Papers by Keyword: Atomistic Modeling

Abstract: . 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.

Abstract: Molecular dynamics is employed to investigate carbon diffusion in cementite. An approximation that carbon atoms can interact with each other only indirectly (via neighbouring iron atoms) is used. The interstitial mechanism of carbon diffusion in cementite is elucidated. The formation energy of defects (a carbon atom on an interstitial position and a vacant site on a regular carbon position) as well as the migration energy of carbon atoms are estimated in the temperature range 1273–1373 K.

Abstract: A theoretical and atomistic study of diffusion and stability of a pure element hollow nanosphere and nanotube is performed. The shrinkage via the vacancy mechanism of these hollow nano-objects is described analytically. Using Gibbs-Thomson boundary conditions an exact solution of the kinetic equation in quasi steady-state at the linear approximation is obtained. The collapse time as a function of the geometrical sizes of the hollow nano-objects is determined. Kinetic Monte Carlo simulation of the shrinkage of these nano-objects is performed: it confirms the predictions of the analytical analysis. Next, molecular dynamics simulation in combination with the embedded atom method is used to investigate diffusion by the vacancy mechanism in a Pd hollow nanosphere and nanotube. It is found that the diffusion coefficient in a Pd hollow nanosphere and nanotube is larger near the inner and external surfaces compared with the middle part of a nanoshell. The molecular dynamics results provide quite a strong but indirect argument that a real pure element hollow nanosphere and nanotube may not shrink as readily via the vacancy mechanism as compared with the predictions of the analytical analysis and kinetic Monte Carlo simulations.

Abstract: Molecular dynamics simulation using the embedded-atom method is applied to study defect formation and distribution in a hollow Pd nanosphere. It is established that besides vacancies, which can nucleate on the inner or external surfaces, at the external surface, other defects (Shockley partial dislocations, twins and stacking faults) form due to its significant reconstruction by means of a/6〈112〉 shears of atomic rows. The density of the defects on the external surface grows with decreasing nanoshell size. It is demonstrated that Shockley partial dislocations can act as vehicles for the transfer of material from the external surface to the inner surface of the nanoshell thus leading to shrinking. It is shown that the vacancy concentration is higher near both surfaces than in the bulk of the nanoshell.

Abstract: The mmolecular dynamics method is applied to investigate carbon interstitial diffusion in austenite at low carbon content. An approximation that carbon atoms can interact with each other only indirectly (via neighbouring iron atoms) is used. Sets of Arrhenius parameters of interstitial carbon jump frequencies identified by the four-frequency model are determined. Comparison of the molecular dynamics results with experimental data analysis in the context of the four-frequency model is performed. It is shown that the four-frequency model may not be adequate to describe the carbon diffusion process. To improve the analytical model the specific role of the transition probabilities during association and dissociation of the first nearest neighbour carbon pairs through the second neighbour sites should be considered. The direct repulsion between the carbon first neighbour positions should be also taken into account in molecular dynamics simulation.