Shock-induced twinning in polycrystalline vanadium: I. twinning stress

The shock-induced twinning at room temperature was studied using 3 mm thick vanadium samples of commercial purity (99.8 wt%), softly recovered after planar impact loading by copper impactors with velocities ranging from 262 to 610 m/s. Microscopic (Light and Scanning Electron) examinations of the samples' cross-sections revealed twins in a strip of vanadium grains, located 100–900 μm apart from the impacted sample surface. Transmission electron microscopy characterization allowed concluding that these twins are produced by a/6 〈111〉{211} glide in subsequent {211} planes. The number of twins, N_tw, per unit area varied from its maximum value, measured at the distance h = 0.2–0.3 mm from the impacted sample surface, to N_tw = 0 at h = 0.7–0.9 mm apart from the surface. Juxtaposition of the presently obtained N_tw(h) dependencies with previously reported spatial distribution of the shear stress, τ(h), in shock-loaded vanadium samples made it possible to determine twinning stress in vanadium as: τ (N_tw = 0) = τ_tw = 0.68(±0.03) GPa.