Thermo-kinetic modeling of Mg-Si couple formation as a precursor to precipitation in Al-Mg-Si

Abstract

As the first step a thermodynamic model describing the Gibbs energy of the system accounting for Mg-Si co-cluster formation in an Al-Mg-Si matrix is developed. The equilibrium concentration of co-clusters is obtained by minimizing the Gibbs energy of the system for a given temperature, chemical composition and the trapping energy, describing the decrease of energy due to Mg-Si cluster formation relatively to isolated Mg and Si atoms. In the second step, the evolution equations describing the kinetics of dimer formation are derived by means of the Onsager (Ziegler) thermodynamic extremal principle. The studies based on this model, can significantly contribute to a better understanding of the early stages in nucleation of complex precipitates. The predictions of our model are compared to similar approaches in open literature, such as the models of Howard and Lidiard, and of Starink et al., which use different approaches to the treatment of the configurational entropy.