Chemical and structural aspects of phase transformations in pyroxenes and aluminum-based alloys.

摘要

The research presented in this dissertation focuses on the characterization of secondary phases in pyroxenes, a single-chain silicate abundant in the crust and upper mantle of the earth, and two Al-based alloys, Al-Ag and Al-Cu-Mg-Ag. The study of two considerably different materials aids understanding of phase transformations across a wide range of physical properties. Minerals commonly slowly cool close to thermodynamic equilibrium, have low symmetry, and have sluggish diffusion rates (∼1018–22 cm2/sec). Alloys, on the other hand, are often rapid heated or cooled, have high symmetry, and have relatively rapid diffusion rates (∼108–14 cm2/sec).; The primary focus of the research is to examine the chemical aspects of solid→solid phase transformations. Historically, the atomic structure of transformations has been more heavily examined than the chemical reactions that occur. This is because, until recently, the technological hardware capable of allowing one to examine the chemistry of materials at or near the atomic scale was not available. Recent advances in the field of electron microscopy are making this type of research not only possible, but routine. Throughout this dissertation, chemical reactions (as well as structural transformations) occurring in the solid state will be examined using these new and powerful techniques to better understand fundamental aspects of phase transformations through a variety of materials.