A thermokinetic model for Mg–Si couple formation in Al–Mg–Si alloys

Abstract

Mg–Si couples formed from atomic Mg and Si represent the first step in Mg–Si cluster formation in a dilute Al–Mg–Si system. Based on the thermodynamic extremal principle, a kinetic model for Mg–Si couple formation is developed. The model utilizes the trapping concept for the calculation of Gibbs energy of the non-equilibrium system and provides a generalized (multiplicative) form of the Oriani equation for description of the equilibrium state. The dissipation in the system accounts for diffusion of both Mg and Si atoms in the lattice.The model is compared with the classical Lidiard and Howard equilibrium theory. Some demonstrative examples are presented. Finally the model is applied to an experimentally studied system. Good quantitative agreement with quenching experiments is obtained, if, simultaneously, the impact of excess quenched-in vacancies and their gradual annihilation in the system, which has been already treated in a previous paper, are accounted for. The model is generally applicable for any couple (and pair) formation.