Effect of Aging Treatment on the Microstructure and Mechanical Properties of Ti-3 Al-8V-6 Cr-4 Mo-4 Zr Alloy  
[PERSON]  
Department of Materials Science and Metallurgical Engineering, Sunchon National University,  
The stable phase of titanium alloys at room temperature varies depending on the type and amount of alloying elements added. For example, \(\beta\) alloys with a BCC structure at room temperature exhibit better fatigue resistance and lower elastic modulus than \(\alpha\) alloys. They have excellent formability and are easy to process for parts with complex shapes [10; 11]. In addition, they generally have a lower elastic modulus (55-91 GPa) than \(\alpha\) and \(\alpha+\beta\) alloys [12]. Owing to these advantages, their application in automotive springs is expanding. Automotive suspension springs made of \(\beta\)-titanium alloys are approximately 53% lighter than those made of steel materials with the same fatigue life, and are being proposed as a way to improve the fuel efficiency in the automotive industry [13]. However, research on the interrelationship between the mechanical properties and microstructure of \(\beta\)-titanium alloys is scarce, compared to that on \(\alpha\) and \(\alpha+\beta\) alloys [14; 15].  
The hardness and tensile strength of \(\beta\)-titanium alloys can be improved by precipitating the \(\alpha\) phase in the \(\beta\) phase matrix through aging treatment and by controlling the fraction and size [16; 17; 18]. When the alloying elements added are lean, the \(\omega\) phase precipitates, and when they are rich, phase separation occurs as \(\beta+\beta^{\prime}\). This shows the precipitation strengthening effect [19]. However, the excessive precipitation of the \(\alpha\) phase in the \(\beta\) phase matrix can increase the elastic modulus; therefore, proper microstructure control is required to balance the strength and elastic modulus [20].  
It is possible to increase the strength of the Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy, a metastable \(\beta\)-titanium alloy, by controlling the grain size and \(\alpha\) precipitation phase through heat treatment. In addition, it is known to have excellent processability due to its low Cr content and low possibility of segregation [21]. Therefore, this alloy is being studied for application in aircraft structural materials and automobile engine springs [14; 15]. However, most of the previous studies on Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloys have focused on heat treatment to improve the mechanical properties. Research on the interrelationship between the microstructure and mechanical properties after heat treatment is lacking. [PERSON] et al. [22] showed that the increase in density of the \(\alpha/\beta\) interfacial area after the aging of the metastable \(\beta\) titanium alloy Ti-3 Al-8V-6 Cr-4 Mo-4 Zr effectively enhances hardness and tensile strength. [PERSON] et al. [15] showed that the tensile strength of the Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy slightly increases with the aging temperature. In the case of a tension coil spring, it is subjected to tensile or compressive stress simultaneously with torsion. However, research on the compressive properties of the Ti-3 Al-8V-6 Cr-4 Mo-4 Zr alloy with respect to aging treatment is insufficient. Therefore, in this study, the microstructure changes associated with aging treatment conditions of a metastable Ti-3 Al-8V-6 Cr-4 Mo-4 Zr \(\beta\)-titanium alloy were observed. In addition, the correlation between hardness and room temperature compression properties was investigated.