V. G. Vaks

Effects of the Interaction between Order Parameter and Concentration on the Kinetics of Antiphase Boundary Motion

The microscopic mean-field kinetic equation introduced previously is used to investigate the motion of an antiphase boundary (APB) after the quench of a disordered alloy below the ordering spinodal. A strong interaction between order and concentration fields in non-stoichiometric alloys results in a peculiar evolution of the structure of an APB and in unusual concentration, temperature and time dependence of the kinetic coefficients for its motion.

Possible effects of bandstructure in properties of quasicrystals

Abstract We note that the electron per atom ratios in the known quasicrystal alloys of simple and noble metals obey rather accurately the Hume-Rothery-type relations which are characteristic for the “band” mechanism of the structural stability. If this mechanism is pronounced in the quasicrystal alloy then a number of its characteristics such as thermopower, shear constant, low-temperature thermal expansion etc. must vary sharply and peculiarly with the alloy composition. Thus experimental investigations of such concentration dependences can provide direct information on the importance of the bandstructure effects in properties of quasicrystals.

On the theory of open systems: statistical thermodynamics and decomposition type phase transitions for the model of an alloy under irradiation

Abstract We discuss a generalization of statistical thermodynamics to the open system case using as an example the model of an alloy under irradiation. The statistical properties of the stationary states are described with the use of generalized thermodynamic potentials and “quasi-interactions” determined from the master equation for probabilities. Methods for solution of this equation are illustrated by mean-field type calculations of correlators, thermodynamic potentials and phase diagrams for disordered alloys.

Inter-ionic potentials and structure factors in liquid alkali metals

The inter-ionic potentials phi (r) in the alkali metals and the possibilities of estimating them from data on the structure factors S(k) are discussed. The dependences of phi (r) on the density and on the form of the pseudopotential model used are investigated. The structure factor is calculated in the Percus-Yevick (PY) approximation which has been shown previously to describe the region of the main peak of S(k) very accurately. phi (r) and S(k) are found to be rather sensitive to the choice of the pseudopotential and the approximation for screening. The form of S(k) is determined primarily by the effective packing fraction, i.e. by the ratio of the effective volume of the repulsive core of phi (r) to the atomic volume, and is less sensitive to variations of the attractive part of phi (r). Potentials phi (r) are presented corresponding to the pseudopotential model discussed earlier which gives a very accurate description of all the atomic properties of solid alkali metals. The S(k) calculated with these phi (r) are shown to be in good agreement with the available data for liquid alkali metals over wide ranges of temperatures and densities.

The master equation approach to configurational kinetics of alloys via the vacancy exchange mechanism: general relations and features of microstructural evolution

The earlier-suggested generalized Gibbs distribution approach to the configurational kinetics of non-equilibrium alloys is extended to the case of many-component alloys and the realistic vacancy-mediated atomic exchange mechanism is incorporated. Exact and approximate equations for the temporal evolution of atomic distributions as well as for the free energy of a non-equilibrium alloy are presented. It is shown that the evolution of the main alloy component distribution for the nearest-neighbour vacancy exchange model can usually be described in terms of an equivalent direct exchange model. This conclusion is illustrated with the computer simulation of decomposition and ordering via the vacancy exchange mechanism for a two-dimensional alloy model. The simulation also reveals the localized ordering phenomenon at very early stages of ordering, in agreement with previous experiments and Monte Carlo simulation, but with no interfacial vacancy trapping which was suggested in previous works in order to explain this phenomenon.

An experimental and theoretical study of martensitic phase transitions in Li and Na under pressure

By using the methods of acoustic emission and neutron-structure analysis, the effect of pressure p on low-temperature phase transitions in Li and Na is investigated. In Li, within the investigated interval of p <or approximately=3 GPa, the transition temperature rises with pressure, and the low-temperature phase has a 9R structure. In Na, on the contrary, pressure suppresses the transition, so that it disappears even at p=0.1-0.2 GPa but the 9R phase is the low-temperature one too. The shape of acoustic emission signals suggests the presence of pretransition phenomena and differences in the kinetics of transitions in Li and Na. Calculations performed explain the difference in the influence of pressure on structural stability of Li and Na by the effect of proximity of the Fermi level to peaks in the electronic density of states, which appears under pressure in Li but is absent from Na. The same effects account for a premartensitic softening of the shear constant, observed under compression of Li.

