动态异质结、协同运动和Johari-Goldstein $β$ -弛豫在金属玻璃形成材料中表现出从脆性到强的转变
Dynamic Heterogeneity, Cooperative Motion, and Johari-Goldstein $β$-Relaxation in a Metallic Glass-Forming Material Exhibiting a Fragile to Strong Transition
摘要：
研究了金属玻璃形成（GF）材料模型中的Johari-Goldstein（JG）$β$弛豫过程（Al 90 Sm 10），以前广泛研究的频率依赖性力学测量和模拟研究致力于平衡性能，通过基于验证和优化的原子间势的分子动力学模拟，其主要目的是更好地理解这种普遍弛豫过程的性质，动态异质性（DH）观点。目前相对较低的温度和长时间的模拟揭示了JG $β$ -弛豫时间$τ_{JG}$和移动粒子团簇$τ_M$的寿命之间的直接对应关系，如在以前的DH研究中定义的，一个对偶的关系，相应的以前观察到的$α$ -弛豫时间$τ_α$和不动粒子团簇$τ_{IM}$的寿命之间的关系。此外，我们发现，平均扩散系数D几乎符合D $_{Al}$，较小的原子物种（铝），和跳跃时间与D符合$τ_{JG}$的数值不确定性，这两个趋势是符合实验研究。这表明，JG $α$ -弛豫是由较小的原子种类和观察到的直接关系，在GF材料中的分子扩散速率在低温下，其中JG $β$ -弛豫成为普遍的模式的结构弛豫的松弛过程。作为一个意想不到的方面，我们的研究，我们发现，Al 90 Sm 10表现出脆弱的强（FS）玻璃形成，在许多其他金属GF液体中发现，但这一事实并没有大大改变DH在这种材料的几何性质和DH的动态性能的关系。
英文摘要：
We investigate the Johari-Goldstein (JG) $β$-relaxation process in a model metallic glass-forming (GF) material (Al90Sm10), previously studied extensively by both frequency-dependent mechanical measurements and simulation studies devoted to equilibrium properties, by molecular dynamics simulations based on validated and optimized interatomic potentials with the primary aim of better understanding the nature of this universal relaxation process from a dynamic heterogeneity (DH) perspective. The present relatively low temperature and long-time simulations reveal a direct correspondence between the JG $β$-relaxation time $τ_{JG}$ and the lifetime of the mobile particle clusters $τ_M$, defined as in previous DH studies, a relationship dual to the corresponding previously observed relationship between the $α$-relaxation time $τ_α$ and the lifetime of immobile particle clusters $τ_{IM}$. Moreover, we find that the average diffusion coefficient D nearly coincides with D$_{Al}$, of the smaller atomic species (Al), and that the hopping time associated with D coincides with $τ_{JG}$ to within numerical uncertainty, both trends being in accord with experimental studies. This indicates that the JG $α$-relaxation is dominated by the smaller atomic species and the observation of a direct relation between this relaxation process and rate of molecular diffusion in GF materials at low temperatures where the JG $β$-relaxation becomes the prevalent mode of structural relaxation. As an unanticipated aspect of our study, we find that Al90Sm10 exhibits fragile-to-strong (FS) glass-formation, as found in many other metallic GF liquids, but this fact does not greatly alter the geometrical nature of DH in this material and the relation of DH to dynamical properties.
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