The initiation and arrest mechanisms of small internal cracks in very high cycles fatigue regime in titanium alloys

最近，当载荷循环次数超过107次时，在确定材料的常规疲劳极限时，已经发现疲劳断裂仍然发生在金属材料中，并且断裂模式从表面起源转移到内部起源。本文利用多尺度SR-CT技术对自然萌生的内部疲劳裂纹进行了全寿命扩展行为的观测。裂纹萌生和扩展分别占疲劳寿命的57%和43%。试样断裂后，不同循环次数下的裂纹前缘叠加在断口上。裂纹扩展过程包括多个小平面的形成和随后在基体中对应于光滑区域的扩展。此外，约95%的疲劳寿命被尺寸小于100 μ m的晶体学水平的裂纹扩展消耗。这一结果表明了微小尺寸内裂纹的重要性，对构件设计和工业维护具有重要意义。此外，发现β钛合金内部裂纹扩展速率比表面裂纹慢，但比应用最广泛的（α + β）钛合金内部裂纹扩展速率快20~100倍。

Recently, when the number of load cycles exceeds 107 where the conventional fatigue limit of the material is determined, it has been discovered that fatigue fractures still occur in metallic materials, and the fracture mode shifts from surface to interior originated. However, studies on internal fatigue fracture are facing great challenges due to its “invisible” feature.In the present study, a full-life growth behavior of a naturally initiated internal fatigue crack was observed by multiscale SR-CT. Crack initiation and propagation contributed to 57% and 43% of the fatigue life, respectively. After specimen fracture, the crack fronts at various cycles were superimposed on the fracture surface. The crack propagation process consisted of multiple facets formations and subsequent growth in the matrix corresponding to the smooth area.Moreover, around 95% of the fatigue life was consumed by the crack growth at a crystallographic level below a size of 100 um. This result indicates the significance of the very small size internal crack, which is meaningful for component design and industrial maintenance. Meanwhile, the growth behaviors were observed to have strong relations with the fracture surface feature, which could provide valuable evidence in the fracture analysis field such as accident investigation.In addition, the internal crack propagation rate in beta titanium alloy was found to be slower than its surface crack, but was 20~100 times faster than the internal crack in the most widely used (alpha+beta) titanium alloys.