Oxygen diffusion in bcc Fe under the influence of foreign atoms and vacancies

A multiscale approach including Density Functional Theory (DFT) and Atomistic Kinetic Monte Carlo (AKMC) simulations is applied to investigate the diffusion of oxygen in bcc Fe under the influence of substitutional foreign atoms or solutes (Al, Si, P, S, Ti, Cr, Mn, Ni, Y, Mo, W) and vacancies. The solutes can be assumed to be immobile since their diffusion coefficient is much smaller than that of oxygen. On the other hand, the vacancy mobility must be considered in the calculations because it is comparable to that of oxygen. The most stable state of oxygen in pure bcc Fe is the octahedral interstitial configuration. Recently, jumps of oxygen in pure bcc Fe, between first-, second-, and third-neighbor octahedral interstitial sites were investigated by DFT. It was found that the first-neighbor jump is most relevant. The second-neighbor jump consists of two consecutive first-neighbor jumps whereas the barrier of the third-neighbor jump is too high to be significant for the diffusion process. In this work DFT is used to determine the modified migration barriers in the presence of solutes. It is found that Si, P, Ni, Mo and W have some effect on the migration barriers of oxygen and their interaction with O is mainly repulsive. Al, Cr and Mn have a significant influence on the barriers and they exhibit strong attractive interactions with O. The most important modification of the barriers is found for S, Ti, and Y where deep attractive states exist. The barriers for oxygen jumps near a vacancy and barriers for vacancy jumps in the environment of oxygen are also calculated by DFT. Based on the migration barriers obtained by DFT, AKMC simulations on a rigid lattice are employed to determine the diffusion coefficient of oxygen in a dilute iron alloy containing different substitutional foreign atoms. It is found that Si, P, Ni, Mo, and W have almost no influence on the diffusivity of O. The presence of Al, Cr, Mn, S, Ti, and Y causes a significant reduction of the mobility of oxygen. Another version of the AKMC code is applied to investigate the mutual influence of oxygen and vacancy diffusion as well as the migration of the oxygen-vacancy pair.