## 3 Results and Discussion
### Microstructure and Phase Analysis after the Single and Two-Stage Aging Processes
#### 3.2.2 EDS and Phase Analysis after Aging Treatments  
The SEM-EDS analysis of the as-compressed ZM6 after the aging process is shown in Figure 7.  
As seen in Figure 7a, the microstructure is composed of two parts: the gray portion is the \(\alpha\)-Mg matrix, that is, the solid solution Zn, Nd, and other elements in the \(\alpha\)-Mg matrix, and the grain is primary spherulite, which is present in a small amount in the eutectic phase and has an island-like distribution at the grain boundary [19]. The divorced eutectic \(\alpha\)-Mg phase is dependent on the \(\alpha\)-Mg growth that takes place during the primary phases. Meanwhile, the \(\beta\) phase (Mg\({}_{12}\)Nd) and the Zr-containing particles (Zn\({}_{2}\)Zr\({}_{3}\)) exist alone at the grain boundary [20]. From Figure 7b,c, after the pre-aging process, the precipitation phase has a coherent relationship with the matrix, and then the secondary aging phase increases and becomes semi-coherent with the matrix. Figure 7e shows that the as-compressed ZM6 is composed of the \(\alpha\)-Mg matrix and Mg\({}_{12}\)Nd phase. In XRD pattern in  
Figure 7: SEM-EDS and XRD analysis of as-compressed ZM6 alloy in discontinuous precipitated phases after aging process: (**a**) microstructure after the SSAT process; (**b**) EDS of sample after the SSAT process; (**c**) microstructure after TSAT process; (**d**) EDS of sample after the TSAT process; (**e**) XRD of sample after the TAST process; (**f**) XRD of sample after the SSAT and TSAT processes. magnesium alloys, the \(\alpha\)-mg Matrix has a dense hexagonal structure, with the precipitation of different rare earth phases and alloy phases, thus the matrix of the magnesium alloys can be strengthened [21; 22; 23; 24; 25]. From Figure 7f, the peak that is obtained by the TSAT process is higher than that that was obtained by the SSAT process, and the time that was needed to reach the peak was shorter due to the fact that there was a higher density of G.P. zones and a higher dispersion after the TSSAT process. The EDS analysis in Figure 7 after the SSAT and TSAT processes is listed in Table 3.  
As per the ESD analysis in Table 3, the second phase under the T6 condition is mainly a fine and dispersed \(\mathrm{\SIUnitSymbolAngstrom}\) phase [26; 27] when the peak hardness of the aging state is reached. Zr is used as a grain refiner in the ZM6 alloy, and the elements Zn, Nd, Zr, and Mg are present at the spot positions of the smaller particles. In order to further analyze the phase precipitates, the TEM, XRD, and EDS for the phase analysis before and after the two-stage aging process are shown in Figure 8.  
Figure 7a shows the TEM image and EDS energy spectrum of the grain boundary precipitates. The precipitates are combined with the matrix, and the precipitates are massive, ranging from about 200-300 nm in size, and the grain size varies greatly, with some sub-crystals appearing. In addition to the second phase at the grain boundary, a small number of lamellar and granular phases were found in the inner (IC) and grain boundary precipitates (GBP). Most of the lamellar phase is suspended in the crystal. There are two types of granular phases that are present in Figure 7a: one is a disorderly distribution in and around the grain boundary, where the size is less than 200 nm, and the other is a directional arrangement along or near the grain boundary in the G.P. zones and is needle-shaped. The grain boundary precipitate phase exists between the intracrystalline precipitates during the two-stage aging process. A flaky phase, spotted-state phase, and half-elliptical phase also exist. The pre-deformed specimen consists of not only the \(\beta^{\prime\prime}/\beta^{\prime}\) precipitates that are distributed, it also includes the globular or rod-shaped \(\beta_{1}\) (FCC) precipitates on the dislocations in the matrix [8]. From Figure 7b, after pre-aging appears, the G.P. zones are totally coherent with the matrix, and these G.P.