1. Types of low-temperature titanium alloys
Titanium alloys possess characteristics such as low density, high specific strength, strong corrosion resistance, and high temperature resistance, making them suitable as cryogenic materials for applications in aerospace, marine, and superconducting fields. However, at low temperatures, the plasticity and toughness of titanium alloys decrease to some extent, exhibiting a significant low-temperature ductile-brittle transition. Interstitial elements such as C, H, and O, as well as substitutional elements like Al, significantly reduce the low-temperature plasticity of titanium alloys, greatly impacting their cryogenic performance. Therefore, current research on cryogenic titanium alloys requires controlling the content of these interstitial or substitutional elements to achieve plasticity preservation and reinforcement.
Titanium alloys can be classified into α titanium alloys, β titanium alloys, and α+β titanium alloys (including near-α titanium alloys). Among them, α titanium alloys with a close-packed hexagonal structure generally have a very low ductile-brittle transition temperature and good plasticity at low temperatures; while β titanium alloys with a body-centered cubic structure have a higher ductile-brittle transition temperature, and their plasticity and toughness decrease as the temperature decreases, generally making them unsuitable for low-temperature use. Currently, commonly used low-temperature titanium alloys are all α titanium alloys or α+β titanium alloys. The most common grade of low-temperature α titanium alloy is Ti-5Al-2.5Sn ELI (extra low interstitial, i.e., ultra-low C, H, O interstitial element content). By controlling the extremely low interstitial element content, its low-temperature performance is improved, and it can be used at temperatures as low as −253 °C. Common grades of α+β titanium alloys include Ti-6Al-4V ELI, Ti-6Al-6V-2Sn, Ti-3Al-2.5V, and Ti-6Al-2Sn-4Zr-2Mo-0.08Si. The most commonly used Ti-6Al-4V ELI titanium alloy can be used at ultra-low temperatures of around −250 °C.
2. Mechanical Properties of Low-Temperature Titanium Alloys
Titanium alloys have a density less than half that of steel, and their yield strength is significantly higher at low temperatures, being three times that of stainless steel. As a lightweight and high-strength material, they are ideally suited for use in critical load-bearing components at low temperatures.
Compared with ordinary titanium alloys, titanium alloys with ultra-low interstitial element (ELI) can still maintain a high elongation at low temperatures, showing that the control of interstitial elements has a significant impact on the low-temperature plasticity of titanium alloys.
3 Applications and Prospects of Low-Temperature Titanium Alloys
In terms of the application and development of low-temperature titanium alloys, the Soviet Union developed the Al-free AT2 series alloys to reduce the Al content in titanium alloys; the United States developed titanium alloys such as Ti-5Al-2.5Sn ELI, Ti-6Al-4V ELI, and Ti-8Al-1Mo-1V during the Apollo program, which are still at an advanced level in the world; Japan, based on the alloys researched by the United States, designed and developed a new low-temperature titanium alloy LT700, which has excellent strength and toughness.
Compared to steel, titanium alloys have higher low-temperature yield strength and are lighter. Furthermore, while possessing similar ductility to aluminum alloys, their low-temperature strength can be several times that of aluminum, and they lack a distinct ductile-brittle transition boundary, making them ideal materials for aerospace cryogenic applications.
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