Sequential multiplication of dislocation sources along a crack front revealed by high-voltage electron microscopy and to
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Grace S. Liu Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801
Hiroto Nakamura and Kenji Higashida Department of Materials Science and Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
Ian M. Robertsonb) Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801 (Received 23 June 2010; accepted 10 December 2010)
The three-dimensional structure of crack tip dislocations in single crystal silicon was observed by combining high-voltage electron microscopy and tomography. It was revealed that dislocations cross-slipped proximal to the crack tip even in the initial stages of plastic deformation. The local stress intensity factor along the crack front was calculated by taking into account the experimentally determined dislocation character. Based on these observations and calculations, a model to account for the sequential multiplication of dislocation sources along the crack front is proposed.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stages. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr_policy DOI: 10.1557/jmr.2010.99
because determining the spatial distribution experimentally was prohibitively difficult. Such simulations showed the formation of dislocation junctions, such as Lomer–Cottrell locks, around the crack tip that require the activation of dislocations on different slip systems so that the dislocations interact to form sessile junctions. For example, if a line dislocation is anchored by two sessile segments, the intermediate glissile segment can serve as a Frank–Read source. This will provide a mechanism to increase the dislocation source density, which is needed to produce the large number of dislocations for dislocation shielding to operate. The presence of these sources, however, has not been verified experimentally and a paucity of information remains about dislocation sources at crack tips. A technique to visualize the spatial distribution of such dislocations is electron tomography. This involves reconstructing a three-dimensional model from many micrographs of the same region that are recorded at regular intervals over a wide angular range. Although widely applied in the life sciences, electron tomography is only emerging in the study of defects in crystalline materials where it was first used to reveal the morphology of precipitates, second phases, and so forth in crystalline materials.10–14 For forming reconstructions, diffraction contrast imaging conditions are generally avoided as it is difficult to maintain the exact same Bragg condition over the requisite angular range and the resulting variations in image contrast can cause difficulties in the reconstruction process. Nevertheless, there are limited examples in which diffraction contrast images have been used to form a tomogram of dislocation
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