Abstract: Titanium alloy weldments in hydrogen environment are facing with the risk of hydrogen embrittlement, and the fracture toughness is very important for the damage tolerance design of titanium alloy. The effects of hydrogen content on fracture toughness, fracture morphology and fracture mode of titanium alloy welded joints were investigated. Micro-mechanisms of fracture toughness evolution induced by solid solution hydrogen and the hydride for welded joints were revealed. Fracture toughness K_Q values of the hydrogenated 0-0.21wt.% CT specimens were decreased from 39.6 MPa· m^1/2 to 22.1 MPa·m^1/2, the fracture morphology of expansion zone was changed from a large number of dimples + a small amount of local brittle fracture to brittle intergranular fracture, and fracture toughness loss and brittle fracture induced by hydrogen for welded joints were increased. Hydrogen rich air masses were produced by local enrichment of solid solution hydrogen induced by stress-induced hydrogen diffusion. The existence of internal pressure shear component in hydrogen rich air masses resulted in the decrease of external stress required for local plastic deformation, the decrease of apparent yield stress and the promotion of microcracks inoculation at a lower K_I level. The dislocation movement could not directly cut the large size of the hydride, and the dislocation plug was generated at the hydride interface, which caused local stress concentration, and the microcracks were incubated at the hydride interface. Besides, the hydride was a brittle phase in the soft matrix metal, which caused high strain due to stress concentration, the hydride itself broke, and the microcracks were incubated inside the hydride. The accelerated cracking of welded joints was caused by solid solution hydrogen and the hydride at once.

Key words: titanium alloy, hydrogenated welded joint, fracture toughness, fracture morphology, evolution mechanism