Distinct microstructural and mechanical responses of Ti-6321 alloy to gaseous and electrochemical hydrogen charging

In this study, two hydrogen charging methods, gaseous charging and electrochemical charging, were used to investigate the hydrogen embrittlement behavior of Ti-6321 alloy. The microstructures, hydrogen concentration distributions, and mechanical properties of specimens subjected to each method were systematically examined. The results show that δ-hydrides readily form following electrochemical charging, whereas no hydride formation is observed after gaseous charging despite comparable hydrogen contents. Gaseously charged specimens exhibit a uniform hydrogen distribution from surface to center, while electrochemically charged specimens display a gradient hydrogen concentration distribution. Hydrogen charging increases the hardness of the α phase due to the solid solution strengthening effect, with gaseously charged specimens exhibiting higher α-phase hardness owing to higher hydrogen content in solid solution in the α phase. In contrast, hydrogen ingress leads to softening of the β phase, because of reduced cohesion strength at α/β interfaces and the embrittlement of the β phase. This softening phenomenon is more pronounced in electrochemically charged specimens. Owing to increased generation of dislocations, dislocation networks, and subgrain boundaries during charging, the electrochemically charged specimens show higher mechanical strength compared to the gaseously charged specimens. However, their ductility is lower, which is attributed to reduced dislocation slip activity and increased occurrence of secondary cracks at α/β interfaces, primarily arising from the hydrogen-enhanced interfacial decohesion mechanism. These findings provide new insights into the distinct effects of hydrogen charging routes on the microstructural evolution and hydrogen embrittlement behavior in titanium alloys.