Phase Stability and Formation in Mg-Gd-Zn Alloys-Key Data for ICME of Mg Alloys

Thermodynamic databases of multicomponent alloy systems are the indispensable basis to apply and develop ICME of Mg alloys. Among the multicomponent Mg alloys with rare earths (RE) and Zn the Mg-Gd-Zn system is found to be a prime example of complex phase formation, both under stable and metastable conditions. This alloy system was studied by a combination of dedicated experiments and thermodynamic modeling. Two metastable phases, I and H2, have been identified in as-cast Mg-rich Mg-Gd-Zn alloys by TEM characterization. Quantitative thermodynamic descriptions of the metastable Mg-GdZn phases and metastable phase diagrams are developed, embedded in a complete Calphad modeling of all stable phases. Dedicated thermodynamic calculations using constrained Scheil solidification simulation reveal conditions for I and H2 phase formation and transformation.