Low temperature titanium alloy material application status

On the one hand, spacecraft structural materials must have sufficient strength and toughness and excellent thermal properties at low temperatures; on the other hand, considering the complexity of the shape of the spacecraft structural parts, the material must have good machinability.

      Compared with traditional cryogenic materials, titanium alloys have higher yield strength at low temperatures, more than 3 times that of stainless steel, while their density is only 1/4 to 1/2 that of stainless steel. in addition, titanium alloys also have a series of advantages such as low thermal conductivity, low coefficient of expansion, non-magnetic, etc., so they are very suitable as new cryogenic materials for space applications.

      At present, cryogenic titanium alloy has been initially applied in the field of liquid rocket engines, mainly as structural materials for hydrogen and oxygen engine storage tanks, hydrogen pump impellers, etc., which has significantly improved the thrust-to-weight ratio, working life and reliability of liquid rocket engines. The problem of low-temperature titanium alloy application is that the elongation and fracture toughness of titanium alloy in low-temperature environment decreases greatly, showing obvious low-temperature brittleness, therefore, how to reduce the low-temperature brittleness of titanium alloy and improve the toughness and plasticity of titanium alloy under low-temperature conditions become the top priority of low-temperature titanium alloy research.

      Scholars at home and abroad have conducted a lot of research to solve this problem and found that two methods can effectively improve the low-temperature properties of titanium alloys by reducing the content of C, H, O and other interstitial elements, and reducing the content of aluminum elements. Through these two methods, a series of new low-temperature titanium alloys with excellent performance have been developed at home and abroad.

      The former Soviet Union was committed to the development and application of low-temperature titanium alloys. By reducing the content of aluminum elements, the former Soviet Union developed a series of low-aluminum cryogenic titanium alloys, among which OT4 and BT5-1 were widely used. OT4 alloy was used in spacecraft orbital docking parts, liquid rocket piping and combustion chamber structural parts; BT5-1 alloy was used in the manufacture of liquid hydrogen containers. In order to further improve the pulse propulsion ratio of liquid rocket motors, a Russian research institute has conducted research and development of high-strength, high plasticity low-temperature titanium alloys suitable for extremely low temperatures of -253°C. 

      The research on low-temperature titanium alloys in the United States has focused on the α-type titanium alloy TA7 ELI (Extra low interstitial) and the α+β-type titanium alloy TC4 ELI. TA7 ELI is a near-α type titanium alloy with good toughness, low thermal conductivity and notch sensitivity at 20 K. It has been successfully used in cryogenic vessels, cryogenic pipes and liquid rocket motor impellers. In the Apollo program, TC4 ELI was used as the main material for liquid hydrogen vessels and liquid hydrogen conduits and achieved good results. In addition, American scholars have also carried out fundamental research on the fracture mechanism and hydrogen embrittlement of cryogenic titanium alloys, and obtained data on the mechanical properties and fracture mechanism of TA7 ELI, TC4 ELI and other cryogenic titanium alloys, which laid the foundation for the further development and application of cryogenic titanium alloys.

      In the field of low-temperature titanium alloy research and development, compared with the United States and Russia and other developed countries, China started late and relatively backward technology. In recent years, with the development of aerospace industry, China began to carry out research on low-temperature titanium alloys. During the "Ninth Five-Year Plan" period, China has carried out the research and development of Ti-2Al-2.5Zr, Ti-3Al-2.5Zr, CT20 and other low-temperature titanium alloys, and the performance of the low-temperature titanium alloys developed in China is shown in Figure 3. CT20 alloy is a low-temperature titanium alloy with all our own property rights, which can be used at very low temperature of 20 K. The alloy has good mechanical properties at low temperatures, with strength greater than 1100 MPa and elongation greater than 10% at 20 K. The alloy also has excellent forming properties and can be processed into bars, plates, tubes and wires. So far, CT20 alloy has been successfully used in a spacecraft cryogenic piping. At the same time, the influence of interstitial elements on the low-temperature mechanical properties of CT20 alloy has been investigated to provide a reference for the further improvement of CT20 titanium alloy performance.

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