## 3 Results and Discussion
### Analysis of the Alloy Phase Diagram and Initial Microstructures
#### 3.1.1 Alloy Phase Diagram Analysis of Mg-Nd-Zn-Zr Alloy  
The representative two binary phase diagrams of the Zn-Mg, Nd-Mg [13], Zr-Mg [14], and Mg-Nd-Zn-Zr alloys are shown in Figure 2.  
From Figure 2a, according to the Mg-Zn binary phase diagram, the maximum solubility of Zn in Mg is 6.18 wt% under equilibrium conditions. As the composition of the Zn element in the ZM6 (Mg\({}_{2.6}\)Nd\({}_{0.4}\)Zn\({}_{0.4}\)Zr) alloy was 4%, which is under the equilibrium solidification condition, all of the Zn was dissolved in the Mg matrix, and the solidification rate of Zn was much higher than that of equilibrium solidification, with a small Mg-Zn mesophase being present along the grain boundary [10]. As seen in Figure 2b, the formed precipitated-phase Mg\({}_{12}\)Nd contained a large number of Mg atoms with a high melting point and good thermal stability [10; 13]. Generally, the melting points of rare earth elements are relatively high, reaching 798-1663 \({}^{\circ}\)C, and their diffusion in the Mg matrix is relatively slower [2; 3]. The strength of the Mg-Nd-Zn-Zr alloy increased by 0.5-1 times, and the limiting temperature increased to 350 \({}^{\circ}\)C compared to the AZ31 alloy [10]. Nd can improve the creep strength of the ZM6 alloy [10]. Figure 2c shows the Mg-Zr phase diagram and demonstrates that the peritectic reaction took place at 649 \({}^{\circ}\)C, when the Zr content was 0.58. It should be noted that the Zr content at the peritectic reaction point varied from 0.50 to 0.76 in different alloys, demonstrating an obvious grain refinement effect [14]. However, this diagram cannot explain the alloy that had a Zr content of less  
Figure 2: Solidification of binary and ternary phase diagrams: (**a**) Phase diagram of Mg-Zn alloy; (**b**) phase diagram of Mg-Nd alloy; (**c**) phase diagram of Mg-Zr alloy; (**d**) phase diagram of ZM6 (Mg\({}_{2.6}\)Nd\({}_{0.4}\)Zn\({}_{0.4}\)Zr) alloy.  
than 0.58%. The Zr addition amount should be 3-5 times that of the dissolved Zr, that is, the amount of Zr that is added in the ZM6 alloy should be 1.32-2.2% during the casting process. As seen in Figure 2d, the ZM6 alloy phase diagram is composed of the liquid zone, the Mg matrix, \(\alpha\)-Zr, Mg\({}_{41}\)RE\({}_{5}\), and Mg\({}_{12}\)RE. For the ZM6 alloy, the composition of the magnesium-rich compound REMg\({}_{12}\) for the light rare earth metals and RE\({}_{5}\)Mg\({}_{24}\) for the trivalent heavy rare earth metals formed a eutectic equilibrium with Mg. The Mg\({}_{12}\)RE phase exists below 234.6 \({}^{\circ}\)C. The pouring temperature should be above the liquid point of 549.5 \({}^{\circ}\)C.  
In order to reduce the hot cracking tendency that Mg-Zn-Zr alloys have, the rare earth metal Nd was added into the ZM6 alloy, which dissolved in the Mg matrix. At the magnesium-rich end, the peritectic reaction and the solid-phase transformation occurs more easily, so a metastable alloy can be easily formed at this end. As the melting points of the rare earth metals increase, the phase diagram shows a smooth change, and several rare earth compounds gradually disappear. The addition of Nd (2.0-3.0%) greatly reduces the solid solubility of zinc in magnesium, and the eutectic that is distributed in the grain boundary is brittle. The higher the content of rare earth elements in the alloy is, the better the casting properties are, but the mechanicals properties will be lower.