The carbon-carbon interaction energy in alpha Fe-C alloys

Abstract

The experimental data of Lobo and Geiger on the activity of carbon in ferrite have been re-analyzed in detail using the statistical thermodynamic relationships of Darken-Smith/Kaufman-Radcliffe-Cohen, Lacher-Fowler-Guggenheim and McLellan-Dunn. Indirect regression analyses had usually to be employed in order to extract a value for the interaction energy between nearest neighboring carbon atoms, ω α . However, the Lobo-Geiger data reported at only two temperatures fulfilled the requirements for performance of a valid indirect analysis using the three expressions correlating activity with composition and ω α . At both temperatures, all three analyses yielded ω α < 0, i.e. an attractive interaction between adjacent carbon atoms; ω α values obtained ranged from −27 to −42 kJ/mol. Published reports that ω α > 0 were shown to have resulted from failure to fulfill the requirements for a valid indirect regression analysis. Using a relationship for the electrostatic interaction energy between carbon atoms due to Blandin, Deplante, Friedel and Machlin, and an equation for elastic strain energy interaction due to Eshelby, it was concluded that the former energy is the more important contributor to both ω α < 0 and ω γ > 0; in both ferrite and austenite, the elastic interaction and electrostatic interaction energies are of opposite sign.