Regensburg 2007 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 33: Diffusion and point defects II

MM 33.4: Talk

Thursday, March 29, 2007, 12:30–12:45, H6

Ab initio study of the carbon-carbon interaction in iron — •Olga Kim, Martin Friák, and Jörg Neugebauer — Max-Planck-Institut für Eisenforschung, Max-Planck-Strasse 1, 402 37, Düsseldorf, Germany

The difference in solubility of carbon between the austenite and ferrite is the basis for the solid solution hardening of steels. The C-solubility is determined by the C-C interaction energy that influences the distribution of carbon atoms in both phases. The interaction energy in austenite is easily measurable and well known. However, in ferrite it could not be deduced experimentally due to the very low solubility of carbon in ferrite. In order to gain a detailed insight into this issue we have theoretically determined the dependence of the C-C interaction energy in ferrite as function of the C-C separation by employing density functional theory (DFT) in the generalized gradient approximation (GGA). Our results show that interstitial carbon atoms strongly repel each other. This effect gives rise to the experimentally observed low solubility of carbon in ferrite. The theoretical data provides also direct insight into ordering tendencies of carbon atoms and thermodynamic stability. In a second step the analogous calculations have been performed for austenite. Using these data the temperature dependent carbon activity in austenite is derived and compared to results obtained by the quasi-chemical approximation (R. B. McLellan and W. W. Dunn: J. Phys. Chem. Sol. 30, (1969) 2631) and with available experimental data.