Effect of microstructure on high cycle fatigue behavior of Ti–5Al–5Mo–5V–3Cr–1Zr titanium alloy

High-cycle fatigue (HCF) behavior of Ti–5Al–5Mo–5V–3Cr–1Zr (Ti-55531) alloy with both lamellar microstructure (LM) and bimodal microstructure (BM) was studied at room temperature. The results indicate that BM presents much higher strength, lower ductility and slightly higher HCF strength (10⁷ cycles, R = −1) than those of LM. Typical dislocation structures including straight prismatic slip lines, curved dislocation lines, dislocation tangles and twins can be discovered in fatigued specimens with two different microstructures. Primary α (α_p) particles and secondary α (α_s) lamellae accommodate more cyclic deformation than retained β (β_r) laths. Grain boundary (GB) α layers have more effect on promoting crack initiation in LM than that in BM. As a result, fatigue microcracks mainly initiate at the interface between GB α films and prior β grains or at the α_s/β_r interphase for LM. However, microcracks primarily nucleate at the α_p/β_trans (β transformed microstructure) interface or at α_p particles in BM. The combination of transgranular and intergranular crack propagation could be observed in the two microstructures. Crack front profile of macrocrack in LM is rougher than that of BM during the stable propagation region.