Vitek, Vaclav
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Publication Strain-Rate Dependence of the Brittle-to-Ductile Transition Temperature in TiAl(2000-11-27) Khantha, Mahadevan; Vitek, Vaclav; Pope, David PThe brittle-to-ductile transition (BDT) and the strain-rate dependence of the brittle-to-ductile transition temperature (BDTT) have been recently investigated in single crystals of TiAl [1]. It was found that the activation energy associated with the BDTT is 1.4 eV when the slip is dominated by ordinary dislocations and 4.9 eV when it is dominated by superdislocations. Despite this difference in the activation energies, the BDTT, while varying with the strain-rate, remains in the same temperature range, viz., between 516-750C and 635-685C for ordinary and superdislocations, respectively. In this paper, we examine how the activation energy of the BDTT can vary with the type of dislocation activity and explain why it can attain values which are clearly much larger than the activation energy for dislocation motion. We describe a strain-rate dependent mechanism of cooperative dislocation generation in loaded solids above a critical temperature and use it to explain the characteristics of the BDT in TiAl. We show that the activation energy associated with the BDTT is a composite value determined by two or more inter-dependent thermally activated processes and its magnitude can be much larger than the activation energy for dislocation motion in certain materials. The predictions of the model are in good agreement with observations in TiAl.Publication A Combined Ab Initio and Bond-Order Potentials Study of Cohesion in Iridium(2003-04-23) Cawkwell, Marc J; Nguyen-Manh, Duc; Vitek, Vaclav; Pettifor, David GThe extremely high melting point and excellent resistance to oxidation and corrosion offered by iridium suggest numerous applications of this transition metal in static components at high temperatures and in aggressive environments. However, the mechanical and physical properties of f.c.c. Ir exhibit numerous anomalies when compared to other metals that crystallize in the f.c.c. structure. Notable examples include a negative Cauchy pressure, 1/2 (C12 – C44), brittle transgranular cleavage after a period of plastic flow even in pure single crystals and anomalous [zeta zeta 0] branches in the phonon spectra. Atomistic studies of extended defects are needed to elucidate the origin of anomalous mechanical properties, such as brittleness. For this purpose we developed a Bond-Order Potential (BOP), an O(N) tight-binding formalism, employing physically transparent parameterizations that use experimental and ab initio data, generated in this study using the Full Potential Augmented Plane Wave plus Local Orbitals (APW+lo) method. The constructed BOP reproduces then both equilibrium as well as a variety of non-equilibrium properties of Ir and represents an excellent description of cohesion in f.c.c. Ir. This description of interatomic interactions is imminently suitable for studies of defects, such as dislocations and grain boundaries, that control plastic deformation and fracture.Publication Ab initio simulation of a tensile test in MoSi2 and WSi2(2000-11-27) Friák, M.; Šob, M.; Vitek, VaclavThe tensile test in transition metal disilicides with C11b structure is simulated by ab initio electronic structure calculations using full potential linearized augmented plane wave method (FLAPW). Full relaxation of both external and internal parameters is performed. The theoretical tensile strength of MoSi2 and WSi2 for [001] loading is determined and compared with those of other materials.Publication A Bond-Order Potential Incorporating Analytic Screening Functions for the Molybdenum Silicides(2004-11-29) Cawkwell, Marc J; Mrovec, Matous; Nguyen-Manh, Duc; Pettifor, David G; Vitek, VaclavThe intermetallic compound MoSi2, which adopts the C11b crystal structure, and related alloys exhibit an excellent corrosion resistance at high temperatures but tend to be brittle at room and even relatively high temperatures. The limited ductility of MoSi2 in ambient conditions along with the anomalous temperature dependence of the critical resolved shear stress (CRSS) of the {110)<111], {011)<100] and {010)<100] slip systems and departure from Schmid law behavior of the {013)<331] slip system can all be attributed to complex dislocation core structures. We have therefore developed a Bond-Order Potential (BOP) for MoSi2 for use in the atomistic simulation of dislocations and other extended defects. BOPs are a real-space, O(N), two-center orthogonal tight-binding formalism that are naturally able to describe systems with mixed metallic and covalent bonding. In this development novel analytic screening functions have been adopted to properly describe the environmental dependence of bond integrals in the open, bcc-based C11b crystal structure. A many-body repulsive term is included in the model that allows us to fit the elastic constants and negative Cauchy pressures of MoSi2. Due to the internal degree of freedom in the position of the Si atoms in the C11b structure which is a function of volume, it was necessary to adopt a self-consistent procedure in the fitting of the BOP. The constructed BOP is found to be an excellent description of cohesion in C11bMoSi2 and we have carefully assessed its transferability to other crystal structures and stoichiometries, notably C40, C49 and C54 MoSi2, A15 and D03 Mo3Si and D8m Mo5Si3 by comparing with ab initio structural optimizations.