Ultra-high Cycle Fatigue Characterization and Ultra-slow Crack Growth of Titanium Alloys

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In ultra-high cycle fatigue failure, ultra-slow crack growth of small cracks has great contribution to fatigue life. Micron-sized notches were prefabricated on the specimen surface by focused ion beam (FIB) technology. Ultrasonic fatigue tests (20 kHz) were suspended at specific life intervals and field emission scanning electron microscope (SEM) was used to carefully observe the fatigue crack growth behavior at end of the notches. Two types of bimodal microstructures and different cyclic stress amplitudes were employed to investigate the ultra-slow crack growth behavior of the alloys. The path of small fatigue cracks is relatively straight at the primary alpha grain, whilst it is more tortuous at the colony. The colony has a higher resistant to the growth of small fatigue cracks than the primary alpha grain in the bimodal microstructure. If small fatigue cracks pass through very few colonies, fatigue crack growth rate will be higher, even if a lower cyclic stress was applied. Owing to the difference in local microstructure characteristics, the growth rate data of small fatigue cracks show obvious dispersity. The higher volume fraction of colonies should be beneficial to improve the growth resistance of small fatigue cracks. In addition, we also found that deformation twins that were induced by the laser shock peening, can retard the growth of small fatigue cracks. These results may provide insightful ideas for the anti-fatigue microstructure design of titanium alloys.