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Factors that determine the life of titanium alloy rods
The fatigue and fracture properties of titanium alloy rods occupy a very important position in determining the service life of the alloy. Over the years, research has been carried out at home and abroad to explore the fatigue and fracture mechanism of titanium alloy rods, and the relationship between them and various metallurgical factors. Due to the complexity of the problem itself, the dispersion of the initial test data, some substantive issues are still not very clear, and the views are not completely unified, and only some representative conclusions are presented here.
The fatigue life of titanium alloy rods, like other materials, depends on the nucleation probability of fatigue cracks and the rate of crack propagation. According to the study of pure titanium, fatigue cracks are mostly nucleated in the slip zone and at the twin interface, but for Ti-6AI-4V alloys, under low stress conditions, the interface of a phase and phase b is the weakest link, only at high stress levels, the probability of endogenous nuclei in the slip zone is significantly increased, and increasing the test temperature also has a similar effect, in order to improve fatigue performance, for a ten-b titanium alloy rod, it is usually desirable to obtain fine equiaxed a ten-b structure, and the b phase is best in a free state to reduce the a/b phase interface area. In addition, the fine grain slip is relatively uniform, and the slip free path is short, which can reduce the stress concentration caused by dislocation plugging. At the same time, fine crystals also have a greater restraining effect on twins. Conversely, coarse grain Weissen tissue has low fatigue strength due to easy nucleation of fatigue cracks. Effect of grain size on fatigue properties of smooth specimens and notched specimens of Ti-6A1-4V alloy.
In the case of notch and stress concentration, the influence of grain size decreases, because the fatigue life mainly depends on the crack growth rate, while the experimental results of pure titanium show that the crack growth rate is not closely related to the grain size.
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