Carbon atoms are always present in Fe-based materials, either as impurities even in high
purity samples or as an intrinsic constituent in steels. Density Functional Theory calculations have
been performed to study the interaction between C atoms and vacancies (V) in α-Fe. We find that
the formation of VCn complexes is energetically favourable for n ≤ 3, with VC2 being the most
stable one. The energy gain corresponding to the clustering reaction VCn-1 + C → VCn depends
mainly on the strength of C-C covalent bonds. The vacancy diffusivity is shown to be significantly
modified by the formation of vacancy-carbon complexes, exhibiting non-Arrhenius behaviour.
Effective vacancy diffusion coefficients in α-Fe are calculated as a function of temperature and
carbon content using a simplified thermodynamic model. The results are discussed in detail in the
limiting case of excess of C with respect to vacancies.
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