Phase equilibria and transformations in ternary Mg-Gd-Zn alloys

Phase equilibria and transformations in ternary Mg-Gd-Zn alloys were studied experimentally by investigating 22 samples with up to 40 at.% Gd and 60 at.% Zn. These results, combined with a critical review of literature data, were used to develop thermodynamic descriptions of the Gd-Zn and the Mg-Gd-Zn systems and to calculate the Mg-Gd-Zn phase diagram in the entire composition range. In the ternary system the hexagonal long-period stacking ordered structure 14H, the hexagonal phase H1, Gd11Mg24Zn64, and five solid solution phases with ternary compositions, GdMg5, GdMg3, GdMg2, Gd(Mg,Zn), and W are found to be stable, whereas the icosahedral phase I, GdMg3Zn6, and other hexagonal phases are metastable. In addition, a new stable phase Gd20Mg19Zn81 with a face-centered cubic structure is identified in the Mg-Gd-Zn system. Based on the thermodynamic description obtained in this work, the microstructures and phase transformations previously observed in the as-cast, solution heat treated and aged conditions are discussed. It is shown that the current thermodynamic predictions are in good accord with the experimental findings, suggesting the viability of such an approach for the microstructure design of Mg-Gd-Zn alloys.