Effect of High Temperature and Nitrogen on the Mechanical Properties of Long-Term Strength of Ti–6Al–4V Titanium Alloy

Until recently, it was believed that the applicability of particular metallic material in practice is determined only by the material maximum application temperature. However, as it turned out later, the applicability of metallic material at high temperatures also depends on the threshold stress, at which the creep strain rate tends to zero. At a high threshold temperature of 300°C, Ti–6Al–4V titanium alloy is already used in the manufacture of steam turbine blades with a steam temperature of up to 590°C. The paper presents various empirical dependences that make it possible to calculate the threshold stress and short-term strength limit by minimizing the total error of the difference between the approximating and experimental times at moment of destruction (or strain rates under steady-state creep conditions). The paper considers the experimental data obtained in uniaxial tension of Ti–6Al–4V titanium alloy samples in a nitrogen environment and in air at 500, 600, and 700°C. The analysis of the calculated mechanical characteristics of long-term strength showed that the nitrogen environment leads to an increase in the threshold stress and the short-term strength limit of the Ti–6Al–4V titanium alloy; and the lower the nominal stress and the higher the temperature, the greater the difference between the times at moment of destruction in the nitrogen and in the air environment. Based on the calculated dependence of threshold stress on the high temperature, it is stated that at temperatures above 650°C, the Ti–6Al–4V titanium alloy cannot be recommended for practical use.