## 3 Results and Discussion
### Mechanical Properties  
From the discussion of the microstructures, it can be learned that the Al-4.5 Zn-1.5 Mg alloys which were subjected to the two-stage homogenization heat treatment could obtain finer Al\({}_{3}\)Zr and Al\({}_{3}\)Sc dispersed grain phases than those that underwent the two-stage homogenization heat treatment. The Al-4.5 Zn-1.5 Mg alloys also could restrain the recrystallization ratios of the alloys in the T6 state. The remaining textures made the mechanical strength of the alloy higher than the recrystallized grains, but it reduced the ductility of the alloy. From the DSC analysis, it can be inferred that the Sc-containing alloy had more precipitation amount of the metastable state \(\eta\)' phase, so its precipitation strengthening was better, and the mechanical strength of the alloy was improved. Consequently, the mechanical strength of the alloys through the two-stage homogenization heat treatment was higher than that through the one-stage homogenization heat treatment. The Al-4.5 Zn-1.5 Mg alloys were doped with different elements and subjected to different heat treatments. Their mechanical properties are summarized in Table 3. Regardless of alloy A(0.1 Zr) or alloy B(0.05 Sc), their T6 hardness, yield strength and tensile strength after the two-stage homogenization heat treatment were better than those after the one-stage homogenization heat treatment, with about 2.7-4% and 1.5-2.7% increases, respectively. However, the ductility decreased by 5.7% and 1.1%, respectively. In addition, after different homogenization heat treatments, the recrystallization degree (\(\sim\)34%) of alloy B(0.05 Sc) in the T6 state was significantly lower than that of alloy A(0.1 Zr). Therefore, from Table 3, it can also be clearly seen that the strength and hardness of alloy B(0.05 Sc) were higher than those of alloy A(0.1 Zr), while the ductility of alloy B(0.05 Sc) was lower. It shows that the mechanical strength of alloy B(0.05 Sc) with a small amount of Sc (0.05 wt%) was better enhanced than that of alloy A(0.1 Zr) with a relatively high content of Zr(0.1 wt%). The tensile fracture surface of the T6 alloy is shown in Figure 9. Whether it was alloy A(0.1 Zr) or alloy B(0.05 Sc), through the one-stage or two-stage homogenization heat treatment, most of the fracture surfaces of the alloys were tough dimple-like fractures. A few brittle fracture cleavages were also observed. With the careful observation and analysis \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Heat Treatment Alloy** & **Mechanical Properties** & \begin{tabular}{c} **One-Stage** \\ **Homogenization (\(a\))** \\ \end{tabular} & \
\begin{tabular}{c} **Two-Stage** \\ **Homogenization (\(b\))** \\ \end{tabular} &
\begin{tabular}{c} **(b \(-\) a)/a \(*\) 100\%** \\ \end{tabular} \\ \hline \multirow{3}{*}{ Alloy A(0.1 Zr)} & Hardness (HV) & 126.7 (1.2) & 131.8 (1.7) & 4.00\% (0.10) \\  & UTS (MPa) * & 226.7 (1.7) & 232.8 (1.1) & 2.70\% (0.05) \\  & YS (MPa) & 364.5 (2.7) & 375.6 (1.9) & 3.10\% (0.07) \\  & EL (\%) & 10.5 (0.5) & 9.9 (0.7) & \(-\)5.70\% (0.20) \\ \hline \multirow{3}{*}{ Alloy B(0.05 Sc)} & Hardness (HV) & 143.3 (1.3) & 147.1 (2.1) & 2.70\% (0.06) \\  & UTS (MPa) * & 238.3 (1.7) & 242.0 (1.9) & 1.50\% (0.10) \\ \cline{1-1}  & YS (MPa) & 384.3 (2.8) & 390.5 (3.2) & 1.60\% (0.02) \\ \cline{1-1}  & EL (\%) & 8.9 (0.6) & 8.8 (0.3) & \(-\)1.10\% (0.04) \\ \hline \hline \end{tabular}  
* ( ) Standard deviation, UTS: ultimate tensile strength, YS: yield strength, EL\%: elongation.  
\end{table} 
Table 3: Al–Zn–Mg mechanical properties of alloy A(0.1 Zr) and alloy B(0.05 Sc) by different homogenization conditions. of the sizes of the dimple-like structures as shown in Figure 9d, alloy B(0.05 Sc) had the smallest dimple-like structure and its cleavages were more obvious through the two-stage homogenization heat treatment. By contrast, through the one-stage homogenization, alloy A(0.1 Zr) had the thickest dimples, as shown in Figure 9a, and the sizes of the other two dimple-like fractures were medium, as shown in Figure 9b,c.