Due to the great defects of titanium itself, titanium alloy has broken through the "thermal barrier" of 600 degrees, and the service temperature has reached a higher level. It is still very difficult to replace nickel-based superalloy, and better new materials and new processes are needed. What are the reasons that limit the use of high temperature titanium alloys at higher temperatures? The rapid decrease of surface oxidation resistance with increasing temperature is an important reason. Stainless steel, as we know it, relies on the formation of an oxide film on its surface to prevent further oxidation. Titanium also combines with oxygen to form an oxide film that binds tightly to the substrate. This layer of oxide film is the key to the oxidation resistance of titanium alloy. The oxide film is very stable below 500 degrees, but with the increase of temperature, the solubility of oxygen in titanium increases, which promotes the titanium dioxide reaction to generate low-price oxide. The density of the oxide layer on the surface increases while the specific volume decreases, and the oxide will fall off and the oxide film loses its protective effect. Even over the years there have been occasional reports that some titanium alloys have superior high temperature properties and have the potential to serve at higher temperatures. But under high temperature application of titanium alloy, there is also a mountain, that is, "titanium fire", that is, titanium alloy ignition combustion event.
Nickel-based superalloys are twice as heavy as titanium alloys, and the difference in the thermal expansion coefficient between the two materials creates a number of technical difficulties and increases the cost of assembly and connection. If use titanium alloy to replace, not only the weight immediately halved, and also does not exist thermal expansion coefficient difference problem, why not? The problem is that the high-temperature properties of titanium alloys are not as glamorous as they seem. The melting point of titanium is 1668 degrees, but the working temperature is higher than 400 degrees, it can become a high temperature titanium alloy, and 600 degrees has been called the "thermal barrier" temperature of titanium alloy, few of the successful pass. Since the 1970s, no titanium alloy whose working temperature exceeds 600 degrees has been put into service for more than 40 years.
In order to reveal the fatigue failure mechanism of titanium alloy TC17, a fatigue life prediction model was established for the fatigue study of titanium alloy TC17. Ultrasonic fatigue test technology was used to carry out fatigue tests on titanium alloy TC17 under different stress amplitudes, and the surface morphology of the specimen was quantitatively detected. The fatigue strength of TC17 was obtained to be 615MPa. Based on the classical two-parameter model (Basquin model) and the fatigue test results of TC17, the stress-life (S-N) curve of TC17 under ultra-high frequency loading is proposed. Scanning electron microscope (SEM) was used to observe the fracture features of specimens. The influence of surface roughness on the high cycle and ultra-high cycle fatigue failure mechanism of titanium alloy TC17 and the degradation of fatigue properties were analyzed. The results show that surface roughness is the main factor that causes the fatigue failure of TC17. The research has positive significance for maintenance and life prediction of titanium alloy TC17 parts.
