The macroscopic deformation of a polycrystalline solid due to local deformation events in the core of grain boundaries was analyzed. The central result was an equation which decomposed the effective macroscopic strain into contributions from three deformation modes: elastic strain in the bulk of the crystallites, the results of dislocation glide and climb processes and deformation events in the grain-boundary core. The latter process was represented by jumps in the displacement vector field that could be decomposed into tangential (so-called slip) and normal (so-called stretch) components. The relevant measure for the grain-boundary-mediated deformation was not the displacement jump vector but a grain-boundary discontinuity tensor that depended upon the displacement jump and upon the orientation of the grain boundary normal. Accommodation processes at triple junctions do not contribute significantly to the macroscopic strain. By means of example, the theory was applied to the effective elastic response of nanocrystalline materials with an excess slip compliance at grain boundaries. The predictions, specifically on the size dependence of the Poisson ratio, agreed with recent experiments on nanocrystalline Pd. The value of the slip compliance for grain boundaries in Pd was obtained as 18pm/GPa.

Kinematics of Polycrystal Deformation by Grain Boundary Sliding. J.Weissmüller, J.Markmann, M.Grewer, R.Birringer: Acta Materialia, 2011, 59[11], 4366-77