The fatigue and fracture properties of medical titanium rods play an important role in determining the service life of alloys. For many years, researches have been carried out at home and abroad to explore the fatigue and fracture mechanism of titanium alloy bars, as well as their relationship with various metallurgical factors. Due to the complexity of the problem itself and the dispersion of the preliminary test data, 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 bars, like other materials, depends on the probability of fatigue crack nucleation and crack growth rate. According to the research on pure titanium, most fatigue cracks nucleate in the slip band and at the twin interface. However, for Ti-6AI-4V alloys, under low stress conditions, the interface between phase a and phase b is the weak link. Only at high stress levels, the probability of nucleation in the slip band increases significantly. Increasing the test temperature has a similar effect. In order to improve the fatigue performance, For a+b type titanium alloy bars, it is usually desirable to obtain fine equiaxed a+b structure, and the b phase is preferably free to reduce the interface area of a/b phase. In addition, the slip of fine grains is relatively uniform, and the free path of slip is short, which can reduce the stress concentration caused by dislocation packing; At the same time, the fine grains also have a greater constraint on twins. On the contrary, the fatigue strength of coarse-grained Widmanstatten is low because of the easy nucleation of fatigue cracks.
In the case of notch 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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