CALPHAD modeling of metastable phases in the Al–Mg–Si system

In the framework of the CALPHAD approach, a thermodynamic assessment of the metastable phases in Al–Mg–Si alloys is presented. The early Mg–Si co-clusters are described as a regular solid solution phase. A split model of fcc-based ordering is used for Guinier–Preston (GP)-zones as suggested by the assessment of microstructural data. The model parameters are optimized based on new thermodynamic first-principles data of the binary subsystems. CALPHAD parameters of Mg5Si6 and Mg1.8Si are refined for application in thermo-kinetic simulations. Mg5Si4Al2 is modeled as an Al-containing form of β″. B′, U1 and U2 are described as line compounds, with parameters derived from first-principles molar enthalpies. Vibrational entropies are determined by combination of first-principles calculations, experimental solvus and heat flux data of continuous-heating DSC. Decreasing entropies of formation of intermetallic Mg–Si and Al–Mg–Si phases correlate with decreasing enthalpies of formation, as predicted from the modeling. First-principles heat capacities are included in the optimization. Application of the assessed thermodynamic parameters in precipitation kinetics simulations is demonstrated in the computational evaluation of continuous-heating experiments of a quenched Al–Mg–Si alloy. The simulations show good accordance with the experimentally known evolution of metastable phases.