Effect of hydrogen and strain rate upon the ductility of mill-annealed Ti6A14V

An extensive comparison of both smooth and fatigue-cracked compact tension (CT) specimens of mill-annealed Ti6Al4V bar after hydriding to various hydrogen levels up to 3000 wppm has isolated several different regimes of mechanical behaviour. Smooth specimens show only slight changes in ductility until the hydrogen concentration reaches 1500 to 3000 wppm, when there is an increasingly pronounced embrittlement attributable to hydride precipitation at alpha/beta interfaces. On the other hand, CT specimens have much greater sensitivity to embrittlement by hydrogen and exhibit both slow and fast crack propagation, except in a narrow range between 60 and 200 wppm hydrogen where only fast cracking was detected. This range appears to separate two distinct cracking regimes: at lower hydrogen concentrations the slow cracking that precedes final failure seems to be creep-related whereas hydride precipitation at alpha/beta interfaces increasingly dominates the fracture process above the critical range. The combined results are discussed in terms of the various theories advanced for embrittlement of titanium alloys by absorbed hydrogen and the particular fracture processes involved. This entails consideration of critical stress intensity factors and related crack velocities monitored during the slow cracking stage. (C) 1998 Elsevier Science Ltd. All rights reserved.