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

在超高周疲劳破坏中，小裂纹的超慢裂纹扩展对疲劳寿命有很大贡献。采用聚焦离子束（FIB）技术在试样表面预制微米级缺口。在特定寿命间隔暂停20 kHz的超声疲劳试验，利用场发射扫描电镜（SEM）仔细观察缺口末端的疲劳裂纹扩展行为。采用两种类型的双峰组织和不同的循环应力幅值来研究合金的超慢裂纹扩展行为。小疲劳裂纹的路径在初级α晶处比较直，而在集落处比较曲折。在双峰组织中，集落对小疲劳裂纹的生长具有比初生α晶粒更高的抵抗能力。如果小的疲劳裂纹穿过很少的菌落，即使施加较低的循环应力，疲劳裂纹的扩展速度也会更高。由于局部组织特征的差异，小裂纹的扩展速率数据表现出明显的分散性。较高的菌落体积分数有利于提高小裂纹的抗生长能力。此外，我们还发现激光冲击强化引起的变形孪晶可以延缓小疲劳裂纹的扩展。研究结果可为钛合金抗疲劳组织设计提供参考。

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.