Abstract:Deformation and fracture behavior of Ti-55531 alloy with lamellar microstructure (LM) and bimodal microstructure (BM) under static loading were analyzed by the in-situ SEM tensile method. The results show that the characteristic parameters of the alpha phase have a strong influence on the deformation, crack initiation and propagation behavior of the alloy under static loading. Dislocation initiated at secondary alpha (αs) interior under pressure, because the thick αs plate is softer than residual beta (βr) lath in LM. Dislocation moves and accumulates at the αs/βr interphase during deformation, which induces that cracks initiate at and propagate along the αs/βr interphase during the process of fracture. However, beta transaction microstructure (βtrans) is greatly harder than equiaxed primary α (αp) in BM, moreover, αpwith large size could support longer slip length. Both of above reasons leads to a large number of multi-direction slips initiate at αp during deformation of BM. The intersection of slip lines in different directions promotes stress concentration at αpinner, and some dislocations accumulate at the αp/βtrans interface to produce stress concentration. These behavior results in the initiation of microcracks within αp and the αp/βtrans interface, and propagation along the weak interface of αp/βtrans, or the aggregation site of αp.