Dislocation structures associated with advancing cracks in gallium arsenide and magnesium oxide
Dewald, Dale Keith
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Permalink
https://hdl.handle.net/2142/20896
Description
Title
Dislocation structures associated with advancing cracks in gallium arsenide and magnesium oxide
Author(s)
Dewald, Dale Keith
Issue Date
1991
Doctoral Committee Chair(s)
Birnbaum, Howard K.
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Engineering, Materials Science
Language
eng
Abstract
The purpose of this research was to determine which dislocation mechanisms were the most important in the fracture processes of semi-brittle materials. In situ TEM deformation experiments were conducted to observe the dislocation structures ahead of propagating cracks in GaAs and MgO. Images of the dislocation structures and arrangements were recorded, both photographically and on video tape, analyzed, and compared to the structures predicted by existing theories. Qualitative knowledge about the role of dislocations in the fracture process was obtained and several new aspects of the behavior of dislocations near the crack were identified. Dislocation structures observed ahead of and in the (011) planes of arrested cracks in GaAs were either the remains of a shear type opening mechanism for the crack, or were emitted from the crack tip as it arrested. The evidence of slip activity and unreacted dislocations ahead of the network structure, and the direct observation of their glide on (011) planes, supported a hypothesis that they are not generated entirely by a crack healing mechanism. These dislocations were later absorbed into the propagating crack, and provided energy for accelerating the propagation rate. The absorbed dislocations were repelled by the crack tip stress field, but their velocity was less than the crack propagation rate. For the case of MgO, dislocations were rapidly nucleated from the crack, but were impeded in their outward motion by interactions with defects and other dislocations. TEM foils of MgO with (001) orientation exhibited considerable ductility whereas (011) foils fractured easily. The active slip planes and dislocation structures in the (001) foils shielded the crack from mode I stresses. The emitted dislocations in (011) foils provided mixed mode shielding which did not sufficiently inhibit crack propagation.
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