The point defect structure of cementite (Fe3C) was investigated by using a combination of the statistical mechanical Wagner–Schottky model and first-principles calculations within the generalized gradient approximation. Large 128-atom super-cells were used to obtain fully convergent point defect formation energies. The present study unambiguously showed that carbon vacancies and octahedral carbon interstitials were the structural defects in C-depleted and C-rich cementite, respectively. The predominant thermal defects in C-depleted and stoichiometric cementite were found to be carbon Frenkel pairs. In C-rich cementite, however, the primary thermal excitations were strongly temperature-dependent: inter-branch, Schottky and Frenkel defects predominated successively with increasing temperature. Using the nudged elastic band technique, the migration barriers of major point defects in cementite were also determined and compared with available experiments in the literature.

Point Defect Thermodynamics and Diffusion in Fe3C - a First-Principles Study. C.Jiang, B.P.Uberuaga, S.G.Srinivasan: Acta Materialia, 2008, 56[13], 3236-44