Evolution Of Lamellar Structures In Al-ag Alloys

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.

Author dc:creator

• Senapati, Sephalika

Contributors dc:contributor

• Heinrich, Helge

• TEM
• HRTEM
• HAADF-STEM
• Engineering
• Materials Science and Engineering

Language dc:language

English

Identifier

CFE0000874

OAI identifier oai:identifier

oai:stars.library.ucf.edu:etd-1616