This work deals with the prediction of grain-size dependent hardening in FC
C and BCC polycrystalline metals at moderately high strains (2-30%). The mo
del considers 3-D, polycrystalline aggregates of purely viscoplastic crysta
ls, and simulates quasi-static deformation histories with a hybrid finite e
lement method implemented for parallel computation. The hardening response
of the individual crystals is considered to be isotropic, but modified to i
nclude a physically motivated measure of lattice incompatibility which is s
upposed to model, in the continuum setting, the resistance to plastic flow
provided by lattice defects. The length-scale in constitutive response that
is required on dimensional grounds appears naturally from physical conside
rations. The grain-size effect in FCC polycrystals and development of Stage
IV hardening in a BCC material are examined. Though the grain-size does no
t enter explicitly into the constitutive model, an inverse relationship bet
ween the macroscopic flow stress and grain-size is predicted, in agreement
with experimental results for deformation of FCC polycrystals having grain-
sizes below 100 microns and at strains beyond the initial yield (>2%). The
development of lattice incompatibility is further shown to predict a transi
tion to Stage IV (linear) hardening upon saturation of Stage III (parabolic
) hardening. (C) 2000 Elsevier Science Ltd. All rights reserved.