Research progress on fatigue behavior of ultra-high strength titanium alloys
Yuan Jingjing, Zhang Xinwei, Jing Jiarui, Wu Xiaowen, Fan Yalong, Lai Minjie, Li Jinshan
(State Key Laboratory of Solidification Technology, Northwestern Polytechnical University, Xi'an, Shaanxi 710072)
Abstract: Ultra high strength titanium alloy has the advantages of high specific strength, high hardenability, damage resistance, and excellent corrosion resistance, and is widely used in the preparation of high-strength structural components such as aircraft landing gear, fuselage frame, and fasteners. In actual service, these components often experience fatigue failure under cyclic loading. Therefore, in-depth research on the fatigue failure laws and mechanisms of ultra-high strength titanium alloys is of great scientific significance and can provide guidance for practical engineering applications. This study reviews the current research on the fatigue behavior of ultra-high strength titanium alloys and explores the influence of microstructure controlled by deformation and heat treatment on the fatigue damage mechanism of alloys. We specifically focused on the mechanism by which characteristic parameters such as the orientation volume fraction, size, and distribution of the alpha phase affect the deformation behavior and damage mode of alloys during the two stages of fatigue crack initiation and propagation in both binary and fully layered structures. In addition, attention was also paid to the fatigue performance strengthening methods of ultra-high strength titanium alloys, and the future research directions of fatigue of ultra-high strength titanium alloys were discussed.
Keywords: ultra-high strength titanium alloy; Fatigue behavior; Microstructure; Crack initiation; crack propagation
Main charts in the text
(The serial number is the one in the text)

Figure 1 Failure Case of Ti-1023 Helicopter Hub Central Component

Figure 2 Crack initiation location in Ti-5551 alloy

图3经超高周疲劳加载后TC17钛合金中αp的KAM图

Figure 4: Crack initiation location in β - CEZ alloy

Figure 5 facets at crack initiation site in Ti-22V-4Al alloy

Figure 6: Tilted facet clusters in the early stage of crack propagation in TC17 titanium alloy

Figure 8 Crack propagation path in β - C alloy

Figure 9: The propagation process of fatigue small cracks in TB6 titanium alloy
Conclusion
This study provides a review of the relevant research on the fatigue behavior of ultra-high strength titanium alloys, especially the mechanism of fatigue crack initiation. The summary is as follows:
(1) The equiaxed primary alpha phase, needle shaped secondary alpha phase, or lamellar secondary alpha phase can all have a significant impact on the fatigue crack initiation mechanism.
(2) When the volume fraction of the primary alpha phase is high, fatigue cracks usually initiate in the primary alpha phase; When the volume fraction of the primary alpha phase is low, coarse alpha lamellar bundles and grain boundary alpha phases often become crack initiation sites.
(3) When a large number of small needle shaped secondary alpha phases are dispersed in the beta matrix, crack initiation is instead influenced by beta grains.
(4) In the process of fatigue crack propagation, the crack propagation path is influenced by the microstructure, and only microstructures equivalent to the size of the crack plastic zone will have a significant impact on the crack propagation path. Generally, the more tortuous the crack propagation path, the longer the crack propagation life.
(5) Reasonable surface strengthening processes can generally effectively improve the fatigue performance of ultra-high strength titanium alloys. The mechanism of action is usually to introduce compressive stress in the surface layer to offset some of the tensile stress during fatigue loading; Or increase the degree of crack closure, thereby reducing the crack propagation rate. However, unreasonable surface strengthening processes can deteriorate fatigue performance.
At present, most of the research on the crack initiation behavior of ultra-high strength titanium alloys is still limited to the description of phenomena, lacking in-depth discussions on their dominant microscopic mechanisms and the competitive relationships between multiple mechanisms. For special phenomena such as sub surface crack initiation, there has not yet been a widely accepted explanation. In addition, due to the small volume fraction of α p and the small and uniformly dispersed distribution of α s phase in ultra-high strength titanium alloys, it is necessary to study the role of β phase and dislocation transfer at the α/β interface in crack initiation. In the study of the influence of process and microstructure on the fatigue performance of ultra-high strength titanium alloys, the essential micro factors should be identified from the perspective of micro mechanisms, and attention should be paid to the establishment of quantitative relationships between microstructure and fatigue performance in order to accurately predict the fatigue behavior of ultra-high strength titanium alloys.