Molecular dynamics studies of surface dynamics and dynamic heterogeneity of supercooled metallic liquid

Molecular Dynamics;Dynamic Heterogeneity;Metallic Liquid

Zhai, Yanqin ; Allain, Jean Paul ; Zhang ; Yang

关键词: Molecular Dynamics;    Dynamic Heterogeneity;    Metallic Liquid;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/105586/ZHAI-THESIS-2019.pdf?sequence=1&isAllowed=y

美国|英语

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

【 摘 要 】

Glasses play a significant roll in both modern manufacturing industry and scientific researches. However, the physical nature of glass, especially the unique issues on its surfaces, has remained unsolved for decades. In this work, we performed MD simulations on Zr50Cu50 supercooled metallic liquid in the bulk, near the free surfaces and the constrained surfaces at a target temperature of 900 K. The goal is to unveil the mechanism of enhanced mobility near the free surface. We examined the atomic motions by decomposing them into in-plane motions parallel to the interface and out-of-plane motions perpendicular to the interface. We found that the in-plane dynamics near the free surface is faster than the bulk, and surprisingly, the local structure near the free surface can be even more compact. Besides, the investigation on the exchange rate of atoms shows that the out-of-plane dynamics on the free surface was almost the same or even weaker than that in the bulk. The mean square displacement and the velocity autocorrelation function indicate an earlier termination of the cage effect and larger atomic arrangements near the free surface, which attenuates the phonons with higher energy and shifts the phonon density of states towards lower energy domain. In contrast, when the vacuum space is filled with pinned atoms, the mobility near the interface decreases to zero rapidly. These observations suggest that the enhanced dynamics near the free surface does not originate from larger free volumes, or, the atoms occasionally extracting themselves from the bulk and moving rapidly on the free surface. Instead, with missing coordination atoms in the vacuum space, the in-plane friction induced on those atoms on the free surface is halved, which finally strengthens the in-plane collective motions on the free surface.Furthermore, we examined the temperature-dependence of dynamics and dynamic heterogeneity of Zr50Cu50 supercooled metallic liquid near the free surface. Dynamic heterogeneity can be displayed by the dynamic correlation length directly. RFOT theory proposed that the enhanced dynamics near the free surface is induced by the decreased dynamic correlation length. Here, the out-of-plane dynamic correlation length extracted from fitting the mobility gradient near the free surface shows little temperature dependence, which demonstrates the unique behavior of the outmost atomic layer on the free surface. However, the in- plane dynamic correlation length characterized by 2-dimensional α2,max was found to be larger on the free surface than in the bulk. Considering the enhanced in-plane dynamics, we realize that the assumption made by RFOT theory that the dynamic rearranging region is hemispherical near the free surface is oversimplified. Instead, we propose that the dynamic rearranging region is distorted from 3-dimension to 2-dimension from the bulk to the free surface, which results in the counter-intuitive simultaneous enhancement of both dynamics and dynamic heterogeneity on the free surface.