Shock-induced twinning in polycrystalline vanadium: II. Surface layer

In the course of study of shock-induced twinning in commercially pure (99.8 wt%) polycrystalline vanadium, some unexpected metallurgical features were found. In all vanadium samples softly recovered after planar impact loading by copper impactors with velocities ranging from 262 to 610 m/s, the domain of twinned grains (located at the distance 100–900 μm from an impacted sample surface) preceded by a relatively narrow strip, 60–100 μm, densely filled by martensite lenticles of micron size. The distribution of shock-induced twins and the stress, required for their nucleation, were considered in the Part I of the present paper series. Part II of this series is focused on the Transmission Electron Microscopy study of the lenticles, formed in immediate proximity to the impacted surface. It was found that these lenticular particles are oblate ellipsoids of micron size filled with the stacks of 10–30 nm thick planar slabs, which possess tetragonal crystal structure. The slabs have alternating orientation of tetragonal axes c while the parameters of their unit cell are derivatives of cubic lattice parameter of vanadium, a_V, namely a = b = 2a_V and c = a_V. Possible model, based on a sequence of glides, capable to generate such microstructure, and the cause for the disappearance of the lenticles beyond 100 μm apart from the impacted surface are discussed.