The nature of amorphous alloys in an immiscible system.

摘要

While amorphous alloys have been studied extensively in binary systems with a negative heat of mixing, such as Ni-Zr and Cu-Ti, amorphous phases in immiscible systems are reported only occasionally and poorly understood. This thesis investigates the nature of such amorphous alloys using Ag-Ni as a model system. The goal is to understand what kind of nonequilibrium alloys can be created in such a system with a large positive heat of mixing, and how different the amorphous alloys obtained are when compared with those well known in easy glass-forming systems. To this end, a detailed analysis of the amorphous alloys obtained, with emphasis on their atomic-level structures, has been carried out using a variety of experimental and computer simulation techniques. The structural features uncovered are directly correlated with the thermodynamic properties measured.; Ag-Ni amorphous alloys have been prepared through the vapor quenching route. Evidence of amorphous phase formation has been obtained through a combination of x-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), extended x-ray absorption fine structure (EXAFS), and magnetic property measurements. The Ag-Ni amorphous alloys are found in unexpectedly low enthalpy states with low crystallization temperatures. EXAFS, combined with reverse Monte Carlo and molecular dynamics (MD) simulations, reveals spinodal-like structures on an extremely fine scale. It appears that spontaneous spinodal decomposition cannot be fully suppressed when the alloy vapor is rapidly solidified in such a system that is highly phase-separating in both the liquid and solid states, while the nucleation of crystals can be avoided. A common-neighbor analysis of the MD configurations indicates significant local icosahedral ordering, which is characteristic of many known amorphous structures and contributes to the further lowering of the amorphous energy state. The simulations demonstrate that it is the nature of the atomic level structure that is directly and quantitatively responsible for the thermodynamic properties observed for these unconventional amorphous alloys.