  In this study, we use discrete dislocation dynamics (DDD) simulation to
investigate the effect of heterogeneous dislocation density on the transition
between quasi-elastic deformation and plastic flow in face-centered cubic
single crystals. By analyzing the stress-strain curves of samples with an
initial, axial dislocation density gradient, we arrive at the following
conclusions: (i) in the regime of quasi-elastic deformation before the onset of
plastic flow, the effective elastic modulus of the simulated samples falls
significantly below the value for a dislocation-free crystal. This modulus
reduction increases with decreasing dislocation density gradient: crystals with
homogeneous dislocation distribution are thus weakest in the quasi-elastic
regime; (ii) the transition towards plastic flow occurs first in regions of
reduced dislocation density. Therefore, the overall yield stress decreases with
increasing dislocation density gradient; (iii) crystals with dislocation
density gradient exhibit a more pronounced hardening stage during which stress
is re-distributed onto stronger regions with higher dislocation density until
the sample flows at a constant flow stress that is approximately independent on
dislocation density gradient. We interpret these findings in terms of a
continuum dislocation dynamics inspired model of dislocation density evolution
that accounts for inversive dislocation motions. The transition between
quasi-elastic and plastic deformation is interpreted as a transition from
inversive to non-inversive dislocation motion, and the initial differences in
elastic modulus are related to a density dependent polarizability of the
dislocation system. The subsequent plastic flow behavior is analyzed in terms
of a modified version of Mughrabi's composite model.
