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EVOLUTION OF LAMELLAR STRUCTURES IN AL-AG ALLOYS

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Date Issued:
2005
Abstract/Description:
In the present study, the formation and the evolution of lamellar structures in different Al-Ag alloys were investigated by transmission electron microscopy (TEM). Plates of the hexagonalƒÁ phase form semi-coherently on the {111} planes of the face centered cubic lattice of the alloy after the formation of Guinier-Preston zones. Guinier-Preston zones are metastable coherent preprecipitates which are silver rich in the aluminum-rich Al-Ag alloys. The decomposition of aluminum rich Al-Ag alloys, particularly the sequence of the later stages of precipitate formation was studied. With scanning electron microscopy and high-resolution transmission electron microscopy the development of the ƒÁ phase was investigated. Samples cut from different Al-Ag alloys were homogenized at temperatures between 530‹C to 560‹C to obtain a single phase f.c.c solid solution. The samples were then quenched to room temperature, followed by heat treatments at temperatures between 140‹C and 220‹C for varying lengths of times. While Guinier-Preston zones increase in diameter with increasing aging duration, silver rich platelets of the ƒÁ Œ phase form. The ƒÁ Œ phase is the next metastable phase in the decomposition sequence before finally the ƒÁ Œ phase transforms to the stable silver-rich phase, termedƒÁ . For samples with silver contents above 12 at.% a parallel lamellar alignment of fine ƒÁ-plates and Alrich matrix is found after extended heat treatments. For all alloys with Ag concentrations below 12 at.% individual ƒÁ plates are found on all four possible (111) planes of the ƒ¿ matrix. iv A method is presented to calibrate the medium-magnification high-angle annular dark-field contrast in scanning transmission electron microscopy. This calibration allows for the quantitative measurement of plate thicknesses from high-angle annular dark-field scanning transmission electron micrographs of Ag2Al plates inclined to the electron beam. Results from these measurements are in good agreement with direct bright-field micrographs of plates viewed edge-on.
Title: EVOLUTION OF LAMELLAR STRUCTURES IN AL-AG ALLOYS.
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Name(s): Senapati, Sephalika, Author
Heinrich, Helge, Committee Chair
University of Central Florida, Degree Grantor
Type of Resource: text
Date Issued: 2005
Publisher: University of Central Florida
Language(s): English
Abstract/Description: In the present study, the formation and the evolution of lamellar structures in different Al-Ag alloys were investigated by transmission electron microscopy (TEM). Plates of the hexagonalƒÁ phase form semi-coherently on the {111} planes of the face centered cubic lattice of the alloy after the formation of Guinier-Preston zones. Guinier-Preston zones are metastable coherent preprecipitates which are silver rich in the aluminum-rich Al-Ag alloys. The decomposition of aluminum rich Al-Ag alloys, particularly the sequence of the later stages of precipitate formation was studied. With scanning electron microscopy and high-resolution transmission electron microscopy the development of the ƒÁ phase was investigated. Samples cut from different Al-Ag alloys were homogenized at temperatures between 530‹C to 560‹C to obtain a single phase f.c.c solid solution. The samples were then quenched to room temperature, followed by heat treatments at temperatures between 140‹C and 220‹C for varying lengths of times. While Guinier-Preston zones increase in diameter with increasing aging duration, silver rich platelets of the ƒÁ Œ phase form. The ƒÁ Œ phase is the next metastable phase in the decomposition sequence before finally the ƒÁ Œ phase transforms to the stable silver-rich phase, termedƒÁ . For samples with silver contents above 12 at.% a parallel lamellar alignment of fine ƒÁ-plates and Alrich matrix is found after extended heat treatments. For all alloys with Ag concentrations below 12 at.% individual ƒÁ plates are found on all four possible (111) planes of the ƒ¿ matrix. iv A method is presented to calibrate the medium-magnification high-angle annular dark-field contrast in scanning transmission electron microscopy. This calibration allows for the quantitative measurement of plate thicknesses from high-angle annular dark-field scanning transmission electron micrographs of Ag2Al plates inclined to the electron beam. Results from these measurements are in good agreement with direct bright-field micrographs of plates viewed edge-on.
Identifier: CFE0000874 (IID), ucf:46648 (fedora)
Note(s): 2005-12-01
M.S.
Engineering and Computer Science, Department of Mechanical, Materials, and Aerospace Engineering
Masters
This record was generated from author submitted information.
Subject(s): TEM
HRTEM
HAADF-STEM
Persistent Link to This Record: http://purl.flvc.org/ucf/fd/CFE0000874
Restrictions on Access: public
Host Institution: UCF

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