Ground and Flight Based Studies of Nucleation and Thermophysical Properties in Metallic Glass Forming Systems PDF Download

Author: Christopher Eric Pueblo
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 185

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Book Description
Metallic glasses are formed by cooling a liquid from high temperature to below the glass transition without crystallization. Due to their soft magnetic properties, high strength and elasticity, and superb corrosion resistance, metallic glasses are excellent candidate materials for technological and industrial applications. These desirable qualities ultimately arise from the atomic structure, so precise knowledge of how chemical and structural ordering influence the liquid's physical properties, such as density, viscosity, and specific heat, can improve the understanding of metallic glass formation. This dissertation presents results of structural and thermophysical property measurements conducted on metallic liquids and glasses, using differential scanning calorimetry (DSC), electrostatic levitation (ESL), and electromagnetic levitation (EML) techniques. The kinetic properties of glasses and liquids are often characterized by their fragility, which is generally defined from the temperature dependence of the viscosity. A study of fragility in both the low-temperature glass and the high-temperature liquid finds that the activation energy for crystallization correlates with the liquid's viscosity behavior, and hence with fragility. A survey of a broad range of thermophysical properties and their relationship to glass-forming ability (GFA) is conducted, and finds that densely packed, highly viscous liquids tend to more easily form glasses. The nature of the repulsive component of the liquid's interatomic pair potential, as determined from high-energy synchrotron X-ray scattering, is found to also directly correlate with fragility, providing a clear connection between the liquid's structure and dynamics. The temperature, TA, which corresponds to the onset of cooperative dynamical processes, is proposed as a more appropriate scaling temperature than the glass transition temperature, Tg. Viscosity, specific heat, and nucleation measurements, performed using EML techniques in microgravity aboard parabolic flights and the International Space Station (ISS), are compared with terrestrial-based ESL measurements to assess the influence of gravity on the physical properties of the liquid and on the nucleation step in crystallization. A specific heat signature consistent with the onset of cooperativity is observed at TA, further supporting the claim that it corresponds to a fundamental crossover in the liquid.