A theoretical model was proposed that described the generation of deformation twins near brittle cracks of mixed I and II modes in nanocrystalline metals and ceramics. In the framework of the model, a deformation twin nucleates through stress-driven emission of twinning dislocations from a grain boundary distant from the crack tip. The emission was driven by both the external stress concentrated by the pre-existent crack and the stress field of a neighbouring extrinsic grain boundary dislocation. The ranges of the key parameters, the external shear stress and the crack length were calculated within which the deformation-twin formation near pre-existent cracks was energetically favourable in a typical nanocrystalline metal (Al) and ceramic (3C-SiC). The results of the proposed model accounted for experimental data on observation of deformation twins in nanocrystalline materials reported in the literature. The deformation-twin formation was treated as a toughening mechanism effectively operating in nanocrystalline metals and ceramics.

Crack-Stimulated Generation of Deformation Twins in Nanocrystalline Metals and Ceramics. M.Y.Gutkin, I.A.Ovidko, N.V.Skiba: Philosophical Magazine, 2008, 88[8], 1137-51