The Role of Microscopic Structures on the Atomic Dynamics of Metallic Glass-forming Systems

Abstract

Metallic glasses have the potential to become next-generation materials because of their fascinating mechanical properties compared to traditional metals. However, industry is facing the problems of the limitation of casting size and the stability of metallic glasses.
Metallic glasses are made by the fast quenching of glass-forming melts. If the atoms inside a liquid do not have sufficient time to move to their crystal lattice points before the system solidifies, a glass will be formed. Therefore, the atomic dynamics of the glass-forming liquids is one of the key objectives in understanding the glass-forming ability (GFA). Among the binary glass-forming systems, Cu–Zr has outstanding GFA. By slightly changing the composition, this system shows big differences in GFA. The GFA of Cu–Zr is further increased by a small addition of Al. However, the GFA drops when the content of Al exceeds 8%. The variations in GFA, together with the simplicity of the systems, make Cu–Zr and Cu–Zr–Al the ideal candidates for this study. In this thesis, the microscopic dynamics of the systems were analyzed by quasi-elastic neutron scattering and molecular dynamics simulation. The self-diffusion coefficients of the liquids at several hundred Kelvin above the melting temperatures were studied and different compositions compared. A change from Arrhenius to non-Arrhenius behavior in the diffusion coefficients as function of temperature could be found. The dynamic heterogeneity in Cu–Zr–Al liquids was accessed by the four-point dynamic susceptibility and the dynamic length-scale. The dynamic properties in the two systems were linked to their atomic structures obtained from X-ray diffraction experiments and numerical calculations. A strong correlation could be found between the dynamic heterogeneity and the atomic structure.
The mechanical properties of the metallic glasses change because of atomic rearrangements in the glass. This rearrangement is also known as physical aging. To understand the aging in metallic glasses, the relaxation processes of Cu–Zr in the glassy state were studied using X-ray photon correlation spectroscopy. The relaxation times of different compositions measured at a range of temperatures were compared. In addition, the waiting-time dependence of the relaxation processes was also addressed. No systematic change in relaxation times was found in this system. Meanwhile, the microscopic structures of Cu–Zr glasses were also investigated by molecular dynamics simulation and reverse Monte Carlo calculation, together with diffraction experiments. The icosahedral cluster showed a pronounced increase with the increase of Cu content in the Cu–Zr system. However, no direct link could be found for the aging with the atomic structure.

Date of Award	17 Aug 2017
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Suresh MAVILA CHATHOTH (Supervisor)