A new method coupling electronic structure calculations with Monte Carlo simulations has been developed to determine surface compositions in Cu-Ni alloys. The calculations are based on an effective Ising model with parameters as defined within the framework of the Generalized Perturbation Method (GPM) and as calculated by means of the tight-binding version of the linear muffin-tin orbital method. The composition profiles are obtained for the fcc(OOl) surface for three bulk compositions, namely Cu75Ni25, Cu50Ni50, and Cu25Ni75 and compared with available experimental data.

We extend the method of Bozzolo, Ferrante and Smith (BFS) for the study of alloy energetics to include a description of the local environment in specific ordered structures. The concept of bond-diagrams is introduced and applied to fcc binary compounds. A simple example of the parameterization of the bond-diagrams is done with reference to available first-principles calculations of Ni-Pt ordered alloys.

We present a study of the hexagonal close-packed (hcp) Ising model for binary alloys within the cluster variation approximation. Groundstates of order stabilized by nearest-neighbor (NN) pair, triplet, and tetrahedron interactions were determined using the cluster configuration polyhedron method; no previous hcp ground-state study has considered all of these interactions. We predict physically realizable groundstates with stoichiometries A, AB (3 distinct structures), A2B, A3B, and A4B3. The previously unpredicted A4B3 structure is stabilized by multiatom (i.e. triplet, tetrahedron) interactions, while the others are stabilized by the two NN pair interactions. The Cluster Variation Method (CVM) was used to calculate the finite-temperature phase-equilibria for prototypical binary alloys. We present the first ordering phase diagrams computed with the CVM which contain all relevant groundstates for both isotropic and anisotropic NN pair interactions.

An explanation of the occurrence of unphysical solutions in the cluster variational method is given. A simple algorithm for the construction of an optimal maximal cluster, i.e., a cluster that guarantees correct results for any set of interatomic interactions, is suggested. Examples of optimal maximal clusters for various two- and three-dimensional lattices are presented.

Phase stability properties of bcc based Fe-Cr alloys are examined in the framework of the first-principles KKR-CPA-GPM formalism and Monte Carlo simulations. For Fe-rich alloys, ordered configurations are found stable with respect to the random state of the alloy, but unstable with respect to the pure Fe and Cr metals. The results are compared with the ones obtained by using energy parameters extracted from experimental diffuse scattering data.

Bi1-xSbx alloys undergo a semimetal-semiconductor transition at x = 0.065 and 0.22 and are semiconducting between these limits. We investigate this system by means of femtosecond time-resolved experiments, and observe an optical response similar to those in Bi, Sb, Te and Ti2O3. A previously-reported model describes the mechanism of generation of coherent lattice vibrations via the optically excited carriers in this class of materials. As predicted by the model, the modulated reflectivity signal ΔR/R in Bi.88Sb.12 shows a beating oscillatory behavior with the Bi-Bi and Bi-Sb A1g vibrational frequencies. Model-based calculations of the ΔR background relaxation time at different Sb compositions provide understanding of the carrier and lattice dynamics throughout the transition.

We have modeled the Ni-P amorphous alloy system by a set of supercells on which we have carried out calculations of electronic and transport properties. The thermopower is determined by calculating the energy dependence of the dc electrical conductivity from the Kubo formalism. At 15% P we find -0.015 μV/K2 for the thermopower divided by the temperature, while for 25% P the calculated value is +0.008 μV/K2, in reasonable agreement with experiment, particularly with regard to the change of sign with concentration.

Ag activities in the Ag-Au-Ge solution were determined by the mass spectrometric analysis of effusates from a unique valved Knudsen cell at 1416 K. A mathematical model was established for Ag activity coefficient as a function of alloy composition. Activities of Au and Ge were calculated by numerical ternary Gibbs-Duhem integration on a spreadsheet. The data obtained from this investigation show that the activities on all three binary systems occurring in the ternary system are in good agreement with accepted literature values. The behavior of all pseudo binaries was found to be consistent with Darken's quadratic formalism. Darken surface of Ag is shown.

A cluster expansion for energetics is combined with a direct, real-space method of studying the electronic structure of ordered and disordered ternary intermetallics. The electronic structure calculations are based on an explicit averaging of local quantities over a small number of randomly chosen configurations. Quantities such as densities of states, one-electron energies, etc., are computed within the framework of the first-principles tight-binding linear muffin-tin orbital method (TB-LMTO). Effective pair interactions, which describe the ordering tendencies of the alloy, are computed for the full ternary alloy. With this technique, then, the effects on ordering trends of ternary additions to a binary alloy may be obtained. Results for Ag-Pd-Rh and Ni-Al-Cu are shown. The self-consistency of these calculations is checked against the fully self-consistent ordered LMTO calculations.

Electronic structure and stability properties of ternary-metal alloys are examined using an extension of the Coherent Potential Approximation -Generalized Perturbation Method approach within the Tight-Binding description of the chemically random alloy. In particular, we report on calculations of density of states, mixing energies and effective cluster interactions which build up the ordering energy. The study focuses on the Ti-V-Fe system and its binary components.

