  The two-phase composite approach of Estrin et al. (1998) describes an
evolving dislocation cell structure. Mckenzie et al. (2007) enhanced the model
to capture the effects of hydrostatic pressure and temperature during severe
plastic deformation. The goal of the present study is to incorporate this
microstructural model into the macroscopic viscoplasticity framework proposed
by Shutov and Krei\ss ig (2008a). Thereby, the two-phase composite approach is
examined carefully. Both physical and numerical drawbacks are revealed and
possible solutions are presented, thus leading to a refined micro model.
Moreover, some improvements concerning reliable parameter identification are
suggested as well. The material parameters of the refined micro model are
identified for an aluminum alloy using TEM cell size measurements. Then, an
extension to non-proportional deformation is performed in such a way that the
evolution of dislocation densities becomes sensitive to load path changes.
Experimental findings suggest that such deformation modes can significantly
influence the evolution of microstructure, including the dissolution of cells
and the reduction of total dislocation density shortly after the load path
change. In order to capture such effects, some tensor-valued state variables
are introduced which couple the refined micro model with the macroscopic
viscoplasticity model. As a result, a new system of constitutive equations is
obtained. In order to demonstrate its capability to respond to load path
changes, load cases as typical for Equal Channel Angular Pressing (ECAP) are
considered. The obtained evolution of dislocation populations differs
signficantly depending on which ECAP route is applied.
