Investigation of fatigue behavior of the Ti-15V-3Al-3Sn-3Cr material under different conditions

β titanium alloys are widely used in many fields such as aerospace, the energy industry, and biomedical thanks to their high strength, high heat treatment efficiency, and low Elasticity Modules. β titanium alloys are classified as β matrix, near β, metastable β, and stable β depending on molybdenum efficiency (MoE) and β phase stability ratio. Ti-15V-3Al-3Sn-3Cr alloy, one of the metastable β titanium alloys, is used in parts of great importance, such as aircraft's main body and fasteners. Considering the application areas, it becomes essential to know and develop the tensile properties of Ti-15V-3Al-3Sn-3Cr alloy and its fatigue behavior under constant and variable amplitude loading. For this reason, single-step and duplex aging treatments were applied to the alloy at low and high temperatures. Low temperature aging treatments were applied between 300-450°C, high temperature aging treatments were conducted at 550°C and for 5, 10, 20, and 40 hours. Duplex aging treatments were applied in two steps: pre-aging treatment and second step aging treatment. Two different pre-aging treatments were applied as 10 hours at 300°C and 24 hours at 250°C during duplex aging treatment. After pre-aging, second-step aging treatments were applied at low and high temperatures for 5, 10, and 20 hours. Microstructure analysis, phase analysis, tensile test, and hardness test were applied to the solution treated and aged samples. Besides, constant amplitude fatigue tests were applied as stress ratio R = 0.1. Variable amplitude fatigue cracks propagation tests were carried out by applying a single overload at 1.3, 1.8, 2, and 2.3 stress rates. As a result of the study, the alloy's microstructural and mechanical properties changed depending on the applied heat treatments at a high rate. It has been observed that increasing aging treatment temperature and time increase the α phase volume fraction and decreases the precipitation-free zones. Besides, in accordance with the literature, with duplex aging treatments, α phases precipitated finer, higher volume fraction, and uniformly distributed. When examining the effect of pre-aging treatments on the microstructure, it was seen that α phases were precipitated more finely in samples pre-aged at 250°C for 24 hours. Maximum elongation against maximum strength was obtained after 20 hours at 450°C in single-step aging treatments and was obtained 24 hours at 250°C followed by 10 hours at 450°C in duplex aging treatments. While fatigue crack propagation behaviors were determined by the grain size and grain boundary angles at the solution treated sample, it was determined by the secondary phases and the precipitation free zones at the aged samples. Especially at the duplex aging treatment, fine and uniformly distributed α phases increased the alloy's tensile strength while increased the fatigue crack propagation rate. Besides, it was observed that the threshold stress intensity factor change (ΔKth) increased with the aging treatment. While the lowest ΔKth value was obtained at the solution treated samples, the highest ΔKth value was obtained at the sample that was applied duplex aging treatment at 250°C for 24 hours and then at 450°C for 20 hours. During variable amplitude fatigue tests, secondary cracks occurred at the solution-treated sample's crack tip after overloading. In contrast, the single-step and duplex aged samples were broken after the overload.