In this paper, very high cycle fatigue (VHCF) behavior of an additively manufactured (AM) Ti-6Al-4V by selective laser melting process and post-heat treated by hot-isostatic pressing is investigated by ultrasonic frequency fatigue test and rotating bending fatigue test. It is shown that the fatigue crack initiation is related to loading types in VHCF regime. Under rotating bending fatigue test, the fatigue crack initiates from specimen surface. While for ultrasonic frequency fatigue test, both the surface and the interior crack initiations are observed. For interior crack initiation, the fracture surface presents fish-eye like pattern and fine granular area (FGA) morphology. Electron backscatter diffraction and transmission electron microscopy observations indicate that there are discontinuous refined grain regions beneath the fracture surface in crack initiation and early growth region (i.e. FGA). It is proposed that the mechanism of crack initiation and early growth of titanium alloys in VHCF regime is attributed to the grain refinement caused by dislocation interaction over a number of cyclic loadings followed by cracks combined with the cracks formed at defects, α-phase, interfaces, etc. during cyclic loadings. The paper also indicates that the fatigue performance of the present AM Ti-6Al-4V is comparable to that of the conventionally processed Ti-6Al-4V.

在本文中，通过超声频率疲劳试验和旋转弯曲疲劳研究了通过选择性激光熔化工艺和热等静压后热处理的增材制造 (AM) Ti-6Al-4V 的超高周疲劳 (VHCF) 行为。测试。结果表明，疲劳裂纹萌生与 VHCF 状态下的加载类型有关​​。在旋转弯曲疲劳试验中，疲劳裂纹从试样表面开始。而对于超声频率疲劳试验，观察到表面和内部裂纹的萌生。对于内部裂纹萌生，断裂表面呈现鱼眼状图案和细颗粒区 (FGA) 形态。电子背散射衍射和透射电镜观察表明，在裂纹萌生和早期生长区（即FGA）断裂面下方存在不连续的细化晶粒区域。提出 VHCF 状态下钛合金裂纹萌生和早期生长的机制归因于在多次循环载荷下位错相互作用引起的晶粒细化，随后裂纹与缺陷、α 相、界面处形成的裂纹相结合等在循环加载期间。该论文还表明，目前的 AM Ti-6Al-4V 的疲劳性能与常规加工的 Ti-6Al-4V 的疲劳性能相当。提出 VHCF 状态下钛合金裂纹萌生和早期生长的机制归因于在多次循环载荷下位错相互作用引起的晶粒细化，随后裂纹与缺陷、α 相、界面处形成的裂纹相结合等在循环加载期间。该论文还表明，目前的 AM Ti-6Al-4V 的疲劳性能与常规加工的 Ti-6Al-4V 的疲劳性能相当。提出 VHCF 状态下钛合金裂纹萌生和早期生长的机制归因于在多次循环载荷下位错相互作用引起的晶粒细化，随后裂纹与缺陷、α 相、界面处形成的裂纹相结合等在循环加载期间。该论文还表明，目前的 AM Ti-6Al-4V 的疲劳性能与常规加工的 Ti-6Al-4V 的疲劳性能相当。