Mechanical response, deformation and damage mechanisms in dual-phase cobalt upon plate impact

Shock compression and spallation damage of a dual-phase polycrystalline cobalt material is investigated via plate impact experiments along with free-surface velocity measurements. The as-received and postmortem samples are characterized with X-ray diffraction measurement and electron back-scatter diffraction. Free-surface velocity histories, the Hugoniot equation of state and spall strength at different peak shock stress are determined. Multiple deformation mechanisms are found. Except for the dislocation slip in both phase, \(\{10{\bar{1}}2\}\) $\{10\overline{1}2\}$ and \(\{11{\bar{2}}1\}\) $\{11\overline{2}1\}$ deformation twinning in the hexagonal close-packed (HCP) phase, and the face-center cubic (FCC) to HCP phase transition are also observed. The \(\{10{\bar{1}}2\}\) $\{10\overline{1}2\}$ twin density at the impact surface increases with increasing shock stress, but is less than twin density at the spall plane for the same shot. Ductile fracture is the main damage mode, and voids are nucleated preferentially within the HCP phase because of the strain localization. The Johnson–Cook constitutive model can describe the dynamic responses of the dual-phase Co. With this model, finite element modeling is consistent with the shock compression part of experimental observations well.