Metastable phases in the aluminum-germanium alloy system. Synthesis by mechanical alloying and pressure induced transformations

Citation

Yvon, Pascal (1994) Metastable phases in the aluminum-germanium alloy system. Synthesis by mechanical alloying and pressure induced transformations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/a9ct-na12. https://resolver.caltech.edu/CaltechETD:etd-12122007-111950

Abstract

Amorphous alloys have been prepared by a variety of techniques from rapid solidification to solid state crystal-to-glass transformations. In this work we report the use of pressure to obtain amorphous alloys in the aluminum-germanium alloy system. Aluminum and germanium form a simple equilibrium eutectic with limited mutual solubility and no intermetallic intermediate phases. We used a regular solution approach to model the effects of pressure on the Al-Ge binary phase diagram. The main effects of pressure are to extend the solubility of germanium in aluminum, to displace the eutectic composition towards the germanium-rich side, and to slightly decrease the eutectic temperature. Using this modeled phase diagram, we designed thermobaric treatments to induce crystal-to-glass transformations in fine grain mixtures of aluminum and germanium.

We used Merrill-Bassett diamond anvil cells to perform experiments at high pressures. We designed and built an x-ray apparatus to enable us to determine the structure of the alloys at pressure and from cryogenic temperatures to 4000C. Two-phase A1-Ge samples with fine microstructures were prepared by splat-quenching and mechanical alloying. We observed a crystal-to-glass transformation at about 80 kbar. The amorphous phase formed was metastable at ambient temperature after the pressure was released. This result was confirmed by transmission electron microscopy (TEM) studies. The amorphous phase obtained by pressurization was found to have a liquid-like structure and was metallic. This is the first time such an amorphous phase is reported in the aluminum-germanium alloy system. In the TEM samples we also observed the presence of a second amorphous phase that was formed upon release of the pressure. This second phase had a tetrahedrally-bonded continuous random network structure, similar to that of semi-conductig amorphous germanium.

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