Study of atomic scale dynamics of glass-forming metallic liquids using neutron scattering experiments and molecular dynamics simulations

bulk metallic glass;quasi elastic neutron scattering;molecular dynamics;simulations;metallic liquids;dynamics;Arrhenius crossover;dynamic heterogeneity;collective dynamics;unsupervised learning;inelastic neutron scattering

Jaiswal, Abhishek

关键词: bulk metallic glass;    quasi elastic neutron scattering;    molecular dynamics;    simulations;    metallic liquids;    dynamics;    Arrhenius crossover;    dynamic heterogeneity;    collective dynamics;    unsupervised learning;    inelastic neutron scattering;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/95584/JAISWAL-DISSERTATION-2016.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Bulk metallic glasses are not the archetypal glass-former due to their large propensity for crystallization. They are characterized by complex multi-body interactions; yet have a simple structure that lacks internal degrees of freedom usually present in prototypical network or molecular glass-formers. Nevertheless, with several empirical design principles it is possible to prepare them in ‘bulk’ sizes with exceptional properties such as corrosion resistance, high hardness, biocompatibility etc. Many past research efforts have focused on the structure-property relaxations in these materials. However, very little is known about the behavior of liquids from which they are quenched to their glassy states. Besides, from a fundamental science perspective, they serve as intriguing systems to explore the universality of liquid dynamics and glass-transition. This research aims to quantify the underlying nature of atomic-scale dynamics using experimental neutron scattering techniques and computational molecular dynamics simulations accompanied by machine learning.Specifically, this work focuses on identifying a key crossover point that marks the onset of glassy behavior in the liquid state from both experiments and simulations. Onset of cooperative dynamics has been observed in many molecular liquids, colloids, and granular materials in the metastable regime on approaching their respective glass or jamming transition points, and is considered to play a significant role in the emergence of the slow dynamics. By measuring the mean diffusion coefficient that characterizes the relaxations at long- time scales of a model metallic glass-former using quasi-elastic neutron scattering, experimental evidence of cooperative dynamics in multicomponent glass-forming metallic liquids is obtained. The onset temperature T_A of cooperativity is found to occur in the equilibrium liquid state and approximately at twice of the glass transition temperature T_g, unlike many molecular systems. Therefore, the metallic system allows examination of intriguing coordinated dynamics unambiguously in the stable liquid phase, which is usually buried in the metastable supercooled regime in many van der Waals molecular liquids. Dynamical crossover from Arrhenius to super-Arrhenius behavior is also observed in other transport properties (self diffusion coefficient, self relaxation time, and shear viscosity) in computer simulations, consistent with experimental observations. A non-parametric, unsupervised machine learning technique is used to directly characterize and visualize the correlated dynamics.The significance of this observation is further established by comparing the Arrhenius crossover in the high-temperature liquid of several metallic, molecular, and network glass-formers to their low-temperature glassy characteristics: kinetic fragility index m and T_g . A simple correlation is predicted, using existing theories of liquid-glass transition, between the fragility index m and the reduced Arrhenius crossover temperature \theta_A = T_A/T_g in the three class of glass-forming liquids. These observations provide a way to estimate the low-temperature glassy characteristics from the high-temperature liquid quantities, and advance the unified understanding of liquid dynamics and glass-transition.Furthermore, the differences in the collective dynamics between simple model liquids and glass-forming metallic liquids and its relation to the Arrhenius crossover is established. The complex interactions and mixing entropy enhance the collective relaxation time to nearly self-relaxation timescale even at large wave-lengths, unlike in simple liquids where they differ by 2–3 orders of magnitude. Fast structural relaxations corresponding to rattling of atoms in cage formed by nearest neighbors reveals an anomaly in its temperature dependence that coincides with specific heat capacity peak as well as the Arrhenius crossover. Experiments and simulations suggest the presence of multiple acoustic excitations in such metallic glasses and liquids at finite wavelengths. The localization of longitudinal acoustic excitations is found to determine the Arrhenius crossover regime in liquids. The wavelength of localized longitudinal excitations reaches the random structure limit at T_A. The work presented here paves the way to rationalize the phenomenon of Arrhenius crossover in such complex systems and elucidates the underlying physical mechanisms causing it.