ON THE ROLE OF NONEQUILIBRIUM GRAIN-BOUNDARY STRUCTURE IN THE YIELD AND FLOW-STRESS OF POLYCRYSTALS

At the beginning of the plastic deformation of polycrystals, dislocati ons are mainly accumulated in grain boundaries (GBs), forming extrinsi c grain-boundary dislocations (EGBDs). EGBDs create long-range stress fields which in turn considerably affect the dislocation glide in grai ns. On the basis of dislocation pile-up theory, the role of these stre sses in the deformation behaviour of polycrystals is studied. Two type s of internal stress induced by non-equilibrium GBs are considered: th e stresses of triple-line disclination dipoles and those of disordered EGBD arrays. It is shown that these two kinds of stress, which differ in their scales, contribute to the flow stress in different ways. The former, scaling with the grain size, leads to an increase in the para meter sigma0(epsilon) in the Hall-Petch relation, while the latter, sc aling with the average EGBD spacing, increases the parameter k(epsilon ). These parameters are calculated for several metals and good agreeme nt with experimental data is obtained.