Vitek, Vaclav
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Publication Explanation of the discrepancy between the measured and atomistically calculated yield stresses in body-centered cubic metals(2007-02-01) Vitek, Vaclav; Groger, RomanWe propose a mesoscopic model that explains the factor of two to three discrepancy between experimentally measured yield stresses of BCC metals at low temperatures and typical Peierls stresses determined by atomistic simulations of isolated screw dislocations. The model involves a Frank-Read type source emitting dislocations that become pure screws at a certain distance from the source and, owing to their high Peierls stress, control its operation. However, due to the mutual interaction between emitted dislocations the group consisting of both non-screw and screw dislocations can move at an applied stress that is about a factor of two to three lower than the stress needed for the glide of individual screw dislocations.Publication Construction, assessment, and application of a bond-order potential for iridium(2006-02-08) Cawkwell, Marc J; Nguyen-Manh, Duc; Pettifor, David G; Vitek, VaclavA tight-binding based bond-order potential (BOP) has been constructed for the fcc transition metal iridium that includes explicitly only d orbitals in the evaluation of the total energy. We show that hybridization between the nearly free electron sp band and the unsaturated covalently bonded d orbitals is important in determining the relative stabilities of the close-packed structures and that this effect can be accurately captured through the use of a central force term. The BOP is found to provide an excellent description of the equilibrium properties of iridium, including its negative Cauchy pressure that is fitted using a many-body repulsive term. The transferability of the BOP is assessed by calculating energy differences between different crystal structures, the energetics of the tetragonal and trigonal deformation paths, the phonon spectra, stacking fault, and vacancy formation energies. Comparison of the results of these studies with either experiments or first principles calculations is found to be good. We also describe briefly the application of the constructed BOP to the atomistic simulation of the core structure of the screw dislocation that led to an explanation of the anomalous deformation and fracture behavior exhibited of iridium.Publication Bond-order potential for simulations of extended defects in tungsten(2007-03-30) Mrovec, M.; Gröger, Roman; Bailey, A. G; Nguyen-Manh, D.; Elsässer, C.; Vitek, VaclavWe present a bond-order potential (BOP) for the bcc transition metal tungsten. The bond-order potentials are a real-space semiempirical scheme for the description of interatomic interactions based on the tight-binding approximation. In the hierarchy of atomic-scale-modeling methods the BOPs thus provide a direct bridge between electronic-structure and atomistic techniques. Two variants of the BOP were constructed and extensively tested against accurate first-principles methods in order to assess the potentials' reliability and applicability. A comparison of the BOP with a central-force potential is used to demonstrate that a correct description of directional mixed covalent and metallic bonds is crucial for a successful and fully transferable model. The potentials are applied in studies of low-index surfaces, symmetrical tilt grain boundaries, and dislocations.