Carbon diffusion in cementite was explored (table 14). It was assumed that carbon atoms interacted with each other only indirectly, via neighbouring iron atoms. An interstitial mechanism of carbon diffusion in cementite was revealed. Carbon diffusion occurred via interstitial sites which formed four positions per Fe3C unit cell: (0.0, 0.0, 0.0), (0.5, 0.5, 0.0), (0.0, 0.0, 0.5) and (0.5, 0.5, 0.5) in units of the lattice parameters, a, b and c. The principal tracer diffusion coefficients and activation parameters of carbon diffusion in cementite were calculated for 1273 to 1373K and compared with available experimental data. It was argued that carbon diffusion was predominantly a consecutive chain of jumps: original carbon site → interstitial carbon site → original carbon site → ... The principal tracer diffusion coefficients of carbon atoms for this mechanism were obtained in a form predicted from random-walk theory. The formation energy (∼0.3eV/atom) of defects (carbon atom on an interstitial position and vacant site on original carbon position) as well as the migration energy (∼1.3eV/atom) for an elementary carbon atom jump in cementite were estimated for 1273 to 1373K from the molecular dynamics data.Molecular Dynamics Simulation and Theoretical Analysis of Carbon Diffusion in Cementite. Levchenko, E.V., Evteev, A.V., Belova, I.V., Murch, G.E.: Acta Materialia, 2009, 57[3], 846-53

Table 14

Average tracer diffusivity of C in Fe3C