J. D. Cotton, The Boeing Company, Seattle, WA; D. J. Bryan, T. Bayha, ATI Allvac, Monroe, NC; M. Leder, I. Levin, VSMPO-AVISMA, Verkhnaya Salda, Russia
Ti-5Mo-5V-5Al-3Cr (Ti-5-5-5-3) is an emerging near-β Ti alloy of significant commercial interest as a viable replacement for Ti-6Al-4V, Ti-10Al-2V-3Fe and some high strength steels in a variety of aerospace applications. Ti-5-5-5-3 offers significantly improved thick section hardenability in a product capable of being extruded, rolled, forged, and/or cast. In addition, Ti-5-5-5-3 can be heat treated in several ways to achieve high strength, high fracture toughness, high fatigue resistance, or a good combination of all of those properties. Recently, evidence has emerged that hydrogen content strongly influences the fracture toughness, a critical aerospace design parameter, at all levels below typical specification limits for titanium alloys. Positive identification of hydrogen as a prominent factor in fracture toughness control could have an impact in alloy specification limits, heat-treatment requirements, additional processing, and new alloy grades, all of which could lead to significant cost and/or value added for low hydrogen content material. A wide variety of characterization tools, including light microscopy, SEM fractography, and TEM, were employed to explore the effects of hydrogen on the microstructure of Ti-5-5-5-3 and the resulting fracture toughness. Hydrogen has been verified to have a strong effect, especially at low levels, on the fracture toughness of Ti-5-5-5-3. The reduced fracture toughness appears to be largely associated with an increase in the amount and nature of boundary type fracture modes. The exact mechanism for how hydrogen modifies boundary fracture behavior is still under investigation; however changes in α phase morphology have been observed as well, and may provide a key to further understanding.
Summary: Ti-5Mo-5V-5Al-3Cr (Ti-5-5-5-3) is an emerging near-â Ti alloy of significant commercial interest. Hydrogen has been verified to have a negative effect, especially at low levels, on the fracture toughness of Ti-5-5-5-3. The mechanism is still under study investigation.
