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A developed and verified thermodynamic model based on the atom and molecule coexistence theory (AMCT) is employed to predict activities relative to pure liquids in standard state in Mg-Al, Mg-Zn, Al-Zn and Mg-Al-Zn melts through the calculated mass action concentrations of structural units, i.e., Ni. According to AMCT, Ni can be extrapolated and calculated by the chemical equilibrium constant of a structural molecule, i.e., Ki, in the Mg-Al-Zn ternary system and binary subsystems. In this paper, the standard Gibbs free energy function, for reported activities and mixing thermodynamic properties in Mg-Al, Mg-Zn and Al-Zn melts, was regressed and optimized. The results showed that Ki and Ni were deduced by Gibbs free energy function at the studied temperature. The results of calculating thermodynamic properties in the full composition range for liquid Mg-Al-Zn from 880 to 1100 K, as well as Mg-Al from 923 to 1073 K, Mg-Zn from 880 to 973 K and Al-Zn from 1000 to 1073 K, are presented in the paper by coupling with Ni and AMCT. An excellent agreement is noticed between the calculated values of this study and measured thermodynamic data from the references, suggesting that the AMCT can be well applied to describe and predict the activities of the Mg-Al-Zn system and its subsystems.

The thermodynamic properties of the full composition range of liquid Ni-Ti, Ni-Al, and Ti-Al alloys were determined using the atom and molecule coexistence theory (AMCT), which is an established thermodynamic model that was used to calculate the mass action concentration (activity) of the structural units in the abovementioned three alloy systems. The temperature dependence of the activity coefficients γ i θ of Ti and Al dissolved in molten nickel to form dilute solutions (0 ≤ x i ≤ 0.01) relative to pure liquid were estimated from the calculated activity coefficients of Ti and Al at different temperatures. The standard Gibbs free energy changes Δ sol G m , i θ of dissolving Ti and Al in nickel and the enthalpies of mixing Δ mix H m, i of liquid Ni-Ti, Ni-Al, and Ti-Al were also obtained. The results indicated that the determined thermodynamic properties, such as the Δ sol G m , i θ , and Δ mix H m, i of the liquid binary alloys reasonably agreed with the experimental data previously reported in the literatures. Thermodynamic properties were predicted for three important subsystems, which included the Ni-Ti, Ni-Al, and Ti-Al alloys, and therefore, the activity a i and Δ mix H m, Ni‐Al‐Ti of the nickel-rich liquid Ni-Al-Ti ternary alloys were also obtained at 1873 K. Furthermore, the values calculated using the classical geometric models, including the Toop, Kohler, and Muggianu models, were compared with those determined by the GSM in this study. • Atoms and molecules coexist in the liquid Ni-Al-Ti ternary alloys. • Thermodynamic properties of liquid Ni-Ti, Ni-Al, Ti-Al binary alloys • The enthalpies of mixing of nickel-rich liquid Ni-Al-Ti ternary alloys at 1873 K. [ABSTRACT FROM AUTHOR]