Evaluation of chemical nucleation driving force for stoichiometric compounds with variable composition

Abstract

This work puts in evidence some inconsistencies in the method currently used in the literature for estimating the driving force for nucleation of stoichiometric phases with variable chemical composition, such as carbonitrides of Ti, Nb, etc. in steel. An alternative approach, properly accounting for the capillarity contribution to the Gibbs free energy change associated to the formation of the critical nucleus, is proposed. The chemical composition of the critical nucleus, which maximises the driving force for precipitation and allows the calculation of the critical radius, is found. Some examples of application are presented and the results are compared to those from the classical approach and discussed. The proposed method eliminates the contradiction encountered in previous approaches found in the literature and demonstrates that for these compounds the usual thermodynamic solution of the chemical equilibrium, which refers to flat surfaces, can not be directly extrapolated to describe the nucleation.