A theoretical study of the interaction between fibers and a matrix is performed, which is based on several approaches that are connected with the calculation of mixing energy in multicomponent alloys or with the calculation of the diffusion coefficients in such systems. As an example, interatomic potentials for the alloys (Al,Si)-B, (Al,Mg)-B, (Al,Cr)-B, (Al,Mn)-B and (Al,Ga.)-B are calculated. The values of the potential barrier Δu for binary alloys are compared with those in ternary systems. It is shown that alloying of a binary alloy by Si reduces the height of potential in comparison with the binary alloy. Other elements make this barrier higher.

We propose a dynamical system theory of triaxial-test deformationS and localization bifurcation in brittle media. We apply it to predict that localization may occur in a packing looser than “critical” and that the general localization shape is a spiral staircase in axisymmetric 3-D cells. These two facts have recently been confirmed experimentally. This theory provides a framework for understanding the development of complex deformation patterns from the mechanics of localization and rupture.

The vacancy formation thermodynamics in six fcc metals Ag, Au, Cu, Ni, Pd and Pt are determined from atomistic simulations as a function of temperature. This investigation is performed using the Embedded Atom Method interatomic potentials and the finite temperature properties are determined within the local harmonic and the quasiharmonic frameworks. We find that the temperature dependence of the vacancy formation energy can make a significant contribution to the vacancy concentration at high temperatures. An additional goal of the present study is to evaluate the accuracy of the local harmonic method under circumstances in which the excess entropy associated with the formation of a defect is very small. Our data demonstrate that while the errors associated with determining the vacancy formation entropy in the local harmonic model are large, a simple extension to the local harmonic method yields thermodynamic properties comparable to that obtained in the quasiharmonic model, but with much higher computational efficiency.

It is shown that in addition to Frenkel’s and Schottky’s defects there is another type of equilibrium defects, such as an unstable pairs (UP) : vacancy and interstitial atom. UP has the life time of the order of 10-1010-12s, independent of temperature. The review of the unstable pairs physics is presented. Numerous experimental evidences of equilibrium UP in semiconductors, insulators, especially in superionic crystals, and metals are discussed.

A simulation procedure based on the spring network model has been developed for studying the fracture behavior in brittle, polycrystalline solids. In this 2-D model, the effect of crystalline symmetry is accounted for by imparting to individual bonds a constitutive relationship using the material compliance matrix. Using this model, it was found that the fracture morphology becomes more intragranular in nature with increasing modulus anisotropy. Careful analysis suggests that this is due to the wider stress distributions, and the resulting larger number of cracks generated in the interior of anisotropic grains.

By considering a drained cohesionless granular sample made up of rigid grains and submitted to a triaxial test, we derive an equation relating the dilatancy K, the deviatoric stress q and the confining pressure p to the energy losses D plasticdue to plastic yielding. We demonstrate that the system is contracting (K ≤ 0) at q=0, when q is increasing and that spontaneous uncontrolled yielding begins occurring when dilatancy K is maximum. We also demonstrate the existence of the characteristic state introduced by Luong and Habib and the existence of the critical state of Schofield and Wroth. We give at last a method to determine the plastic losses during a triaxial cell test using the experimental data.

The structure and elastic properties of Ni/Cu and Ni/Au multilayer systems are investigated as a function of the number of Ni monolayers built into the systems. We employed lattice statics simulations with the interatomic potentials described by the embedded-atom method. For the Ni/Cu systems, coherent interfaces and FCC structure are maintained, and no elastic anomaly is found. For the Ni/Au systems, when the Ni layers are thick enough, they undergo a strain-induced phase transformation from FCC to HCP structure. An enhancement of Young’s modulus of these systems is found to be associated with this structural change.

Lattice vibrations and atomic level elastic moduli have been studied for a bicrystal containing a fully relaxed symmetrical tilt boundary in Au. Central force many body potentials have been employed to describe atomic interactions. In the long-wavelength limit the phonons localized at grain boundaries can be identified with Stoneley waves known from continuum analyses. These waves are localized at the grain boundary and their velocity agrees well with that evaluated using the local effective elastic moduli of the interfacial region. However, the usually used continuum model assuming an ideal match across the interface is not sufficient to analyze these waves fully and an explicit description of interfacial properties need to be included into the continuum models of interfaces.

Most atomistic studies of grain boundaries have been carried out using central forces to describe atomic interactions. However, in transition metals with unfilled d-bands the angular dependence of interatomic forces may be important. The purpose of this paper is to investigate the significance of angular forces in the case of Tungsten. The calculations have been performed for the Σ5(210) symmetrical tilt grain boundary using two alternate approaches. First are the central-force many-body potentials of the Finnis-Sinclair type. The second are the angular dependent potentials obtained via a moment analysis of the electronic density of states. The results of these two approaches are compared by analyzing the boundary structures, the relative displacements of the adjoining grains and the expansion. Differences in structural characteristics are discussed in terms of the effect of angular forces.

A new method to calculate the phonon dispersions for simple metals is first introduced, based on the Mobius transform. The cohesive energy can be obtained from experiment or theory, then the pair and three—body potentials between atoms in metals can be obtained by use of the modified Mobius theorem. Based on only the pair potentials the phonon dispersions for Cu, A1 and Ni have been obtained without any adjustable parameters. The calculation of elastic constants has involved both the 2—and 3—body interactions, and the restriction of the Cauchy relation has been erased. The above result represents a significant improvement over the Carlsson — Gelatt — Ehrenreich method.