**1. Introduction**

As the lightest green engineering structural material, magnesium alloy has many advantages, including high specific strength and stiffness, good shock and noise reduction performance, electromagnetic shielding, and easy processing and forming, etc. It has broad application prospects in transportation, aerospace, and military industries [1]. Shechtman et al. [2] first found the quasicrystalline phase in an Al-Mn quench alloy system in 1984. Unlike traditional crystals, quasicrystals have special symmetry of five or more times [3–6]. This structural particularity also makes them have high hardness and strength, as well as low friction coefficients, strong thermal stability, and corrosion resistance [7–9]. Therefore, introducing quasicrystals as a dispersion strengthening phase into magnesium alloys can theoretically compensate for the shortcomings of traditional magnesium alloys [10].

Quasicrystalline alloys have received more and more attention and recognition in recent decades [11,12]. In addition, studying the factors that affect the formation of quasicrystals for the synthesis and application of quasicrystals is significant. There are many factors affecting the formation of quasicrystals according to former studies, including cooling rate, composition, electronic structure, and melt treatment [13].

The quasicrystalline alloys of Mg-Zn-Re (where Re = rear earth element) have been extensively researched. It has been confirmed that the Zn/Y ratios of I-phase (Mg3Zn6Y1) and W-phase (Mg3Zn3Y2) are 6 and 1.5, respectively [14]. These ratios are in accordance with those for Mg-Zn-Gd alloys. In addition, W'-phase, H-phase, and other phases, which have different structures, exist in quasicrystalline alloys. Therefore, making a profound study of the structures and relationships between different phases will aid in obtaining a good understanding of the interlink among phases, which is of grea<sup>t</sup> significance to the study of the atomic structure of strengthened phases in high-performance magnesium alloys.

For Mg-Zn-Gd alloys, the icosahedral quasicrystal phase (I-phase) has been confirmed as being able to be made as an equilibrium phase during solidification or crystallization in a certain range of elemental components and holding time [15,16]. When the components or holding time change, the composition, structure, and symmetry of phases may change. In this paper, we prepared different Mg(70-x)Zn30-Gdx(x=3,4,5) (at. %) alloys and investigated the composition, structure, and symmetry of phases. This work focuses on the metastable phases in the Mg-Zn-Gd system; the formation mechanism of the symmetrical rod phase is also discussed.

### **2. Materials and Methods**

The experimental ternary alloys with nominal component Mg(70-x)Zn30Gdx(x=3,4,5) (at. %) were prepared by melting high-purity Mg (99.98 wt %), Zn (99.96 wt %), and master alloy Mg-Gd (30.21 wt %) in an electric resistance furnace. Firstly, Mg and Mg-30.21 wt % Gd alloys were placed into a graphite-clay crucible. When the temperature reached 720 ◦C and the alloys previously added had melted, Zn was added into the molten metal. After all the alloys had become molten, the melt was kept at 720 ◦C for minutes. The melt was then poured into a 200 ◦C preheated steel mold and cooled slowly in the atmosphere. Protective gas composed of CO2 and SF6 was always in the process of smelting to prevent evaporation and oxidation of components during smelting.

X-ray diffraction (Bruker D8 Advance) employing Cu Kα radiation was used to determine the constitution of phases with a scanning rate of ~5◦ min−1. The microstructure and morphology were characterized by SEM (FEI-QUANTA FEG250) equipped with EDS (X-MAX50) for analyzing the local chemical compositions of different phases and TEM (JEM-2100). The thermal stability and microhardness of phases was researched by DSC (HCT-1) and a Rockwell hardness tester. Sample preparations for TEM observations were made up of mechanical polishing and ion-beam thinning (GATAN-691).
