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The hydrogenation behavior of Ti6Al4V alloy was investigated at different hydrogenation temperatures, holding time and hydrogen pressures. The distribution of hydrogen in titanium alloy was studied by OM. Results show that the hydrogen content of Ti6Al4V alloy is controlled by hydrogenation temperature, hydrogen pressure and holding time. Hydrogen content increases first and then decreases with the increasing of hydrogenation temperature. Hydrogen content increases linearly with the increasing of hydrogen pressure. The process of hydrogenation is diffusion of hydrogen into titanium alloy, and the distribution of hydrogen becomes uniform with the increase of holding time.

The hydrogen absorption characteristics and microstructural evolution of TC21 titanium alloy were investigated by kinetic model analysis, optical microscopy (OM) and X-ray diffraction (XRD). The results show that the hydrogen absorption reaction occurred during the hydrogen absorption process of TC21 titanium alloy can be divided into two different stages according to the hydrogen absorption kinetics. After hydrogenation, the microstructure of TC21 titanium alloy changes obviously. Just a little hydrogen will change the contrast of transformed β phase. The contrast of α phase darkens when the hydrogen content in TC21 titanium alloy exceeds 0.5% (mass fraction). The phase/grain boundaries become ambiguous or even vanished, and β phase becomes the main phase instead of α phase when the hydrogen content reaches 0.625%. Moreover, α phase disappears when the hydrogen content reaches 1.065%. Additionally, the XRD analysis shows that α′ martensite and FCC δ hydride appear in the hydrogenated alloy. According to the microstructures and XRD analysis, the schematic diagrams of hydrogen diffusion process in TC21 titanium alloy were established.

TC21 alloy was hydrogenated at different temperatures, and its hydrogen absorption characteristic, hydrogen content, hydrogen absorption kinetics, microstructural evolution and mechanism of hydride formation were investigated. Results show that under the same initial hydrogen pressure, hydrogen absorption rate of TC21 alloy is very slow and hydrogen content of the alloy is close to zero when hydrogenation temperature is lower than 500 °C. The initial hydrogen absorption rate achieves the maximum value at 600 °C. Hydrogen content increases first, then increases up to a maximum at 600 °C and decreases last with the increase of temperature. Hydrogen absorption reactions reach equilibrium faster when temperature increases. Hydrogen absorption reaction occurred during hydrogen absorption process of TC21 alloy is a complicated process which contains different stages according to hydrogen absorption kinetics. Hydrogenation temperature has a significant effect on microstructural evolution of TC21 alloy. With the addition of 0.68 wt.%H in the alloy, β transus temperature drops below 750 °C but still higher than 650 °C. HCP α2 phase and FCC δ hydride appear in the specimens hydrogenated at 550 °C and 650 °C according to XRD and TEM analyses. The morphologies of δ hydrides in the alloy hydrogenated at different temperatures are not identical, acicular δ hydrides exist in the alloy hydrogenated at 550 °C, while large amount of lamellar δ hydrides exist in the alloy hydrogenated at 650 °C. Furthermore, the formation mechanism of δ hydride is discussed, results reveal that δ hydrides precipitate directly in β phase at high temperature, and the most appropriate temperature for the precipitation of δ hydrides in β phase is 600 °C.