Micro-plastic deformation behavior and life enhancement mechanism of additively manufactured Ti-6Al-4V titanium alloy during high cycle fatigue

The high cycle fatigue (HCF) properties of laser directed energy deposition (L-DED) Ti-6Al-4V titanium alloy at different maximum cyclic stresses (σ_max) were studied, with a particular focus on revealing the micro-plastic deformation behavior and the enhancement mechanisms of HCF life under a relatively high maximum cyclic stress of σ_max=600 MPa. The results indicate that as σ_max increases, the cyclic plastic deformation mechanism of L-DED Ti-6Al-4V titanium alloy undergoes significant transformations. At σ_max=480 MPa, the dominant deformation characteristic is twinning deformation. When σ_max further increases, the deformation mode gradually transitions into dislocation slip in β (510 MPa), dislocation pile-up in α′ (540 MPa), and dislocation entanglement in α′ (570 MPa). Notably, at σ_max=600 MPa, the novel granular α′ phase precipitates within the microstructure, leading to a substantial reduction in both the number of dislocations and micro-plastic deformation compared to the matrix. Additionally, the short needle-like α′ can directly transform into the deformation-induced recovery microstructure. As a result, the proportion of recrystallized microstructure increases, facilitating fatigue damage recovery. Consequently, the HCF life of L-DED Ti-6Al-4V titanium alloy is improved at σ_max=600 MPa.