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