Abstract
A theoretical model is proposed that describes 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 is 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, τ, and the crack length, L, are calculated within which the deformation-twin formation near pre-existent cracks is energetically favourable in a typical nanocrystalline metal (Al) and ceramic (3C-SiC). The results of the proposed model account for experimental data on observation of deformation twins in nanocrystalline materials reported in the literature. The deformation-twin formation is treated as a toughening mechanism effectively operating in nanocrystalline metals and ceramics.
Acknowledgements
The authors greatly thank Dr. A.G. Sheinerman for helpful discussions. The work was supported, in part, by the Office of US Naval Research (grant N00014-07-1-0295), the Russian Federal Agency of Science and Innovations (Contract 02.513.11.3190 of the Program “Industry of Nanosystems and Materials” and grant MK-5122.2007.1 of the President of the Russian Federation), the Russian Federation of Basic Research (Grants 08-01-00225-a and 08-02-00304-a), the National Science Foundation Grant CMMI #0700272, Russian Academy of Sciences Program “Structural Mechanics of Materials and Construction Elements”, and St. Petersburg Center of Russian Academy of Sciences.