Deciphering the impact toughening mechanism of α+β titanium alloy with lamellar microstructure: From crack initiation and propagation perspectives

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$\overline{1}$2} <$\overline{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.