A NEW MODEL FOR DIFFUSIONAL GROWTH WITH NONEQUILIBRIUM INTERFACE CONDITIONS

A new model for diffusional growth including massive transformations is presented. The thermodynamic stability of a thin layer at the interface determines if a transformation can occur. A new mass balance relation for the phase transformations at the interface is introduced. The interface velocity is modeled as a function of the interphase structure and of the Gibbs molar free energy difference for the transformation of the layer. The interface compositions are determined by the dynamics of the system, i.e., no assumption about the interface composition, e.g., local equilibrium, is made. No special treatment is necessary to cover, e.g., para-equilibrium, as it is automatically included by the general formalism. Transinterphase diffusion is included via a simple description. The model can simulate both diffusion control and interface control. The model is demonstrated for a simple model system and the austenite-to-ferrite transformation in an Fe-0.11 pct C alloy at 770-degrees-C.