Adapting the condensation and evaporation model to the study of kinetics of phase transformations in binary metal systems

Abstract

A binary system in solid solution when is quenched at a temperature under the solubility limit, shows a structural instability that disappears when said system reaches equilibrium. In the process of reaching stability, a number of microstructural modifications or fluctuations in concentration occur, which tend to generate a decrease in the free energy of the system. Critical fluctuations arise, prompting the association of critical size clusters, from which the growth of the phases responsible for final equilibrium is produced. Such proposal falls completely under the nucleation and growth theory where the first stage --before critical fluctuation appears, occurs at nucleation, whereas the latter stage corresponds to growth and coalescence of the clusters. The evaporation and condensation theory has been adapted so that the process of phase transformation is simulated while considering different evolution pathways from solid solution to the phase of equilibrium. Because the nucleation phase is energetically unstable, it is characterized by each cluster in that stage, while every cluster in the metastable or stable growth phase is grouped via the corresponding transformed fraction, which includes every metastable or stable cluster formed for a given aging time and temperature, thus allowing phase transformation to be expressed as one equation per phase plus the nucleation equations instead of an equation per cluster of different size. This paper expands on the calculation of the problem, determining the effect of the different parameters affecting phase kinetics in the Fe-C system for very low concentrations of C.