  The nonequilibrium dynamics of diffusion-mediated plasticity and creep in
materials subjected to constant load at high homologous temperatures is studied
atomistically using Phase Field Crystal (PFC) methods. Creep stress and grain
size exponents obtained for nanopolycrystalline systems, $m \simeq 1.02$ and $p
\simeq 1.98$, respectively, closely match those expected for idealized
diffusional Nabarro-Herring creep. These exponents are observed in the presence
of significant stress-assisted diffusive grain boundary migration, indicating
that Nabarro-Herring creep and stress-assisted boundary migration contribute in
the same manner to the macroscopic constitutive relation. When plastic response
is dislocation-mediated, power law stress exponents inferred from dislocation
climb rates are found to increase monotonically from $m \simeq 3$, as expected
for generic climb-mediated natural creep, to $m \simeq 5.8$ as the dislocation
density $\rho_d$ is increased beyond typical experimental values. Stress
exponents $m \gtrsim 3$ directly measured from simulations that include
dislocation nucleation, climb, glide, and annihilation are attributed primarily
to these large $\rho_d$ effects. Extrapolation to lower $\rho_d$ suggests that
$m \simeq 4-4.5$ should be obtained from our PFC description at typical
experimental $\rho_d$ values, which is consistent with expectations for power
law creep via mixed climb and glide. The anomalously large stress exponents
observed in our atomistic simulations at large $\rho_d$ may nonetheless be
relevant to systems in which comparable densities are obtained locally within
heterogeneous defect domains such as dislocation cell walls or tangles.
