Abstract: This study deciphered the influence of lamellar α ( α l ) colony parameters on impact toughness of α + β titanium alloy with lamellar microstructure. α + β titanium alloy Ti-5Al-7.5V-0.5Mo-0.5Zr-0.5Si was β treated and cooled under different cooling rates to obtain α l colony with diverse morphology and size. U-notch Charpy impact test revealed that the impact toughness increased with decreased cooling rate and consequent α l colony coarsening. Impact load-displacement curves demonstrated that α l colony coarsening simultaneously enhanced both impact crack initiation energy and crack propagation energy. In the crack initiation region near U-notch, slip trace analysis indicated that coarse α l colony extended dislocation mean free path and triggered multiple slips, which facilitated plasticity prior to U-notch cracking and enhanced W i . Furthermore, as evidenced by Focus Ion Beam-Transmission Electron Microscopy, the nucleation of {10 $\bar{1}$2} <$\bar{1}$011> twin in coarse α l colony mitigated deformation heterogeneity, acted as prismatic slip pathway, and provided sustainable <c + a> dislocation sources, thereby further delaying U-notch crack initiation and enhanced W i . Conversely, fine α l colony restrained dislocation mobility and inhibited twin nucleation, leading to inferior U-notch plasticity and resultant low W i . From U-notch cracking to final fracture, the sustained crack blunting due to substantial plastic deformation of coarse α l along the crack path, as well as crack deflection and branching between adjacent coarse α l colonies, synergistically enhanced W p . Conversely, fine α l colony impaired the plasticity along the crack path and restrained crack deflection, which was inconducive to W p . In summary, α l colony coarsening played a crucial role in activating multiple toughening mechanisms during both crack initiation and propagation to achieve desirable impact toughness in α + β titanium alloys with lamellar microstructure.

Key words: α+β titanium alloy, Lamellar microstructure, Impact toughness, Dislocation slip, Deformation twinning, Crack deflection