Formation and characterization of Mg-based bulk metallic glasses and nanocrystalline materials

Citation

Liu, Wenshan (1998) Formation and characterization of Mg-based bulk metallic glasses and nanocrystalline materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/27c9-s363. https://resolver.caltech.edu/CaltechETD:etd-01232008-083911

Abstract

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The first metallic glass was discover by Duwez and his colleagues at Caltech in 1960. The recent development of new metallic alloys which form bulk metallic glasses at low cooling rates has led the metallic glasses research to significant advances. For the first time it has become possible to carry out measurements of thermophysical properties such as specific heat, viscosity, atomic diffusion, and crystal nucleation rates for liquid alloys over the full temperature range of the undercooled melt. These new materials exhibit a rich variety of phenomena, such as liquid/liquid phase separation and nanocrystalline phase formation, and have interesting potential as engineering materials.

This thesis reports the systematic studies of Mg-base amorphous, one of the best bulk metallic glass formers. The formation of coexistent amorphous and nanocrystalline phases in slowly quenched alloy of [...] containing 3 to 15 at.% of Li is reported. The grain size of the nanocrystal ranges from 2 to 20 nm increasing with Li content. The transmission electron microscopy investigations showed evidence of phase separation in [...] alloy. The thermodynamics and kinetics of [...] alloy were studied. Absolute values of specific heat capacities were measured with respect to sapphire standards for amorphous, crystal, superercooled liquid, and melt in differential scanning calorimetry. The differences in enthalpy, entropy and Gibbs free energy between crystal and superercooled liquid were calculated using the measured specific heat capacity and the heat of fusion. The three-point beam-bending method was used to measure the viscosity of [...] alloy in amorphous and supercooled liquid using thermal mechanical analyzer. Strong glass behavior and glass forming ability were discussed. The phase separation and crystallization of the [...] alloy were studied with x-ray, differential scanning calorimetry, transmission electron microscopy, and small angle scattering. The results suggested that the decomposition in the undercooled liquid state triggers the crystallization as well as limits the crystal growth.

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