Design magnesium alloys: How computational thermodynamics can help

Thermodynamics has often been viewed applicable to states near equilibrium only although irreversible thermodynamics was already developed in 1950s. The CALPHAD technique of computational thermodynamics developed since early 1970s has helped to change this view. This technique couples the phase diagram and thermochemical properties to explicitly characterize all phases in a system, including stable, metastable, and unstable phases over a wide range of temperature, pressure and composition. The modeling of Gibbs energy of individual phases enables the calculation of driving forces between any intermediate non-equilibrium states for simulating dynamic microstructure evolutions. As same as the most commercial alloys, magnesium alloys are multi-component in nature with many intermetallic phases. To develop robust alloys that are less sensitive to process variability, phase relations under both equilibrium and non-equilibrium conditions are extremely valuable for the design of alloy compositions and processing procedures. In this presentation, the CALPHAD technique will be discussed. Particular attention will be paid to the phase relations in the Mg-Al-Zn ternary alloys.