Simulation of Tantalum Nanocrystals Under Shock-Wave Loading: Dislocations and Twinning

We simulate strong shock waves in nanocrystalline tantalum using atomistic molecular dynamics simulations, for particle velocities in the range 0.35-2.0 km s(-1), which induce pressures in the range 20-195 GPa. Our simulations explore strain rates in the range 108 s(-1) - 1010 s(-1), and lead to a peak strength in the range 3-15 GPa. Nanocrystalline tantalum exposed to strong shock waves demonstrates deformation enabled by concomitant dislocations, twinning, and grain boundary activity at a variety of particle velocities. Twinning is observed for a mean grain size of 7 nm, starting at around 32 GPa, in disagreement with models which predict a Hall-Petch behavior for twinning, i.e. a twinning stress scaling with grain size d as d(-0.5), and supporting the presence of an inverse Hall-Petch effect for twinning at small grain sizes.