Part II. Metallurgical factors governing the H-assisted intergranular cracking of peak-aged Ti-3Al-8V-6Cr-4Mo-4Zr (beta-
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I. INTRODUCTION
THE focus of this study is IG cracking since that is the dominant fracture morphology observed in aqueous testing of STA Beta-C.[2] Aqueous testing of fatigued precracked specimens[2] and tensile testing of notched specimens with predissolved hydrogen (H) in air[3] have shown that the STA condition of Beta-C Ti is susceptible to H-assisted IG cracking, while the ST condition is not. This situation is not particular to STA Beta-C. The IG environmentally assisted cracking (EAC) of STA beta-Ti alloys is highlighted as one of the outstanding unresolved issues in a review by Wanhill.[4] Previous work suggests that irreversible trapping and/ or bulk equilibrium hydriding of the alpha phase during precharging does not contribute to H-assisted IG fracture initiation.[1] In addition, immunity to H-assisted IG fracture initiation of an all-beta ST microstructure cold worked (CW) to a similar strength level as the STA condition indicates that yield strength alone does not govern IG fracture initiation in the STA condition.[1] Possible factors governing IG fracture initiation include the following: (1) microstructural features such as grain-boundary alpha films or platelet colonies, (2) segregation of a critical species to or from the grain boundary MICHELLE A. GAUDETT, Materials Engineer, is with the Fatigue and Fracture Branch, Carderock Division, Naval Surface Warfare Center, West Bethesda, MD 20817-5700. JOHN R. SCULLY, Associate Professor, is with the Department of Materials Science and Engineering, School of Engineering and Applied Science,University of Virginia, Charlottesville, VA 22903-2442. Manuscript submitted January 12, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
during the aging treatment, or (3) a localized deformation mode in the STA condition. The intent of this work is to examine the effect of these factors on IG fracture initiation by examining various metallurgical conditions that attempt to isolate these factors. A brief background discussion of each factor is provided subsequently. A terracelike fracture morphology observed in acicular alpha/beta alloys Ti-6A1-4V and Ti-5A1-2.5Sn exposed to H2 at 680 to 760 torr can be explained quantitatively by a model that involves the formation and rupture of a brittle alpha-hydride film at the a/b boundaries.[5,6] Therefore, the presence of alpha films at b/b grain boundaries, the likelihood of hydride formation upon charging, and their effect on IG fracture initiation in an aged b 1 a microstructure will be considered here. As a part of this scenario, the grainboundary alpha films, if present, must be characterized. In previous aqueous stress-corrosion cracking (SCC) studies of STA Beta-21S (Ti-15Mo-3Nb-3A1-0.2Si), the EAC susceptibility was correlated to the presence of large platelet colonies of the alpha phase at the grain boundaries.[7] A resolutionizing treatment at higher temperatures and/or longer times can be used to promote the precipitation of these large colonies in aged Beta-C[8] and the effect of H on this condition will be examined
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