CALPHAD Modeling and Microstructure Investigation of Mg-Gd-Y-Zn Alloys

In this study, CALPHAD (CALculation of PHAse Diagrams) modeling was used to design and optimize Mg-Gd-Y-Zn alloys containing long period stacking order (LPSO) phases. The selected compositions were evaluated using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction to identify major phases and determine their area fractions. It was seen in as-cast samples that a blocky LPSO 14H phase formed at the grain boundaries while a filament-type LPSO 14H formed in the Mg grains. As the rare earth (RE) and Zn concentrations increased, eutectic Zn-rich intermetallics and more of the RE-rich blocky LPSO formed along grain boundaries. After annealing, an increase in the Zn-rich intermetallic area fraction, decrease in bulky LPSO area fraction, and increase in filament-type LPSO were observed. In higher alloyed samples, a Zn- and Y-rich phase was observed that was not consistent with the predicted or reported phase. These results indicate the present CALPHAD databases well represent the LPSO 14H formation in the Mg-Gd-Y-Zn system studied and can be used to tailor the microstructure to potentially improve the strength and ductility in these alloys. Further investigation is needed to determine if the existing reliably databases model the other secondary phases.