Effect of microstructure on impact fracture mechanism of a high-strength Ti-5Al-7.5V-0.5Si-0.25Fe-0.2O alloy

The impact property of high-strength Ti-5Al-7.5V-0.5Si-0.25Fe-0.2O alloy (Ti575) with trimodal microstructure (TM) and lamellar microstructure (LM) were investigated by combining instrumented Charpy U-notch impact test with microstructure characterization. Impact load-displacement curves illustrated that the higher impact toughness of LM (47.8 J/cm²) than TM (38 J/cm²) were attributed to both intrinsic toughening during crack initiation and extrinsic toughening during crack propagation. In crack initiation stage of LM, electron back-scattered diffraction (EBSD) evidenced the nucleation of numerous {10–12} <–1011> tensile twins in coarsened lamellar α (α_l) in vicinity of the crack notch. The high intrinsic toughness of LM was due to the dynamic microstructure refinement and facilitation of prismatic/pyramidal slip induced by twins during impact. For TM in contrast, smaller-sized primary α(α_p) and α_l suppressed the nucleation of twins, exacerbated stress concentration and reduced plastic deformation near the crack notch. In crack propagation stage, the kinking, shearing of α_l, crack deflection at α_l/β interfaces and secondary crack in α_l colony borders contributed to a tortuous crack path and large energy dissipation in LM, whereas crack could bypass α_p and cut through thin α_l in TM easily, resulting in a flat crack path and poor extrinsic toughness of TM. In summary, this work provided a basic understanding of the impact fracture mechanism of Ti575 with different microstructures.