  Viscoplastic flow of polycrystalline metallic materials is the result of
motion and interaction of dislocations, line defects of the crystalline
structure. In the microstructural (physics-based) constitutive model presented
in this paper, the main underlying microstructural processes influencing
viscoplastic deformation and mechanical properties of metals in cold and warm
regimes are statistically described by the introduced sets of postulates/axioms
for continuum dislocation dynamics (CDD). Three microstructural (internal)
state variables (MSVs) are used for statistical quantifications of different
types/species of dislocations by the notion of dislocation density. Considering
the mobility property of dislocations, they are categorized to mobile and
(relatively) immobile dislocations. Mobile dislocations carry the plastic
strain (rate), while immobile dislocations contribute to plastic hardening.
Moreover, with respect to their arrangement, dislocations are classified to
cell and wall dislocations. Cell dislocations are those that exist inside
cells/subgrains, and wall dislocations are packed in (and consequently formed)
the subgrain walls/boundaries. Therefore, the MSVs incorporated in this model
are cell mobile, cell immobile and wall immobile dislocation densities. The
evolution of these internal variables is calculated by means of adequate
equations that characterize the dislocation processes dominating material
behavior during cold and warm monotonic viscoplastic deformation. The
constitutive equations are then numerically integrated; and the constitutive
parameters are determined/fitted for a widely used ferritic-pearlitic steel
(20MnCr5).
