PSI - Issue 40

Vladlen Nazarov et al. / Procedia Structural Integrity 40 (2022) 325–333 Vladlen Nazarov / Structural Integrity Procedia 00 (2022) 000 – 000

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Table 2. Experimental dependence Nazarov (2015a) of the ultimate elongation on the nominal stress for the VT6 (English equivalent is Grade−6 ) titanium alloy at 650 o C.

Stress at the initial creep time ( MPa )

25

30

35

40

45

50

55

60

65

Ultimate elongation taking into account the neck ( mm )

51.0 44.2 40.3 47.8 48.2 44.4 44.6 24.9 43.3

Stress at the initial creep time ( MPa )

70

75

80

85

90

100

110

120

120

Ultimate elongation taking into account the neck ( mm )

16.2 25.0 23.2 25.0 25.8 19.6 14.9 23.3 15.5

The results of a similar study had been also obtained for the Ti−600 ( structure is represented nearly by the   phase) titanium alloy at 650 o C Zeng et al. (2007) and Zeng et al. (2018) where it is established that an increase of the nominal stress from 300 to 350 MPa leads to an increase for the ultimate strain from 24 to 38%. The results of the high cycle fatigue tests of the Ti−600 titanium alloy at ambient temperature at a frequency of 120 – 130Hz and with two kinds of load ratios was given Zeng et al. (2011). The high cycle fatigue strength for the solutioned and aged alloy is found to be 475MPa fatigued with a load ratio of 0.1, and whic h is 315MPa with a load ratio of −1. Since the segment of the creep curve (the dependence of strain on time) after the time of appearance of the neck does not make sense to use the criterion Nazarov et al. (2020) for the appearance of the neck was proposed for describing the creep process. Titanium alloys, which are inert at normal temperatures, exhibit high chemical activity Kolachev et al. (2001) when heated in a number of gas environments, which include hydrogen, nitrogen and air. The heating temperature and the composition of the gas medium have a significant effect on mechanical properties of titanium alloys. Hydrogen forms solid solutions with titanium and 2 TiH titanium hydride. At temperatures above 320 °C titanium hydride completely dissolves in titanium and passes into a solid solution with a hydrogen content of up to 1.5 %. With a temperature drop below 200 °C the solubilit y of hydrogen in the   phase of titanium drops sharply. The decrease in solubility is especially great at 100−150°C. In titanium alloys, when cooled below these temperatures, 2 TiH hydride is released. During quenching, the hydride is released in the form of highly dispersed particles and the form of plates during slow cooling. Diffusion of nitrogen deep into titanium at temperatures below 550 °C proceeds slowly, but is sharply activated at 700 °C. The thin nitride layer appears at 8 00−1000°C. Air o xygen at temperatures below 300 °C forms 3 5 Ti O type compounds w ith titanium at 400−800 °C is formed mainly 2 TiO titanium dioxide and at above 800°C TiO and 2 3 Ti O oxides are detected. Compounds obtained by heating titanium in air, nitrogen and hydrogen environments, remaining in it after cooling, significantly degrade its plasticity and increase its tendency to embrittlement. For experimental data Zamaraev et al. (2017) on the effect of nitrogen, air and hydrogen and helium on the strain rate at secondary creep for titanium alloys VT1−0 and VT5 at 400−1050 o C follows that nitrogen and helium lead to a noticeable increase for the strain rate, while hydrogen (at temperature higher 500 o C), on the contrary, leads to a decrease for the strain rate. Table 3. Experimental data Nazarov (2012) for the VT5 (English equivalent is Grade−5) titanium alloy at 600 o C (obtained on the cylindrical specimens (Fig. 4) with 5 mm diameter and 25 mm working length).

Concentration of hydrogen (% weight )

Stress at the initial creep time ( MPa )

Strain rate at the secondary creep ( %/h )

Rupture time ( h )

Logarithmic ultimate strain taking into account the neck ( % )

0.0 0.1 0.0 0.1

5.2 3.6

4.4 9.5 1.4 4.9

48 53 28 30

150

11.9

225

3.8

The fact that hydrogen is able to slow down the creep process under conditions of stationary axial force at high temperature is observed in experimental data for titanium alloy VT6 Lokoshchenko et al. (2008) and Lokoshchenko et al. (2008) and titanium alloy VT5 at 600 o C Nazarov (2012). For experimental data (Table 3) follows that the hydrogen concentration significantly affected the strain rate at secondary creep and time at the rupture moment, but at the same time, it did not have a noticeable effect on the ultimate strain. It is concluded from the analysis Nazarov