The Hydride Precipitation Mechanisms in the Hydrogenated Weld Zone of Ti–0.3Mo–0.8Ni Alloy Argon-Arc Welded Joints

A review of the microstructural evolution and phase transformation in the hydrogenated weld zone of Ti–0.3Mo–0.8Ni alloy argon-arc welded joints has been considered. The role of crystallographic, microstructural, and precipitation mechanisms on the defect-free properties of the hydrogenated weld zone has been analyzed, and hydride phase formations have been revealed by the influence of hydrogen on the microstructural characteristics of the weld zone. The results show face-centered cubic (FCC) δ and face-centered tetragonal (FCT) γ hydride phase formations are found in the hydrogenated 0.21 wt% H weld zone. Large lamellar, slender plate δ and long needle γ hydrides can only precipitate from the alpha lamellae, and not from the transformed beta phase due to the high hydrogen solubility found in the beta phase. Formation of the δ and γ hydrides are the result of α_H phase separation reaction: α_H → α (H lean region) + δ (H rich region) and α_H → γ (H rich region), respectively. The precipitation mechanisms and characteristics of the δ and γ hydrides formed in alpha phase are discussed in detail. Dislocation multiplication around the hydrides is promoted effectively by hydrogen addition, the fact that the quantity of dislocations around the δ hydride increased obviously compared to γ hydride indicated the α_H → δ phase transformation result in a greater volume expansion rate.