|Alternative Title||The research on the behavior of crack initiation and early growth in high-cycle and very-high-cycle fatigue regimes for titanium alloys|
|Place of Conferral||北京|
|Keyword||超高周疲劳 应力比 裂纹萌生 微结构 钛合金|
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。
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.
|潘向南. 钛合金高周和超高周疲劳的裂纹萌生与初始扩展行为研究[D]. 北京. 中国科学院大学,2020.|
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