Investigation of the metallurgical factors affecting hydrogen embrittlement of solution heat treated and aged Ti-3Al-8V-6Cr-4Mo-4Zr (wt.%) employing fracture, microscopy and desorption methods

The effects of pre-dissolved hydrogen (H) on room temperature fracture initiation in Ti-3Al-8V-6Cr-4Mo-4Zr (wt%) have been investigated. The peak aged condition (STA) shows enhanced H embrittlement susceptibility compared to the solutionized condition (ST) as measured by reductions in the fracture initiation stress with H content and the introduction of intergranular (IG) fracture. Fracture initiation testing of cold worked, ST condition suggests that yield strength alone does not control H-assisted IG fracture initiation of the STA condition.; Stroke rate studies, thermal desorption, transmission electron microscopy and x-ray diffraction investigations of H partitioning suggests that equilibrium hydriding and/or irreversible trapping do not singularly control IG fracture. Examination of additional metallurgical conditions show that grain boundary alpha colonies or films are not responsible for IG fracture. Deformation studies suggest that plastic deformation is concentrated at or near the grain boundaries in the STA condition. An increase in IG cracking with aging time is consistent with the segregation of a critical species to the grain boundaries. Therefore, a H-induced IG initiation mechanism that includes the deleterious effects of grain boundary segregation and deformation on the grain boundary fracture stress is proposed.; Thermal desorption spectroscopy was employed to compare the hydrogen detrapping and desorption characteristics of the ST and STA conditions. Due to the oxide barrier that prevents H egress until high temperatures ({dollar}sim{dollar}350-400{dollar}spcirc{dollar}C) and a dominant H desorption peak due to lattice H, resolution of possible trapping states was difficult. Modeling of H desorption showed that the ST and STA conditions do not have any trapping states of a binding energy greater than 65 kJ/mol.; The limitations of the thermal desorption spectroscopy technique led us to examine and model the mechanism by which H desorbs from Ti-3Al-8V-6Cr-4Mo-4Zr. In the temperature range of 325-490{dollar}spcirc{dollar}C, hydrogen release was not observed until the oxide film was dissolved and sufficient O diffusion into the matrix occurred. Mixed diffusion-surface recombination controlled desorption was observed with apparent activation energies for desorption that agree well with theory. At intermediate temperatures (490-620{dollar}spcirc{dollar}C), isothermal egress showed a change in the rate controlling process from diffusion control to surface recombination control.