Hydrogen embrittlement of solution heat-treated and aged β-titanium alloys Ti-15%V-3%Cr-3%Al-3%Sn and Ti-15%Mo-3%Nb-3%Al

Effects of electrochemically introduced hydrogen on the room-temperature mechanical properties of two β-titanium alloys were compared. Solution heat-treated (SHT), single-step peak-aged (PA), and duplex-aged (DA) conditions were investigated using notched tensile bars and Bridgman's analysis of longitudinal stress and average effective diametral plastic strain. Ti-15%Mo-3%Nb-3%Al (referred to as 21S) was more susceptible to hydrogen embrittlement than Ti-15%V-3%Cr-3%Al-3%Sn referred to as 15-3 based on reductions in longitudinal stress, plastic strain, and fractography at H concentrations > 1,000 ppm by weight. Hydriding of the α or β phase was not observed over the range of H concentrations investigated. Instead, changes in fracture behavior with H were correlated with deformation behavior and α precipitation. Susceptibility of 21S was attributed to a high-temperature, long solution treatment that affected deformation behavior in the SHT condition and promoted grain boundary α precipitation in the PA condition. The high-temperature reduced α nucleation sites in grain interiors and promoted planar slip. Subsequent α precipitation occurred preferentially on β grain boundaries and, lastly, in grain interiors, resulting in fine intragranular precipitates. In contrast, a lower temperature of shorter duration for 15-3 resulted in wavy slip after SHT and more homogeneous, slightly coarser α precipitates upon aging, which may have been less prone to slip localization by dislocation shearing. Localized planar slip and grain boundary α colonies were believed to promote internal H embrittlement and aqueous environmentally assisted cracking. These factors were linked more closely to thermomechanical processing than to alloy composition in the present study