Atomistic study of diffusion-mediated plasticity and creep using phase field crystal methods

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[Asymptotically = to]1.02 and p[Asymptotically = to]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[Asymptotically = to]3, as expected for generic climb-mediated natural creep, to m[Asymptotically = to]5.8 as the dislocation density pd is increased beyond typical experimental values. Stress exponents m[> ~]3 directly measured from simulations that include dislocation nucleation, climb, glide, and annihilation are attributed primarily to these large pd effects. Extrapolation to lower pd suggests that m[Asymptotically = to]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 pd may nonetheless be relevant to systems in which comparable densities are obtained locally within heterogeneous defect domains such as dislocation cell walls or tangles.