Modeling short-range ordering in liquids: The Mg–Al–Sn system

A liquid solution of components A and B may often exhibit a tendency towards short-range ordering (SRO). This may be modeled by the Modified Quasichemical Model (MQM) which attributes the SRO to the preferential formation of nearest-neighbor A–B pairs or, alternatively, by an associate model which attributes the ordering to the formation of AnBm associates or molecules. Although both models can often provide similar and equally good fits to experimental thermodynamic and phase equilibrium data in a binary system, the MQM provides significantly better predictions of the thermodynamic properties of ordered ternary liquid phases A–B–C solely from the optimized model parameters of the A–B, B–C and C–A binary sub-systems. This is illustrated through coupled thermodynamic/phase diagram optimization of the Mg–Al–Sn system. A similar example for the Mg–Al–Sc system is also presented.