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

Abstract

The low cycle fatigue (LCF) of Ti-6Al-3 Nb-2Zr-1Mo and Ti-6Al-4 V ELI with a bimodal structure were compared at different total strain amplitudes (∆εt/2) from 0.5 % to 1.0 %. Because the Ti-6Al-3 Nb-2Zr-1Mo exhibited a higher tensile elongation, but a lower yield strength than Ti-6Al-4 V ELI, it displayed a slightly longer fatigue life than Ti-6Al-4 V ELI when ∆εt/2 was larger than 0.7 % (plastic deformation predominated), while its fatigue life became shorter when ∆εt/2 was less than 0.7 % (elastic deformation predominated). Both Ti-6Al-3 Nb-2Zr-1Mo and Ti-6Al-4 V 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 ∆εt/2 was consistent with the variation of fatigue life depended on ∆εt/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-3 Nb-2Zr-1Mo than that in the Ti-6Al-4 V 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.