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In this study, single and duplex aging treatment was applied to the metastable beta titanium alloy of titanium-15 vanadium-3 aluminum-3 chromium-3 tin with cryogenic treatment. The single-step aging treatment was applied for 20 h at 450°C and 10 h at 550°C. The cryogenic treatment was carried out at −196°C for 24 h, followed by single-step or duplex aging treatment. The results obtained show that the cryogenic treatment causes the formation of martensitic alpha phases. Following the cryogenic treatment, martensitic alpha phases were observed in the sample, which was subjected to aging treatment at 450°C for 20 h, whereas these phases disappeared completely in the sample treated at 550°C for 10 h. Preaging treatment applied after cryogenic treatment prevented the formation of precipitate-free zones by providing a finer and uniform distribution of α phases. Cryogenic treatment followed by preaging treatment at 250°C for 24 h led to the formation of dense metastable prephases. This resulted in a 10% increase in tensile strength, 4% elongation, and 5% hardness compared to preaging at 300°C for 10 h after cryogenic treatment. The optimum mechanical properties in single-step sample groups were obtained in samples aged 20 h at 450°C. In duplex-aged samples, optimum mechanical properties were obtained in cryogenic treatment followed by 250°C/10 h + 450°C/20 h duplex aging treatment.

In this study, the effect of notch orientation and a single overload on the fatigue crack propagation behavior of Ti-6Al-4V alloy was investigated. The notches were formed in two different forms: symmetrical and asymmetrical. Two types of fatigue crack propagation tests were applied to the specimens, with constant amplitude and overload. It was determined that the increased notch angle delays the formation of crack at notch tip and causes the crack propagation rate to decrease. In asymmetric notched specimens, crack formation and crack propagation rate vary depending on the notch angle. At the edge where the notch angle is small, the crack is formed in a shorter time and the crack propagation rate is also high. The K I values decreased with increasing the crack angle, while the K II values showed only change in the threshold region due to crack angle. The crack growth rate in all specimens decreased with the application of a single overload, and the rate of crack retardation after overload increases with increasing notch angle.