  The influence of grain size on the flow stress of various FCC polycrystals
(Cu, Al, Ag and Ni) has been analyzed by means of computational homogenization
of a representative volume element of the microstructure using a FFT approach
in combination with a strain gradient crystal plasticity model. The density of
geometrically necessary dislocations resulting from the incompatibility of
plastic deformation among different crystals was obtained from the Nye tensor,
which was efficiently obtained from the curl operation in the Fourier space.
The simulation results were in good agreement with the experimental data for
Cu, Al, Ag and Ni polycrystals for grain sizes > 20 microns and strains < 5%
and provided a physical explanation for the higher strengthening provided by
grain boundaries in Al and Ni, as compared with Cu and Ag. The investigation
demonstrates how the combination of FFT with strain gradient crystal plasticity
can be used to include effect of grain boundaries in the mechanical behavior of
polycrystals using realistic representative volume elements of the
microstructure.
