Role of String-like Collective Atomic Motion on Diffusion and Structural Relaxation in Glass Forming Cu-Zr Alloys
Jack F. Douglas, Xuhang Tong, Cheng Zhong, Xiaodong Wang, Qingping Cao, Dongxian Zhang, Jian-Zhong Jiang
We investigate Cu-Zr liquid alloys using molecular dynamics (MD) simulation and well-accepted EAM potentials over a wide range of chemical composition and temperature as a model metallic glass-forming (GF) liquid. As with other types of GF materials, the dynamics of these complex liquids is characterized by dynamic heterogeneity in the form of transient polymeric clusters of highly mobile atoms that are composed in turn of atomic clusters exhibiting string-like cooperative motion. In accordance with the string theory of relaxation (STR), an extension of the Adam-Gibbs (AG) model, changes in the activation energy with temperature of both the Cu and Zr diffusion coefficients D and the alpha structural relaxation time can be described to a good approximation by changes in the average string length L. In particular, we confirm that the strings are a concrete realization of the abstract cooperatively rearranging regions of AG model. We also find coexisting clusters of relatively immobile atoms that exhibit predominantly icosahedral local packing rather than the low symmetry packing of the mobile atoms. These two distinct types of dynamic heterogeneity are then associated with different fluid structural states. Glass-forming liquids are thus analogous to polycrystalline materials if the icosahedrally-packed regions are taken as corresponding to crystal grains while the strings reside exclusively in the relatively disordered GB-like regions exterior to these locally relatively well-ordered regions. A dynamic equilibrium between localized (immobile) and wandering (mobile) particles exists in the liquid so that the dynamic heterogeneity can be considered to be type of self-assembly process. We also characterize changes in the local atomic free volume in the course of string-like atomic motion to better understand the initiation and propagation of these fluid excitations.
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Role of String-like Collective Atomic Motion on Diffusion and Structural Relaxation in Glass Forming Cu-Zr Alloys, Journal of Chemical Physics
(Accessed August 9, 2022)