  Along with high strength, plasticity is what makes metals so widely usable in
our material world. Both strength and plasticity properties of a metal are
defined by the motion of dislocations - line defects in the crystal lattice
that divide areas of atomic planes displaced relative to each other by an
interatomic distance. Here we present first fully dynamic atomistic simulations
of single crystal plasticity in metal tantalum predicting that above certain
maximum rate of straining - the ultimate limit - the dislocations can no longer
relieve mechanical loads and another mechanism, twinning, comes into play and
takes over as the dominant mode of dynamic response. At straining rates below
the ultimate limit, the metal attains a path-independent stationary state of
plastic flow in which both flow stress and dislocation density remain constant
indefinitely for as long as the straining conditions remain unchanged. In this
distinct state tantalum flows like a viscous fluid while still remaining a
strong and stiff metal.
