Hybrid calphad DFT modelling of the Mg–Al–Ca system

Abstract

The Mg–Al–Ca system is an important system for Mg alloys. Al is the most common alloying element for Mg alloys, and additions of Ca can be used to produce ductile alloys and alloys with good creep properties. The Mg–Al–Ca system has been investigated extensively in the past, in particular the Mg-rich corner. Also, calculations using density functional theory (DFT) have been extensively applied in the past. DFT calculations generally show high accuracy and consistency in this system. The Mg–Al–Ca system contains all three Laves phases (C14, C15 and C36), but existing thermodynamic models of this system only contain rather simplified and limited models for the Laves phases. In this work, DFT calculations are used to provide energies of formation for all possible end-members of the Laves phases and the Mg17Al12 (A12) phase. This is combined with a thermodynamic modelling of the system to provide a thermodynamic description of the system where the Laves phases and the Mg17Al12 phase have reasonable Gibbs energies for the complete composition space. The new model description also provides a good representation of most existing experimental data, in particular in the important Mg-rich corner.