Factors affecting the service life of titanium rods

The fatigue and fracture properties of titanium alloy rods occupies a very important position in determining the service life of the alloy. Over the years, domestic and foreign studies have been conducted to explore the fatigue and fracture mechanisms of titanium alloy rods and their relationship with various metallurgical factors. Due to the complexity of the problem itself and the scattered data from the preliminary experiments, some substantive issues are still not very clear, and the views are not completely unified. Only some representative conclusions are introduced here.

The fatigue life of titanium alloy rods, like other materials, depends on the probability of fatigue crack nucleation and the rate of crack growth. According to the study of pure titanium, fatigue cracks mostly nucleate in the slip zone and at the twin interface, but for Ti-6AI-4V alloys, under low stress conditions, the a-phase and b-phase interfaces are the weakest It is only at high stress levels that the probability of nucleation in the slip zone increases significantly. Increasing the test temperature has a similar effect. In order to improve the fatigue performance, it is usually desirable to obtain a small equiaxed titanium alloy rod. b organization, and 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 free path of slip is short, which can reduce the stress concentration caused by dislocation plugging; at the same time, the fine-grain also has a greater constraint on twins. Conversely, the coarse-grained Widmanstatten structure is easy to nucleate fatigue cracks, so the fatigue strength is low.

In the presence of notches and stress concentration, the effect of grain size is reduced, because the fatigue life at this time 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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