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Title:Time-resolved and three-dimensional study of dislocation-particle interactions in aluminum and copper alloys
Author(s):Liu, Grace
Director of Research:Robertson, Ian M.
Doctoral Committee Chair(s):Robertson, Ian M.
Doctoral Committee Member(s):Bellon, Pascal; Zuo, Jian-Min; Johnson, Duane D.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Precipitate Strengthening
Transmission electron microscopy (TEM)
in situ straining
3D Imaging
Electron Tomography
Abstract:Dislocation-particle interactions in three alloys systems, Al-Cu, Al-Mg-Sc, and Cu-Co have been investigated using in situ straining experiments inside the transmission electron microscope, post mortem analysis, and electron tomographic reconstructions. Specifically, diffraction-contrast electron tomography was developed during the course of this work as a 3D imaging technique for crystalline systems such that the dislocation and defect arrangement could be fully characterized and spatially-resolved. Micrographs imaged using two-beam and kinematical bright-field and weak-beam dark-field conditions were acquired over an angular range then used to reconstruct the tomograms. Supplemental advances to the technique include: Visualization of dislocations in color based on Burgers vector or other relevant characteristics; determination of the specimen coordinate system in the tomogram; and overcoming the g ∙ b invisibility condition by dual-axes tomography. In the Al-Cu system, which contained Al2Cu plate-shaped particles residing on {001}Al habit planes, in situ straining experiments revealed glide of lattice dislocations in the coherent side of the particle-matrix interface. The confined dislocations escaped the interface by cross-slip after reaching one end of the Al2Cu plate or by shearing the particle, as observed during post mortem analysis. Dislocations bypassed the particles by shear at multiple locations along their length, suggesting that a shearing site becomes more unfavorable with each dislocation passage such that adjacent slip systems must be activated in order to continue shear. When highly deformed, the inter-particle region was shown by electron tomography to be populated with a complex configuration of lattice dislocations pinned at both ends on a particle interface as well as debris created from dislocation-dislocation interactions. In the Cu-Co system, semicoherent Co-rich octahedral particles normally unshearble by lattice dislocations were observed to be sheared by small twins emanating from the Cu matrix. This new deformation behavior was attributed to the high strain rates associated with twinning and a small increase in interfacial energy associated with shearing of Co particles, which has the same stacking as the Cu matrix. Interactions with lattice dislocations, on the other hand, exhibited similar behavior during in situ and post mortem observations as the Al-Mg-Sc system, which contained Al3Sc particles. In the semi-coherent regime, between 40 nm and up to around 200 nm for Al3Sc and 100 nm for Co, an initial elastic interaction from the misfit strain field gave way to a novel bypass mechanism involving the creation of half-loops attached on one side to the particle interface via bowing and cross-slip while locally pinned at on the particle interface. Bypass of such a particle also involved interfacial dislocations, which increased the complexity of the particle-matrix interface and impeded subsequent bypass of the same particle. In all cases, multiple interactions with lattice dislocations resulted in the evolution of defect structures around a particle over time and the interactions examined were more complex than single dislocation-single particle interactions depicted in classical theoretical models.
Issue Date:2011-08-25
Rights Information:© 2011 Grace Liu. All rights reserved.
Date Available in IDEALS:2011-08-25
Date Deposited:2011-08

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