Equilibrium Versus Paraequilibrium Precipitation of Cementite in Ternary Alloys: Thermodynamic and Kinetic Interpretations

The precipitation of cementite on decomposition of austenite is examined by an edgewise layer growth model of the carbide in Fe-1.2C wt% steels. The interface boundary can migrate into the supersaturated matrix isothermally at 400-600 A degrees C, similar to tempering. The present paper thus explores precipitation mechanisms by two different kinetics. Intitially, an equilibrium growth, by the diffusional transport of atoms is simulated with the Dictra package accessing thermodynamic parameters from Thermo-calc. An extremely slow rate for the growth of the carbide platelet seems to be implausible due to the presence of the ternary [M = Mn, Si, Al] of 2 wt%. To overcome this, the investigation then finds an alternative, bringing diffusion constraints of the substitutional atoms under consideration in a paraequilibrium growth study. The SGTE database of the Thermo-calc however lacks paraequilibrium composition of Al in the carbide, thus Thermo-calc cannot be used thoroughly for this study. A first-principles calculation therefore is necessary which in turn evaluates the time-lag in equilibrium versus paraequilibrium growth kinetics. It reveals that the paraequilibrium precipitation of cementite is decisive due to Si (or Al) as the ternary, whereas, in the case of manganese steel, an opposite trend can be seen that the equilibrium partition of Mn reduces the Gibbs free energy during the interface study.