Kinetics of formation of twinned structures under L10-type orderings in alloys

The earlier-developed master equation approach and kinetic cluster methods are applied to study the kinetics of L10-type orderings in alloys, including the formation of twinned structures characteristic of cubic-tetragonal-type phase transitions. A microscopical model of interatomic deformational interactions is suggested which generalizes a similar model of Khachaturyan for dilute alloys to the physically interesting case of concentrated alloys. The model is used to simulate A1→L10 transformations after a quench of an alloy from the disordered A1 phase to the single-phase L10 state for a number of alloy models with different chemical interactions, temperatures, concentrations, and tetragonal distortions. We find a number of peculiar features in both transient microstructures and transformation kinetics, many of them agreeing well with experimental data. The simulations also demonstrate the phenomenon of an interaction-dependent alignment of antiphase boundaries in nearly equilibrium twinned bands which seems to be observed in some experiments.

On the significance of local lattice distortions for configurational interactions in substitutional alloys

Abstract We present quantitative estimates of the local lattice distortion (“deformational”) contribution to the configurational interactions in disordered substitutional alloys which determine their ordering, segregation and related phenomena. We show that, unlike to a widespread opinion, the deformational terms are often quite large, thus taking them into consideration is necessary for the quantitative theories of these alloys.

The effect of proximity of the Fermi level to singularities in the electron state density on elastic and thermodynamic properties of metals and alloys

The authors investigate the effect of the proximity of the Fermi surface (FS) to a singularity in the electron state density on atomic properties of a nearly free-electron metal. Expressions for thermodynamic potentials in the improved version of the pseudopotential perturbation theory are given which generalise the conventional formulae to the case of small distances eta between the FS and the Brillouin zone (BZ) face and also take into account the Fermi liquid interaction effects. The case of the FS proximity to the BZ edge is also considered. The character of the eta dependence of atomic properties is illustrated by model calculations for disordered Li1-xMgx alloys in the BCC, FCC and HCP structures. They calculate the concentration dependencies of the elastic constants cik and their volume derivatives delta cik/ delta u, the low-temperature phonon and electron contributions to the specific heat Cv and to the thermal expansion coefficient beta T, and the equilibrium lattice parameters. All the quantities have peculiar anomalies at small eta which can be rather large, especially in delta cik/ delta u and beta T. The results are compared with the available experimental data.

On the accuracy of the liquid theory approximate methods for the description of liquid metal thermodynamics

The accuracy of the various liquid theory methods for calculating the thermodynamic properties of liquid metals is investigated. The calculations have been made for liquid Na, Rb, Mg and Al over wide ranges of temperature T and density n using (i) the variational method and hard-sphere reference system (the HSV method), (ii) the Barker-Henderson (BH) method, (iii) the Weeks-Chandler-Andersen (WCA) method and (iv) the expressions for energy and pressure in terms of the radial distribution function found in the Percus-Yevick approximation (the PYEP method). The results of the calculations are compared with each other and with experiment. The pseudopotential model is used which describes all the atomic properties of the alkali metals to a high precision, those of Mg rather well, and of Al fairly well. It has been shown that the simple HSV approximation suitable for a semi-quantitative description of the liquid metal properties is insufficient for quantitative calculations of differential characteristics, such as melting temperatures, entropies of fusion, entropies of liquid alloy mixing, etc. The PYEP and BH methods allow the authors to calculate some thermodynamic potentials rather precisely but the computational difficulties prevent them from being used for calculating other thermodynamic properties. The WCA method has been found to combine high accuracy with sufficient simplicity of calculations and seems, therefore, to be the most promising one for quantitative calculations of liquid metal thermodynamics. They also discuss the problems of applicability of the methods considered at various T and n (particularly in the vicinity of the critical point), of the dependence of their accuracy on the metal valency and the sensitivity of the results to the type of pseudopotential model used.