Grain size, stress, and creep in polycrystalline solids

If a stress σ is applied to a polycrystal of grain size L, the mode of creep deformation depends on the answers to the following questions: (I) Does σ exceed the Peierls stress σp; (II) Does L exceed the dislocation spacing in a Taylor lattice stabilized by σp; (III) Does Lσ exceed the value required for a Frank-Read or Bardeen-Herring source to operate within the grain? (IV) Does L1/2σ exceed the Hall-Petch value required for slip to propagate across a grain boundary? The (L, σ) plane is thus partitioned into regions in which different creep modes predominate.