Strain rate dependence of tension and compression behavior in nano-polycrystalline vanadium nitride

We performed molecular dynamics (MD) uniaxial tension and compression simulations of nano-polycrystalline (npc) vanadium nitride (VN) with different strain rates to investigate strain rate effects and tension-compression asymmetry in npc VN. The Zener's familiar anisotropy index A of the polycrystalline VN model is 0.957, very closed to 1.0, demonstrating its elastic isotropy. The Young's modulus increases with the increase of strain rate. The yield strain, yield stress and flow stress increase in power form with the increase of strain rate. As the strain rate varies in the range between 5E8 and 1E9 s−1, strain rate effect becomes insignificant, and the corresponding loading can be regarded as quasistatic or low strain rate loading. Tension-compression asymmetry was observed and analyzed systematically. The elastic tension-compression asymmetry should be ascribed to the higher friction in compression, the asymmetry of interatomic potential, and the effect of cutoff distance in a MD simulation. While the plastic tension-compression asymmetry should mainly result from the different kinds of atomic interaction, intergranular fracture and dislocation glide. No distinct grain boundary sliding was found.