Effects of vacancy-solute clusters on diffusivity in metastable Fe-C alloys

Citation Kabir, Mukul et al. " Effects of vacancy-solute clusters on diffusivity in metastable Fe-C alloys. Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Diffusivity in defected crystals depends strongly on the interactions among vacancies and interstitials. Here we present atomistic analyses of point-defect cluster ͑PDC͒ concentrations and their kinetic barriers to diffusion in ferritic or body-centered-cubic ͑bcc͒ iron supersaturated with carbon. Among all possible point-defect species, only monovacancies, divacancies, and the PDC containing one vacancy and two carbon atoms are found to be statistically abundant. We find that the migration barriers of these vacancy-carbon PDCs are sufficiently high compared to that of monovacancies and divacancies. This leads to decreased self-diffusivity in bcc Fe with increasing carbon content for any given vacancy concentration, which becomes negligible when the local interstitial carbon concentration approaches twice that of free vacancies. These results contrast with trends observed in fcc Fe and provide a plausible explanation for the experimentally observed carbon dependence of volume diffusion-mediated creep in ferritic ͑bcc͒ 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 concentrations͒ and saddle points ͑activation barriers governing mobility͒.