## 3 Results and Discussion
### Experimental Tensile Properties under Various Heat Treatments
#### 3.3.2 Effect of Aging Time on Tensile Properties of As-Cast and As-Compressed ZM6  
After the solid solution process (525 \({}^{\circ}\)C \(\times\) 4 h), for the T6_SAST process, the aging temperature region was 275 \({}^{\circ}\)C, and the aging time was between 8 h and 20 h. Moreover, for the T6_TAST process, the regulation of the pre-aging temperature region was 275 \({}^{\circ}\)C, the pre-aging time was 6.0-15.0 min, and the final aging temperature was at 200 \({}^{\circ}\)C with a holding time of 8.0 h. The effects of the aging time on the mechanical properties that were affected by the aging process are shown in Figure 12.  
After solution treatment, the length of time that was required for the subsequent aging process had an important effect on mechanical properties. From Figure 12a,b, the mechanical properties that were achieved by the T6_SSAT process reached their peak values  
Figure 11: Mechanical properties and grain size of ZM6 alloys under various aging temperatures: (**a**) tensile and yield strength by SAST process; (**b**) hardness, elongation, and grain size by SAST process; (**c**) tensile and yield strength by TAST process; (**d**) hardness, elongation, and grain size by TAST process. after 16 h of aging, demonstrating a tensile strength of 230.32 MPa, a yield strength of 149.65 MPa, and hardness of 70.5 HV. For the T6_SSAT process, the highest tensile strength appears at the pre-aging time of 12 min, where \(\sigma_{\text{b}}=294.46\) MPa, \(\sigma_{0.2}=188.72\) MPa, and \(\delta=10.93\%\). When the aging time exceeds 16 h, the tensile strength decreases, elongation increases, and over-aging occurs. During the aging process, the changes in the precipitation phase directly affect the changes that take place in the mechanical properties of the alloy. When the alloy is aged, the strength is increased, and the plasticity is decreased due to the precipitation of the a\" (Mg3 Nd) and a (Mg12 Nd) phases [37; 38; 39]. When the aging time is 16 h, the UTS reaches 294.46 MPa, and the elongation decreases to 10.93%. From Figure 12c,d, as the pre-aging time increases, the strength and hardness increase first and then decrease after the final aging period, and reach their maximum values at 275 \({}^{\circ}\)C, where the effect of pre-aging at 275 \({}^{\circ}\)C for 10 min is the best. For the two-stage aging process at 525 \({}^{\circ}\)C \(\times\) 4 h + 275 \({}^{\circ}\)C \(\times\) 12 min + 200 \({}^{\circ}\)C \(\times\) 8 h, the comprehensive mechanical properties of the as-compressed ZM6 alloy are the best, demonstrating a yield strength of 188.72 MPa, a tensile strength of 294.46 MPa, and a hardness of 82.56 HV compared to the single-stage aging process. There is a very good short-term (12 min) aging limit between 250 \({}^{\circ}\)C and 325 \({}^{\circ}\)C.  
The comparative mechanical properties of the as-compressed ZM6 alloy that were achieved by the T5 and T6_SAST processes are shown in Figure 13.  
Figure 12: Mechanical properties and grain size of ZM6 alloys under various aging times: (**a**) tensile and yield strength by SAST process; (**b**) hardness, elongation, and grain size by SAST process; (**c**) tensile and yield strength by TAST process; (**d**) Hardness, elongation, and grain size by TAST process. From Figure 13a, the strength increases gradually and then decreases slightly as the aging time increases. After solution treatment and aging for 16 h, the strength of the as-compressed ZM6 alloy reached its highest value of 268.84 MPa when the aging time was 16 h, and the elongation of the alloy also drops to the lowest point, but the elongation of 11.54% is still higher than that of the as-cast alloy. From Figure 13b, when the aging time was 16 h by T6_TSAT process, the elongation decreased from 11.95% to 10.62%. After aging for 16 h, during the longer aging time, the mechanical properties of the alloy did not change much. When the aging time for T6_TSAT the process is prolonged from 16 h to 64 h, the strength only changes by 3.1 MPa, and the elongation decreases by 0.5%.