Bond-Order Potentials for Molybdenum and Niobium: An Assessment of Their Quality

1998 ◽  
Vol 538 ◽  
Author(s):  
M. Mrovec ◽  
V. Vitek ◽  
D. Nguyen-Manh ◽  
D. G. Pettifor ◽  
L. G. Wang ◽  
...  
Keyword(s):  
Bond Order ◽  
Tight Binding ◽  
Direct Calculation ◽  
Real Space ◽  
Central Force ◽  
Full Potential ◽  
Extended Defects ◽  
Many Body ◽  
Deformation Path ◽  
Bond Order Potentials

AbstractThe bond-order potentials (BOP) have been constructed for Mo and Nb. These potentials are based on the real-space parametrized tight-binding method in which diagonalization of the Hamiltonian is avoided by direct calculation of the bond-order. In this scheme the energy consists of three parts: The bond part that comprises contributions of d electrons and introduces into the scheme the covalent character of bonding, the central-force many-body part that reflects the environmental dependence of sp overlap repulsion and a pair-wise contribution. The potentials were tested by calculation of energy differences between the bcc and several alternate structures and by investigating the trigonal deformation path. These calculations have been made in parallel using BOP and the full-potential linearized augmented plane-wave method. The central-force many-body Finnis-Sinclair type potentials have also been included into the study of the deformation path. This evaluation of BOP reveals that the potentials reproduce very closely the ab initio results and are, therefore, very suitable for atomistic studies of extended defects in the transition metals.

scholarly journals A Combined Ab Initio and Bond-Order Potentials Study of Cohesion in Iridium

2003 ◽  
Vol 779 ◽  
Author(s):  
Marc J. Cawkwell ◽  
Duc Nguyen-Manh ◽  
Vaclav Vitek ◽  
David G. Pettifor
Keyword(s):  
Ab Initio ◽  
Bond Order ◽  
Tight Binding ◽  
Full Potential ◽  
Extended Defects ◽  
Cauchy Pressure ◽  
Deformation And Fracture ◽  
Phonon Spectra ◽  
Resistance To Oxidation ◽  
Bond Order Potential

AbstractThe 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 [ΆΆ0] 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 nonequilibrium 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.

scholarly journals A Bond-Order Potential Incorporating Analytic Screening Functions for the Molybdenum Silicides

2004 ◽  
Vol 842 ◽  
Author(s):  
Marc J. Cawkwell ◽  
Matous Mrovec ◽  
Duc Nguyen-Manh ◽  
David G. Pettifor ◽  
Vaclav Vitek
Keyword(s):  
Crystal Structure ◽  
High Temperatures ◽  
Bond Order ◽  
Tight Binding ◽  
Dislocation Core ◽  
Real Space ◽  
Internal Degree ◽  
Extended Defects ◽  
Ambient Conditions ◽  
Bond Order Potential

ABSTRACTThe 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 C11b MoSi2 and we have carefully assessed its transferability to other crystal structures and stoichiometries, notably C 40, C 49 and C 54 MoSi2, A15 and D03 Mo3Si and D8m Mo5Si3 by comparing with ab initio structural optimizations.

Study of the Mechanical Behavior of BCC Transition Metals Using Bond-Order Potentials

1999 ◽  
Vol 578 ◽  
Author(s):  
M. Mrovec ◽  
V. Vitek ◽  
D. Nguyen-Manh ◽  
D. G. Pettifor ◽  
L. G. Wang ◽  
...  
Keyword(s):  
Transition Metals ◽  
Bond Order ◽  
Tight Binding ◽  
Dislocation Core ◽  
Body Centered Cubic ◽  
Alternative Structures ◽  
The Core ◽  
Deformation Properties ◽  
Directional Bonding ◽  
Bond Order Potentials

AbstractDeformation properties of body-centered-cubic transition metals are controlled by the core structure of screw dislocations and their studies involve extensive computer simulations. In this paper we present the recently constructed bond-order potentials (BOP) that are based on the realspace parametrized tight-binding method. In order to examine the applicability of the potentials we have evaluated the energy differences of alternative structures, investigated several transformation paths leading to large distortions and calculated phonon dispersions. Using these potentials we have calculated γ-surfaces that relate to the dislocation core structures and discuss then the importance of directional bonding in studies of dislocations in transition metals.

