Atomistic analyses were made of point-defect cluster concentrations and their kinetic barriers to diffusion in ferritic or body-centered-cubic iron supersaturated with carbon. Among all possible point-defect species, only monovacancies, divacancies, and the point-defect cluster containing one vacancy and two carbon atoms were found to be statistically abundant. It was found that the migration barriers of these vacancy-carbon point-defect clusters were sufficiently high compared to that of monovacancies and divacancies. This leads to decreased self-diffusivity in body-centered cubic Fe with increasing carbon content for any given vacancy concentration, which becomes negligible when the local interstitial carbon concentration approached twice that of free vacancies. These results contrast with trends observed in face-centered cubic Fe and provide a plausible explanation for the experimentally observed carbon dependence of volume diffusion-mediated creep in ferritic (body-centered cubic) Fe-C alloys. Moreover, this approach represents a general framework to predict self-diffusivity in alloys comprising a spectrum of point-defect clusters based on an energy-landscape survey of local energy minima (formation energies governing concentrations) and saddle points (activation barriers governing mobility).

Effects of Vacancy-Solute Clusters on Diffusivity in Metastable Fe-C Alloys. M.Kabir, T.T.Lau, X.Lin, S.Yip, K.J.Van Vliet: Physical Review B, 2010, 82[13], 134112