Dong-Geun Lee

Cost-effective β-titanium alloys were developed via the hydrogenation–dehydrogenation (HDH) process using low-cost β-stabilizers Mo and Fe. Ti-5Mo-xFe (x = 2, 4 wt%) alloys were fabricated by powder metallurgy and subjected to six post-heat treatment conditions to reduce porosity and improve properties. The as-sintered alloys exhibited high porosity (15–20%), which adversely affected mechanical and corrosion performance. Heat treatment above the β-transus significantly reduced porosity, with Ti-5Mo-4Fe treated at 900 °C for 2 h showing the greatest reduction. Microstructures evolved from α + β lamellar Widmanstätten to equiaxed β with TiFe precipitates. Increased Fe content and heat-treatment temperature enhanced strength, while TiFe precipitates degraded corrosion resistance. Thus, optimized post-heat treatment improves strength and corrosion performance, although Fe content must be controlled.

Ti-xMo-2Fe alloys with high specific strength were designed by adding Mo and Fe as β-stabilizing elements. The influence of cold swaging on the martensitic transformations in Ti-xMo-2Fe (x = 3.4, 5, 9.2 wt.%) alloys was investigated. In these alloys, appropriate chemical compositions promote a stress-induced phase transformation from the β phase to orthorhombic α″ martensite, which improves elongation while maintaining high strength. As the Mo content increases from 3.4 to 5 wt.%, the amount of β-stabilizing elements increases and the β stability is enhanced, thereby altering the phase transformation mechanism. In the Ti-9.2Mo-2Fe alloy, both α″ martensite and a very hard ω phase were identified by X-ray diffraction and transmission electron microscopy. The hard and brittle ω phase causes premature brittle fracture prior to macroscopic yielding. Among the investigated alloys, the Ti-5Mo-2Fe alloy exhibits the best overall combination of high tensile strength, elongation to failure, and high fatigue strength.

Cast β titanium alloys exhibit low mechanical properties and are therefore suitable for various thermomechanical processing methods, such as forging, hot rolling, and extrusion, before practical applications. Depending on β stability, different deformation mechanisms, including slip, stress-induced martensite (SIM), and twinning, can occur, leading to diverse mechanical properties. Thus, controlling β stability and thermomechanical processing conditions are crucial to tailoring the properties of β titanium alloys. In this study, Ti-5Mo-xFe (x = 1, 2 wt%) alloys were investigated. The β stability was adjusted by varying the Fe content, and hot rolling was performed at 900°C with a 60% reduction in thickness. In the Ti-5Mo-1Fe alloy, due to its relatively low β stability, dynamic strain-induced transformation of the α phase predominantly occurred, accompanied by deformation mechanisms involving slip as well as tensile twins with an 86° misorientation and compressive twins with a 64° misorientation. Conversely, in the Ti-5Mo-2Fe alloy, which exhibited higher β stability, dynamic recrystallization of the β phase was the dominant deformation mechanism, and {332} twins along with α'' martensite formed by stress-induced martensitic transformation adjacent to the twin boundaries were observed. These findings demonstrate that precise control over β stability through compositional tuning effectively tailors the deformation mechanisms, thereby significantly influencing the microstructural evolution and mechanical properties of β titanium alloys.

Ti-2Al-9.2Mo-2Fe (2A2F) alloy is a low-cost β-Ti alloy in which the expensive β-stabilizing elements (Ta, Nb, W, Ni) are replaced with relatively inexpensive Mo and Fe for use in low-cost applications in various industries. The 2A2F alloy exhibits excellent mechanical properties such as high specific strength and low elastic modulus compared to conventional steel alloys but is prone to brittleness owing to the formation of the ω phase when heat-treated at relatively low temperatures. Therefore, an appropriate aging treatment should be performed to control the precipitation of the isothermal ω phase and secondary α phase. This study aims to derive the appropriate aging-treatment conditions following a solution treatment at 790 °C for 1 h, which is below the β-transus temperature of 815 °C. The aging treatments are conducted at holding temperatures in the range of 450–600 °C and holding times between 1 and 18 h. At relatively low aging temperatures of 450 °C and 500 °C, the precipitation of the isothermal ω phase resulted in significantly high hardness and compressive strength. As the aging temperature and holding time increased, the ω phase gradually transformed into the secondary α phase, leading to a balanced combination of strength and ductility. However, at excessively high aging temperatures and prolonged durations, excessive precipitation and growth of secondary α phases occurred, which caused a reduction in hardness and compressive strength, accompanied by an increase in ductility. In this study, the effects of precipitation evolution on mechanical properties such as tensile strength and hardness under various heat treatment conditions were comparatively analyzed.

