Abstract
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}α type twins in fatigued specimens. Primary αp particles and secondary αs lamellae accommodate more cyclic deformation than retained βr laths. Furthermore, the dislocation free zone can be observed in the αp/βtrans (β transformed microstructure) boundary. As a result, microcracks primarily nucleate at the αp/βtrans interface or at αp particles. Moreover, a few microcracks initiate at the αs/βr interface or at αs plates of βtrans microstructure. These fatigue crack initiation behaviors promote the fracture of Ti–55531 alloy.
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