55% of the solid density. We calculate the electronic sputtering induced by a fast, penetrating ion, using a thermal spike representation of the deposited energy. We find that sputtering for this porous solid is, surprisingly, the same as that for a full-density solid, even though the sticking coefficient is high.