Kinetic and molecular dynamics analysis of carbon diffusion in austenite

A kinetic 10-frequency model for interstitial diffusion via octahedral interstices in the fcc lattice is developed. In this model, the specific role of the transition probabilities during association and dissociation of the first nearest neighbour interstitial pairs through the second nearest neighbour sites is considered. Application of the model is made to carbon diffusion in austenite. Molecular dynamics is used to investigate carbon interstitial diffusion in austenite at low carbon contents. The assumption that carbon atoms can interact with each other only indirectly (via neighbouring iron atoms) is used. The Arrhenius parameters of interstitial carbon jump frequencies consistent with the 10-frequency model are determined. Comparison of the molecular dynamics results with experimental data at 1273?K in the context of the 10-frequency model is performed. It is shown that a small direct repulsion between carbon atoms at first nearest neighbours should be included. It is found that the initial increase (with increasing carbon content) in both the tracer and the chemical diffusion coefficients is shown to be a result of increased rates of dissociation of carbon from first and second nearest neighbour pairs to third nearest neighbour sites.     

First–principles study of potassium adsorption and diffusion on graphene

Potassium–ion batteries (KIBs) are a new–type of energy storage devices that have attracted increasing attention due to their low cost and the abundant resource of K in the Earth’s crust. Monolayer and multilayer graphene are promising electrode materials for KIBs. Herein, the adsorption and diffusion of potassium atoms on the surface of graphene were studied using the first–principles calculations including the van der Waals interaction. It was determined that K atoms can stably adsorb on the surface of graphene. The climbing image nudged elastic band method was employed to calculate the diffusion barriers of a single K atom and two K atoms on the surface of graphene. The results demonstrated that the diffusion barrier of a single K atom on graphene was low. The interaction between K atoms was considered and it facilitates the K atom diffusion to the second and third nearest–neighbour site of the K adatom, but prevents the K atom diffusion to the far nearest–neighbour site of the K adatom. Moreover, the difference in charge density demonstrates that there was a significant charge transfer from two K adatoms to its nearest–neighbour carbon atoms.     

Monte Carlo simulation of phosphorus diffusion in α-iron via the vacancy mechanism

Monte Carlo simulations of the vacancy and phosphorus (P) atom diffusion in body centred cubic (bcc) iron are presented. The input parameters for the calculations, namely the activation energies of atomic jumps, have been obtained using a potential set developed recently for a dilute Fe–P alloy using ab initio data. The diffusion coefficients entering equations for the fluxes of vacancies and solute atoms are evaluated. The results show that, in the temperature range of practical importance for P segregation, P atoms move down the vacancy gradient; hence, under irradiation conditions, vacancies should drag P atoms towards sinks of point defects. This is because of the high binding energy between a P atom and a vacancy in the first and second nearest neighbour sites from each other, which allows a vacancy to move around a P atom without loss of bonding and, hence, co-migrate with it.     

Cems study of the carbon distribution in austenite

CEMS resolution allows a careful study of Fe−C pure austenite spectra with high carbon content. Three Fe environments are detected which are ascribed to Fe atoms with one carbon first nearest neighbour and zero carbon second nearest neighbour and two environments with no carbon first nearest neighbour but zero carbon second nearest neighbour and one to four carbon second nearest neighbours respectively. This confirms the repulsive interaction between carbon interstitials and the tendency towards Fe₈C ordering is suggested.     

Diffusion mechanisms in the Fe_3Si alloys

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Application of a solvation shell analysis to the diffusivity of an interstitial species in binary substitutional solutions

A model is presented for the effect of substitutional solute elements on the diffusion coefficient of an interstitial species for low concentrations of both solutes. The model is based on the theory of absolute reaction rates and a Wagner random, noninteractive model for the substitutional solution, with Maxwell-Boltzmann statistics for the distribution of the interstitial atoms. Simplifying assumptions are made concerning the configuration of activated complexes, treating them as formed by the interstitial atom and the two nearest lattice atoms. The calculations are compared with experimental data reviewed for the diffusivity of carbon in Fe-Cr, Fe-Mn, Fe-Co and Fe-Ni alloys.     

The density dependence of the diffusion constant in interacting lattice gases: Application to surface diffusion of 0 onW(110)

The influence of interactions between adsorbed particles on their diffusion constant is investigated by kinetic Ising models with independent nearest neighbour hops. This leads to expressions for the ratio of the diffusion constant at arbitrary coverage to its value at =0 as a function of the interaction energies relative to temperature. It is shown that under certain conditions this quantity obeys a particle-hole symmetry. Exact results in the whole range of densities are given in one dimension for nearest neighbour interaction. They already yield a qualitative agreement with experimental results and are also compared to corresponding numerical simulations. The introduction of a next nearest neighbour interaction is shown to produce drastic changes in the density dependence of the diffusion constant in some of the cases. A generalized quasichemical approximation and a virial expansion are made in two dimensions, leading to a better agreement with the measurements.     

