On the role of non-equilibrium grain-boundary structure in the yield and flow stress of polycrystals

Abstract

At the beginning of the plastic deformation of polycrystals, dislocations are mainly accumulated in grain boundaries (GBs), forming extrinsic grain-boundary dislocations (EGBDs). EGBDs create long-range stress fields which in turn considerably affect the dislocation glide in grains. On the basis of dislocation pile-up theory, the role of these stresses in the deformation behaviour of polycrystals is studied. Two types of internal stress induced by non-equilibrium GBs are considered: the 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 parameter σ₀(η) in the Hall-Petch relation, while the latter, scaling with the average EGBD spacing, increases the parameter k(η). These parameters are calculated for several metals and good agreement with experimental data is obtained.