Three-Body Correlation in the Diluted Generalized Hubbard Model

1997 ◽  
Vol 491 ◽  
Author(s):  
O. Navarro ◽  
M. Avignon
Keyword(s):  
Nearest Neighbor ◽  
Dimensional Space ◽  
Linear Chain ◽  
Tight Binding ◽  
Real Space ◽  
Many Body ◽  
Higher Dimensional ◽  
Generalized Hubbard Model ◽  
Three Body ◽  
Body Correlation

ABSTRACTA real-space method has been used to solve the generalized Hubbard Hamiltonian for a system with few electrons. The method is based on mapping the correlated many-body problem onto an equivalent tight-binding one in a higher dimensional space. For a linear chain, we have obtained an exact solution of the problem of three non-parallel electrons. The three-body correlation are studied by examining the binding energy in the ground state, for different values of the hopping parameters and of the on-site (U) and nearest-neighbor (V) interactions.

Transition Metal Impurity-Dislocation Interactions in NiAl: Dislocation Friction and Dislocation Locking

2000 ◽  
Vol 646 ◽  
Author(s):  
O. Yu. Kontsevoi ◽  
Yu.N. Gornostyrev ◽  
A.J. Freeman
Keyword(s):  
Transition Metal ◽  
Central Atom ◽  
Tight Binding ◽  
Electronic States ◽  
Dislocation Core ◽  
Real Space ◽  
Solid Solution Hardening ◽  
Full Potential ◽  
Recursion Method ◽  
Transition Metal Impurity

ABSTRACTThe energetics of the interaction of the <100>{010} edge dislocation in NiAl with early 3d transition metal (TM) impurities was studied using the ab initio real-space tight-binding LMTO-recursion method with 20,000 atom clusters and up to 1,000 non-equivalent atoms in the dislocation core. The coordinates of the atoms in the core were determined within the Peierls-Nabarro (PN) model with restoring forces determined from full-potential LMTO total energy calculations. TM impurities were then placed in different substitutional positions near the dislocation core. For most positions studied, the interaction between impurities and the dislocation is found to be repulsive (dislocation friction). However, when the impurity is in the position close to the central atom of the dislocation core, the interaction becomes strongly attractive, thus causing dislocation locking. Since the size misfit between the Al atom and the substituting TM atom is very small, this locking cannot be explained by elastic (or size misfit) mechanisms; it has an electronic nature and is caused by the formation of the preferred bonding between the electronic states of the impurity atom and the localized electronic states appearing on the central atom of the dislocation core. The calculated results are then discussed in the scope of experimental data on solid solution hardening in NiAl.

Surface Relaxations of Aluminum Simulated by Bond Order Potentials

1999 ◽  
Vol 578 ◽  
Author(s):  
S. R. Nishitani ◽  
S. Ohgushi ◽  
H. Adachi ◽  
M. Aoki
Keyword(s):  
Bond Order ◽  
Interatomic Potential ◽  
Phonon Dispersion ◽  
Tight Binding ◽  
Dispersion Curves ◽  
Experimental Results ◽  
Phonon Dispersion Curves ◽  
Bond Order Potentials

AbstractAn interatomic potential for aluminum was developed, which is based on empirical tight binding approximations. The model successfully reproduced the shear constants, structure energy differences, and phonon dispersion curves. This transferable potential was applied on static surface relaxations, and shows good agreements with experimental results on the oscillatory damped behavior of the multilayer relaxations and the expansion of the (111) surface.

Development of Bond-Order Potentials for BCC Transition Metals

2016 ◽  
Vol 258 ◽  
pp. 3-10
Author(s):  
Vaclav Vitek ◽  
Yi Shen Lin ◽  
Matous Mrovec
Keyword(s):  
Bond Order ◽  
Phonon Dispersion ◽  
Tight Binding ◽  
Screw Dislocations ◽  
Compressive Stresses ◽  
Bcc Iron ◽  
Tight Binding Method ◽  
Formation Energies ◽  
Phonon Dispersion Relations ◽  
Bond Order Potentials

In this paper we present bond-order potentials (BOPs) based on the tight-binding method. The potentials have been developed for bcc non-magnetic metals of group V.B (V, Nb, Ta) and group VI.B (Cr, Mo, W) as well as for the ferromagnetic bcc iron. The testing of the transferability of BOPs involves energies of alternate structures, formation energies of vacancies and self-interstitials, transformation paths between different structures and phonon dispersion relations. An example of the application of these potentials is modeling of the structure and glide of 1⁄2<111> screw dislocations under the effect of applied shear and tensile/compressive stresses.

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