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The nonequilibrium dynamics of diffusion-mediated plasticity and creep inmaterials subjected to constant load at high homologous temperatures is studiedatomistically using Phase Field Crystal (PFC) methods. Creep stress and grainsize exponents obtained for nanopolycrystalline systems, $m \simeq 1.02$ and $p\simeq 1.98$, respectively, closely match those expected for idealizeddiffusional Nabarro-Herring creep. These exponents are observed in the presenceof significant stress-assisted diffusive grain boundary migration, indicatingthat Nabarro-Herring creep and stress-assisted boundary migration contribute inthe same manner to the macroscopic constitutive relation. When plastic responseis dislocation-mediated, power law stress exponents inferred from dislocationclimb rates are found to increase monotonically from $m \simeq 3$, as expectedfor generic climb-mediated natural creep, to $m \simeq 5.8$ as the dislocationdensity $\rho_d$ is increased beyond typical experimental values. Stressexponents $m \gtrsim 3$ directly measured from simulations that includedislocation nucleation, climb, glide, and annihilation are attributed primarilyto 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 typicalexperimental $\rho_d$ values, which is consistent with expectations for powerlaw creep via mixed climb and glide. The anomalously large stress exponentsobserved in our atomistic simulations at large $\rho_d$ may nonetheless berelevant to systems in which comparable densities are obtained locally withinheterogeneous defect domains such as dislocation cell walls or tangles.

An Atomistic Study of Diffusion-Mediated Plasticity and Creep using Phase Field Crystal Methods