  Here we use large-scale molecular dynamics (MD) simulations of the high-rate
deformation of nanocrystalline tantalum to investigate the processes associated
with plastic deformation for strains up to 100%. We use initial atomic
configurations that were produced through simulations of solidification in the
work of Streitz et al [Phys. Rev. Lett. 96, (2006) 225701]. These 3D
polycrystalline systems have typical grain sizes of 10-20 nm. We also study a
rapidly quenched liquid (amorphous solid) tantalum. We apply a constant volume
(isochoric), constant temperature (isothermal) shear deformation over a range
of strain rates, and compute the resulting stress-strain curves to large
strains for both uniaxial and biaxial compression. We study the rate dependence
and identify plastic deformation mechanisms. The identification of the
mechanisms is facilitated through a novel technique that computes the local
grain orientation, returning it as a quaternion for each atom. We find both
dislocation and twinning processes are important, and they interact in the weak
strain hardening in these extremely fine-grained microstructures.
