Papers by Author: Graeme E. Murch

Abstract: In this paper, the diffusion isotope effect and the Manning factor are investigated by means of Molecular Dynamics simulations in liquid Cu-Ag alloys. The values for the diffusion isotope effect parameter allow for the estimate of the number of atoms that are moving cooperatively in a basic diffusion event as ‘seen’ by a given (tracer) atom. On average, in the considered alloys and considered temperatures, this is limited to between 5 and 15. This is consistent with results of Molecular Dynamics simulations on the average coordination number calculations. This would suggest that, together with a given atom, a majority of the neighbouring atoms are involved in a basic diffusion event. Results for the Manning factor (MD simulation) for Cu-Ag liquid alloys are seemingly in agreement with the direct exchange mechanism where only two atoms are involved in the elementary diffusion event. This is not in apparent agreement with the isotope effect results. It was shown, however, that any ring mechanism, or, more complex, cage mechanism are, in fact, a combination of several simultaneously happening direct exchanges. Any other possible mechanisms for diffusion in liquids is most likely a combination of direct exchanges as well. It can be seen then that the collective nature of all considered mechanisms is very similar and follows the direct exchange signature properties.

Abstract: In this paper, the diffusion isotope effect and diffusion mechanism are investigated by means of molecular dynamics simulations in two liquid alloys, Ni-Ag and Ni-Cu. The values for the diffusion isotope effect parameter allow for the estimate of the number of atoms which are moving cooperatively in a basic diffusion event as experienced by a given atomic species. It is shown that the composition dependence of N_D is typically very small. However, the temperature dependence of this parameter is much more pronounced. In addition, it is shown that, on average, in these alloys and temperatures considered, N_D is limited to the range: 5<N_D<17. This is consistent with results of molecular dynamics simulations on the average coordination number calculations. This would suggest that, together with a given atom, depending on temperature, the neighbouring atoms are all involved in the basic diffusion event.

Abstract: A novel study of interdiffusion analysis in multicomponent alloys is presented. A custom written Matlab fitting program (MFP) is used as the main tool for the present study. The interdiffusion matrices are obtained using a newly developed mathematical approach based on the fitting into the closed form solution for the composition profiles. Overall, the new fitting method gives very good outcomes and allows the probing of multiple solutions (validated by back tests) that exist when a single diffusion couple is used in a multicomponent system. An important finding of the present investigations is that small differences in composition profiles may lead to large differences in the interdiffusion coefficients.

Abstract: A novel study of analysis interdiffusion in multicomponent alloys is investigated by means of closed form solutions and numerical simulations. Quaternary as well as selected CoCrFeMnNi (HEAs) quinary metallic systems are analysed using one, two and three diffusion couples with the full set of interdiffusion coefficients being calculated. A custom written Matlab fitting program (MFP) is used as the main tool for the simultaneous fitting into multiple composition profiles in both systems. The retrieved interdiffusion matrices are obtained using a newly developed approach that is interlinked with composition vectors, eigenvalues and eigenvector. On average, it can be concluded that the accuracy of the obtained matrices steadily improves with the increase of the number of couples used in the analysis.

Abstract: In this paper, a brief history of the contributions of many of the major researchers in the field of solid state diffusion is presented starting from 1829 up to the present day. People who are still making significant contributions to the field are mentioned. The authors are well aware that such an attempt is necessarily incomplete and inevitably based on personal knowledge and flavour.

Abstract: The kinetic properties such as diffusivity and viscosity of the metal melt are the foundations to reveal the structure evolutions and the glass formation abilities during solidification of the investigated alloy, thus, to control the microstructures, defects and properties of materials. In this work, ab initio molecular dynamics simulations were utilized to investigate the kinetic and thermodynamic properties and the structural relaxations of Fe-X (X = 10-15 wt% Al, Cr, Mn and Ti, or 1-2wt% B and C) melts under various temperature and external pressure, which are in line with the interested concentration range of multi-component Fe-based alloys. The kinetics and structural relaxations are characterized by mean squared displacement, velocity autocorrelation function and self-intermediate scattering function. The thermodynamics properties including entropy and heat capacity are calculated by combining the vibrational and electronic contributions based on vibrational and electronic density of states. The predicted kinetics and thermodynamics properties under high temperature and pressure agree well with the experimental and theoretical results while the connection among structural relaxations and diffusion are revealed based on the Stokes-Einstein relation and the Hall-Wolynes (HW) relation. This work provides an insight into the structure-property relationships of metal melts, which are essential in the development of advanced multi-component Fe-based alloys.

Abstract: We review the results of our Monte Carlo simulation studies carried out within the past two decades in the area of atomic-migration-controlled phenomena in intermetallic compounds. The review aims at showing the high potential of Monte Carlo methods in modelling both the equilibrium states of the systems and the kinetics of the running processes. We focus on three particular problems: (i) the atomistic origin of the complexity of the ‘order-order’ relaxations in γ’-Ni₃Al; (ii) surface-induced ordering phenomena in γ-FePt and (iii) ‘order—order’ kinetics and self-diffusion in the ‘triple-defect’ β-NiAl. The latter investigation demonstrated how diverse Monte Carlo techniques may be used to model the phenomena where equilibrium thermodynamics interplays and competes with kinetic effects.

Abstract: One of the most technologically beneficial engineering ceramics is yttria stabilized zirconia (YSZ). As a result, research interest about YSZ has been intensive for many years. In this study, the lattice thermal conductivity and oxygen diffusion coefficient of YSZ are investigated at different temperatures (from 700 K to 1300 K) and zero pressure with the Green-Kubo formalism. We find that the lattice thermal conductivity decreases as the temperature increases, particularly at low temperatures and it shows a slightly temperature independence at high temperatures. The results demonstrate that the YSZ has quite a low thermal conductivity compared with pure zirconia. We also show that the oxygen tracer diffusion coefficient, as calculated from the mean square displacements, has an activation energy of 0.85eV.

Abstract: Zirconia has a number of remarkable properties, including a very low thermal conductivity. In this research, the phonon thermal conductivity of two phases (cubic and monoclinic) of zirconia (ZrO₂) are calculated. For this purpose, an equilibrium molecular dynamics simulation employing the Green-Kubo formalism is used. The results are presented in detail over a wide temperature range, from 100 K to 2400 K and 100 K to 1400 K for the above-mentioned structures, respectively, with a 100K temperature step. The temperature dependence of the equilibrium atomic volume demonstrated a reasonable agreement with the experimental data. Moreover, the lattice thermal conductivity was calculated by analysing the heat current autocorrelation function. The results showed that zirconia has a low thermal conductivity that is dependent on the temperature. It was also shown that the lattice thermal conductivity of the two phases of zirconia can be decomposed into three contributions due to the acoustic shortrange and long-range phonon and optical phonon modes. Finally, the results from this research are compared with the available experimental data.

Abstract: The wide range of industrial applications is the main reason for an increased interest in dioxides such as HfO₂. In this study, classical molecular dynamic simulations were performed to calculate the lattice thermal conductivity of the cubic phase of HfO₂, over a temperature range of 100-3000 K, based on the Green-Kubo fluctuation method. In this research, the heat current autocorrelation function and lattice thermal conductivity were calculated in the a-direction. The lattice thermal conductivity of the cubic phase of HfO₂ was found to be a result of three contributions. These were the optical and acoustic short-range and long-range phonon modes. Comparisons between the results of the research and experimental data when available indicate good agreement. Keywords: lattice thermal conductivity, molecular dynamics, Green-Kubo formalism, heat current autocorrelation function, hafnium dioxid