Low cycle fatigue behavior of near-alpha titanium alloys used in deep-driving submersible: Ti-6Al-3 Nb-2Zr-1Mo vs. Ti-6Al-4 V ELI

The low cycle fatigue (LCF) of Ti-6Al-3Nb-2Zr-1Mo and Ti-6Al-4V ELI with a bimodal structure were compared at different total strain amplitudes ( ∆ ε t / 2 ) from 0.5% to 1.0%. Because the Ti-6Al-3Nb-2Zr-1Mo exhibited a higher tensile elongation, but a lower yield strength than Ti-6Al-4V ELI, it displayed a slightly longer fatigue life than Ti-6Al-4V ELI when ∆ ε t / 2 was larger than 0.7% (plastic deformation predominated), while its fatigue life became shorter when ∆ ε t / 2 was less than 0.7% (elastic deformation predominated). Both Ti-6Al-3Nb-2Zr-1Mo and Ti-6Al-4V ELI showed a continuous cyclic softening behavior due to the rearrangement or mutual annihilation of initial dislocations during cyclic deformation, and the variation of cyclic softening rate depended on ∆ ε t / 2 was consistent with the variation of fatigue life depended on ∆ ε t / 2 . In cyclic deformation, the prismatic and basal slips were easier to activate, and the dislocation bands were denser and more homogeneous in the Ti-6Al-3Nb-2Zr-1Mo than that in the Ti-6Al-4V ELI due to the actual Al content in former was smaller than that in latter, which was beneficial to reduce local stress concentration and prolong fatigue life. • The LCF properties of Ti-6Al-3Nb-2Zr-1Mo and Ti-6Al-4V ELI depend on ∆ ε t / 2 , when ∆ ε t / 2 is larger than 0.7%, the former fatigue life is slightly longer than the latter. However, when ∆ ε t / 2 is less than 0.7%, the former fatigue life becomes shorter than the latter. • The work provided data for the material selection of the pressure hull of deep-diving submersibles.