Vitek, Vaclav
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Publication Atomic Structure of a Grain Boundary in a Metallic Alloy: Combined Electron Microscope and Theoretical Study(1991-09-30) Luzzi, David E; Yan, Min; Vitek, Vaclav; Å ob, MojmÃrA synergistic high-resolution electron microscopy (HREM) and theoretical analysis of the structure of a grain boundary in copper containing bismuth is presented. The calculation of the structure of the boundary were carried out using N-body empirical potentials constructed using results of ab initio full-potential linear-muffin-tin-orbital calculations. Excellent agreement between the calculated and observed structures is shown by comparing a through-focal series of observed and calculated images. It is shown for the first time that HREM combined with computer modeling employing realistic empirical potentials can decipher with a great accuracy the structure of boundaries containing multiple atomic species.Publication Radial Distribution Function and Structural Relaxation in Amorphous Solids(1981-12-15) Srolovitz, David J; Vitek, Vaclav; Egami, TakeshiA method of interpreting radial distribution functions (RDF) of amorphous metals is proposed in which the role of the local atomic structure is emphasized. It is found that the width and height of the peaks of the RDF are related to the second moment of the atomic-level hydrostatic stress distribution ⟨p2⟩. The results of this analysis are then used to explain the details of the changes that occur in the RDF when structural relaxation takes place. The theoretical â–µRDF is found to be in excellent agreement with the results of a computer study and previous experimental results. It is further proposed that changes in ⟨p2⟩ may be most easily accounted for in terms of changes in the density of the structural defects defined in terms of the local fluctuations in the hydrostatic stress. In this way the changes that occur in the structure of amorphous metal during structural relaxation, as represented by the RDF, may be explained in terms of the motion and annihilation of these structural defects. It is concluded that the number density of defects which could account for the observed changes in the experimental RDF is 10%. It is also found that while the hydrostatic stress distribution may be significantly changed during structural relaxation, the distribution of the atomic-level shear stresses remains unaltered.Publication Ab Initio Study of the Ideal Tensile Strength and Mechanical Stability of Transition-Metal Disilicides(2003-11-01) Friák, Martin; Å ob, MojmÃr; Vitek, VaclavThe ideal tensile test in transition metal disilicides MoSi2 and WSi2 with a C11b structure is simulated by ab initio electronic structure calculations using the full-potential linearized augmented plane wave method. The theoretical tensile strength for [001] loading is determined for both disilicides and compared with that of other materials. A full relaxation of all external and one internal structural parameter is performed, and the influence of each relaxation process on energetics and strength of materials studied is investigated. Differences in the behavior of various interatomic bonds including tension-compression asymmetry are analyzed and their origin in connection with the changes of the internal structural parameter is traced. For comparison, the response of bonds in MoSi and CoSi with B2 structure to the [001] loading is also studied.Publication Ab Initio Calculation of Phase Boundaries in Iron Along the bcc-fcc Transformation Path and Magnetism of Iron Overlayers(2001-02-01) Friák, Martin; Å ob, MojmÃr; Vitek, VaclavA detailed theoretical study of magnetic behavior of iron along the bcc-fcc (Bain’s) transformation paths at various atomic volumes, using both the local spin-density approximation (LSDA) and the generalized gradient approximation (GGA), is presented. The total energies are calculated by the spin-polarized full-potential linearized augmented plane waves method and are displayed in contour plots as functions of tetragonal distortionc/aand volume; borderlines between various magnetic phases are shown. Stability of tetragonal magnetic phases of γ-Fe is discussed. The topology of phase boundaries between the ferromagnetic and antiferromagnetic phases is somewhat similar in LSDA and GGA; however, the LSDA fails to reproduce correctly the ferromagnetic bcc ground state and yields the ferromagnetic and antiferromagnetic tetragonal states at a too low volume. The calculated phase boundaries are used to predict the lattice parameters and magnetic states of iron overlayers on various (001) substrates.Publication Atomistic Studies of Deformation and Fracture in Materials with Mixed Metallic and Covalent Bonding(2007-03-09) Vitek, Vaclav; Cawkwell, Marc J; Gröger, RomanMaterials with high melting temperatures (over 2000°C) tend to be brittle at ambient and even relatively high temperatures. High melting temperatures originate in strong interatomic bonding arising from formation of dd or dp bonds that also affect and/or control crystal structures and properties of extended defects, such as dislocations, grain boundaries. These, in turn, govern plastic deformation and fracture. General goal: Establish relationship between electronic structure and mechanical behaviorPublication Prediction and Observation of the bcc Structure in Pure Copper at a Σ3 Grain Boundary(1995-09-11) Schmidt, C.; Ernst, F.; Finnis, M. W; Vitek, VaclavWe