Optimized low-cycle fatigue behavior and fracture characteristics of Ti–6Al–4V alloy by Fe microalloying

In the present work, low cycle fatigue (LCF) behavior and fracture characteristic of Ti–6Al–4V-0.55Fe alloy with bimodal microstructure, consisted of equiaxed primary α (α_p), lamellar α (α_l) and β matrix, are systematically investigated at room temperature. Results indicate that Ti–6Al–4V-0.55Fe alloy mainly exhibits a continuous softening behavior both at high and low strain amplitudes, due to the interaction of back stress(σ_b) and friction stress(σ_f) mainly related to the plastic deformation heterogeneity and precipitates shearing, respectively. Compared with Ti–6Al–4V alloy, LCF life of Ti–6Al–4V-0.55Fe is similar at high strain amplitudes (Δε_t/2 > 1.0%), while much higher at low strain amplitudes (Δε_t/2 < 1.0%), which could be attributed to the extra resistance to dislocation movement and crack propagation produced by Fe microalloying. The fractography shows remarkably different characteristics for these imposed strain amplitudes. At low strain amplitude, there is only one crack initiation site caused by dislocations pile-ups and stress concentration on the specimen surface, accompanied with narrower fatigue striation, while more cracks initiated on the surface with wider fatigue striation due to strain accumulation at high strain amplitudes.