PSI - Issue 82

Juraj Belan et al. / Procedia Structural Integrity 82 (2026) 119–124 Juraj Belan et al. / Structural Integrity Procedia 00 (2026) 000–000

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2. Experimental material and methods A two-phase, a + b alloy, Ti6Al4V, heat No. 066-174 from supplier BIBUS Metals AG, Brno, CZ, was used as the experimental material. The alloy was supplied in an annealed state with the chemical composition and mechanical properties listed in Table 1.

Table 1. Chemical composition (wt. %) and selected mechanical properties of experimental material.

Ti6Al4V

Fe

C

N

H

O

Al

V

Ti

0.16

0.025

0.009

0.0049

0.164

6.112

4.105

Balance

UTS [MPa]

YS [MPa]

Elongation [%]

Hardness HRC

Hardness HV

Long. (not applicable to hardness measurements) Trans. (not applicable to hardness measurements) * value from material list; ** own hardness measurement

1023 1185

994

12.13 11.73

32.5* 42**

315*

1152

412**

Block-shaped samples with a base measuring 10 mm x 11 mm x 55 mm were heat-treated by oxidation annealing at a temperature above the β-transus, 1050°C, with a holding time of 0.5 h, 1 h, and 1.5 h, and cooled in water (water quenched, cooling rate » 500 °C . s -1 ). Metallographic samples were prepared using standard procedures for titanium alloys on a Struers TegraSystem device and then chemically etched with 1.5 ml HF + 2 ml HNO 3 + 10 ml H 2 O. The microstructure after oxidation annealing and cooling in water was observed using a Neophot 32 optical microscope (OM) and a Tescan Vega II scanning electron microscope (SEM). Hardness measurements were also performed on experimental samples using the Rockwell (STN EN ISO 6508) and Vickers (STN EN ISO 6507) methods. The measurements were performed for two main reasons: i) to verify, at least approximately, the reliability of the mechanical properties data specified in the material sheet (HRC, HV, UTS, and YS), and ii) to select a suitable load for fatigue testing to ensure that the first samples would fail "with certainty." The HRC and HV hardness values specified by the supplier and measured are shown in Table 1. The results show a significant difference between the declared hardness and the measured hardness, HRC » + 30% and HV » + 31% compared to the values specified by the supplier. Fatigue tests using three-point bending were performed on a Zwick/Roell Amsler 150HFP 5100 device at an ambient temperature of 22 °C ± 5 °C. The test parameters (with respect of hardness measurements) mean stress s m = 311 MPa and load amplitude s a = 155.5 ÷ 272 MPa, loading frequency f » 60 Hz ÷ 70 Hz, and cycle asymmetry parameter R < 1. The number of cycles to failure N f = 2.10 7 was considered to be the so-called run-out. The samples were cooled by an external fan during the tests. The fatigue tests were evaluated via S-N curves. 3. Results and discussion The microstructure of the Ti6Al4V alloy in its initial state consists of uniform lamellae of the a -phase surrounded by the original b -phase. The microstructure corresponds to the state after annealing without significant polyhedral grains, Fig. 1a. Applied oxidation annealing to a temperature above b -transus with different isothermal holding times followed by cooling in water emphasized the polyhedral structure formed by the original b -phase, where areas with a significant occurrence of the so-called basket-weave microstructure oriented perpendicular to the original b -phase grains, Sieniawski et al. (2013), Haohua et al. (2025) and Wei et al. (2023), are observed at the grain boundaries. With increasing cooling rate, this microstructure tends to grow and form a Widmanstätten structure or a¢ -martensite needles, Fig. 1b. Due to the high affinity of Ti alloys to oxygen, an a -case layer formed on the surface during oxidation annealing. During shorter isothermal holding times (0.5 h), an a -case layer was formed, consisting of quasi-rounded a -phase structures with a minor occurrence of short surface cracks and an average thickness of » 122 µ m, Fig. 2a. With increasing isothermal holding time, the thickness of the a -case layer increased to » 204 µ m (after 1 h) and » 266 µ m (after 1.5 h, Fig. 2b), respectively. At the same time, the a -phase transformed into more elongated and sharply pointed