Due to their high specific strength, magnesium alloys have become increasingly attractive for automotive structural applications. It has previously been shown that Zr is a very effective grain refiner of Mg-Zn alloys. In this work, we present a thermodynamic model of the Mg-Zn-Zr system based on the CALPHAD method. Using this model, the influence of Zn additions on the grain size of Mg-Zr alloys is discussed based on the thermodynamic properties of the Mg-Zn-Zr ternary. Calculations of the liquidus surfaces and equilibrium solidification are performed and related to experimental observations. The possible formation of metastable intermetallic phases during solidification and heat treatment is also discussed. Solubility of Zr in Mg-Zn-Zr alloys as well as phase fractions of intermetallic compounds are systematically studied as these parameters greatly affect grain refining.
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