Phenomena Associated with Melting and Super Cooling in A1 and the Solid-Liquid Interface in an A1-Si Base Alloy Investigated by In-Situ Analytical Transmission Electron Microscopy

Palanisamy, Prakash, Department of Materials Science and Engineering, University of Virginia
Howe, James, Department of Materials Science and Engineering, University of Virginia
Soffa, William, Department of Materials Science and Engineering, University of Virginia
Shiflet, Gary, Department of Materials Science and Engineering, University of Virginia
Zhigilei, Leonid, Department of Materials Science and Engineering, University of Virginia
Stach, Eric

Crystal growth is an important technique for controlling the microstructure of materials, and hence, studies pertaining to crystal growth are crucial for developing new materials with novel properties. The present research explores the behavior of the core and valence electrons, and the nearest-neighbor atomic distance during heating and cooling of pure Al through the melting temperature, as well as the properties associated with the solid-liquid interface in a commercially important Al-Si-Cu-Mg alloy, using in-situ analytical transmission electron microscopy (TEM). Electron energy-loss spectroscopy (EELS) in a TEM was used to follow changes in the valence electron density through the melting temperature and during supercooling in pure Al particles. A non-linear plasmon energy change observed during heating solid Al is due to a phonon anharmonicity that is not present in liquid Al during heating. Similarly, a non-linear plasmon behavior observed during supercooling liquid Al could due to local ordering in the liquid. Comparing the full-width at half-maximum of the plasmon peaks show that damping of plasmons is faster in liquid than in solid Al due to electron-phonon interactions and/or Anderson localization. The extended energy-loss fine structure (EXEFLS) in the EELS spectrum is sensitive to the nearest-neighbor atomic distance and coordination in a material. A comparison of the nearest-neighbor distance and the inverse volume plasmon energy change through the melting temperature shows good agreement in the solid Al, whereas the correlation is less good in liquid Al because thermal vibrations (Debye-Waller factor) dampen the EXELFS oscillation at high temperatures. A qualitative comparison of the energy-loss near edge spectra (ELNES) of solid and liquid Al in both superheated and supercooled states shows that their partial electron density of states are quantitatively different and that the presence or absence of crystallinity is the most important factor contributing to the ELNES. The partitioning of solute elements was investigated by measuring the Cu concentrations in solid Si, liquid Al and at the solid-liquid interface at 585 °C, where the particle is partially molten, and subsequently undercooling to 565 and 470 °C. The Cu concentration after fluorescence correction was compared with thermodynamic calculations. The results shows that Cu segregation during undercooling assists in nucleating Al 2 Cu prior to Mg 2 Si phase at a high-index Si facet-liquid Al interface under non-equilibrium conditions. The heterogeneous segregation of Cu at the interface was determined to be a thermodynamically driven process by measuring the Cu concentration in liquid Al and at the solid-liquid interface for prolonged times at temperature. The plasmon at a solid-liquid interface in an Al-Si alloy particle was investigated by stepping a 0.6 nm diameter electron beam at 0.8 nm increments across a singular Si{111}-liquid Al interface in sub-eV, sub-Å microscope (SESAM). Low energy-loss spectra acquired across solid-liquid interface were compared with calculations using dielectric theory. The result shows that a unique plasmon resonance is present at the interface between the crystalline Si and liquid Al, thereby giving rise to a plasmon peak at 15.5 eV between the Si (16.3 eV) and liquid Al (14.2 eV) plasmons in the experimental EEL spectra. This result was corroborated with energy-filtered TEM. The intensity profile across the solid-liquid interface in the energy-filtered image shows that the interface plasmon signal is delocalized to within ~5.5 nm of the interface. APPROVAL SHEET The dissertation is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science and Engineering. PRAKASH PALANISAMY This thesis dissertation has been read and approved by the examining committee: Dr. James M. Howe Dissertation Advisor Dr. William A. Soffa Committee Chairman Dr. Gary J. Shiflet Committee Member Dr. Leonid V. Zhigilei Committee Member Accepted for the School of Engineering and Applied Science: Dr. Eric A.

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PHD (Doctor of Philosophy)
crystal growth, materials, microstructure
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