Metastable phase formation in ternary Mg-Gd-Zn alloys

Two metastable phases have been identified in as-cast Mg-rich Mg-Gd-Zn alloys by TEM characterization. The I-phase was found to have a composition close to Gd₁₁Mg₂₉Zn₆₀ with an icosahedral structure (a_R = 0.524 nm). The H2-phase, with a composition close to Gd₁₆Mg₁₄Zn₇₀, was identified to have a hexagonal structure (a = 0.937 nm and c = 0.977 nm). An extension of the thermodynamic description of the ternary Mg-Gd-Zn phase diagram to incorporate the formation of these metastable phases has been modelled. Thermodynamic simulation of metastable phase formation has been conducted by suppressing the formation of more stable phases in a constrained Scheil solidification simulation to form I and H2 phases. The results of these solidification simulations are compared with reported as-cast alloy structures. Limiting values for the Gd/Zn atomic ratio, ∼<0.55 and ∼>1.4, are found to be crucial for phase selection. An application to Mg-Zn-Gd-Y alloys is discussed briefly.