Chemical reactions that convert a solid oxide into new fluid and solid products with compositions that differ from the initial oxide are considered to be "incongruent" (e.g., incongruent melting, incongruent dissolution). While some types of incongruent reactions are well understood, the basic kinetic mechanisms by which oxides undergo "incongruent reduction" with molten metals are not. The objective of this research is to develop a fundamental understanding of the kinetic mechanisms (rate-limiting steps, microstructural evolution) by which a solid oxide undergoes incongruent reduction with a reactive metallic liquid. In this work, the incongruent reduction of aluminum oxide in contact with Mg-Al liquids (which involves the formation of solid magnesium aluminate spinel) will be examined. Novel experiments with in-situ x-ray diffraction will be used to track, in real time, the formation of spinel on alumina surfaces immersed under molten Mg-Al layers. The kinetics of alumina weight change upon immersion in Mg-Al melts will also be studied under steady-state conditions. These kinetic measurements (phase change, mass change) will be used along with microchemical and microstructural analyses of the alumina/melt interface to identify the rate-limiting step(s) of incongruent alumina reduction. %%% The rate of reaction between ceramics and liquid metals is of vital importance for a number of advanced technologies, such as: 1) novel, low-cost, reactive infiltration processing of ceramic/metal composites for automotive applications (e.g., cylinder liners, brake components, bearing sleeves), and 2) investment casting of advanced titanium alloys for lightweight and strong structural components in aircraft. The purpose of this research project is to develop a better fundamental understanding of the mechanisms of reactions between solid ceramics and liquid metals, so that the rate of such reactions can be better predicted and controlled. This research will involve the use of state-of-the-art analytical equipment to track the progress of such reactions in real time at interfaces between solid ceramics and liquid metals.
