PSI - Issue 59

Sviatoslav Motrunich et al. / Procedia Structural Integrity 59 (2024) 58–65 Sviatoslav Motrunich et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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0.8- 3 μm (Fig. 5.c) . The intragranular structure of the metal of the deep layers is lamellar, consisting of α -phase colonies of different sizes. Layers of the β -phase are located between the plates and has width of 0.3- 1 μm (Fig. 5.c). In the body of the grain, the formation of lamellar (Widmanstetten ) morphology of α and β phase secretions is observed. The length of the packets of α -phases inside the grains is 10- 40 μm (Fig. 5.b). Thus, the study of the microstructure gives reason to believe that the alloy of the above composition is a two- phase titanium α+β -alloy. Tensile testing shows that obtained Ti-6Al-7Nb alloy has high plasticity indicators (elongation and reduction of area) while maintaining high ultimate strength. This can be explained by the formation of an equiaxed fine-grained structure in ingots obtained by the EBCHM. As can be seen from the Table 3, the values of the mechanical characteristics of the obtained samples fully meet the requirements of the international standard UNS R56700, which indicates the high quality of the metal obtained by the developed technology. Nevertheless, tensile and impact strength characteristics are comparable to widely used Ti-6Al-4V. Table 3. Tensile and impact strength characteristics of obtained Ti-6Al-7Nb alloy ingot. Alloy Production Yield Strength (R 0,2 ), MPa Tensile Strength (R m ), MPa Elongation (A), % Area reduction (Z), % Impact strength (KCV), J/cm 2 Ti-6Al-7Nb EBCHM 840.8 905.1 13.5 35.8 48.8 UNS R56700 793 862 >10 >25 - Ti-6Al-4V UNS R56400 880 950 >14 >36 17

Ti-6Al-4V [18] Ti-6Al-7Nb

800

700

600

500

Stress range (2S a ), MPa

R s = 0.1

400

10 3

10 4

10 5

10 6

10 7

Cycles to failure (N f )

Fig. 6. Experimental fatigue data of Ti-6Al-7NB specimens compared to literature data for Ti-6Al-4V, reported by Wang (2014).

Initial study of fatigue behavior of obtained Ti-6Al-7Nb alloy shows good results and could be compared with Ti-6Al-4V alloy (Fig. 6). But still more deep and precise analysis of durability and obtaining of basic S-N curves at low and high cycle fatigue region of obtained alloy is needed. The fracture analysis of the surface of the tested specimens reveals that the initiation and propagation of fatigue cracks at different levels of loading are typical for α - β titanium alloys, without any abnormalities. (Fig. 7). Fractographic studies have shown that the examined samples fracture transgranularly and have a significant proportion of the viscous component in the fracture. This ensures high ductility (relative elongation) of titanium alloy Ti-6Al-7Nb after EBCHM. Thus, based on obtained results, it can be stated that proper EBCHM technology was developed suitable for the production of the medical titanium alloy Ti-6Al-7Nb and the quality of which in terms of chemical composition, structure and mechanical properties fully meets the requirements of the standards.