Graduate Thesis Or Dissertation
 

Thermodynamis and kinetics of Zr₅₈̣₅Cu₁₅̣₆Ni₁₂̣₈Al₁₀̣₃Nb₂̣₈ bulk metallic glass forming alloy

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  • Among recently found multi-component Zr-based bulk metallic glasses (BMG), Zr₅₈.₅Cu₁₅.₆Ni₁₂.₈Al₁₀.₃Nb₂.₈ is characterized by its lower critical cooling rate requirement and absence of beryllium as one of the constituent element which makes its handling more convenient. Its large supercooled liquid regions made possible the study of thermodynamics and kinetics of this alloy. Precise thermal measurements were performed using Differential Scanning Calorimeter (DSC) and Differential Thermal Analysis (DTA). Specific heat capacity measurement in the glass, supercooled liquid and crystalline region were performed with reference to sapphire to evaluate the thermodynamic functions. Enthalpy, entropy and Gibbs free energy difference between supercooled liquid and crystal were calculated as a function of temperature. Smaller Gibbs free energy difference upon undercooling reflects the high glass forming ability of this alloy. Isothermal relaxation and crystallization studies of Zr₅₈.₅Cu₁₅.₆Ni₁₂.₈Al₁₀.₃Nb₂.₈ bulk metallic alloy have been performed in the glass transition and the supercooled liquid region. The temperature range in relaxation experiments is usually lower than Tg due to its low thermal stability with respect to crystallization at higher temperatures. An experimental method of enthalpy relaxation has been developed to study the isothermal relaxation kinetics. The relaxation data obtained for temperatures below the onset of the glass transition temperature was fitted using a stretched exponential function. Results reveal that enthalpy does relax in an Arrhenius fashion. The activation energy of the Zr₅₈.₅Cu₁₅.₆Ni₁₂.₈Al₁₀.₃Nb₂.₈ obtained from the Arrhenius fit can be correlated with the activation energy for diffusion of constituent species. Medium sized and small sized atoms are likely to diffuse through collective hopping mechanism. Yet further investigation is required to understand the diffusion mechanism at lower temperatures. Also, the stretching exponents are close to unity, which indicates that the alloy is a rather strong glass former. This is confirmed by Vogel Fulcher Tammann (VFT) fits of the heating rate dependence of the glass transition. The Time Temperature Transformation (TTT) diagram determined close to the glass transition by heating the sample to isothermal temperature being in good agreement with that of the C. C. Hays TTT diagram, which is obtained by cooling the alloy from stable melt. This surprising agreement reveals the stable and homogeneous nature of Zr₅₈.₅Cu₁₅.₆Ni₁₂.₈Al₁₀.₃Nb₂.₈ glass forming liquid and its regular nucleation and growth mechanism at any temperatures.
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