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The paper presents the test results obtained for fatigue crack growth in Ti–6Al–4V titanium alloy subjected to bending. The tests were performed in plane specimens with the stress concentrators being a one‐sided sharp notch. The tested specimens were made of the oxygenated Ti–6Al–4V subjected to various variants of heat treatment. The tests were carried out at the fatigue test stand MZGS‐100 under loading frequency 28.4 Hz. From the obtained results of fatigue and structural tests it appears that schemes of crack propagation and fatigue lives of the considered alloy are different depending on a structure obtained as a result of a given heat treatment.

The paper contains the results of tests on the fatigue crack growth in Ti-6Al-4V titanium alloy. Plane specimens of 4 mm in thickness with a unilateral notch were used. An analysis of crack growth was carried out for specimens without and after 2 and 4 h oxygenation process as well as fatigue crack growth rate was determined. From the test results it appears that the fatigue crack growth in the specimens without thermo-chemical treatment differs from the fatigue crack growth in the specimens after oxygenation. It has been also found that the applied surface treatment increases fatigue life of the considered alloy. Keywords: titanium alloy, oxygenation, fatigue crack growth, bending, notch.Titanium and its alloys are the materials having very good mechanical, physical and chemical properties. Titanium alloys are mainly used in aerospace, armaments, shipbuilding and medical industries, or other fields where the structure weight, high strength and high corrosion resistance are very important. They are applied in production of heat exchangers, water desalting lines and many other products. However, application of titanium is limited, because its wear resistance is low, and its susceptibility to seizing under friction is high. Thus, development and modification of titanium-based materials [1,2] should include not only a proper selection of the alloying elements, but changes of the surface layer properties by suitable surface treatment, as well. Oxygenation of titanium is one of such methods. It uses susceptibility of oxygen to formation of the solid solution in Tiα under high temperature. The authors of this paper discussed that problem [3]. The regularities of titanium alloys surface hardening under saturation in oxygen-containing media were described in [4,5]. It is also important, how the applied modification of the surface layer influences fatigue strength and crack growth in the material. Tests of the fatigue crack growth in two-phase titanium alloys are described [6]. Influence of microstructure on the fatigue crack growth was analyzed for turbine blades made of Ti-6Al-4V titanium alloy. It was found that the grain size strongly influenced the crack growth.The aim of this paper is to determine the influence of the oxygenation time on changes in the material and the fatigue crack growth in Ti-6Al-4V titanium alloy.Material and testing procedures. The crack growth was considered in Ti-6Al-4V alloy [3,7] (brand name IMI318, σ y = 930 MPa, σ u = 977 MPa, micro-hardness is 373HV 0.1 ) in the structural aspect of the material. This alloy is a typical two-phase α+β structure. Aluminium is dissolving in α-phase and stabilizing it, and vanadium stabilizes β-phase. Plane specimens were made of the as softened bar (hot rolled and after stabilizing annealing), 16 mm in diameter with a width of 16 mm, thickness of 4 mm and the unilateral side notches of the root rounding 0.2 mm and 2 mm in depth.The specimens with notches were subjected to oxygenation at a temperature of 900°C by 2 and 4 h in the electric furnace in hot air...

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