Microstructure of solidified Mg-Gd binary alloys: Unveiling nature and heredity of controversial cubic phase

The Gd-rich cubic phase, which is ubiquitous in Mg-Gd alloys, plays a crucial role in determining their mechanical properties. However, its chemical composition has long been debated, and it has often been misidentified as MgGd or GdO. This study focuses on the origin and hereditary characteristics of the cubic phase. Through multiscale characterization techniques such as X-ray diffraction analysis, scanning electron microscopy, focused ion beam, and transmission electron microscopy, the microstructural evolution of three as-cast Mg-Gd binary alloys and their diluted counterparts (Mg-10 wt% Gd) was systematically investigated. The spatial distribution, crystal structure, and chemical composition of the cubic phase were examined. Microstructural analysis reveals that the cubic phase in the Mg-Gd binary alloy exhibits a face-centered cubic structure (lattice parameter a ≈ 0.5328 nm), and it is identified as GdH 2 . Additionally, trace amounts of α-Gd are detected. Upon dilution to Mg-10 wt% Gd, the GdH 2 phase remains largely undissolved, demonstrating stable hereditary characteristics, whereas the submicron α-Gd phase completely dissolves. The hereditary characteristics of GdH 2 are closely associated with the H content in the melt and the initial microstructure of the Mg-Gd binary alloy. This study resolves the existing controversy regarding the cubic phase, establishing a theoretical foundation for optimizing the design of Mg-Gd alloys and enhancing the strength–ductility synergy.