Keywords

TEM, HRTEM, HAADF-STEM

Abstract

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 hexagonal [gamma] 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 [gamma] phase was investigated. Samples cut from different Al-Ag alloys were homogenized at temperatures between 530 degrees C to 560 degrees 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 degrees C and 220 degrees C for varying lengths of times. While Guinier-Preston zones increase in diameter with increasing aging duration, silver rich platelets of the [gamma] phase form. The [gamma] phase is the next metastable phase in the decomposition sequence before finally the [gamma] phase transforms to the stable silver-rich phase, termed [gamma]. For samples with silver contents above 12 at.% a parallel lamellar alignment of fine [gamma] plates and Alrich matrix is found after extended heat treatments. For all alloys with Ag concentrations below 12 at.% individual [gamma] plates are found on all four possible (111) planes of the [alpha] matrix. 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.

Notes

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Graduation Date

2005

Semester

Fall

Advisor

Heinrich, Helge

Degree

Master of Science (M.S.)

College

College of Engineering and Computer Science

Department

Mechanical, Materials and Aerospace Engineering

Degree Program

Materials Science and Engineering

Format

application/pdf

Identifier

CFE0000874

URL

http://purl.fcla.edu/fcla/etd/CFE0000874

Language

English

Release Date

January 2006

Length of Campus-only Access

None

Access Status

Masters Thesis (Open Access)

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