Cyclic softening behavior of TC17 titanium alloy fabricated by laser directly energy deposition

In aero-engines, cyclic softening can cause premature fracture of components, which is regarded as a challenge for the security of titanium alloy parts in service. In this work, the cyclic deformation behavior of LDEDed and beta-forging TC17 titanium alloys with different morphology of alpha phase was explored, cyclic softening mechanism was investigated by using transmission electron microscopy (TEM) observation of dislocation morphology. The experimental results demonstrate that cyclic softening occurs when the total strain range increases to 2.0 %. The LDEDed specimen has a smaller stress amplitude and a larger plastic strain during cyclic softening than wrought specimen. In the softened deformation microstructure of LDEDed specimen, the size of alpha p decreases with the increase of strain, but the size and shape of alpha p and alpha s in wrought specimen do not change much. The main dislocation forms that dominate the cyclic softening in the alpha phase and the beta matrix are slip bands and dislocation tangles, respectively, and the slip band in the alpha phase cannot pass through the secondary phase in the beta matrix for slip transfer. Under the same strain, the lower dislocation density in the LDEDed microstructure is due to the larger directional back stress produced by the forward loading, which causes the dislocation rearrangement and annihilation under the reverse loading.