Papers by Author: Graeme E. Murch

Abstract: In crystalline solids, during such processes as chemical interdiffusion in alloys, ionic conductivity and the annealing out of radiation damage there will inevitably be a net flux of vacancies. In most cases, when different species of atoms have different jump rates with vacancies within a net flux of vacancies, the phenomenon of the vacancy-wind effect will occur. This effect was first discovered in the 1960s by the late Dr John Manning. It is a subtle phenomenon that comes about because of the local redistribution of the equilibrium concentration of vacancies with respect to two or more species of drifting atoms in a driving force. The effect is captured in various ‘vacancy-wind factors’ (some of which are now sometimes called Manning factors) which formally arise from non-zero off-diagonal Onsager phenomenological transport coefficients and non-unity values of the tracer correlation factors. In this paper, the effect is reviewed and discussed.

Abstract: High entropy alloys (HEAs) are composed of five or more principal elements with equal (or nearly equal) compositions. In this paper, interdiffusion phenomenon in the HEAs is investigated. Two composition dependent (as well as composition independent) interdiffusion matrices have been used for detailed studying of the diffusion behaviour in CoCrFeMnNi HEAs. These matrices are calculated according to the Darken and Manning formalisms and are used in combination with the explicit finite difference method (EFDM) to obtain interdiffusion profiles. First, the interdiffusion profiles are calculated for the case of the terminal binary diffusion couple. A significant difference in the composition profiles is found between predictions according to the Darken and Manning formalisms. Next, the interdiffusion problem in the 5-component alloy is addressed numerically by considering the interdiffusion coefficients as constant, independent of composition, in CoCrFeMnNi alloys for several diffusion couples (mainly quasi-binary and quasi-ternary). The simulated composition profiles are found to be in a very good agreement with the available experimental results [1, 2]. It should be pointed out that the independence on composition of the interdiffusion matrix should be used for diffusion couples under two conditions: relatively small changes in composition, and the non-zero/non-dilute terminal compositions. The composition dependent interdiffusion matrix should be used in the diffusion couple if the composition differences are large and/or zero/dilute terminal compositions. In this paper, the Darken and Manning formalisms are used for modelling the composition dependent interdiffusion matrices. The purpose of this modelling is to systematically investigate interdiffusion in CoCrFeMnNi alloys in diffusion couples with substantial changes in composition. The main application of the present research is in the prediction of possible interdiffusion profiles in the framework of the random alloy model.

Abstract: In this study, the phonon-based thermal conductivity of magnesite (MgCO₃) and dolomite (CaMg(CO₃)₂) is calculated and compared with an earlier recent calculation on calcite (CaCO₃). Equilibrium molecular dynamics simulation by way of the elegant Green-Kubo formalism is used for calculating the thermal conductivity. The thermal conductivity is investigated over a wide temperature range (from 200 K to 800 K) for all of the above mentioned materials. The most reliable potential parameters are used for characterising the interatomic interactions. In all of the models, two independent mechanisms are considered. The first is temperature independent, which is relevant to the acoustic short-range and optical phonons, and the other is temperature dependent, which is linked to the acoustic long-range phonons. In the study, the heat current autocorrelation function (HCACF) is calculated over the averages of the NPT, NVT and NVE ensembles in the x- and z- directions. In addition, it is shown that the optical, acoustic short- and long-range phonon modes are the main contributors to the decomposition model of the thermal conductivity. In a further investigation, the effects of the computational cell sizes on the thermal conductivity are investigated with five different simulation blocks containing 30, 240, 810, 1920 and 6480 atoms. Finally, this research provides a comparison of the thermal conductivity from this study and experimental studies: they are in good agreement.

Abstract: The influence of composition on thermotransport (coupling between mass and heat transport) in Ni-Al melts is investigated by making use of equilibrium molecular dynamics simulations in conjunction with the Green-Kubo formalism. To describe interatomic interactions in Ni-Al melt models, we employ the embedded-atom method potential developed in [G.P. Purja Pun, Y. Mishin, Phil. Mag., 2009, 89, 3245]. It is demonstrated that the employed interatomic potential gives good agreement with the recent experimental study [E. Sondermann, F. Kargl, A. Meyer, Presented at the 12th International Conference on Diffusion in Solids and Liquids (DSL-2016), 26-30 June 2016, Split, Croatia] regarding the direction of thermotransport in Al-rich liquid Ni-Al alloys. Moreover, the predicted values of the reduced heat of transport (the quantity which explicitly characterizes both the magnitude and direction of thermotransport) in Ni-Al melts, reveal fairly weak composition dependence while being practically independent of temperature at all. Accordingly, in the presence of a temperature gradient, our simulation results for the models of liquid Ni₂₅Al₇₅, Ni₅₀Al₅₀ and Ni₇₅Al₂₅ alloys predict consistently Ni and Al to migrate to the cold and hot ends, respectively. Meanwhile, the highest value, about eV, of the reduced heat of transport is observed for Ni₅₀Al₅₀ alloy model and it slightly decreases towards Al-rich and Ni-rich compositions.

