## 3 Results and Discussion  
### Microstructure Changes for Different Aging Conditions  
The XRD patterns, optical micrographs, and EDS spectra of specimens subjected to stress-relief heat treatment at 950 \({}^{\circ}\)C for 25 s and air cooled are presented in Figure 2,  
Figure 1: Schematic heat treatment process of Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy. accompanied by the corresponding data in Tables 2 and 3. The XRD results contained \(\alpha\)-phase and \(\beta\)-phase peaks, and the microstructure observations confirmed the formation of fine precipitates in the equiaxed crystal structure and \(\beta\) matrix. EDS analysis was performed to clearly identify the phases of the precipitate through composition analysis, and the EDS spectrum of the white precipitate showed a relatively high content of Si and Zr (Figure 2c). The EDS spectrum of the \(\beta\) base indicated a lower content of Si, compared to that in the white precipitate (Figure 2d), thus confirming that the white precipitate present in the \(\beta\) matrix was the Ti\({}_{\mathrm{x}}\)Zr\({}_{\mathrm{y}}\)Si\({}_{\mathrm{z}}\) phase. These precipitates appear when Zr is added to the titanium alloy; Zr reduces the solubility of Si, which promotes the formation of precipitates [21; 28]. It has also been reported that the Ti\({}_{\mathrm{x}}\)Zr\({}_{\mathrm{y}}\)Si\({}_{\mathrm{z}}\) phase does not precipitate when Zr is not added [29; 30]. The Ti\({}_{\mathrm{x}}\)Zr\({}_{\mathrm{y}}\)Si\({}_{\mathrm{z}}\) phase precipitates in the matrix because of the rapid cooling rate after heat treatment, and the size and fraction of the phase increases with increasing heat-treatment temperature [29; 30]. By controlling the aging conditions to properly distribute the Ti\({}_{\mathrm{x}}\)Zr\({}_{\mathrm{y}}\)Si\({}_{\mathrm{z}}\) phase within the \(\beta\) matrix, the grain growth can be hindered, resulting in grain refinement [21; 31]. To analyze the influence of the aging-treatment conditions on the secondary \(\alpha\) phase, optical micrographs of the microstructures of specimens subjected to 2 h to 24 h of aging treatment at 400 \({}^{\circ}\)C to 550 \({}^{\circ}\)C are presented in Figure 3. The microstructure change with the increase in aging time at a temperature of 400 \({}^{\circ}\)C was insignificant. This was because of the low driving force for the secondary \(\alpha\)-phase precipitation due to the relatively slow diffusion rate at low aging temperatures [32]. During 2 h of aging at 450 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, and 550 \({}^{\circ}\)C temperatures, \(\alpha\)GB was obtained because of the preferential precipitation of the secondary \(\alpha\) phase on the \(\beta\) grain boundary. \(\alpha\)GB was formed by the migration of the \(\alpha\) stabilizing elements (O and Al) present inside the \(\beta\) phase toward the \(\beta\) grain boundary  
Figure 2: Microstructural properties of Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy after stress relieving at 950 \({}^{\circ}\)C for 25 s and air cooling; (**a**) XRD profile, (**b**) optical microstructure, (**c**) EDS spectrum of Ti\({}_{\mathrm{x}}\)Zr\({}_{\mathrm{y}}\)Si\({}_{\mathrm{z}}\) precipitation, and (**d**) EDS spectrum of \(\beta\) matrix. The precipitation mechanism of the secondary \(\alpha\) phase according to aging treatment temperature is shown in Figure 4. As mentioned earlier, at 400 \({}^{\circ}\)C, the diffusion rate of alloying elements slows and the growth of precipitates decreases, so a very long aging treatment time is required to complete the phase transformation to the secondary \(\alpha\) phase [34]. Therefore, the microstructure change with increasing aging time is minimal. In addition, the diffusion rate of atoms increased with the increase in aging temperature. The time for precipitation and the growth of the secondary \(\alpha\)-phase occurred rapidly as the aging temperature increased from 450 \({}^{\circ}\)C to 550 \({}^{\circ}\)C, confirming the coarsening of the secondary \(\alpha\) phase.