Thermodynamic calculation of phase equilibria in the Bi–Mg–Zn ternary system

Abstract

AbstractOn the basis of the experimental phase equilibria of the Bi–Mg–Zn ternary system, the thermodynamic assessment of the Bi–Mg–Zn ternary system was carried out by means of the CALPHAD (CALculation of PHAse Diagram) technique. The Gibbs free energies of the solid solution phases (Rhombohedral_A7, Hcp_A3 and Hcp_Zn) were described by the substitutional solution model, and that of the liquid phase was described by the associate model with the constituent species Bi, Bi2Mg3, Mg and Zn. The non-stoichiometric compound, α-Bi2Mg3 as well as its high temperature allotrope β-Bi2Mg3 were described by the sublattice models, (Bi,Zn,Va)2(Mg,Zn)3 and (Bi,Zn)1(Bi,Zn,Va)3(Mg,Zn)6, respectively. In order to obtain a set of self-consistent thermodynamic data, the thermodynamic parameters of the phases, including liquid, α-Bi2Mg3 and β-Bi2Mg3, of the Bi–Mg–Zn ternary system were optimized while the thermodynamic data of other phases were adopted directly from the literature reports of the constituent binary systems. With this newly developed thermodynamic database, the CALPHAD results can well reproduce the experimental phase diagram information of the Bi–Mg–Zn ternary system. The solidification processes of a typical Bi–Mg–Zn alloy are calculated and analyzed using the phase equilibrium approach and the Gulliver–Scheil model in comparison with the experimental investigation.