Research On Properties Of TC4 Alloy Treated By Cyclic Thermo-Hydrogen Treatment

Thermo-hydrogen processing (THP), which is a unique technology in which hydrogen is used as a temporary alloying element in titanium alloy, allows to change the phase composition, microstructure of titanium alloy, consequently improves its mechanical properties. Hydrogenation behavior of TC4 titanium alloy was investigated at different hydrogenation temperatures, holding time and hydrogen pressures, including: the effects of hydrogenation temperature, hydrogen pressure and holding time on hydrogen absorption law of titanium alloy; the relationship between hydrogen distribution and holding time. Compression test was performed to study the effect of heat treatment methods on deformational properties of TC4 alloy hydrogenated for one time and TC4 alloy hydrogenated for different times, in order to determine the cyclic thermo-hydrogen process which is beneficial to the plasticity of TC4 alloy at room temperature. The effect of cyclic thermo-hydrogen treatment on microstructural evolution, phase composition and morphology of fracture of TC4 alloy were investigated by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM).Result shows that the absorption of hydrogen into titanium alloy is an exothermic and diffusion process. Hydrogen content increases first and then decreases with the increasing of hydrogenation temperature, increases linearly with the increasing of hydrogen pressure. With the increasing of holding time, Hydrogen pressure decreases severely when the holding time is less than 10 min and decreases slowly when the holding time exceeds 10 min. Hydrogen content in TC4 alloy increases rapidly at the beginning and slowly with the increasing of holding time, finally becomes stable.The process of cyclic thermo-hydrogen treatment which is beneficial to improving the properties of TC4 alloy at room temperature can be described as follows: Hydrogenation temperature in each procedure is 750℃ and the holding time is 120min. Solution treatment+quenching would be done after the cyclic thermo-hydrogen treatment, and the solution temperature is 850℃, holding time is 30min followed by quenching.Microstructure of TC4 alloy treated by cyclic thermo-hydrogen treatment changes obviously. Tiny acicular martensite a’ and wider acicular martensite a" appear in each TC4 alloy specimen hydrogenated for different times. Besides, face-centered cubic (FCC) 8 hydride appears and the content of β phase increases. In summary, the grain size is finest and stabilized β phase is the most when TC4 alloy hydrogenated for two times.Cracks appear in TC4 alloy hydrogenated for different times after compression test at room temperature. Ultimate compression increases first and then decreases with the increasing of hydrogenation times, but they are all higher than that of alloy without hydrogen. The ultimate compression of alloy hydrogenated for two times is the highest, reaches 42%, and increases 22.1% comparing with the alloy without hydrogen. Yield strength decreases first and then increases slightly, but they are all lower than that of alloy without hydrogen. Yield strength of TC4 alloys hydrogenated for two times is lowest, decreases 11.1% comparing with alloy without hydrogen. The compressive strength basically remains unchanged and elastic modulus presents a tendency of decrease.All in all, cyclic thermo-hydrogen treatment has a good effect on the compression properties of TC4 alloy at room temperature. When TC4 alloy hydrogenated for two times, its compression properties are the best.