PSI - Issue 82

Juraj Belan et al. / Procedia Structural Integrity 82 (2026) 119–124

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Juraj Belan et al. / Structural Integrity Procedia 00 (2026) 000–000

This is also confirmed by SEM fractography analysis of samples after fatigue testing. In all cases, initiation occurred on the surface of the samples. In the starting stage, the fatigue crack initiated by a slip mechanism from the surface and is characterized by cleavage facets (Fig. 4a). The propagation of the fatigue crack was subsequently carried out by a transcrystalline cleavage mechanism with significant striations, and a few secondary cracks have also occurred (Fig. 4b). The nature of initiation after oxidation annealing is influenced by the formation of an a -case layer. In this case, the crack initiated at the interface of sharply pointed a -phase formations, where significant surface cracks formed in the a -case layer (Fig. 4c). The fatigue crack propagation area is also significantly different with regard to the change in the microstructure of the matrix ( a¢ -martensite). Intercrystalline cleavage failure prevails with significant stages of brittle cleavage (formation of a basket weave microstructure at the boundary of the polyhedral grain, as seen in Fig. 1b) and an increased number of secondary cracks oriented at an angle of ≈45° to the direction of propagation of the main fatigue crack (Fig. 4d). Fatigue striations on the fracture surface of oxidation annealed samples were not observed. 4. Conclusions Oxidation annealing of Ti6Al4V alloy at a temperature above the β-transus with different isothermal holding times (0.5 h, 1 h, and 1.5 h) and cooling in water caused the formation of an α-case layer on the surface. Based on the experiments, it can be concluded that the cooling rate from the annealing temperature changed the original lamellar microstructure to a polyhedral microstructure with basket-weave at the boundaries of the b -phase grains and a¢ - martensite inside the original b -phase grains. The thickness of the a -case layer depends on the isothermal holding time, and its thickness increased from ≈122 µm (0.5 h) to ≈266 µm (1.5 h). The formation of the a -case layer is also associated with a significant reduction in the fatigue life of the alloy due to the presence of surface cracks, which accelerate the initiation and propagation of fatigue cracks. For this reason, determining the fatigue life of an alloy with an a -case layer is very complex and difficult to predict. Acknowledgements The authors acknowledge the KEGA projects No. 004ŽU-4/2023 and No. 009ŽU-4/2023 for the financial support Afroz, S., Santosh, K., Ashish, D., Shreyas, K., Atul, P., Rajkumar, S. 2019. Effect of Temperature and Cooling Rates on the α+β Morphology of Ti-6Al 4V Alloy. Procedia Structural Integrity 14, 782-789. https://doi.org/10.1016/j.prostr.2019.07.056. ASM Hondbook vol. 3. Alloy phase diagrams. 1998. ASM International. pp. 126. Baillieux, J., Poquillon, D., Malard, B. 2015. Observation using synchrotron X-ray diffraction of the crystallographic evolution of α-titanium after oxygen diffusion. Philosophical Magazine Letters 95 (5), 245-252. http://dx.doi.org/10.1080/09500839.2015.1014876. Berthaud, M., Popa, I., Chassagnon, R., Heintz, O., Lavková, J., Chevalier, S., 2020. Study of titanium alloy Ti6242S oxidation behaviour in air at 560°C: Effect of oxygen dissolution on lattice parameters. Corrosion Science 164, 108049. https://doi.org/10.1016/j.corsci.2019.06.004. Boonchuduang, T., Bootchanont, A., Klysubun, W. et al., 2020. Formation of Alpha-Case Layer During Investment Casting of Pure Ti and Ti-6Al 4V Using Comparative XRD and EXAFS Investigation. Metallurgical and Materials Transactions A 51, 586–596. https://doi.org/10.1007/s11661-019-05541-1. Gaddam, R., Safer, B., Pederson, R., Antti, M. L., 2015. Oxidation and alpha-case formation in Ti-6Al-2Sn-4Zr-2Mo alloy. Materials Characterization 99, 166-174. Haohua, S., Sensen, H., Yingjie M., Min, Q., Qian, W., Dongmei, Ch., Haitao, L., Jianke, Q., Jiafeng, L., Rui, Y. 2025. Quantitative investigation of the effects of basketweave microstructure on mechanical strength of α+β titanium alloy. Journal of Materials Research and Technology 37, 4991-5002. https://doi.org/10.1016/j.jmrt.2025.07.121. Konda, N., Verma, R., Jayaganthan, R. 2023. Estimation of high cycle fatigue life of additively manufactured Ti6Al4V using data analytics. Procedia Structural Integrity 46, 87-93. https://doi.org/10.1016/j.prostr.2023.06.015. Lütjering, G., Williams, J. C. 2007. Titanium, In: Engineering Materials and Processes (2nd edition), Springer-Verlag, Berlin, pp. 449. Sieniawski J, Ziaja W, Kubiak K, et al., 2013. Microstructure and Mechanical Properties of High Strength Two-Phase Titanium Alloys. Titanium Alloys - Advances in Properties Control. InTech. pp. 69-80. Available at: http://dx.doi.org/10.5772/56197. Wei, L., Xu, X., Zhao, Y., Yan, X., Zhou, Y., Wu, Z., Yu, Y., 2023. High Thermal Stability of a Colony and Basket-Weave Mixed Microstructure in Selective-Laser-Melted Ti-6Al-4V Alloy Induced by Electropulsing. Metals 13, 538. https://doi.org/10.3390/met13030538. Yan, X., Kou, H. C., Han, F. B., Tang, B., Zhang, L. J., Li, J. S., 2016. Effect of Alpha Phase Characteristics on the High Cycle Fatigue Properties of Ti-6Al 4V Alloy. Materials Science Forum 849. 259–265. of this work. References