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 Atomistic Studies of Dislocation Glide in gamma-TiAl(2002-12-02) Porizek, Radoslav; Znam, Stefan; Vitek, Vaclav; Nguyen-Manh, Duc; Pettifor, David G.Computer simulation of the core structure and glide of ordinary 1/2<110] dislocations and <101] superdislocations in L10 TiAl has been performed using the recently constructed Bond-Order Potentials. This description of atomic interactions includes explicitly, within the tightbinding approximation, the most important aspects of the directional bonding, namely d-d, p-p and d-p bonds. The ordinary dislocation in the screw orientation was found to have a non-planar core and, therefore, high Peierls stress. The superdislocation was found to possess in the screw orientation either a planar (glissile) or a non-planar (sessile) core structure. However, the glissile core transforms into the sessile one for certain orientations of the applied stress. This implies a strong asymmetry of the yield stress and the break down of the Schmid law when the plastic flow is mediated by superdislocations. At the same time, this may explain the orientation dependence of the dislocation substructure observed in the single-phase gamma-TiAl by electron microscopy.Publication Bond-Order Potentials with Analytic Environment-Dependent Tight-Binding Integrals: Application to BCC Molybdenum(2000-11-27) Mrovec, Matous; Nguyen-Manh, Duc; Pettifor, David G.; Vitek, VaclavWe present a new Screened Bond-Order Potential (SBOP) for molybdenum in which the environmental dependence of two-center tight-binding bond integrals has been implemented via a recently developed analytic expression. These bond integrals reproduce very well the numerical ab-intio values of screened LMTO bond integrals. In particular, they display the large discontinuity in ddpi between the first and second nearest neighbor of the bcc lattice whereas they do not show any discontinuity in ddsigma. This dependence can be traced directly to the angular character of the analytic screening function and is shown to be critical for the behavior of the second nearest neighbor force constants. The new BOP eliminates the problem of the very soft T2 phonon mode at the N point that is found in most two-center tight-binding models. Preliminary study of the core structure of 1/2<111> screw dislocations performed using SBOP indicates that the core is narrower and less asymmetric than structures found in previous studies, in agreement with recent ab-initio calculations.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 Dislocation/Twin/Interface Interactions during Deformation of PST TiAl Single Crystals, an AFM Study(2002-12-02) Chen, Yali; Pope, David P; Vitek, VaclavPST TiAl crystals oriented such that the deformation axis lies in the (111) interfacial planes have been deformed in compression. This deformation produces so-called "channeled flow" in which the strain perpendicular to the (111) interfaces is zero, while the other two strains are equal and opposite in sign. Thus the sample simply shortens axially and spreads laterally in the channels defined by the (111) interfacial planes. We have examined the fine structure of deformation bands on the free surface of these deformed samples using AFM to see how the deformation processes interact with the boundaries. By measuring the offset angle at the surface we have been able to show that not only is the macroscopic displacement vector parallel to the lamellar boundaries, but the total shear vector in each layer is also parallel to the lamellar boundaries. However these deformation bands have very different characters, requiring complex deformation processes at the boundaries in order to satisfy this requirement. Some consist of either just super dislocations or just ordinary dislocations with Burgers vectors lying in the interface. But others consist of a special combination of twinning and ordinary dislocations in fixed ratio, such that the net shear vector also lies in the boundary, even though the individual twinning and dislocation shear directions are inclined to it. This results in deformation that is homogeneous and completely 'channeled' inside each lamella with no shear vector perpendicular to the lamellar boundaries. We have also shown that the cooperative twinning and slip is homogeneous on the nano-scale, i.e., the twinning and slip occurs in the same volume of material.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.