Characterization of an Al-Y-Zn Intermetallic Particle Phase in Extruded Mg_96−x Al _x Zn₂Y_1.9La_0.1 Alloys

This study focuses on extruded Mg_96−x Al _x Zn₂Y_1.9La_0.1 alloys, which contain the long period stacking ordered Mg₁₂ZnY phase. The presence of Al results in the precipitation of Al-Y-Zn precipitates, the crystal structure of which is not adequately described by ternary Al-Y-Zn phases reported in the literature. Therefore, the purpose of this investigation is to characterize the precipitates morphologically, chemically, and crystallographically. After casting, the precipitates exist as complexly shaped micron-scale clusters within the microstructure. After extrusion, the phase appears as slightly faceted particles with a plate-like morphology. The crystal structure is hexagonal and its space group is \( P6_{3} /mmc \). For the specific composition of Mg_95.50Al_0.50Zn₂Y_1.9La_0.1, the lattice parameters of this ternary phase are a ≈ 8.98 Å and c ≈ 7.1 Å and the stoichiometry is near equiatomic. However, with decreasing Al content of the alloy, the c parameter, degree of atomic ordering, and Zn content of the phase are all observed to increase. Specifically, for Mg_95.75Al_0.25Zn₂Y_1.9La_0.1, c ≈ 7.3 Å, and the stoichiometry is approximately AlYZn₂.