## 3 Results and Discussion
### Microstructure Changes for Different Aging Conditions
This is because higher aging temperatures result in a higher driving force for growth but a reduced driving force for secondary \(\alpha\) nucleation [35]. To analyze the phase changes with aging conditions, the XRD patterns of specimens subjected to 2 h to 24 h of aging at 400 \({}^{\circ}\)C to 550 \({}^{\circ}\)C are presented in Figure 5. For all aging conditions, \(\alpha\) and \(\beta\)-phase peaks were observed, and no additional precipitated phase peaks were observed. To analyze the changes in the phase-volume fractions of the \(\alpha\) and \(\beta\) phases with aging conditions using XRD patterns, Equation (1) was used to calculate the phase volume fraction, and the obtained results are presented in Table 4.  
\[V_{f,\alpha}=\frac{A_{\alpha}}{A_{\alpha}+A_{\beta}}V_{f,\beta}=\frac{A_{\beta} }{A_{\alpha}+A_{\beta}} \tag{1}\]  
At aging temperatures above 450 \({}^{\circ}\)C, the volume fraction of the secondary \(\alpha\) phase increased with the increasing aging time and showed a trend similar to that of the microstructure. At 400 \({}^{\circ}\)C, the volume-fraction change in the secondary \(\alpha\) phase was insignificant because of the slow diffusion rate and lack of driving force for precipitation at a relatively low temperature. At the aging temperature of 500 \({}^{\circ}\)C, the volume fraction of the secondary \(\alpha\) phase increased from 6.79% to 23.62%, with the increase in aging time, and at the aging temperature of 550 \({}^{\circ}\)C, the volume fraction of the secondary \(\alpha\) phase increased from 8.14% to 27.13%, with the highest secondary \(\alpha\)-phase volume fraction (27.13%) obtained at 550 \({}^{\circ}\)C/24 h. As shown, with the increase in aging time at aging temperatures above 500 \({}^{\circ}\)C, additional \(\alpha\)-phase peaks were observed around 20 = 39 \({}^{\circ}\) and 41\({}^{\circ}\), and the volume fraction of the secondary \(\alpha\)-phase increased sharply. This can be seen in Figure 6, which  
Figure 4: Precipitation mechanism of the secondary \(\alpha\) phase of Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy according to aging temperature. shows the time-temperature transformation (TTT) diagram of the Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy, where the \(\beta^{\prime}\) phase, which acts as a nucleation site for the secondary \(\alpha\) phase at aging temperatures above 500 \({}^{\circ}\)C, undergoes a rapid phase separation from \(\beta\rightarrow\beta+\beta^{\prime}\) because of the short aging time [30; 36]. Therefore, the phase transformation from the \(\beta^{\prime}\) to the secondary \(\alpha\) phase is accelerated, which explains the high secondary \(\alpha\)-phase fraction [19]. \begin{table}
\begin{tabular}{c c c c} \hline
**Temp. (\({}^{\circ}\)C)** & **Time (h)** & \(V_{f,\alpha}\)**(\%)** & \(V_{f,\beta}\)**(\%)** \\ \hline \multirow{4}{*}{400} & 2 & 1.45 & 98.55 \\  & 8 & 2.30 & 97.70 \\  & 16 & 0.90 & 99.10 \\  & 24 & 1.17 & 98.83 \\ \hline \multirow{4}{*}{450} & 2 & 1.33 & 98.67 \\  & 8 & 1.73 & 98.27 \\  & 16 & 1.76 & 98.24 \\  & 24 & 7.53 & 92.47 \\ \hline \multirow{4}{*}{500} & 2 & 0.99 & 99.01 \\  & 8 & 5.41 & 94.59 \\ \cline{1-1}  & 16 & 6.79 & 93.21 \\ \cline{1-1}  & 24 & 23.62 & 76.38 \\ \hline \multirow{4}{*}{550} & 2 & 1.25 & 98.75 \\  & 8 & 8.14 & 91.86 \\ \cline{1-1}  & 16 & 20.08 & 79.92 \\ \cline{1-1}  & 24 & 27.13 & 72.87 \\ \hline \end{tabular}
\end{table} 
Table 4: Volume fractions of the aged Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy. Figure 5: XRD profiles of Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy according to aging temperatures; (**a**) 400 \({}^{\circ}\)C, (**b**) 450 \({}^{\circ}\)C, (**c**) 500 \({}^{\circ}\)C, and (**d**) 550 \({}^{\circ}\)C.