Molecular dynamics simulation and theoretical analysis of carbon diffusion in cementite

Carbon diffusion in cementite is explored using molecular dynamics simulation. An assumption wherein carbon atoms interact with each other only indirectly via neighbouring iron atoms is used. An interstitial mechanism of carbon diffusion in cementite is revealed. Carbon diffusion is realized via interstitial sites, which form 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 are calculated for the temperature range 1273–1373 K and compared with the available experimental data. It is argued that carbon diffusion is 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 are obtained in a form predicted from random-walk theory. The formation energy (∼0.3 eV atom−1) of defects (carbon atom on an interstitial position and vacant site on original carbon position) as well as the migration energy (∼1.3 eV atom−1) for an elementary carbon atom jump in cementite are estimated in the investigated temperature range 1273–1373 K from the molecular dynamics data.