## 4 Conclusions  
In this study, a Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy was aged in the range of 400 \({}^{\circ}\)C to 550 \({}^{\circ}\)C, and the microstructure changes and mechanical properties were analyzed. The following conclusions were obtained:  
1. The metastable \(\beta\)-titanium Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy was subjected to stress-relief heat treatment at 950 \({}^{\circ}\)C for 25 s, and a fine Ti\({}_{\text{x}}\)Zr\({}_{\text{y}}\)Si\({}_{\text{z}}\) phase was precipitated in the \(\beta\)-base owing to the rapid cooling rate. Subsequent aging treatments at 400 \({}^{\circ}\)C to 550 \({}^{\circ}\)C for 2 h, 8 h, 16 h, and 24 h resulted in the generation of a secondary \(\alpha\) phase, preferentially on the \(\beta\) grain boundary, in all specimens except that treated at 400 \({}^{\circ}\)C, where the driving force for precipitation was low. The secondary \(\alpha\) phase precipitated inside the \(\beta\) matrix as the aging time increased.
2. At aging temperatures below 500 \({}^{\circ}\)C, the aging time significantly contributed to the amount of secondary \(\alpha\) phase precipitated, which increased with increasing aging time. This improved the hardness and compressive yield strength. At 550 \({}^{\circ}\)C, the \(\beta^{\prime}\) phase, which acted as a nucleation site for the secondary \(\alpha\) phase, rapidly underwent phase separation from \(\beta\rightarrow\beta+\beta^{\prime}\), which accelerated the phase transformation to the secondary \(\alpha\) phase, resulting in the coarsening of the secondary \(\alpha\) phase. The softening phenomenon reduced the hardness and compressive yield strength and increased the compressive elongation.
3. The best mechanical properties (406.3 HV and 1433.2 MPa) were obtained at 500 \({}^{\circ}\)C/24 h because of the increase in the \(\alpha/\beta\) interfacial area as a large amount of fine secondary \(\alpha\) phase was generated. This condition was confirmed to be the optimal process condition.
4. The correlation between the distribution of precipitation phases and mechanical properties during the aging treatment of the Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy was confirmed, and optimal aging process conditions were derived to secure the mechanical properties required for springs and aircraft structural materials.  
Conceptualization, D.-G.L.; methodology, S.-W.L., [PERSON]-M.K., [PERSON]-J.L. and [PERSON]-G.L.; validation, S.-W.L., [PERSON]-G.L. and [PERSON]-J.L.; formal analysis, S.-W.L., [PERSON]-M.K., [PERSON]-J.L. and [PERSON]-G.L.; investigation, S.-W.L., [PERSON]-M.K. and D.-G.L.; writing--original draft preparation, S.-W.L. and [PERSON]-M.K.; writing--review and editing, D.-G.L.; supervision, D.-G.L.; funding acquisition, D.-G.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Korea Evaluation Institute of Industrial Technology (KEIT) (No. 20010047), the Korea Institute for Advancement of Technology (KIAT) (P0023676, HRD Program for Industrial Innovation) grant funded by the Korea Government (MOTIE), and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (RS-2023-00244296).  
Not applicable.  
The datasets generated and/or analyzed during the current study are available for the corresponding author upon reasonable request.  
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.