## 3 Results and Discussion
### Microstructure and Phase Analysis after the Single and Two-Stage Aging Processes
#### 3.2.3 Precipitation Mechanism and Strengthening Mechanism  
After the two-stage aging process, the tensile surface fracture at ambient temperature was determined and is shown in Figure 9.  
Figure 9a shows that the as-cast ZM6 alloy has more obvious ductile fracture characteristics with an obvious tearing line and with spherical particles in the dimples. Moreover, the tensile deformation mechanism of the precipitation phase is mainly conducted by slip and twinning, and the dislocations are shown to demonstrate a continuous flow along the slip surface. Figure 9a shows how cleavage fractures and torn edges appeared; the intergranular fracture is the main mode and is partly along the mixed transgranular. In Figure 9b, the macroscopic fracture of the as-compressed ZM6 alloy is flush, and the granular fracture is depicted with a darker color. As the deformation of the magnesium alloy is not coordinated, and the crack source is easy to produce in the precipitation phase [30; 31], which leads to a decrease in the elongation. In the durable fracture that is caused by aging in Figure 9b, there are many cracks in the grain boundary, and the grain boundary crack is more obvious. Although the tensile fracture surface is ductile, Mg(Zn)Zr compound particles still exist in the deep part of the dimples. In fact, the dislocation slip and deformation twins of wrought magnesium alloys often interact [32; 33], which has an important effect on mechanical properties  
The precipitation formation mechanism that takes place during the aging process is shown in Figure 10.  
Figure 10a shows the schematic diagram of the precipitate transformation mechanism in a single-stage aging process. Generally, the existence of a G.P. zone ranges from room temperature to 180 \({}^{\circ}\)C, a\({}^{\prime\prime}\) ranges from 180 \({}^{\circ}\)C to 260 \({}^{\circ}\)C, and a\({}^{\prime}\) ranges from 200 \({}^{\circ}\)C to 320 \({}^{\circ}\)C. Meanwhile, a exists at a higher temperature range, from 300 \({}^{\circ}\)C to the solvus temperature of about 550 \({}^{\circ}\)C [31; 32]. The precipitation order is found to be [31; 32; 33]: supersaturated solution\(\rightarrow\)G.P. zones\(\rightarrow\)\(\alpha\)(hcp)\(\rightarrow\)\(\beta\)\"(D0\({}_{19}\))\(\rightarrow\)\(\beta\)\"(fcc)\(\rightarrow\)\(\beta\)(bct), and the grainy precipitate is varied: the phaseflaky phase, spotted state phase, and half-elliptical phase. The G.P. zones are needle shaped and are located along the (000l) direction, as shown in Figure 10b. When a single-stage aging treatment is applied at 200 \({}^{\circ}\)C for 8 h, heterogeneous \(\beta\)\" phases [3; 30] are rapidly precipitated from a supersaturated solid solution and grow rapidly due to the high aging temperature and due to early aging. However, in a certain range near the precipitates of the \(\beta\)\" phase in the early stage of aging, a dilution zone of solute atoms will be formed. As the aging time increases, the dilution zone will expand as the \(\beta\)\" phase continues to grow in the depleted zone of solute atom, making it difficult for the \(\beta\)\" phase to grow a nucleus during the later aging period. Therefore, after single-stage aging at 200 \({}^{\circ}\)C for 8.25 h, the size and dispersion of the \(\beta\)\" phase are larger and lower. Figure 10c shows the schematic diagram of the phase transformation and precipitation  
Figure 9: Tensile surface fracture of ZM6 after the two-stage aging process: (**a**) as-cast state; (**b**) as-compressed state. mechanism during the two-stage aging process. Compared to single-stage aging, the \(\beta\)\" phase density becomes larger, the size becomes smaller, and the \(\beta\)\" phase distribution is more dispersed [34; 35; 36]. During the pre-aging process, a large number of dispersed G.P. zones are first formed in the matrix. During the two-stage aging process, heterogeneous nuclei from the \(\beta\)\" phase in the G.P. zones increase and form a large number of dispersed \(\beta\)\" phases.