Atomic Relaxation Process in Bulk Metallic Glass-Forming Alloy Systems

Abstract

Bulk metallic glasses (BMGs) have attracted much attention in the fields of materials science and engineering due to their considerable scientific importance and potential technological applications. The glass-forming ability (GFA) of BMGs is related to the factors that hinder the ability of correlated atomic motions, which undergo during crystallization. Thus, understanding the precise knowledge and exploring the characteristics of molten alloys prior to solidification are mandatory. Therefore, a fundamental understanding of vitrification mechanism as well as GFA requires the knowledge of atomic relaxation dynamics of the glass-forming liquids.

In this thesis, atomic relaxation dynamics of bulk metallic glass-forming alloy systems have been investigated by Quasi-elastic Neutron Scattering (QENS) and X-Ray Photon Correlation Spectroscopy (XPCS) techniques. The systems studied are Ce-based bulk metallic glasses and equal atomic multi-component glass-forming high-entropy alloys.

The Ce-based alloys system is a model bulk glass-forming alloy system that shows the enhanced at first and then diminished GFA on increasing the percentage of micro alloying element Co. The micro alloying does not have any significant impact on the thermodynamic properties. The results of the QENS studies, indicate that atomic caging is the primary factor that influences the GFA. The composition dependence of the atomic caging time is found to be well correlated with GFA of the system.

In the XPCS studies of Ce-based BMGs, a sharp change of the shape parameter from compressed in the glassy state to stretched in the supercooled liquid indicates that there is a remarkable equilibrium vs out-of-equilibrium dynamical crossover around Tg. In the glassy state, the autocorrelation functions of Ce-based BMGs can be well described by compressed correlation function, which is faster than the exponential correlation function. This compressed behavior arises from ballistic-like motion due to the presence of internal stresses inside the glasses in the out of equilibrium state. The time evolution of the microscopic dynamics during aging is directly captured in XPCS by the intensity of the two-time correlation function. The overall broadening of the two-time correlation function intensity profile reflects a slowing down of the microscopic dynamics corresponding to physical aging.

In the glass-forming high-entropy alloys melts, the long-range diffusion processes are extremely sluggish. The Ti20Zr20Cu20Ni20Be20 alloy melt with a faster atomic mobility exhibits a jump diffusion process, while the other high-entropy alloy melts with lower atomic mobility have a collective motion, i.e., a group of atoms moves together in the diffusion process. The intermediate scattering functions obtained from the QENS data show a large stretching in time, which has never been observed in other glass-forming alloy melts. The diffusion rate in these high-entropy melts is much slower as compared to that of the best glass-forming melts at their respective melting temperature.

The QENS and XPCS complementary techniques provide an efficient and practical methodology for investigating the relaxation dynamics and atomic transport mechanism of bulk metallic glass-forming alloy systems.

Date of Award	18 Aug 2017
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Suresh MAVILA CHATHOTH (Supervisor)