疲劳是循环载荷作用下材料性能发生劣化并导致破坏。超高周疲劳是失效周次在10⁷以上的疲劳破坏。在世界范围内，疲劳破坏造成的经济损失约为当年生产总值的4%。钛合金是一种重要的工程材料，常用于制备航空发动机的关键部件，美国的规范要求这些部件至少要有10⁹周次的安全寿命。

裂纹萌生和初始扩展一直是疲劳研究的重要问题，它们承担超高周阶段95%以上的循环寿命。本文以TC4（Ti-6Al-4V）和VT3-1（Ti-6Al-2Mo-1.25Cr）钛合金为实验对象，研究高周和超高周疲劳的裂纹萌生和初始扩展。前者是广泛使用的钛合金；后者被用于俄罗斯D-30航空发动机现役构件。

本文使用20 kHz的超声波轴向加载TC4钛合金，获得不同平均应力和应力比下等轴组织钛合金的高周和超高周疲劳性能。断口学分析显示，存在裂纹萌生特征区RA，包括：表面粗糙源区Sur-RA、亚表面粗糙源区Sub-RA和内部粗糙源区Int-RA。我们发现RA边沿的应力强度因子分别等于相应的长裂纹疲劳扩展门槛值，尤其是对作为表面裂纹的Sur-RA，此结果是第一次报道。我们还归纳了三种破坏模式：表面起源无粗糙区、表面起源有粗糙区和内部起源有粗糙区。这些结果填补了等轴组织钛合金超高周疲劳研究的不足。

本文通过聚焦离子束（FIB, focused ion beam）和透射电镜（TEM, transmission electron microscope）对不同应力比R下的超高周疲劳裂纹萌生区进行微结构表征。在R = –1时，Sur-RA的粗糙断面存在纳米晶表层，首次证明真空环境和类似真空的试样内部并非钛合金超高周疲劳裂纹萌生和微结构细化的必要条件。而R = 0的Sub-RA和R = 0.5的Int-RA均未出现显微组织的细化。对于VT3-1钛合金在R = –1和0.1时的内部裂纹萌生区，只在R = –1时的断面表层发现纳米晶。这两个结果均为首次发现，再次佐证了“大数往复挤压”NCP模型。

VT3-1钛合金具有由粗晶片层组织（LM, lamellar microstructure）和细晶等轴组织组成的复杂微结构，本文对其高周和超高周疲劳的内部裂纹萌生区进行了研究。结果显示，初始裂纹始终起源于具有相同取向的粗晶LM团簇，而非等轴α晶粒，因为前者尺度远大于后者。我们指出，近α和α+β型钛合金的疲劳裂纹萌生倾向于取向相近的大尺度连通区域。这一结果厘清了传统认识的误区。

另外，VT3-1钛合金内部裂纹萌生区的粗糙度、特征形貌和微结构在正、负应力比表现出非常大的不同，显示了裂纹萌生和初始扩展以及微结构演化机制的不同。结合NCP模型，发现这是因为前者的远场载荷没有压应力分量，而后者始终处于循环压应力的作用之中。因此，在高周和超高周阶段，循环载荷的压应力分量有重要的作用，其不仅主导裂纹产生之后裂纹面的闭合和挤压，还能影响裂纹产生之前的萌生行为和扩展路径。这同样是由本文首次提出，应该重视压应力在疲劳中的作用。

Fatigue is the degradation of material properties under cyclic loading, which always leads to final failure. Very-high-cycle fatigue (VHCF) is the fatigue failure with loading cycles beyond 10⁷. Worldwide, the economic loss caused by fatigue damage is about 4% of the GDP (gross domestic product) of that year. Titanium alloy is an important engineering material that is often used to make key components of aero-engines, and the US standard requires these components to have a safe life of at least 10⁹ cycles.

Crack initiation and early growth have always been essential issues in VHCF research, and they consume more than 95% of total fatigue life. In this dissertation, TC4 (Ti-6Al-4V) and VT3-1 (Ti-6Al-2Mo-1.25Cr) titanium alloys are used as test materials to investigate the behavior and mechanism of crack initiation and early growth in high-cycle fatigue (HCF) and VHCF regimes. TC4 is a widely used titanium alloy with an equiaxed microstructure (EM), and VT3-1 is used for key components of the Russian D-30 aero-engine.

In this dissertation, an ultrasonic axial loading method (20 kHz) was used to obtain the HCF and VHCF properties of TC4 titanium alloy with EM under different values of mean stress and stress ratio (R). The revealed fractography shows that there are characteristic regions RA (rough area) of crack initiation, including: Sur-RA (surface RA), Sub-RA (subsurface RA) and Int-RA (internal RA). We first reported that the stress intensity factor at the peripherie of RA region is equal to the threshold value for fatigue crack growth of the corresponding long cracks, especially for the surface crack of Sur-RA. We also summarized three failure types: surface-without-RA, surface-with-RA and interior-with-RA. These provide new knowledge for the VHCF of titanium alloys with EM.

In this dissertation, microstructure characterization of VHCF crack initiation areas at different R values was performed by focused ion beam (FIB) and transmission electron microscopy (TEM). At R = –1, there is a nanograin layer just underneath the surface of Sur-RA region, which for the first time proves that the vacuum or vacuum-like environment is not a necessary condition for the crack initiation with microstructure refinement in VHCF of titanium alloys. However, neither the Sub-RA at R = 0 nor the Int-RA at R = 0.5 showed microstructure refinement. For the internal crack initiation regions of VT3-1 titanium alloy at R = –1 and 0.1, nanograins were found only at the case of R = –1 underneath the fracture surface. Both of these results were found for the first time, and again confirmed the NCP (numerous cyclic pressing) model.

The VT3-1 titanium alloy has a complex microstructure composed of a coarse lamellar microstructure (LM) and a fine EM. In this dissertation, the internal crack initiation areas of HCF and VHCF were analyzed. The results show that the initial fatigue cracks always originated from the coarse LM cluster with the same orientation, rather than the equiaxed α grains, because the former is much larger than the latter. We pointed out that fatigue crack initiation of near-α and α + β-type titanium alloys tends to be large-scale connected regions with similar orientations. This result clarified the misunderstanding of traditional results.

In addition, the roughness, characteristic morphology and microstructure of the crack initiation area of the VT3-1 titanium alloy show very large differences in the positive and negative stress ratios, which suggests different mechanisms in the behaviors of crack initiation, its early growth and the related microstructure evolution. Combined with the NCP model, it was found that this is because the far-field fatigue loading of positive R cases has no compressive component, while the negative R cases are always under the effect of cyclic compression. Therefore, in the HCF and VHCF regimes, the compressive stress component of the cyclic loading plays an important role. It not only dominates the closure and compression of the crack surfaces after crack formation, but also affects the initiation behavior and propagation path before the crack formation. This is also proposed for the first time in this dissertation, and thus the role of compressive stress in fatigue should be emphasized.