Dr Elena Levchenko

Equilibrium molecular dynamics simulation in conjunction with the Green-Kubo formalism is employed to study the transport properties of a model Ni50Al50 melt with the embedded-atom method potential developed in [G.P. Purja Pun, Y. Mishin, Phil. Mag., 2009, 89, 3245]. The principal objective of the work is to quantitatively characterise and analyse thermotransport in the system, i.e. diffusion driven by a temperature gradient. In addition, direct phenomenological coefficients for mass and thermal transport are also evaluated and analysed in the process. Furthermore, the results obtained are compared with previously published data for a different model of Ni50Al50 melt with an alternative embedded-atom method potential for the alloy as well as with experiment where possible. It is found that both potentials are able to consistently predict both direct transport coefficients over a wide temperature range. However, these two potentials are found to be inconsistent in characterising the cross-coupled heat and mass transport, predicting even different directions (sign) of the heat of thermotransport. The origin of this difference is discussed in the paper in detail.

In the present paper (1) the Hall method (HM) (specifically designed for determining the interdiffusion coefficient at the low and high composition limits of the corresponding interdiffusion composition profile) is further developed in order to be applied to the whole composition profile resulting in the Extended Hall method (EHM); (2) A comparative study of the HM, EHM, Boltzmann-Matano (BM) and Sauer and Freise (SF) methods is performed using composition profiles generated by computer simulation. The results clearly indicate that the HM/EHM technique is only applicable when the interdiffusion coefficient is constant (i.e. independent of composition) or almost constant at the low composition regions. In all other cases, the BM and SF methods give the best agreement with the input interdiffusion function even at the ends of the composition profiles.

The phonon-mediated contribution to the thermal transport properties of liquid NiAl alloy is investigated in detail over a wide temperature range. The calculations are performed in the framework of equilibrium molecular dynamics making use of the Green-Kubo formalism and one of the most reliable embedded-atom method potentials for the intermetallic alloy. The phonon-mediated contribution to the thermal conductivity of the liquid alloy is calculated at equilibrium as well as for the steady state. The relative magnitude of the thermal conductivity decrease induced by the transition to the steady state is estimated to be less than 2% below 2000 K and less than 1% at 3000 and 4000 K. It is also found that the phonon-mediated contribution to the thermal conductivity of the liquid alloy can be accurately estimated (well within 1%) on the basis of an approximation which invokes the straightforwardly accessible microscopic expression for the total heat flux without demanding calculations of the partial enthalpies needed for the precise evolution of the reduced heat flux (pure heat conduction). On the basis of these calculations, the correspondence between the experimentally observed and modelled kinetics of solidification due to a difference in thermal conductivity is discussed.

The recently introduced analytical model for the heat current autocorrelation function of a crystal with a monatomic lattice [Evteev et al., Phil. Mag. 94 (2014) p. 731 and 94 (2014) p. 3992] is employed in conjunction with the Green¿Kubo formalism to investigate in detail the results of an equilibrium molecular dynamics calculations of the temperature dependence of the lattice thermal conductivity and phonon dynamics in f.c.c. Ni. Only the contribution to the lattice thermal conductivity determined by the phonon¿phonon scattering processes is considered, while the contribution due to phonon¿electron scattering processes is intentionally ignored. Nonetheless, during comparison of our data with experiment an estimation of the second contribution is made. Furthermore, by comparing the results obtained for f.c.c. Ni model to those for other models of elemental crystals with the f.c.c. lattice, we give an estimation of the scaling relations of the lattice thermal conductivity with other lattice properties such as the coefficient of thermal expansion and the bulk modulus. Moreover, within the framework of linear response theory and the fluctuation-dissipation theorem, we extend our analysis in this paper into the frequency domain to predict the power spectra of equilibrium fluctuations associated with the phonon-mediated heat dissipation in a monatomic lattice. The practical importance of the analytical treatment lies in the fact that it has the potential to be used in the future to efficiently decode the generic information on the lattice thermal conductivity and phonon dynamics from a power spectrum of the acoustic excitations in a monatomic crystal measured by a spectroscopic technique in the frequency range of about 1¿20¿THz.

