The role of the grain boundary in influencing the deformation of a bicrysta
l is explored using a rate-dependent crystal formulation of the Fleck-Hutch
inson strain gradient plasticity theory. The physical basis of the theory i
s the elevated strengthening of a slip system due to geometrically necessar
y dislocations, associated with spatial gradients of slip. The theory is im
plemented within the finite element framework and is used to study the defo
rmation of a bicrystal under in-plane shear loading. Contrary to classical
scale-independent crystal plasticity theories, the strain gradient theory p
redicts that the deformation state depends strongly upon grain size. Strain
gradient effects are pronounced within a narrow layer at the grain boundar
y of a bicrystal, and a significant grain-size dependence of the yield stre
ngth is predicted. (C) 1999 Elsevier Science Ltd. All rights reserved.