## 3 Results and Discussion
### Evaluation of Mechanical Properties Changes  
To observe the changes in the mechanical properties due to the secondary \(\alpha\) phase precipitated during aging, Vickers hardness measurement and room temperature compression tests of the aged specimens were performed and the results are presented in Figure 7. To minimize the influence of the barreling phenomenon during the room-temperature compression test, the average values of the compressive strength and compressive yield strength at 30% compressive strain were calculated and are shown in Table 5.  
For all aging temperature conditions, except 400 \({}^{\circ}\)C, the hardness and compressive yield strength significantly improved. This was due to the increased precipitation of the fine secondary \(\alpha\)-phase with longer aging time, which increased the \(\alpha/\beta\) interfacial area. The \(\alpha/\beta\) interface effectively impedes the movement of dislocations during plastic deformation [37]. Therefore, increasing the \(\alpha/\beta\) interfacial area significantly contributes to  
Figure 6: Time–temperature transformation (TTT) diagram for Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy [29]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Temp. (\({}^{\circ}\)C)** & **Time (h)** & \
\begin{tabular}{c} **Compressive Yield** \\ **Strength (MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Compressive Strength (MPa)** \\ **(at Strain 30\%)** \\ \end{tabular} \\ \hline - & - & \(908.1\pm 4.1\) & \(1378.3\pm 10.2\) \\ \hline \multirow{4}{*}{400} & 2 & \(928.6\pm 5.4\) & \(1442.7\pm 6.9\) \\  & 8 & \(935.0\pm 4.1\) & \(1425.5\pm 21.2\) \\  & 16 & \(947.7\pm 6.2\) & \(1492.4\pm 23.7\) \\  & 24 & \(942.5\pm 13.6\) & \(1425.0\pm 10.1\) \\ \hline \multirow{4}{*}{450} & 2 & \(935.6\pm 7.4\) & \(1386.2\pm 4.5\) \\  & 8 & \(937.8\pm 12.9\) & \(1403.9\pm 47.0\) \\ \cline{1-1}  & 16 & \(1000.3\pm 4.9\) & \(1476.8\pm 12.8\) \\ \cline{1-1}  & 24 & \(1202.1\pm 5.7\) & \(1683.0\pm 20.6\) \\ \hline \multirow{4}{*}{500} & 2 & \(930.1\pm 9.6\) & \(1389.0\pm 67.4\) \\  & 8 & \(1064.0\pm 29.5\) & \(1525.3\pm 73.1\) \\ \cline{1-1}  & 16 & \(1349.8\pm 18.9\) & \(1822.0\pm 43.3\) \\ \cline{1-1}  & 24 & \(1433.8\pm 21.5\) & \(1859.9\pm 11.7\) \\ \hline \multirow{4}{*}{550} & 2 & \(915.0\pm 12.3\) & \(1396.8\pm 54.4\) \\ \cline{1-1}  & 8 & \(981.5\pm 5.8\) & \(1488.4\pm 59.6\) \\ \cline{1-1}  & 16 & \(1110.5\pm 2.6\) & \(1651.6\pm 29.3\) \\ \cline{1-1}  & 24 & \(1143.5\pm 6.1\) & \(1648.6\pm 26.2\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Compressive yield strength and compressive strength of Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy at strain 30%. improving the strength of the alloy. However, the 400 \({}^{\circ}\)C condition did not have enough driving force for the secondary \(\alpha\) phase to precipitate because of the slow diffusion rate caused by the relatively low aging temperature. Therefore, the amount of precipitation of the secondary \(\alpha\) phase was the lowest and the \(\alpha/\beta\) interfacial area was the smallest (\(\alpha\) volume fraction: 0.90% to 2.30%; \(\beta\) volume fraction: 97.70% to 99.10%), and the strengthening effect with increasing aging time was insignificant [32]. For the 450 \({}^{\circ}\)C condition, the change in hardness and compressive yield strength was minimal because of the small amount of secondary \(\alpha\)-phase precipitation until 8 h. Then, as the aging time increased, the amount of fine secondary \(\alpha\)-phase precipitation within the \(\beta\) grain boundary and \(\beta\) matrix increased, resulting in a sharp increase in hardness and compressive yield strength (311 HV to 372.5 HV; 937.8 MPa to 1202.1 MPa), with each value increasing by 19.8% and 28.2%, respectively. For the 500 \({}^{\circ}\)C condition, the hardness and compressive yield strength values increased rapidly (306.1 HV \(\to\) 406.3 HV;  
Figure 7: Mechanical properties of Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy after aging treatments; (**a**) Vickers hardness variation, and (**b**) compressive yield strength variation.