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      Superhard nanocomposite of dense polymorphs of boron nitride: Noncarbon material has reached diamond hardness

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          A maximum in the strength of nanocrystalline copper.

          We used molecular dynamics simulations with system sizes up to 100 million atoms to simulate plastic deformation of nanocrystalline copper. By varying the grain size between 5 and 50 nanometers, we show that the flow stress and thus the strength exhibit a maximum at a grain size of 10 to 15 nanometers. This maximum is because of a shift in the microscopic deformation mechanism from dislocation-mediated plasticity in the coarse-grained material to grain boundary sliding in the nanocrystalline region. The simulations allow us to observe the mechanisms behind the grain-size dependence of the strength of polycrystalline metals.
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            Deformation-mechanism map for nanocrystalline metals by molecular-dynamics simulation.

            Molecular-dynamics simulations have recently been used to elucidate the transition with decreasing grain size from a dislocation-based to a grain-boundary-based deformation mechanism in nanocrystalline f.c.c. metals. This transition in the deformation mechanism results in a maximum yield strength at a grain size (the 'strongest size') that depends strongly on the stacking-fault energy, the elastic properties of the metal, and the magnitude of the applied stress. Here, by exploring the role of the stacking-fault energy in this crossover, we elucidate how the size of the extended dislocations nucleated from the grain boundaries affects the mechanical behaviour. Building on the fundamental physics of deformation as exposed by these simulations, we propose a two-dimensional stress-grain size deformation-mechanism map for the mechanical behaviour of nanocrystalline f.c.c. metals at low temperature. The map captures this transition in both the deformation mechanism and the related mechanical behaviour with decreasing grain size, as well as its dependence on the stacking-fault energy, the elastic properties of the material, and the applied stress level.
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              Materials: Ultrahard polycrystalline diamond from graphite.

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                Author and article information

                Journal
                Applied Physics Letters
                Appl. Phys. Lett.
                AIP Publishing
                0003-6951
                1077-3118
                March 05 2007
                March 05 2007
                : 90
                : 10
                : 101912
                Article
                10.1063/1.2711277
                6346fba1-41b4-4221-a2f6-a6224f092618
                © 2007
                History

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