Abstract: In this study, mass transport properties of liquid Cu-Ag alloys are investigated over wide temperature and composition ranges. The calculations are performed within the framework of the Green-Kubo (GK) formalism by using equilibrium molecular dynamics (MD) simulations along with one of the most reliable embedded-atom method potentials for this system developed by [P. Williams et al.: Modell. Simul. Mater. Sci. Eng. vol. 14 (2006), p. 817]. The approach employed allows for evaluation of the components’ self-diffusion coefficients as well as the phenomenological coefficient for mass transport Lcc. The results obtained in this study can be used to predict the kinetics of solidification of real liquid Cu-Ag alloys.

Abstract: In this chapter, we review the Nernst-Planck equation describing the cation interdiffusion coefficient, the two tracer cation diffusion coefficients and the thermodynamic factor in ionic compounds and how it is related to the analogous Darken-Manning relationship in binary alloy systems. We make use of the Onsager flux equations of non-equilibrium thermodynamics to rigorously address the problem. The recently found correction factor to the Nernst-Planck equation is analyzed visually by means of computer simulation

Abstract: It is generally well recognized that in the course of a grain boundary (GB) diffusion experiment the diffusion of solute atoms in grain boundaries must exhibit a strong time-dependent segregation. But there has been no clear understanding of exactly how this time dependence develops. In this chapter, we review and analyse transient solute GB diffusion by means of the computer simulation technique of Lattice Monte Carlo (LMC). This technique has been successfully used on numerous occasions for the purposes of systematically studying the GB transition regimes that occur between the principal well-defined Harrison GB kinetics regimes (A, B and C-Types). Recently, the analysis using LMC has been extended to the case of solute GB diffusion when the segregation factor is independent of time. In the present paper, we analyse two cases of solute segregation in GB diffusion: first, where the solute atoms are homogeneously distributed along the tracer source plane but their mobility is not high at this plane; and the second, where the mobility of the solute atoms along the tracer source plane is comparable to their mobility along the GB. It is shown that the time dependence of the segregation can contribute significantly into the resulting values of the triple-product that is usually obtained experimentally in the Harrison Type-B kinetics regime.

Abstract: We analyse the formalism of transport in a binary system especially focussing 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 Qc*' which characterizes part of the interdiffusion flux that is proportional to the temperature gradient. In an isothermal system Qc*' represents the reduced heat flow (pure heat conduction) consequent upon unit interdiffusion flux. We demonstrate that Qc*' is independent of reference frame and is practically useful for direct comparison of simulation and experimental data from different sources obtained in different reference frames. Then, we use equilibrium molecular dynamics simulations in conjunction with the Green-Kubo formalism to study the heat transport properties of a model of the liquid Ni₅₀Al₅₀ alloy at three state points within the temperature range 1500 – 4000 K. Our results predict that in the liquid Ni₅₀Al₅₀ alloy 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.

Abstract: This manuscript investigates the compressive properties of Corevo^® foam. Corevo^® foam is a cellular metal manufactured by the infiltration casting of salt dough with aluminium. Corevo^® foam samples with different porosities are tested by using quasi-static compression loading. Their mechanical properties (i.e.: effective Young’s modulus, Poisson’s ratio, initial yield stress and material yield stress) are then compared to reveal the importance of the density difference. In addition, three-dimensional finite element analysis is performed on models generated from micro-computed tomography (μCT). The results of two different pore sizes are obtained and compared in the scope of this work. These numerical results are verified by comparison with the experimental analysis. A sound agreement is found. Numerical analysis in this work also includes the investigation of the mechanical material anisotropy and plastic deformation.

Abstract: Phonon dynamics and phonon thermal conductivity of f.c.c. Cu are investigated in detail in the temperature range 200 1300 K within the framework of equilibrium molecular dynamics simulations making use of the Green-Kubo formalism and one of the most reliable embedded-atom method potentials. It is found that the temporal decay of the heat current autocorrelation function of the f.c.c. Cu model at low and intermediate temperatures demonstrates 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 200 800 K. The intensity of the peak decreases as the temperature increases. At 900 K, it transforms to a shoulder which diminishes almost entirely at 1200 K. It is suggested that the peak may be activated by the influence of the Cauchy pressure in f.c.c. Cu on the phonon dynamics. A decomposition model of the heat current autocorrelation function of a monatomic f.c.c. lattice is introduced. This model can capture all contributions to the function discussed in the literature. It is found that the temperature dependence of the phonon thermal conductivity of the f.c.c. Cu model is in good agreement with previous calculations on the f.c.c. Ar model which follows an exponent close to-1.4, i.e. varies more rapidly than the T-1 law predicted by the theory. The calculated phonon thermal conductivity of the f.c.c. Cu is found to be about one order of magnitude higher than the f.c.c. Ar. This is explained by the inclusion of the electronic contribution to the bulk lattice properties during the fitting of the embedded-atom method potential functions to the experimental or ab initio data. It is demonstrated that the electronic contribution to the total thermal conductivity of f.c.c. Cu dominates over the whole studied temperature range. Nevertheless, the phonon contribution increases as the temperature decreases. The contribution can be estimated to be about 0.5 % at 1300 K and about 5 % at 200 K.