Anisotropic deformation and damage of dual-phase Ti-6Al-4V under high strain rate loading

Plate impact experiments are conducted on a rolled titanium alloy Ti-6Al-4V to investigate the effects of structural anisotropy on dynamic deformation and damage, with the loading axis being parallel to the normal direction or transverse direction, referred to as LA∥ND and LA∥TD, respectively. Free-surface velocity histories are measured to evaluate the mechanical properties; the Hugoniot elastic limits for the LA∥ND and LA∥TD loading are 3.38 GPa and 3.75 GPa, respectively; the spall strengths are 4.54–4.71 GPa for the LA∥ND loading, and 4.69–4.91 GPa for the LA∥TD loading, for the impact velocities explored. The postmortem samples are characterized with scanning electron microscopy, electron backscatter diffraction and synchrotron X-ray computed tomography. The {101¯2} extension twins are activated near spall cracks and their number increases with increasing impact velocity. Microvoids nucleate at both grain (in the matrix) and phase boundaries, and grain boundary nucleation is more likely to occur given the small volume fraction of the β-phase. Cracks for the LA∥ND loading are generally perpendicular to the shock direction, while they are serrated and distributed more discretely for the LA∥TD loading. The damage degree for the LA∥ND loading is larger than for the LA∥TD loading, consistent with the corresponding spall strengths. At incipient spall, the average size of voids/cracks is larger for the LA∥ND loading than for the LA∥TD loading. Compared with the LA∥ND loading, small voids/cracks are more isotropic and large voids are more anisotropic for the LA∥TD loading.