Failure mechanism of a precipitation hardened titanium alloy in very high cycle fatigue regime

Very High Cycle Fatigue (VHCF) is the case of fatigue fracture under more than a billion cycles. It is a rather common fact that there is a decrease in fatigue strength in the VHCF regime for some metals such as aluminum alloys. Consequently, VHCF behavior of materials has been gaining attention owing to an increased demand in fatigue life of machinery components including aero-engine components. In general, fatigue fracture due to VHCF manifests different damage mechanisms with respect to high cycle fatigue. In fact, as a distinctive morphology of interior failure, a fish-eye pattern can usually be observed at fracture surface. Recent publications have confirmed that the crack initiation process can consume more than 90% of fatigue life in the VHCF regime. Consequently, the crack initiation mechanism is an important subject while investigating the fatigue behavior of materials under VHCF loading. At present, titanium alloys are the most important metallic materials used in aeronautics, due to their high strength and stiffness to weight ratios, excellent fatigue and exemplary corrosion resistance. In particular, as a newly developed material, TC11 alloy exhibits higher strength, superior creep resistance and better thermal stability due to the presence of some alloying elements including aluminum, zirconium, molybdenum and silicon.

A detailed review of existing published literature reveals that little has been done to show the roles played by the particles/precipitates on the fatigue properties of TC11 alloys under VHCF loading. To address this, Tao Gao (graduate candidate) and Professor Hongqian Xue of Northwestern Polytechnical University in China together with Dr. Zhidan Sun and Professor Delphine Retraint from the University of Technology of Troyes in France studied in detail the fatigue properties of a TC11 titanium alloy in the VHCF regime. Specifically, their goal was to assess the fatigue properties of this alloy and to reveal the effect of silicide precipitates and α₂ phase (Ti₃Al) on its fatigue behavior including failure mechanism in VHCF regime. Their work is currently published in the research journal, Materials Science & Engineering A.

For this purpose, the research team carried out tension-compression fatigue experiments using an ultrasonic fatigue testing system. In addition, the researchers engaged in fractographic observations at the microscopic scale that were conducted by focusing on crack initiation stage to reveal the effect of α₂ phase and silicides on microcrack initiation and early propagation mechanisms.

Overall, the goal was to provide some valuable elements for the optimization of both chemical composition and thermomechanical processing of this alloy in order to achieve better VHCF performance. Their results showed that the crack initiation regions are characterized by microvoids, dimples and peaks rather than crystallographic facets frequently reported for other titanium alloys. The ductile damage instead of brittle cleavage is the dominant failure mode for this alloy in the VHCF regime.

In summary, the study presents an in-depth investigation of the fatigue properties and the failure mechanisms of a TC11 titanium alloy in the VHCF regime, where particular attention was paid to the effects of silicides and α₂ phase on the failure mechanisms of this alloy. Detailed observation of fracture surfaces revealed that the ductile damage mechanism can be attributed to the combined effect of the silicide precipitates and α₂ phase. Overall, the results showed that the cuboid micro-silicide precipitates significantly deteriorate the fatigue life of the TC11 alloy.