The Influence of Carbon Diffusion on the Character of the γ-α Phase Transformation in Steel

Abstract

During the austenite-ferrite phase transformation the excess amount of carbon that can not be dissolved in the growing ferrite phase accumulates in the austenite phase ahead of the moving interface. This local carbon enrichment along the interface in the austenite decreases the driving force for transformation. Two processes therefore determine the actual interface velocity: the transformation rate of the iron lattice from fcc to bcc and the diffusion of the carbon along its gradient into the austenite grain. If the diffusion of the carbon is the rate-determining process, the transformation occurs in the diffusion-controlled mode. If the rate of transformation is determined by the lattice transformation, this is called the interface-controlled or mean-field mode. When the transformation rate is not determined by a single process, this is referred to as the mixed mode of transformation.
The mixed-mode character can be identified by the carbon concentration in the austenite at the α-γ interface, x_C^{γ, γα}. If x_C^{γ, γα} is close to the concentration given by the local α-γ equilibrium, the character of the transformation is predominantly diffusion controlled. On the other hand, if diffusion is relatively fast, x_C^{γ, γα} is close to the average concentration in the austenite, and the transformation has a interface-controlled character. Due to the build-up of concentration gradients, the transformation character can gradually change during the transformation. In this paper, a two-dimensional study of a mixed-mode transformation is performed for Fe-C alloys of different carbon contents, and the character of the transformation is quantitatively determined.