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Title:Rapid Solidification and Undercooling of Aluminum-Germanium Alloys: Characterization and Control of Microstructure (Metastability, Metallic Glass, Electron Microscopy)
Author(s):Kaufman, Michael Joseph
Department / Program:Metallurgy and Mining Engineering
Discipline:Metallurgical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Metallurgy
Abstract:A determination of the importances of undercooling and cooling rate in the microstructural development of Al-Ge alloys during rapid solidification has been carried out. A new technique has been developed which allows for such determinations to be performed directly in a transmission electron microscope. This technique involves the in-situ melting and solidification of submicron powders, where it is possible to vary the cooling rate from the liquid or semi-solid state over a fairly large range while observing directly the various solidification microstructures. Further, a heating stage may be used either to measure the liquid undercoolings achieved upon slowly cooling or to select substrate temperatures (and so affect the nucleation temperatures) during rapid quenching experiments. In this manner it has been shown that certain combinations of phases form over rather specific ranges of undercooling (substrate temperature).
In addition to the in-situ solidification and undercooling studies, melt spun ribbons, splat quenched foils and amorphous films of similar compositions have been analyzed. The as-produced microstructures were characterized in the microscope and, subsequently, the amorphous films were heated with the beam to cause crystallization, the products of which were found to vary with composition and heating rate. From these observations, it appears that temperature ranges exist, whether approached by down-quenching from the liquid state or up-quenching from the amorphous state, where the formation of the different phases becomes favorable and that the primary function of cooling rate during rapid solidification is to inhibit nucleation processes so that substantial undercoolings may be achieved.
A computer model has been developed in an effort to predict the number of grains which form in highly undercooled Al-30Ge (eutectic) droplets. Specifically, this model utilizes modified nucleation and growth equations for glass forming alloys to predict the number of (alpha)-Al grains which form upon solidification. It is shown that the number of grains increases significantly as the initial undercooling is raised. However, as the undercooling temperatures approach the glass transition temperature, the number of grains is predicted to drop suddenly toward zero resulting in the formation of amorphous (or partially amorphous) microstructures. These predictions are in good agreement with the experimental observations.
Issue Date:1984
Description:191 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1984.
Other Identifier(s):(UMI)AAI8422095
Date Available in IDEALS:2014-12-16
Date Deposited:1984

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