## 2 Materials and Methods
### Experimental Heat Treatment Schedules and Property Measurements  
#### 2.2.1 Heat Treatment Schedules of As-Cast and As-Compressed ZM6  
The as-cast and as-compressed ZM6 were used to prepare specimens using the DK7740 wire cutting machine (Jiangsu Fangzheng CNC Machine Tool Co., Ltd., Taizhou, China), which was operated with a voltage of 5.0 V and a current of 2.2 A. Sample dimensions for the heat treatment and tensile test are shown in Figure 1.  
The tensile sample had the flat section size that is seen in GB/T1177-2018 (the specifications are as in Figure 0(a)), and the tensile rate was \(1\times 10^{-3}\) s\({}^{-1}\). The pre-compressed cylindrical samples had a diameter of \(10\pm 0.05\) mm and a length of \(10\pm 0.05\) mm, as seen in Figure 0(b).  
The ZM6 samples were heated in the KF1600-I box resistance furnace (Tianjin Mafuer Technology Co., Ltd., Tianjin, China) with a heating rate of 7 \({}^{\circ}\)C/min. The protective agent that was used during the heat treatment process was iron sulfide (pyrite). Among processes, the T4 process was treated using a solid solution (525 \({}^{\circ}\)C \(\times\) 4 h), and the T5 process was treated through aging (200 \({}^{\circ}\)C \(\times\) 8 h) without the use of a solid solution. The T6 process was treated using a solid solution at 525 \({}^{\circ}\)C for 4.0 h and through aging at different aging temperatures and aging times. Both the as-cast and as-compressed samples were studied under four different conditions in order to find the effect of the aging temperature and the aging time on their mechanical properties. The heat treatment schedules of the as-cast and as-compressed ZM6 samples that were designed and carried out are listed in Table 2.  
From Table 2, the specimens that were in both states were tested simultaneously, and meanwhile, the T6 single-stage aging treatment (T6_SSAT) samples and T6 with two-stage-aging treatment (T6_TSAT) samples were measured and discussed for comparison. For the TSAT process, the pre-aging process comprised heating at 200-300 \({}^{\circ}\)C with a holding time of 0.05-0.25 h, and the final-stage aging was at 200 \({}^{\circ}\)C with a holding time of 8.25 h. The T6 heat treatment process is usually adopted, and the solid solution process that is  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Tensile Properties**} & **Hardness** \\ \hline  & \(\sigma_{\text{b}}\) **(MPa)** & \(\sigma_{\text{0.2}}\) **(MPa)** & \(\delta\) **(\%)** & **o (\%)** & **HV** \\ \hline As-cast ZM6 & 153.36 & 108.13 & 3.14 & 4.5 & 54.12 \\ As-compressed ZM6 & 232.16 & 139.75 & 10.97 & - & 63.26 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Tensile properties and hardness of as-cast and as-compressed ZM6 alloys.  
Figure 1: Sample dimensions: (**a**) Sample dimensions for tensile test; (**b**) pre-compressed deformation for tensile test and sample dimensions for solid solution and aging process.  
usually chosen for experiments is 525 \({}^{\circ}\)C for 4 h with cooling at 80 \({}^{\circ}\)C or above, which can completely dissolve the second phase into the matrix to obtain a supersaturated solid solution. For the SSAT process, the aging temperature was 100-400 \({}^{\circ}\)C with temperature intervals of 100 \({}^{\circ}\)C that were obtained by air cooling. For the TSAT process, the pre-aging temperature was 200-300 \({}^{\circ}\)C with temperature intervals of 25 \({}^{\circ}\)C that were obtained air cooling, the pre-aging times were 0.05 h, 0.10 h, 0.15 h, 0.20 h, and 0.25 h, and then the secondary aging treatment (200 \({}^{\circ}\)C \(\times\) 8 h) was carried out. The T6_SSAT process took place at 200 \({}^{\circ}\)C \(\times\) 8.25 h, and the T6_TSAT process took place at 200-350 \({}^{\circ}\)C \(\times\) 0.05-0.25 h and 200 \({}^{\circ}\)C \(\times\) 8 h. When both the as-cast T6 and as-compressed T6 samples were aging at 200 \({}^{\circ}\)C for 0-64 h, the aging times were chosen to be 0 h, 4 h, 8 h, 12 h, 16 h, 20 h, 32 h, and 64 h.