## 3 Results and Discussion
### Experimental Tensile Properties under Various Heat Treatments
#### 3.3.3 Comparison of Mechanical Properties under Various Heat Treatment Conditions  
Comparison of mechanical properties as well as grain size in the as-cast and as-compressed alloys under the T5 and T6 conditions are shown in Figure 14.  
Figure 14: Mechanical properties of ZM6 alloys under different conditions: (**a**) mechanical properties under various as-cast treatment conditions; (**b**) mechanical properties under various as-compressed treatment conditions. Figure 13: Comparative mechanical properties of as-compressed ZM6 alloy by T5 and T6_SAST processes: (**a**) tensile and yield strength; (**b**) elongation and hardness. Figure 14a shows the tensile strength at room temperature in different states. Figure 14 shows a comparison of the tensile properties between the as-cast state and the compression state. After the hot compression in Figure 14b, the strength is further increased because the microstructure is further refined and because the grain boundary per unit volume increases. During the tensile process, the non-complete microstructure homogeneity makes it difficult for the external load to be completely distributed and transferred to the neighboring grains, and these large grains become the source of the fracture, leading to the ductility decreasing. Under the same conditions, the mechanical properties of all of the compressed alloys are obviously better than those of the as-cast alloys. The reason for the differences in the mechanical properties is that more dispersed precipitates formed in the G.P. zones after secondary aging, thus strengthening the alloy [40; 41]. Compared to the as-cast samples, the strength and elongation of the as-compressed ZM6 alloy increase. This is due to increases in both grain refinement and the grain boundaries, which can effectively prevent dislocation movement, resulting in increases in both the strength and elongation. Mechanical properties, hardness, and the subsequent ZM6 heat-resistant magnesium alloy phases under different heat treatments are listed in Table 4.  
As seen in Table 4, the ultimate tensile strength and the elongation of the as-cast ZM6 alloys are 155.36 MPa and 3.14%, respectively. After hot compression, the intermetallic compounds that broke down during the compression process also prevent the dislocation from moving, resulting in an increase in the tensile strength. For the as-compressed state, the \(\sigma_{\rm b}\) increased to 294.46 MPa, and the elongation \(\delta\) increased from 3.14% to 10.93% compared to that of as-cast ZM6. The matrix is strengthened by the appearance of a new grain boundary; thus, the strength and ductility are largely improved through hot compression. The ductility of the as-compressed alloys was higher than that of those alloys that were in the as-cast state. The UTS value of the as-compressed alloys obviously decreased, and the elongation increased after T4 treatment. After T5 treatment, the UTS value of the as-compressed alloy increased slightly, and the elongation the at break decreased. The elongation that took place during two-stage aging is lower than the elongation that took place during single-stage aging, where the elongation decreased by 5.12% and the tensile strength increased by 9.73%; thus, the heat treatment period is shortened. For the strengthening mechanisms, S.S.S. represents the solid solution strengthening, P.S. represents the precipitation strengthening, F.G.S. stands for the fine grain strengthening, and M.S. is the matrix strengthening. After the T4 solution aging treatment, a discontinued precipitated phase was produced at the grain boundary at the initial aging stage, and the elastic modulus of the matrix was therefore increased by solid solution strengthening (S.S.S.). After the TAST process, the precipitated-phase Mg\({}_{12}\)Nd with a BCT structure was linear or banded along the base plane of (0001) Mg, and the density near the grain boundary was higher than that in the crystal. The particles with linear distribution had an obvious strengthening effect on the Mg matrix.