Abstract
The effects of electrochemically introduced hydrogen on the room temperature mechanical properties of two B titanium alloys, Ti-15V-3Cr-3Al-3Sn and Ti-15Mo-3Nb-3Al are compared. Solution heat treated (SHT), peak aged (PA), and duplex aged (DA) conditions are investigated using notched tensile bars and Bridgman’s analysis of longitudinal stress and average effective plastic strain. Ti-15Mo-3Nb-3Al is more susceptible to hydrogen embrittlement than Ti-15V-3Cr- 3Al-3Sn based on reductions in longitudinal stress, plastic strain, and changes in fracture mode at hydrogen concentrations above 1000 wt ppm. Hydriding of the α and β phases was not observed over the range of hydrogen concentrations investigated. Instead, changes in fracture paths with hydrogen are correlated with deformation behavior and α precipitation. The susceptibility of Ti- 15Mo-3Nb-3Al is attributed to a high temperature, long time solution treatment which affects deformation behavior in the SHT condition and promotes grain boundary α precipitation in the PA condition. The high temperature solution treatment removes α nucleation sites from grain interiors and promotes planar slip. Subsequent α precipitation occurs preferentially on β grain boundaries and, lastly, in grain interiors resulting in fine intragranular precipitates. It is hypothesized that fine intragranular α plates as well as aligned boundary α colonies are readily sheared and also promote planar slip in the PA condition. In contrast, a lower temperature, shorter duration solution treatment for Ti-15V-3Cr-3Al-3Sn results in wavy slip and more homogeneous, slightly coarser α precipitates upon aging, which may be less prone to slip localization by dislocation shearing. Localized planar slip and grain boundary α colonies are believed to promote both internal hydrogen embrittlement and aqueous environmentally assisted cracking.
