Atomistic simulations of the plasticity behavior of shock-induced polycrystalline nickel

Shock loading single crystalline nickel creates a defected nanostructure dominated by stacking faults and twins. This transformation is caused by a complex interplay between the incident waves, the waves reflected from sample-free surfaces, and the interference between reflected waves. The plasticity behavior of this shock-induced defected nickel was studied using molecular dynamics (MD) simulations. Compared to a perfect single-crystal nickel sample of the same size, the twinned sample has significantly less yield stress in compression, a slightly lower yield stress in tension, and a yield stress about 30 pct higher in shear. Importantly, our simulations reveal the underlying atomistic mechanisms of dislocation nucleation and twin growth. We observe that while strengthening under shear loading involves lattice dislocations cutting through twins, weakening arises from nucleation of dislocations on the twins under tensile and compressive loading. Also, we have discovered precursors to dislocation loop nucleation in these simulations.