The vibrational contribution to the thermal transport properties of liquid Cu is investigated in detail in the temperature range 1300-1800 K. The calculations are performed in the framework of equilibrium molecular dynamics making use of the Green-Kubo formalism and one of the most reliable embedded-atom method potentials for Cu. It is found that the temporal decay of the heat current autocorrelation function of the liquid Cu model can be described by a single exponential function, which is characterized in the studied temperature range by a constant value of the heat flux relaxation time of about 0.059 ps. The vibrational thermal conductivity of the liquid Cu model slightly decreases with temperature from about 1.1 W/(mK) at 1300 K to about 1 W/(mK) at 1800 K. Near the melting temperature it is about 30% lower than the vibrational thermal conductivity of the f.c.c Cu model. The calculated thermal diffusivity of the liquid Cu model is demonstrated to retain a constant value of about 2.7 × 10-7 m2/s in the studied temperature range, which is about two orders of magnitude higher than the atomic diffusivity in the model at the melting temperature. The vibrational contribution to the total thermal conductivity of liquid Cu is found to slightly decrease with temperature, being estimated as about 0.7-0.5% in the temperature range of 1400-1800 K. Furthermore, the applicability of some simple theoretical treatments of vibrational thermal transport in liquid Cu is discussed.

The thermal resistance of a crystal lattice with a monatomic unit cell due to three-phonon scattering processes is investigated in detail theoretically. A general expression for the lattice thermal conductivity is derived from a combined analysis based on: (i) the Boltzmann equation and (ii) data on the heat current autocorrelation function obtained via molecular dynamics simulations in conjunction with the Green-Kubo formalism. It is argued that the phonon gas in a monatomic lattice conducts heat as if it consisted of two distinct parts (two 'thermal fluids'), so that the lattice thermal conductivity can be decomposed into contributions from these two parts. The origin of the behaviour of the phonon gas, which is explored in the present work, is due to an intrinsic interplay between Umklapp and normal three-phonon scattering processes. New insight into the nature of the lattice thermal conductivity is demonstrated and the results of the present work are in agreement with previous studies in this area.

Equilibrium atomic configurations and the kinetics of "order- order" and surface segregation processes in B2-ordering stoichiometric A-50 at.%B binary thin films are investigated by means of Semigrand Canonical Monte Carlo (SGCMC) and Kinetic Monte Carlo (KMC) simulations. The (100)-oriented films are modeled with an Ising-type Hamiltonian with previously evaluated pair-interaction energy parameters yielding the effect of "triple-defect disordering". The SGCMC simulations provide equilibrium vacancy concentration and atomic configuration in the films with B-atom termination of both free surfaces achieved at high temperatures by the generation of an antiphase boundary. Despite strong vacancy surface segregation, the thermodynamic activation energy for their formation inside the films is the same as in the bulk material. KMC simulations implemented with the SGCMC-determined equilibrium vacancy concentration reveal very slow relaxation of the films towards equilibrium configuration. The B-termination of the (100) free surfaces is produced by A-atom diffusion inwards into the films mediated by vacancies segregating on surfaces. © 2014 Elsevier Ltd. All rights reserved.

The formalism of thermotransport in a binary system is analysed. Focus is put on a detailed consideration of the heat of transport parameter characterizing diffusion driven by a temperature gradient. We introduce the reduced heat of transport parameter, which characterizes part of the interdiffusion flux that is proportional to the temperature gradient. In an isothermal system represents the reduced heat flow (pure heat conduction) consequent upon unit interdiffusion flux. It is demonstrated that is independent of reference frame and is useful in a practical way for direct comparison of simulation and experimental data from different sources obtained in different reference frames. In the case study of the liquid Ni50Al50 alloy, we use equilibrium molecular dynamics simulations in conjunction with the Green-Kubo formalism to evaluate the heat transport properties of the model within the temperature range of 1500-4000 K. Our results predict that in the presence of a temperature gradient Ni tends to diffuse from the cold end to the hot end whilst Al tends to diffuse from the hot end to the cold end.