Low energy level spacing distribution in the atomic table ensemble

We have analysed the nearest neighbour spacing distributions for the atomic table ensemble. The analysis carried out indicates that the random matrix theory arguments extend even to the ground state domain of atoms.     

Nearest neighbour contribution to the electric field gradient at Cd in silver-gold alloys

The field gradients produced by minority atoms in Ag-Au alloys have been measured by PAC of ¹¹¹Cd. Due to an In-Au attraction, the fraction of ¹¹¹In atoms having an impurity as nearest neighbour is enhanced in silver-rich alloys.     

One-dimensional effects in the intermetallic compound Al11Mn4

Magnetic susceptibility and electrical resistivity measurements show that the intermetallic compound Al₁₁Mn₄ has a one-dimensional magnetic character. This is attributed to the presence of chains of nearest neighbour Mn atoms together with a strongly depressed interaction between distant Mn atoms.     

Alloying effects and site occupancies of Re in the C14 Cr-based Laves phases: a first-principles study

First-principles calculations based upon the density functional theory have been carried out to investigate the alloying effects and site occupancies of Re in the C14 XCr₂ (X?=?Nb, Ta, Ti, Zr, Hf) Laves phases. The calculated results indicate that Re tends to facilitate and participate in the formation of the Laves phases, generating X₈Cr₁₅Re accordingly. The partial density of states and charge density di?erence were analysed to re?ect the bonding characteristics. For X₈Cr₁₅Re, the bonding between the doped Re and its nearest neighbour Cr atoms all show covalent characteristics, which contribute to the phase stability. The substitution of Re on X sites is energetically unfavourable due to the weak bonding between Re and its nearest neighbour X atoms.     

ENDOR-Untersuchungen an GaP: Fe3+

We report on the first ENDOR-measurements of a defect centre in a III–V compound. The symmetry and the hyperfine parameters of three groups of neighbour atoms of the Fe³⁺-ion were determined. The results suggest the Fe³⁺-ion to be localized interstitially between four nearest phosphorus atoms in a tetrahedral arrangement.     

Mössbauer analysis of Fe−N austenites

Analysis of transmission Mössbauer spectra for high nitrogen Fe?N austenites is done in terms of five iron environments dealing with first and second nearest neighbour shells. The similar analysis for Fe?C austenites which displays only three iron environments led to the conclusion that carbon atoms are ordered whereas nitrogen atoms are more randomly distributed.     

Calculation of enhancement factors for solute diffusion in the fcc dilute random alloy

In this paper, we address the enhancement of solute diffusion by solute addition. We focus on the little studied kinetic effect leading to solute enhancement arising from the kinetics of the encounter of a solute atom with a vacancy in the presence of other solute atoms. The study complements an earlier study by two of the present authors of the enhancement of the solvent diffusivity. In the present paper, the fcc random alloy model is employed at the dilute limit. We employ Monte Carlo techniques to calculate the solute diffusion enhancement factors B ₁ and B ₂. Solute correlation factors, upon which the enhancement factors rely in this model, are calculated to a very high precision using long runs and averaging over a very large number of atoms. There is fair agreement with the kinetic treatments of Manning and of Holdsworth and Elliott, and excellent agreement with the self-consistent theory of Moleko et al.     

Enhancement of solute self-diffusion in fcc alloys

Using a new simulation approach to the calculation of correlation factors in very dilute solutions, the present paper reports on results of the first calculations of the first and second solute enhancement factors for solute self-diffusion in the fourteen-frequency model postulated by Bocquet for describing the diffusion kinetics of paired and unpaired solute atoms in dilute fcc alloys. Bocquet's assumption that correlation effects do not contribute to solute self-diffusion enhancement is shown to have a fairly limited range of validity. Specifically, it is shown that this assumption is reasonable when the solvent–vacancy exchange frequency far from the solute is high compared with the others or when all of the solute–vacancy exchange frequencies are small compared with the others. The assumption is also reasonable when the vacancy is not attracted to the solute atoms and when the vacancy does not often alternate between two solute atoms. The present study would strongly suggest that many of the ratios of exchange frequencies that have been determined from experimental data on the basis of this assumption are likely to be in error.     

The spatial fluctuations of the magnetization in amorphous ferromagnets

The spatial variation of the magnetization of amorphous ferromagnets containing transition metal and metalloid atoms is shown to depend explicitly on that of the alloy concentration and of the nearest neighbour distance. This variation will thus be in general on an atomic scale for these materials.     

Localized vacancies in palladium produced by thermal neutron capture recoil

The environment of recoil atoms in Pd metal after the (n, γ)-process in ¹¹⁰Pd at low temperatures has been studied by perturbed angular correlations of ¹¹¹Cd probes. It shows that 19(1) % of the recoil atoms have a vacancy as nearest neighbour.     

Intensity of the quadrupole structure in the V NMR in transition metal alloys

The intensity variation of the quadrupole satellite of the ⁵¹V magnetic resonance has been studied relative to alloying concentrations in σ-phase V-Co alloys. The nearest neighbour atoms play the most important role in this fine structure.