Beta titanium alloy contains the highest proportion of beta stabilizing elements, which weaken lattice bonding and reduce elastic modulus (which can be as low as 45 GPa). This characteristic makes beta titanium alloy particularly attractive for biomedical implant applications. These elements can also completely preserve the high-temperature beta phase when rapidly cooled (such as water quenching) to room temperature. Subsequent aging treatment can precipitate small alpha phases (and other metastable phases) in the beta phase matrix, thereby providing a strengthening effect. The ultimate tensile strength of aged beta titanium alloys can be significantly higher than that of alpha and alpha+beta titanium alloys, thus they occupy a special position in the overall application of titanium alloys in the aircraft industry, especially in the field of key load-bearing components. For example, Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17) has a yield strength of 1000-1200 MPa and is mainly used as a high-strength fan disk in aircraft engines.
In contrast, the room temperature yield strength of cold rolled and aged Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C) can approach 1500 MPa, lending application in aircraft springs, fasteners and pressure shells. 3 Current studies of β - type titanium alloys are mainly on Ti-6Al-2Sn-4Zr-6Mo (Ti-6246),Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17),Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511), Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531),Ti-5Al-5V-5Mo-3Cr (Ti-5553) and alloy systems such as Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C). The ultimate tensile strength (UTS) of nearly all additive manufactured beta titanium alloys in their as deposited condition are below 1000 MPa while offering very good elongation of ≥ 15%.
Heat treatment after additive manufacturing can significantly alter the tensile properties (especially the tensile strength) of β - titanium alloys. Regardless of whether the solution treatment temperature is above or below the β phase transition point, combined with aging treatment, the alloy strength (yield strength YS and ultimate tensile strength UTS) can be significantly improved, and its strength value can far exceed that of additive manufacturing Ti-6Al-4V alloy.
The laser directed energy deposition (L-DED) Ti-55531 alloy treated by solid solution and aging in the β phase region has a ultimate tensile strength of 1352 megapascals and a total elongation of 5.9%. Laser powder bed melting (L-PBF) Beta-C alloy can achieve a tensile strength (UTS) of over 1600 megapascals and a uniform elongation of about 5% through direct aging treatment, which is currently the highest strength value reported for additive manufacturing of titanium alloys.
However, solid solution+aging and direct aging treatment usually lead to a decrease in plasticity: for example, the elongation of L-PBF Ti-55531 after solid solution+aging treatment is less than 2%. To this end, researchers developed a triple annealing process (solid solution in the β phase region and solid solution+aging in the α/β two-phase region) in L-DED Ti-55531, which increased the total elongation from 5.9% in the solid solution aging state to 8.5%. Research on L-PBF Ti-55511 and L-PBF Ti-17 has shown that increasing the aging temperature can improve plasticity (from brittle fracture before yield at 450 ℃ aging to approximately 8% elongation at 650 ℃ aging). In addition, L-DED Ti-55511 can obtain a UTS of approximately 1100 megapascals and an elongation of approximately 12% after undergoing beta field preheating treatment at 900 ℃ and furnace cooling, followed by annealing.
For alloys with low content of beta stabilizing elements (such as Ti-6Al-2Sn-4Zr-6Mo), annealing at 600-900 ℃ is usually carried out to promote the decomposition of alpha martensite into alpha and beta phases, thereby achieving high yield strength (from about 450 megapascals to about 1050 megapascals) while maintaining good plasticity (>15%).
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