The development of β-type titanium alloy

With the development of the new generation of turbofan aircraft engines and the 3rd generation fighter aircraft in China, the application of titanium alloys in China's aerospace industry has developed rapidly, and higher requirements have been put forward for the properties of titanium alloys. These requirements are mainly manifested in two aspects: one is higher fracture toughness, and the other is to obtain a high strength of over 1100 MPa on the basis of ensuring sufficient fracture toughness.

Previous accident analyses have shown that the aircraft body structure often exhibits a low-stress fracture phenomenon, that is, the working stress of the component is lower than the yield strength of the material and causes brittle fracture. This is due to the presence of microcracks in the component caused by casting, forging, heat treatment, and even mechanical processing. The safety, reliability, and service life of the component using such a cracked component depend on the resistance to crack instability expansion, which is marked by fracture toughness: KIC. Research shows that β-type titanium alloys can have better fracture toughness than α-type titanium alloys and α+β two-phase titanium alloys at higher strength levels, meeting the requirements for improving the structural efficiency and component lifespan of aircraft components. At the same time, with the rapid development of China's aerospace industry, the requirements for new generation standard parts specify that the material strength level should reach above 1100 - 1300 MPa, so the research and development of β-type titanium alloy materials with a strength level of 1100 - 1300 MPa and their processing techniques have become the urgent task for China's material engineers. TB6 titanium alloy (Ti-10V-2Fe-3Al), which is the most widely used high-strength and high-toughness near-β-type titanium alloy to date, is a titanium alloy produced to adapt to the damage tolerance design principle, with high structural efficiency, high reliability, and low manufacturing cost. The main β-stabilizing elements in the alloy are Fe and V. Under normal use conditions, Rm ≥ 1100 MPa, KIC ≥ 60 MPa·m1 /2, and it has been applied to the main beam of the landing gear of Boeing 777 aircraft, the main landing gear support of A380, and other components. The technical difficulty in the development of this alloy is to avoid the segregation of β-stabilizing elements. TC18 titanium alloy (Ti-5Al-5Mo-5V-1Cr-1Fe), which is a near-β-type alloy, has high strength, high toughness, and high hardenability. The strength limit under annealing state can reach 1080 MPa, and under strengthening heat treatment state can reach 1200 MPa or higher, with satisfactory elongation, reduction of area, and impact toughness. The technology difficulty in the development of this alloy is to avoid the segregation of β-stabilizing elements. TC21 (Ti-6Al-2Mo-1.5Cr-2Zr-2Sn-2Nb), jointly developed by Northwest Institute of Nonferrous Metals, Beijing Aviation Materials Research Institute, etc., is the first high-strength and high-toughness damage-tolerant titanium alloy with independent intellectual property rights in China. Currently, Rm ≥ 1100 MPa, KIC ≥ 70 MPa·m1 /2. This alloy has been applied in China's 3rd generation advanced aircraft. After being selected for use in the engine compartment air duct of the Boeing 777 aircraft, it has achieved good results. After a certain domestic aircraft model replaced 30CrMnSiA structural steel with this material, the welded assembly of the fuselage load-bearing frame achieved a weight reduction of 15% to 20%, significantly improving the structural efficiency of the aircraft. Currently, China has been able to produce TB8 titanium alloy rods and wires with a strength higher than 1300 MPa.