Crack-stimulated generation of deformation twins in nanocrystalline metals and ceramics

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.

Keywords:

• nanocrystalline materials
• grain boundaries
• mechanical properties
• twins
• fracture

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.