The current development status of high-strength and high-elastic titanium alloys

The elastic deformation ability of metallic materials is influenced by the yield strength and elastic modulus. The tensile linear elastic limit (ε0.2) is mostly lower than 1%. The strength of traditional titanium alloys varies from 400 to 1500 MPa depending on the alloy grade, and the elastic modulus ranges from 50 to 120 GPa, which is much lower than that of steel (about 210 GPa). The elastic deformation ability of titanium alloys is approximately twice that of steel. The high strength and low elastic modulus of titanium alloys give them excellent elastic deformation ability, which is widely used in aerospace fields as a structural-functional integrated material.

In the 1950s, the United States first used Ti-6Al-4V titanium alloy bolts made of titanium alloy in B-52 bombers, thus initiating the application of titanium alloy fasteners in the aerospace field. With the continuous lightweight requirements in aerospace and weapon equipment, lightweight, high-strength, and high-elastic titanium alloys gradually partially replaced the traditional 30CrMoSiA steel in fasteners, improving the safety and reliability of equipment. Currently, the tensile strength of commonly used α+β and β-type titanium alloys is basically at the 1000 MPa level, such as Ti-6Al-4V, Ti-3Al-5Mo-4.5V, Ti-5Mo-5V-8Cr-3Al, and Ti-15Mo-3Al-2.7Nb-0.3Si (β 21S), etc.

Since the 1970s, McDonnell Douglas began to use Ti-13V-11Cr-3Al to manufacture springs for civilian aircraft, replacing spring steel to achieve a 70% weight reduction. Subsequently, Lockheed, Boeing, and Airbus, etc., began to use β titanium alloy materials to manufacture spring components such as the upper and lower locks of landing gear, hydraulic return, and aircraft control, with representative alloys including Ti-15V-3Cr-3Al-3Sn and Ti-3Al-8V-6Cr-4Mo-4Zr (β-C), whose elastic modulus is approximately 104 GPa and tensile strength is 1300~1450 MPa.

The typical grades applied domestically include TB2, TB3, and TB5, etc. Currently, the α+β and β-type titanium alloys used for springs and fasteners generally adopt the α+β two-phase structure to achieve high strength, and the elastic modulus (90~120 GPa) is also relatively high, resulting in lower elastic performance and difficulty in meeting the requirements for high-strength and high-elastic materials in advanced aircraft. The β-type Ti-45Nb alloy, as a special material for rivets, has been applied in aerospace products at home and abroad. This alloy has the advantages of low elastic modulus, good plasticity, and cold working formability, but its strength, especially the yield strength, is low, and the matching of strength and elastic performance is poor.

Since the 1990s, in order to reduce the elastic modulus of medical titanium alloys, a series of low-elastic modulus metastable β-type titanium alloys have been developed, such as Ti-29Nb-13Ta-4.6Zr and Ti-35Nb-5Ta-7Zr, etc., achieving better elastic performance, but these titanium alloys are developed for the medical field, with low strength, and difficult to meet the requirements for high-strength and high-elasticity of titanium alloys for aerospace fasteners and springs. In 2003, Toyota Central Research Institute in Japan developed a multifunctional titanium alloy (rubber metal) with excellent comprehensive performance (atomic fraction %), with the typical composition of Ti-23Nb-0.7Ta-2Zr-1.2O, after 90% cold rolling deformation, the strength can reach 1200 MPa, the elastic modulus is 55 GPa, and the elastic limit is approximately 2.5%, showing excellent high-strength and high-elasticity matching, and this alloy has constant elasticity over a wide temperature range.