Oxidation behavior of high temperature titanium alloys
Since aeroengines work at high temperatures, the high-temperature titanium alloy components will withstand oxidation corrosion. The high-temperature mechanical properties of titanium alloys are also restricted by their ability to resist oxidation corrosion. The oxidation of titanium alloys will form an oxide film on the surface, which isolates the interior of the metal from the external environment. The properties are relatively stable and will not be further oxidized. However, as the temperature increases, the surface oxide film peels off cyclically. When titanium alloys are used at high temperatures, their high-temperature mechanical properties will be reduced due to the effects of oxidation and corrosion.
The oxidation behavior of high-temperature titanium alloys using temperatures between 500°C and 750°C has been studied. It has been shown that temperature has a great impact on the oxidation rate of high-temperature titanium alloys. In the early stages of oxidation, the oxidation weight gain changes linearly, and as the oxide layer increases, , the chemical reaction rate decreases, the weight increases in a parabolic shape, and the oxide film consists of Al2O3 in addition to TiO2. Zeng Shangwu et al. studied the high-temperature oxidation behavior of TC4 titanium alloy. The study found that TC4 can remain intact during cyclic oxidation in the oxide film at 650°C, but may crack and peel at higher temperatures.
There are currently many methods to improve the oxidation resistance of high-temperature titanium alloys, such as adding other alloying elements to pure titanium to make a variety of new titanium alloys. If aluminum is added, a dense oxide film is formed to protect the titanium alloy from oxidative corrosion, thereby improving the high-temperature oxidation resistance of the titanium alloy. In addition, silicon and chromium can also form oxide films. However, excessive alloying will also affect the physical properties of the material. While improving the oxidation resistance of high-temperature titanium alloys, other high-temperature mechanical properties of high-temperature titanium alloys may also be affected, making them unable to be well used in aerospace engines. Therefore, how to reasonably add alloying elements still requires continued research.
Another method is to fill the surface of titanium alloy with materials that resist high-temperature oxidation. For example, aluminizing the surface of TC4 high-temperature titanium alloy can greatly improve its high-temperature oxidation resistance. In addition, there are also methods such as pre-oxidation. Can improve antioxidant capacity. However, each method currently has certain limitations, and a variety of anti-oxidation measures need to be comprehensively used to ensure the high-temperature mechanical properties of high-temperature titanium alloys.
Fatigue characteristics of high temperature titanium alloys
In aeroengines, structures or parts are mainly subjected to alternating loads, and fatigue failure is the main failure mode. Zhang Yajuan et al. conducted experiments to study the fatigue crack growth characteristics of Ti-6Al-4V titanium alloy. The research showed that as the stress ratio increases, the fatigue crack growth threshold will decrease, and when the stress intensity factor is constant, the crack growth rate is related to the stress ratio. into a positive correlation. The fatigue S-N curve appears an inflection point at about 107 times. Cracks initiate on the surface or subsurface. At the same time, high temperature will promote the expansion of cracks.
Fatigue failure of titanium alloys occurs from time to time, and some anti-fatigue measures need to be taken to improve their fatigue characteristics. Shot peening technology is currently a widely used method to improve the fatigue performance of parts. The principle of this method is mainly to form residual compressive stress on the surface of the specimen to partially offset the effect of the external load, thereby improving the fatigue performance of the part. Research shows that the fatigue limit of materials treated by shot peening can be increased by more than 34%, and shot peening has a very significant effect on improving fatigue performance. In addition to shot peening, extrusion strengthening and ion implantation are also methods to improve the fatigue properties of titanium alloys.
Creep properties of high temperature titanium alloys
Since the components in aeroengines work at high temperatures, creep has a great impact on the service life and safety performance of aeroengines. Creep usually refers to a slow, unrecoverable deformation of materials under high temperature and sustained load.
The research results show that dislocation slip and climb play an important role in the creep deformation process of TA15 alloy within a certain temperature and stress range, and dislocation climb plays a major role above 550°C. Stress, time, temperature, alloy elements, microstructure, etc. all play a certain role in the creep behavior of high-temperature titanium alloys. The research results show that as the test stress increases, the time required for the creep process to reach a steady state will become shorter, and the creep rate will become faster at the steady state. Research on creep should be carried out in conjunction with the interaction of creep fatigue, because failure of components in engines often occurs under the combined action of the two.
