Mutual dependence of oxygen and vacancy diffusion in bcc Fe and dilute iron alloys


Mutual dependence of oxygen and vacancy diffusion in bcc Fe and dilute iron alloys

Wang, X.; Faßbender, J.; Posselt, M.

A combination of density functional theory (DFT) and an efficient calculation method based on Atomistic Kinetic Monte Carlo simulations (AKMC) is used to investigate the interdependence of oxygen (O) and vacancy (v) diffusion in bcc Fe and in dilute iron alloys with the substitutional atoms Y and Ti. Both O and v are considered as mobile while the substitutional atoms are assumed to be immobile. DFT is applied to determine the binding energy between O and v for different distances, the migration barriers for O in the environment of v, and the corresponding barriers of v in the vicinity of O. In agreement with previous work O and v have a very strong binding at the 1st neighbor distance. On the other hand, the calculations show that the Ov pair at the 6th neighbor distance is instable. The newly found simultaneous jumps of both O and v compensate the lack of jump paths that would occur due to this instability. The DFT results are employed to determine the diffusion coefficient of O and v using the AKMC-based calculation method. At first a model system with fixed O and v concentrations is studied. It is found that even a small v content of some ppm can lead to a strong reduction of the O diffusivity. A similar effect is obtained for v diffusion under the influence of O. Furthermore, investigations on the interdependence of O and v diffusion in the first phase of thermal processing of oxide dispersion strengthened iron alloys are performed, and the influence of the substitutional atoms Y and Ti is studied. A simple thermodynamic model is employed to determine the concentration of O, Y, and Ti monomers as well as the total v concentration, for a typical total content of O, Y, and Ti. These results are used in calculations of the diffusion coefficients of O and v. Not only a strong mutual dependence but also a significant influence of Y on O diffusion is found. Finally, O and v diffusivities in a system with an O content close to the thermal solubility are calculated, where the monomer and total concentrations are determined by two different thermodynamic models. Even for such a low amount of O in the alloy the diffusion coefficients differ strongly from those in perfect bcc Fe.

Keywords: Ferritic iron alloys; Mutual dependence of oxygen and vacancy diffusion; First-principle calculations; Atomistic Kinetic Monte Carlo simulations; Influence of substitutional atoms

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