## 3 Results and Discussion
### Microstructure and Phase Analysis after the Single and Two-Stage Aging Processes
#### 3.2.1 SEM and TEM Analysis after Aging Treatments  
Taking the two-step aging process as an example, the microstructures of the as-cast ZM6 alloy under various aging temperatures and aging times are shown in Figure 5.  
Figure 5a-c are the microstructures after pre-aging at 250 \({}^{\circ}\)C, 275 \({}^{\circ}\)C, and 300 \({}^{\circ}\)C with the holding time of 0.20 h, respectively, which are composed of polycrystalline \(\alpha\)-Mg and are unevenly distributed cluster precipitates. A spherical phase exists in the grain. From Figure 5a, the grain shape changes from a chrysanthemum-shaped to hexagonal structure, and the average grain size increases slightly, from 39.6 \(\upmu\)m to 41.0 \(\upmu\)m. Figure 5d-f correspond to microstructures that have been pre-aged at 275 \({}^{\circ}\)C for 0.15, 0.20, and 0.25 h and then re-aged at 200 \({}^{\circ}\)C for 8 h; the microstructures show cluster-like precipitates that are still inhomogeneous and that are dispersed in the \(\alpha\)-Mg solid solution. It can be seen from Figure 4b that that Mg\({}_{12}\)Nd compounds are around the \(\alpha\)-Mg grain boundaries and demonstrate a coarse network morphology with spicule and rod-shaped phases [17; 18]. It can be seen that after the final aging step, the average grain size of the as-cast ZM 6 is 39.6-41.5 \(\upmu\)m, and the average grain size increases slightly when the pre-aging time is prolonged. Moreover, the effect of the pre-aging temperature on the average grain size is greater than that of the pre-aging time.  
The TEM and HRTEM of the as-compressed samples with SSAT at 250-300 \({}^{\circ}\)C for 8.25 h and TSAT at 275 \({}^{\circ}\)C for 0.15-0.25 h + 200 \({}^{\circ}\)C for 8 h were analyzed, as shown in Figure 6.  
From Figure 6a, it can be seen that the \(\beta\)\" (Mg\({}_{3}\)Nd) phase is uniformly distributed in the \(\alpha\)-Mg matrix, but its size is relatively uneven. From Figure 6b, the black flake \(\beta\)\" phase that is larger in size is completely coherent with the matrix, and its length is about 25 nm. Figure 6c shows cluster-like precipitates with uneven distribution. The precipitates are rod-shaped and are 100-1500 nm in length, and a thin-layered (about 400 nm) precipitation-free zone is formed near the grain boundary. The intersecting lamellae are dense point-like phases that are accumulated from the edges. As seen in Figure 6d, after pre-aging at 275 \({}^{\circ}\)C for 0.15 h, a large number of G.P. zones are formed in the \(\alpha\)-Mg matrix. Figure 6c shows two typical dislocations, and a dislocation network can be observed as a characteristic subgrain boundary that is formed by the interaction of those dislocations. Figure 6d is a regularly arranged dislocation wall in which the same number of dislocations repel each other. Figure 6e shows the distribution of the \(\beta\)\" phase in the matrix after the 275 \({}^{\circ}\)C \(\times\) 0.20 h + 200 \({}^{\circ}\)C \(\times\) 8 h aging treatments. The distribution of the \(\beta\)\" phase in the \(\alpha\)-Mg matrix is more dispersed than the one that formed as a result of treatment with the T6_SSAT process. Figure 6f demonstrates that the \(\beta\)\" phase has a length of about 10 nm and a thickness 2 nm after the TSAT process. Compared to the SSAT process, the \(\beta\)\" phase becomes smaller after the TSAT process.