The phonon-mediated thermal conductivity of f.c.c. Cu is investigated in detail in the temperature range 40-1300 K. The calculations are performed in the framework of equilibrium molecular dynamics making use of the Green-Kubo formalism and one of the most reliable embedded-atom method potentials for Cu. It is found that the temporal decay of the heat current autocorrelation function (HCACF) of the Cu model at low and intermediate temperatures demonstrate a more complex behaviour than the two-stage decay observed previously for the f.c.c. Ar model. After the first stage of decay, it demonstrates a peak in the temperature range 40-800 K. A decomposition model of the HCACF is introduced. In the framework of that model we demonstrate that a classical description of the phonon thermal transport in the Cu model can be used down to around one quarter of the Debye temperature (about 90 K). Also, we find that above 300 K the thermal conductivity of the Cu model varies with temperature more rapidly than, following an exponent close to -1.4 in agreement with previous calculations on the Ar model. Phonon thermal conductivity of Cu is found to be about one order of magnitude higher than Ar. The phonon contribution to the total thermal conductivity of Cu can be estimated to be about 0.5% at 1300 K and about 10% at 90 K. © 2013 © 2013 Taylor & Francis.

An analytical treatment of decomposition of the phonon thermal conductivity of a crystal with a monatomic unit cell is developed on the basis of a two-stage decay of the heat current autocorrelation function observed in molecular dynamics simulations. It is demonstrated that the contributions from the acoustic short-and long-range phonon modes to the total phonon thermal conductivity can be presented in the form of simple kinetic formulas, consisting of products of the heat capacity and the average relaxation time of the considered phonon modes as well as the square of the average phonon velocity. On the basis of molecular dynamics calculations of the heat current autocorrelation function, this treatment allows for a self-consistent numerical evaluation of the aforementioned variables. In addition, the presented analysis allows, within the Debye approximation, for the identification of the temperature range where classical molecular dynamics simulations can be employed for the prediction of phonon thermal transport properties. As a case example, Cu is considered.

Self-diffusion of component atoms and order-order relaxations in a B2-ordering binary system AB showing a tendency for triple-defect formation were consistently simulated by means of two Monte Carlo techniques. In view of a strict correlation between antisite-defect and vacancy concentrations the Kinetic Monte Carlo (KMC) simulations were implemented with a temperature-dependent vacancy concentration determined by means of Semi-Grand Canonical Monte Carlo (SGCMC) simulations. The Ising model of the system was completed with local-configuration-dependent saddle-point energy parameters related to vacancy mediated atomic jumps. The simulations elucidated the atomistic origin of the experimentally observed low rate of order-order relaxations in NiAl, as well as reproduced the experimental relation between the activation energies for order-order kinetics and Ni self-diffusion in NiAl. Higher value of the deduced activation energy for atomic migration with respect to the effective energy barriers related to individual atomic jumps indicated their high correlation. © 2013 Taylor & Francis Group, LLC.

In this paper, carbon diffusion in cementite is studied by molecular dynamics simulation. An assumption that carbon-carbon interaction occurs only indirectly via neighbouring iron atoms is used. An interstitial mechanism of carbon diffusion in cementite is revealed. The principal tracer diffusion coefficients and activation parameters of carbon diffusion in cementite are calculated for the temperature range 1223-1373 K and compared with the available published experimental data.

In this work the structural changes in the Pd42.5Cu 30Ni7.5P20 bulk and ribbon metallic glasses near the glass transition temperature have been investigated by a monochromatic high flux, high energy beam from the European Synchrotron Radiation Facilities (ESRF). Structure factors (SFs) and pair distribution functions (PDFs) were calculated using the measured intensities at different temperatures. We used a regularisation algorithm in order to calculate PDFs. Regularisation was carried out firstly by direct smoothing of the experimental SFs using their expansion in terms of odd Hermit functions (which are orthogonal in (0,8) interval) and secondly by Fourier transformation of the smoothed function.

Molecular dynamics was used to study the structural and phase transformations of the model of an initially amorphous Fe95C5 alloy upon ultrarapid heating and cooling at an average rate of ¿ 6.6×1011 K/s. Upon heating from 1140 to 1200 K, the model system was found to undergo crystallization with the formation of an interstitial solid solution of carbon in a-Fe. The model system melts at T = 1800-1900 K. Upon subsequent cooling, the alloy is glassified at T =1020 K. The energy of the homogeneous formation of a crystallization center (¿0.79 eV) in the bulk of the amorphous phase of the Fe95C5 alloy was calculated by the model of the activation-energy spectrum.