Viscosity measurements of bulk metallic glass forming alloys

Citation

Bakke, Eric (1997) Viscosity measurements of bulk metallic glass forming alloys. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5161-9m27. https://resolver.caltech.edu/CaltechETD:etd-01042008-090419

Abstract

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The viscosity of the [...] bulk metallic glass forming alloy in both the supercooled liquid and the equilibrium molten state was measured. Parallel plate rheometry and three-point beam-bending were used to measure the viscosity as a function of temperature in the supercooled liquid, and the technique of capillary flow was used in the molten state. The high thermal stability above the glass transition of this bulk metallic glass former with respect to crystallization allows measuring viscosities 120 K into the supercooled liquid region. Viscosity in the range from [...] to [...] poise has been measured using parallel plate rheometry, a region of viscosities that has not been previously accessible for supercooled metallic melts. The measurements were carried out with different heating rates between 0.0167 K/s and 1.167 K/s as well as isothermally. Using three-point beam bending, viscosity in the range from [...] to [...] poise has also been measured. These two methods, which involve completely different geometries for the measurement of flow, yielded consistent values for viscosity where their applicable regions overlap. The viscosity of the supercooled liquid of this bulk glass former, above the glass transition temperature, can be fit by a Vogel-Fulcher relation which exhibits a small Vogel-Fulcher temperature relative to the glass transition temperature. The values of viscosity measured by capillary flow above the liquidus temperature agree with the extrapolation of the Vogel-Fulcher relation to these temperatures. This bulk metallic glass former exhibits strong glass behavior, similar to silicate glasses. The relatively high viscosity in the supercooled liquid and smaller Gibbs free energy difference compared to the crystal both favor bulk glass formation. This glass forming ability is directly related to the fragility index and the relaxation kinetics as measured by viscosity, as well as the critical cooling rate. Knowledge of the viscosity as a function of temperature is essential for shaping and molding, and has led to possible extended commercial applications of these novel materials which exhibit unique mechanical properties.

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