Part II. Metallurgical factors governing the H-assisted intergranular cracking of peak-aged Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C)

A previous study (Part I) showed that the solution-treated and aged (STA) ( i.e., peak-aged) condition of Beta-C Ti, (sigma(0.2 pct y) = 1260 MPa) poss esses an enhanced hydrogen (H) embrittlement susceptibility compared to the solution-treated (ST) condition, (sigma(0.2 pct y) = 865 MPa), as measured by reductions in the fracture initiation stress with predissolved H conten t and the introduction of an intergranular (IG) fracture mode. It was also shown that yield-strength elevation and the subsequent enhancement in the l ocal hydrostatic stresses within the notch root are not the controlling fac tors in the H-assisted* IG fracture initiation of the STA condition. Previo us work (Part I) implicates a microstructural feature or condition associat ed with the 500 degrees C aging treatment. In this study, it is shown that localized internal hydride precipitation at the grain boundaries or alpha b eta interfaces was not detected by a variety of experimental methods over t he range Of internal H contents for which IG fracture initiation was observ ed. It was also shown that grain-boundary alpha colonies or films are not r esponsible for the IG fracture initiation in the STA condition. A measured increase in hydrogen embrittlement (HE) susceptibility as a function of agi ng time at 500 degrees C is consistent with the segregation or depletion of a critical species at the grain boundary, However, grain-boundary segregat ion/depletion could not be detected with Auger electron spectroscopy(AES) o f specimens fracturing in a vacuum. Compression tests used to characterize and compare the alloys' slip behavior showed that plastic deformation is co ncentrated at or near the grain boundaries in the STA condition, Therefore, a possible intergranular fracture initiation mechanism that includes the e ffects Of hydrogen and localized deformation is discussed.