Predicting carbon diffusion in cementite from first principles

Combining first-principles calculations with recently developed statistical physics tools, we determine carbon diffusion mechanisms and the resulting diffusion coefficients in pure-Fe and weakly alloyed ${M}_{3}\mathrm{C}$ cementites. The predicted coefficients in ${\mathrm{Fe}}_{3}\mathrm{C}$ are in good agreement with experimental measurements of carburization rate in ferritic steels. In our proposed diffusion mechanisms, C migrates by jumps between interstitial sites rather than via the C Frenkel pair mechanism, as proposed by previous studies based on semiempirical simulations. In the alloyed cementites, the C diffusion can be slowed down due to the presence of Mn solutes up to 500 K, while it is mostly unaffected by the addition of Mo or Cr solutes.