Characterization on the three-dimensional morphology and microstructure of eutectics in as-cast Mg-Gd-Y-Zr alloys by using X-ray tomography technique

Abstract The three-dimensional morphology and microstructure of the eutectics in as-cast Mg-Gd-Y-Zr alloy were investigated by using X-ray tomography technique, in which the phase constitution of the eutectics was studied by using SEM and EDS. The primary phase (α), supersaturated Mg phase (α’) and secondary phase (β) were characterized, based on which the boundaries of primary phase/eutectics and α’ / β were clearly identified, and the quantification of the eutectics in the Mg-Gd-Y-Zr alloys was investigated, such as the volume fraction of the eutectics, the fraction of β phase in the eutectics (fβ/eu) and the ratio between the fraction of β and that of α’ phases (fβ/a′). Furthermore, the relationship between the microstructure and quantification of eutectics in the magnesium alloys solidified at different cooling rates were discussed. It is revealed that the eutectics of the studied alloys present complex network topology structure in the three-dimensional space, and the networks of eutectics become denser with the increase of cooling rate. Besides, the study shows that the microstructure of the eutectics, the lamellar or rod-like geometry, in the studied alloys is closely related to fβ/eu and fβ/a′ of the eutectics.