The effects of hydrogen on the deformation and fracture of β-titanium

The hydrogen-induced ductile–brittle transition in the BCC β-titanium alloy, Timetal® 21S, occurs abruptly at a critical hydrogen concentration that decreased with decreasing tensile test temperature. Mechanical property tests showed that solute hydrogen reduced the yield strength of ductile specimens and decreased the fracture stress of brittle specimens. To identify the operative mechanism a series of experiments were performed to test the applicability of the stress-induced hydride mechanism, the hydrogen-enhanced plasticity mechanism, and the decohesion mechanism of hydrogen embrittlement. The experiments showed that no hydrides were associated with the fracture process, indicating that the stress-induced hydride mechanism was not responsible for the observed sharp ductile–brittle transition. In situ straining experiments in a controlled environment transmission electron microscope showed that hydrogen enhanced the mobility of dislocations in both uncharged and hydrogen charged alloys, showing that the hydrogen-enhanced localized plasticity mechanism cannot account for the observed behavior. The experimental results are, however, fully consistent with the decohesion mechanism of hydrogen embrittlement.