The Effect of Hydrogen on the Fracture Toughness of Ti-5Mo-5V-5Al-3Cr

Ti-5Mo-5V-5Al-3Cr (Ti-5553) is a deep-hardenable titanium alloy of commercial importance as a viable replacement for Ti-6Al–4V, Ti-10Al-2V-3Fe and some high strength steels in a variety of aerospace applications. Ti-5553 offers significantly improved thick section hardenability in a product capable of being extruded, rolled, forged, and/or cast. In addition, Ti-5553 can be heat treated in several ways to achieve high strength, high fracture toughness, high fatigue resistance, or a reasonable balance of properties. Production experience has indicated 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. Multiple characterization tools, including light microscopy, SEM fractography, and TEM, were employed to explore the effects of hydrogen on the microstructure of Ti-5553 and the resulting fracture toughness. Much reduced hydrogen content, within common material specification limits, appears have a strong effect on the fracture toughness of Ti-5553. While not unique to Ti-5553, it is an important dependence to understand in terms of property control. Changes in fracture toughness appear to correlate with the amount of boundary fracture, although the exact mechanism for how hydrogen modifies boundary fracture behavior is not understood, and merits additional study.