Effect of 0.12wt% Hydrogen Addition on Microstructural Evolution of Ti-0.3Mo-0.8Ni Alloy Argon-arc Welded Joints

The effects of hydrogen addition (0.12 wt% H) on microstructural evolution in Ti-0.3Mo-0.8Ni alloy argon-arc welded joints have been investigated using optical microscope (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) to reveal the influence of hydrogen on the characteristics of defect-free titanium alloy welded joints. The results show that hydride precipitation changes the initial microstructure of the welded joints. The increase of hydrogen content is favorable for the β phase precipitation. Face centered cubic δ hydride is evenly distributed in the hydrogenated 0.12 wt% H welded joints. The lamellar δ hydride could only precipitate from the lamellae α, and not from the transformed β phase. Formation of δ hydride is associated with the result of α _H phase separation reaction: α _H →α (H lean region) + δ (H rich region), and H rich regions finally transform to the δ phase. The dislocation distribution is heterogeneous and there is a relatively high density of dislocations in the vicinity of the precipitated δ hydride, which are caused by the lattice distortion due to the stress field generated by the hydride precipitation.