A theoretical model was proposed to describe the nucleation of deformation twins at grain boundaries in nanocrystalline materials under the action of an applied stress and the stress field of a dipole of junction or grain-boundary wedge disclinations. The model was used to consider pure nanocrystalline Al and Cu with an average grain size of about 30 nm. The conditions of barrier-free twinning-dislocation nucleation were studied. These conditions were shown to be realistic for the metals under study. As the twin-plate thickness increased, one observes two stages of local hardening and an intermediate stage of local flow of a nanocrystalline metal on the scale of one nanograin. In all stages, the critical stress increased with decreasing disclination-dipole strength. The equilibrium thickness and shape of the twin plate were analyzed and found to agree well with the well-known results of experimental observations.

Mechanism of Deformation-Twin Formation in Nanocrystalline Metals. M.Y.Gutkin, I.A.Ovidko, N.V.Skiba: Physics of the Solid State, 2007, 49[5], 874-82