To evaluate the deformation mechanisms of Ti-5Mo-1Fe alloy, compression tests were performed at strain rates ranging from 10-3/sec to 3×103/sec at room temperature. The results showed that Ti-5Mo-1Fe alloy exhibited different deformation mechanisms depending on the strain rate. Under quasi-static strain rates, the Stress-Induced Martensitic (SIM) transformation from the metastable β phase (BCC) to the α" phase was observed through the double yield phenomenon in the stress-strain curve. This contributed to enhanced compressive strength, work hardening, and ductility by absorbing and dispersing deformation energy. Shear bands (SB) were observed near the fracture zone under quasi-static conditions, since the SIM transformation acts as an obstacle to dislocation movement. In contrast, a dynamic strain rate generated adiabatic shear bands (ASB) due to localized heating, leading to coarsened grains near the fracture zone. With increasing strain rates, significant temperature rises were detected in the specimens, leading to increased β phase stability and a reduced chemical driving force for the α" transformation, thereby suppressing the SIM transformation. Consequently, dislocation slip became the dominant deformation mechanism at high strain rates. This study provides insights into the strain rate sensitivity of metastable β-titanium alloys, offering fundamental information for the development of advanced Ti alloys with high strength, good ductility, and impact resistance.

The mechanical properties of beta titanium alloys can be improved by precipitating the α phase in the β-phase matrix and controlling the microstructure via appropriate aging treatments. In this study, heat treatment in the range of 400 to 550 °C is performed to optimize the aging of Ti-3Al-8V-6Cr-4Mo-4Zr alloys. The increase in the aging temperature and holding time increases the hardness and compressive yield strength owing to the precipitation of the secondary α phase in the β matrix. The precipitation driving force at 400 °C is low because of the slow diffusion rate, and therefore the improvements in the hardness and strength are small. At temperatures above 500 °C, phase separation occurs rapidly (β → β + β′), and the β′ phase acts as a nucleation site for the secondary α phase. The phase transformation from the β′ to the secondary α phase is promoted at 500 °C, resulting in the highest hardness (406.3 HV) and compressive yield strength (1433.8 MPa) at 24 h. At 550 °C, the secondary α phase grows and the hardness and compressive yield strength degrade. These results can be effectively applied to manufacture springs with excellent formability and mechanical properties.

Titanium alloys that are used in biomedical applications must possess biocompatibility and a low elastic modulus so that they protect host bone tissue without causing stress shielding. As the elastic modulus of beta Ti alloys is close to that of bone (10–30 GPa), these alloys are considered potential orthopedic implant materials. The elastic modulus of the single β-phase Ti-39Nb-6Zr (TNZ40) alloy is approximately 40 GPa, whereas the strength is lower than that of other types of Ti alloys. Interstitial oxygen in a Ti matrix is well known to improve the matrix strength by solid-solution hardening. The desired mechanical properties can be optimized using a thermo-mechanical procedure to maintain a low elastic modulus. In order to enhance the strength, TNZ40 alloys were fabricated with different amounts of oxygen. The TNZ-0.16O and TNZ-0.26O alloys were cold swaged into 11 mm diameter bars, subjected to solution treatment at 900 °C and 950 °C for 2 h, and furnace-cooled to room temperature. As a result, recrystallized grains were clearly observed in the β matrix. The TNZ-0.26O alloy that was cold-worked by swaging followed by solution treatment at 900 °C exhibited the best mechanical properties (Vickers hardness: 247 HV, ultimate tensile strength: 777 MPa, elongation at rupture: 18.6%, and compressive strength: 1187 MPa). This study reports the effects of oxygen content on the recrystallization behavior and mechanical properties of these alloys.

Titanium alloys are currently used in offshore industries. The titanium alloys used in offshore plants need to have excellent corrosion resistance and mechanical properties because offshore plants are exposed to harsh corrosive environments. At this time, the Ti-6Al-4V ELI alloy is mainly used in offshore plants because it has excellent mechanical properties and corrosion resistance. However, Ti-6Al-4V ELI alloy has the disadvantage of poor formability. To improve this, studies on metastable beta-Titanium alloys with a BCC structure are being actively conducted. In this study, a metastable beta-titanium alloy was fabricated using Mo, which is inexpensive and improves corrosion resistance, and Fe, which improves strength, among the titanium beta stabilizer elements. After the Ti-6Al-4V ELI and Ti-xMo-2Fe alloys solution treatments, electrochemical corrosion experiments were conducted to analyze corrosion characteristics, and mechanical properties were also analyzed through compression tests at room temperature and Vickers hardness measurement. Corrosion properties and the mechanical properties of Ti-xMo-2Fe alloy were considered in connect experiments such as microstructure analysis and hardness measurement. It was confirmed that the corrosion resistance of the Ti-xMo-2Fe alloy was better than that of the Ti-6Al-4V ELI alloy, and it was confirmed that the compressive strength and elongation rate of Ti-xMo-2Fe with its BCC structure get higher than Ti-6Al-4V ELI alloy.