Towards a better understanding of the hydrogen impact on fatigue behavior of commercially pure titanium

This study examined how hydrogen content (15–920 wppm) affects the low-cycle fatigue behavior of commercially pure titanium at 300 K, under conditions favoring prismatic slip. Three distinct hydrogen content regimes were identified, illustrating the competition between hardening and softening mechanisms, depending on whether hydrogen is present in solid solution or as γ- and δ-hydrides. This competition is related to internal stresses evolutions, driven by dislocation patterns formation and interactions with hydrides and/or dissolved hydrogen. Hydrogen in solution reduces short-range interactions by screening the hardening effect of oxygen. Long-range internal stresses are increased due to hydride formation. The saturation of this increase as a function of hydrogen content is correlated with the observed deformability of these phases. The impact of hydrogen on cyclic damage mechanism was questioned in term of crack initiation and propagation in relation with dislocation and hydride features corresponding to the three hydrogen content regimes. Hydrogen in solution inhibits twinning, thereby promoting crack initiation along prismatic planes, which delays the cyclic number to crack initiation. However, the crack propagation rate is higher due to the activation of secondary cracking at a crack tip. Hydride formation reduces the number of cycles to initiation and localizes damage along facets corresponding to the hydride/α-phase interface. The formation of γ hydrides at the crack tip reduces the propagation rate; however, at higher hydrogen contents, long δ phases promote crack propagation by providing preferential paths.