have used molecular dynamics and simulated annealing to study an asymmetrical Σ3 tilt grain boundary with ⟨211⟩ rotation axis in Cu. The boundary plane was inclined at 84° with respect to the {}(111) plane. A simple central force N-body interatomic potential was used. The most stable configuration shows a broad band of predominantly bcc structure in the boundary region. Samples of the bicrystal with the same misorientation and inclination of the boundary plane were observed in a 1250 kV transmission electron microscope, confirming the predicted structure with atomic resolution.Publication Analytic Environment-Dependent Tight-Binding Bond Integrals: Application to MoSi2(2000-11-06) Nguyen-Manh, Duc; Pettifor, David G; Vitek, VaclavWe present the first derivation of explicit analytic expressions for the environmental dependence of the σ, Ï€, and δ bond integrals within the orthogonal two-center tight-binding approximation by using the recently developed bond-order potential theory to invert the nonorthogonality matrix. We illustrate the power of this new formalism by showing that it not only captures the transferability of the bond integrals between elemental bcc Mo and Si and binary C11b MoSi2 but also predicts the absence of any discontinuity between first and second nearest neighbors for the ddσ bond integral even though large discontinuities exist for ppσ, ppÏ€, and ddÏ€.Publication Dislocation Screening and the Brittle-to-Ductile Transition: A Kosterlitz-Thouless Type Instability(1994-08-01) Khantha, Mahadevan; Pope, David P; Vitek, VaclavWe propose a new model for the brittle-to-ductile transition based on the Kosterlitz-Thouless concept of dislocation screening. In this model, thermal fluctuations assisted by the applied stress drive the spontaneous generation of dislocations and the instability occurs well below the melting temperature. In the limit of zero stress, our model reduces to the Kosterlitz-Thouless theory of the melting transition, and, in the opposite limit of zero temperature, we obtain the Rice-Thomson result for the brittle-to-ductile transition.Publication Local Fluctuations and Ordering in Liquid and Amorphous Metals(1988-02-15) Chen, S. P; Egami, T.; Vitek, VaclavA molecular-dynamics study of the structure and dynamics of monatomic liquids and glasses is presented. The local atomic structure and its development during the quenching process are analyzed in terms of fluctuations of atomic-level stresses and their correlations. This approach extends the basis for the description of the local structure from the usually employed scalar quantity, the local density fluctuation, to a tensorial quantity, the local stress fluctuation. It is shown here that the local stress fluctuations and their spatial and temporal correlations provide a detailed picture of the dynamics of the liquid and of the transition from an ideal fluid to a viscous liquid, and then to a glass. In particular, it is demonstrated that the shear stresses which are spatially uncorrelated at high temperatures become correlated below a temperature, Ts, which is about twice the glass transition temperature. At the same time the dynamic behavior of the liquid, characterized by the diffusivity, viscosity, and phonon states, changes sharply at this temperature. Implications of this apparent structural transition and its origin are then discussed.Publication Bond-Order Potential for Molybdenum: Application to Dislocation Behavior(2004-03-01) Mrovec, Matous; Nguyen-Manh, Duc; Pettifor, David G; Vitek, VaclavThe bond-order potential (BOP) for transition metals is a real-space semiempirical description of interactions between the atoms, which is based on the tight-binding approximation and the d-band model. This scheme provides a direct bridge between the electronic level modeling and the atomistic modeling, where the electronic degrees of freedom have been coarse grained into many-body interatomic potentials. In this paper we construct BOP in which both the attractive and the repulsive contributions to the binding energy are environmentally dependent due to both the nonorthogonality of the orbitals and the breathing of the screening charges. The construction of the BOP is described and tested in detail. First, the energies of alternative crystal structures (A15, fcc, hcp, simple cubic) are calculated and compared with those evaluated ab initio. The transferability of the BOP to atomic configurations that deviate significantly from the bcc lattice is studied by computing the energies along tetragonal, trigonal, and hexagonal transformation paths. Next, the phonon spectra are evaluated for several symmetrical crystallographic directions and compared with available experiments. All these calculations highlight the importance of directional bonding and the investigation of phonons demonstrates that the environmental dependence of the bond integrals is crucial for the phonons of the N branch not to be unphysically soft. Finally, the constructed BOP was applied in the modeling of the core structure and glide of the 1/2⟨111⟩ screw dislocation. The calculated structure of the core agrees excellently with that found in the recent ab initio calculations and the observed glide behavior not only agrees with available ab initio data but is in agreement with many experimental observations and explains the primary reason for the breakdown of the Schmid law in bcc metals.