Effect of high-temperature hydrogen treatment on the microstructure and properties of titanium alloy Ti-6Al-4V

Abstract

The influence of hydrogen on the structure of titanium alloys is a complex phenomenon, which, depending on the circumstances, may be negative or positive. Negative effects of hydrogen on the properties of titanium alloys primarily manifest themselves in a decrease in plastic properties and impact resistance and the formation of cracks in the structure. The positive effect of hydrogen on the titanium alloy properties is manifested in the high-temperature hydrogen treatment (HTM—Hydrogen Treatment of Materials), where hydrogen is a temporary alloying component. Because of the high values of its diffusion coefficient, hydrogen can be easily introduced into the titanium and just as easily removed from it. The HTM plays an important role in the forming and fragmentation of titanium hydride. Because the diameter of a hydrogen atom is very small, the hydrogen atoms occupy the interstitial sites. Titanium and its alloys exhibit a high affinity for hydrogen absorption, about 60 at.% hydrogen at 600 °C. Because the specific volume of titanium hydride is about 13-17% higher compared to that of the α phase, high stresses in the crystal lattice of the titanium hydride phase leads to local plastic deformation and a highly deformed phase. These developments lead to changes in the structure of titanium alloys and their mechanical properties. This paper presents characterization studies of the structure of two-phase titanium alloy Ti-6Al-4V after hydrogen treatment. This hydrogen treatment consisted of three stages: hydrogenation in a hydrogen gas atmosphere at 850 °C, a cyclic hydrogen treatment (1 or 3 cycles at 250 °C) and dehydrogenation in a vacuum (550 °C). It was shown that hydrogen appreciably changed the microstructure of the surface layer of the tested titanium alloy. Qualitative analyses of the structure using secondary electrons in a scanning electron microscope (SEM) were carried out. Image analysis helped to determine a quantitative description of the alloy structure after the hydrogen treatments.