Effect of 0.770 wt%H addition on the microstructure of Ti-6Al-4V alloy and mechanism of δ hydride formation

Ti-6Al-4V alloy was hydrogenated at 750 °C below the eutectoid transformation temperature with 0.770 wt%H, and its microstructural evolution and phase transformations were investigated by the use of optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that, in comparison to the as-received alloy, the contrasts of equiaxed α phase and transformed β phase under optical microscope in the hydrogenated one were reversed. The XRD peaks of β phase after hydrogenation moved to lower angles obviously because of the lattice expansion of β phase with the solution of hydrogen atoms, and the XRD peaks of α phase were broadened due to microscopic internal stress. In addition, XRD and TEM analysis revealed that the face-centered cubic (FCC) δ hydride could precipitate not only in α phase but also in β phase. Furthermore, the mechanisms of δ hydride formation were discussed, and the results showed that the hydrogen diffusion has a significant effect on the δ hydride formation.