1.1 Overview of the development of high-temperature titanium alloys in the United States
In 1954, the United States developed the world's first high-temperature titanium alloy: TC4 (nominal composition Ti-6Al-4V), which opened the research process of high-temperature titanium alloys. After that, many other high-temperature titanium alloys were developed and researched on the basis of TC4. TC4 alloy adds α-stabilizing element Al and β-stabilizing element V to the titanium matrix, and obtains a microstructure in which α phase and β phase coexist. It has good mechanical properties and high temperature resistance and other comprehensive properties, and is used in structural components and high-temperature resistant components. The maximum operating temperature of TC4 is 350℃. In the subsequent development, due to the increasingly higher requirements for materials in the aviation field, the performance disadvantages of TC4: insufficient heat resistance, poor cold workability, complex preparation process, poor hardenability, etc., are not enough to support its application in the aviation field.
For the above reasons, in the 1960s, the United States developed titanium alloys with higher working temperatures: Ti-6246 and Ti-6242, raising the working temperature to about 450°C. In the 1970s, the United States effectively improved the creep strength and fatigue strength of titanium alloys by adding Si elements to the alloy, and developed Ti-6242S alloy with a service temperature of more than 500°C. In the 1980s, the United States developed Ti-1100 alloy with higher high temperature resistance, which has a good application in aviation aircraft.
1.2 Overview of the development of high-temperature titanium alloys in the United Kingdom
After the United States developed TC4, the United Kingdom did not lag behind and successively developed a series of titanium alloys. In the 1950s, the United Kingdom developed IMI550 titanium alloy, which increased its high temperature resistance to more than 400°C by adding Si elements to the alloy, and its high temperature strength was also 10% higher than TC4. In the 1960s, the UK developed IMI679 and IMI685 alloys, with alloy compositions of Ti-2Al-11Sn-5Zr-1Mo-0.2Si and Ti-6Al-5Zr-0.5Mo-0.25Si. These two alloys have low molybdenum content. Compared with TC4, the creep strength of these two alloys has been greatly improved. In terms of operating temperature, the operating temperature of IMI679 reaches 450℃; the operating temperature of IMI685 exceeds 500℃. The welding and processing properties of IMI685 alloy are good, which is conducive to its application in the aviation field.
In the 1970s and 1980s, based on the Ti-Al-Sn-Zr-Mo-Nb-Si alloy, the UK developed IMI829 and IMI834 alloys to improve the fatigue strength of the alloy, with a maximum operating temperature of 600℃. By refining the macro and micro structures, these two alloys have greatly improved creep strength and oxidation resistance. After β solution treatment, the microstructure of IMI829 alloy is transformed into a small amount of β transformation microstructure and needle-shaped α phase, which improves the creep strength and fracture toughness of the alloy; after α+β solution treatment, the microstructure of IMI834 alloy is transformed into needle-shaped transformation β microstructure and a small amount of primary α phase, which improves the fatigue strength and creep strength of the alloy.
1.3 Development of Russian high-temperature titanium alloys
Based on the experience of the United States and the United Kingdom, Russia independently developed and manufactured BT3-1 high-temperature titanium alloy. BT3-1 adds two eutectoid β-stabilizing elements, Cr and Fe, to strengthen the β phase and α phase. The maximum operating temperature can reach 450℃, and its thermal strength and medium-temperature strength are relatively high. In 1958, Russia developed BT8 and BT9 alloys, which effectively improved their heat resistance. The nominal composition of BT8 alloy is Ti-6.5Al-3.5Mo-0.2Si, which belongs to the Ti-Al-Mo-Si series α+β type martensitic alloy. The thermal strength and heat resistance of BT8 are greatly improved compared with BT3-1, and it can work for a long time in an unprotected environment. The use temperature of BT9 alloy can reach 500℃. In the early stage of research, it contained Sn element, which was later replaced by Zr element. The creep strength and endurance strength of BT9 alloy have been greatly improved. BT8 and BT9 have many applications mainly in compressors.
Russia subsequently developed BT18 near-α alloy. BT18 contains a trace amount of β phase and can be used for a short time at 800℃ or for a long time at 550-600℃. Its creep strength and plasticity are slightly insufficient compared with similar alloys. On the basis of BT18, the content of Al element is reduced and Zr is replaced by Sn to obtain BT18Y alloy. After improvement, the thermal stability, impact toughness and creep strength of the alloy are improved. However, its instantaneous tensile properties are slightly reduced at high temperatures. The BT36 alloy developed by Russia is the most heat-resistant titanium alloy, and its service temperature can reach 600℃. On the basis of BT18Y, 1% Nb is replaced by 5% W. Compared with Nb, W has a higher melting point and higher creep limit. Compared with BT18Y, the comprehensive performance of BT36 has been greatly improved, and it is widely used in aircraft engine related parts. Now, the research on titanium alloys with the addition of (0.7%-5.0%) W element has become a new trend in Russia's high-temperature titanium alloys.
