Papers by Author: Taras M. Radchenko

Abstract: The statistical-thermodynamics and kinetics models of atomic ordering in a metal-doped graphene (binary two-dimensional planar graphene-type crystal lattice) at 1/8, 1/4, and 1/2 stoichiometries are proposed. Impossibility of (completely) atomic-ordered distribution at 1/6 and 1/3 stoichiometries is ascertained in a graphene-type crystal lattice (in case of a short-range interatomic interactions at least). If a graphene is doped by the short-range interacting metal atoms, the superstructures described only by a one LRO parameter are possible; and if it is doped by the long-range interacting metal atoms, the new superstructures with the two or three LRO parameters may appear as well. If stoichiometry is 1/4, the structure has a one long-range order (LRO) parameter is more thermodynamically favorable than those have one or two LRO parameters. It is established that kinetics curves of LRO parameters can be non-monotonic for structures where there are two or three LRO parameters (because graphene-type lattice contains two sublattices, and mixing energy is different for each of them). It is shown that the most ordered is structure with equal atomic fractions of carbon and metal atoms, while the least one is structure with a maximal difference of carbon and metal atoms. Kinetics results confirm statistical-thermodynamic ones: firstly, equilibrium values of LRO parameter coincide within the framework of both models, secondly, equilibrium (and instantaneous) value of LRO parameter in a nonstoichiometric binary graphene-type structure (where atomic fraction of a doping component deviates from the stoichiometry to the side of the higher concentrations) may be higher than it is in a stoichiometric one. The dominance of the same physical mechanisms of atomic ordering in both mixed nanosystems and macrosystems is assumed.

Abstract: Within the framework of the lattice-statics and static fluctuation-waves’ methods, the available energiesof strain-induced interaction of interstitial–interstitial, interstitial–substitutional and substitutional–substitutional impurity atomic pairs are collected and analysed for f.c.c.-(Ni,Fe)–C solutionsallowing for discrete atomic structure of the host-crystal lattice. The lattice spacings, elasticity moduliand/or quasi-elastic force parameters of the host-crystal lattice, and concentration coefficients of thedilatation of solid-solution lattice due to the respective solutes are selected as the input numerical experimentaldata used. The above-mentioned interaction energies prove to have non-monotonically decreasing(‘quasi-oscillating’) and anisotropic dependences on discrete interatomic radius-vector, andthemselves are strong and long-range. In all f.c.c.-(Ni,Fe)-base solutions, there is strain-induced attractionin many co-ordination shells. In general, the strain-induced interaction between impurity atomsin γ-Fe is weaker than in α-Ni (but in some solid solutions, it may prove to be of the same order).The verification of applicability of the approximation of strain-induced interaction of impurities forf.c.c.-(Ni,Fe)–C alloys (by means of analysis of thermodynamic C activity and ‘short-range order’ parametersof C-atoms’ distribution revealed by Mössbauer spectroscopy) showed that it must be supplementedwith additional short-range (‘electrochemical’) repulsion in the first co-ordination shell.Nevertheless, in any case, the strain-induced interaction of impurity atoms must be taken into accountfor analysis of structure and properties of f.c.c.-(Ni,Fe)-base solutions.

Abstract: Using the self-consistent field approximation, the static concentration waves approach and the Onsager-type kinetics equations, the descriptions of both the statistical thermodynamics and the kinetics of an atomic ordering of D019 phase are developed and applied for h.c.p.-Ti–Al alloy. The model of order–disorder phase transformation describes the phase transformation of h.c.p. solid solution into the D019 phase. Interatomic-interaction parameters are estimated for both approximations: one supposes temperature-independent interatomic-interaction parameters, while the other one includes the temperature dependence of interchange energies for Ti–Al alloy. The partial Ti–Al phase diagrams (equilibrium compositions of the coexistent ordered α2-phase and disordered α-phase) are evaluated for both cases. The equation for the time dependence of D019- type long-range order (LRO) parameter is analyzed. The curves (showing the LRO parameter evolution) are obtained numerically for both temperature-independent interaction energies and temperature-dependent ones. Temperature dependence of the interatomic-interaction energies accelerates the LRO relaxation and diminishes a spread of the values of instantaneous and equilibrium LRO parameters versus the temperature. Both statistical-thermodynamics and kinetics results show that equilibrium LRO parameter for a non-stoichiometry (where an atomic fraction of alloying component is more than 0.25) can be higher than for a stoichiometry at high temperatures. The experimental phase diagram confirms the predicted (ordered or disordered) states for h.c.p.-Ti– Al.

Abstract: Using the static concentration waves’ method and self-consistent-field approximation, the Onsager-type kinetics equation is solved to describe the L12-type long-range order (LRO) relaxation. To calculate the diffusivities for Permalloy, the experimental diffraction data in respect to LRO-parameter evolution for Ni3Fe are used. Theoretical curves of a time-dependent LRO for Ni– Fe are plotted. Dilution of alloying element (with a deviation from stoichiometry) results in deceleration of the LRO-parameter history (at the initial evolution stage) and in an increase of its relaxation time. Both statistical-thermodynamics and kinetics results show: at high temperatures, equilibrium LRO parameter for non-stoichiometric Permalloys can be higher than for stoichiometric one.

Abstract: Relaxation of diffuse-scattering intensities of various kinds of waves is a phenomenon of an especial interest since its study enables one to obtain the most detailed information on both the equilibrium short-range order (SRO) and the non-equilibrium SRO, and therefore, it is the most convenient instrument for investigating SRO kinetics. The SRO kinetics is studied by the use of available data of measurements of residual electrical resistivity for substitutional f.c.c.-Ni–Al solid solutions during their isothermal annealing. Within the framework of the first-order and (more realistic) second-order kinetics models, the maximum characteristic relaxation times and equilibrium magnitudes of the residual electrical resistivity for these solid solutions at different annealing (1273–1626 K) and quenching temperatures (≥ 1723 K) are evaluated, and the hypothetical values of similar quantities appropriate for the diffuse scattering of radiations are estimated too.