Low-cycle fatigue property and damage mechanisms of Ti-6Al-4V-0.55Fe alloy with lamellar microstructure

The low cycle fatigue property and damage mechanisms of Ti-6Al-4V-0.55Fe (TC4-F) alloy with lamellar microstructure (LM) were systematically investigated under different strain amplitudes. The cyclic stress response and cyclic hardening/softening behavior of LM under selected strain amplitudes are analyzed and compared with bimodal microstructure (BM). The results demonstrate that both the strain amplitude and microstructure have a significant influence on hardening/softening behavior of this alloy. The initial cycle hardening is observed in both LM and BM under the low strain amplitude, while BM exhibits a higher degree of hardening. However, the alloy exhibits a softening behavior throughout the entire cycling process under high strain amplitudes. Both LM and BM exhibit typical characteristics of fatigue striations and secondary cracks in their fracture morphology. The fracture profile shows that the LM trend to crack deflection at triple junctions, and the increase of crack deflection path can slow down the propagation rate of fatigue cracks. The dislocation features indicate that the cyclic hardening/softening behavior of TC4-F alloy with LM mainly relies on the secondary α (α_s) phases, while that of BM primarily relies on primary α (α_p). It is noteworthy that the presence of stacking faults leads to cyclic saturation of LM under low strain amplitudes, whereas twining in LM contributes to cyclic softening under high strain amplitudes.