High cycle fatigue behavior of Ti–5Al–5Mo–5V–3Cr–1Zr titanium alloy with bimodal microstructure

In this study, high–cycle fatigue (HCF) damage behavior of Ti–55531 alloy with bimodal microstructure (BM) was studied at room temperature. Fatigue crack initiation and propagation mechanisms of the alloy were thoroughly investigated by studying fracture morphology, crack front profiles, polished microstructure and dislocation structures beneath fatigue main–crack initiation sites of HCF samples. The results indicate that this alloy presents an excellent HCF strength (10⁷cycles, R = −1) as equal to 656 MPa. Dislocation analyses exhibit that typical dislocation structures include straight prismatic slip lines, curved dislocation lines, dislocation tangles and {1¯011}_αtype twins in fatigued specimens. Primary α_pparticles and secondary α_slamellae accommodate more cyclic deformation than retained β_rlaths. Furthermore, the dislocation free zone can be observed in the α_p/β_trans(β transformed microstructure) boundary. As a result, microcracks primarily nucleate at the α_p/β_transinterface or at α_pparticles. Moreover, a few microcracks initiate at the α_s/β_rinterface or at α_splates of β_transmicrostructure. These fatigue crack initiation behaviors promote the fracture of Ti–55531 alloy.