Abstract

We investigate the Johari–Goldstein (JG) \(\beta \)-relaxation process in a model metallic glass-forming (GF) material (\(\hbox {Al}_{90}\hbox {Sm}_{10}\)), 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 \(\beta \)-relaxation time \(\tau _\mathrm{JG}\) and the lifetime of the mobile particle clusters \(\tau _\mathrm{M}\), defined as in previous DH studies, a relationship dual to the corresponding previously observed relationship between the \(\alpha \)-relaxation time \(\tau _{\alpha }\) and the lifetime of immobile particle clusters \(\tau _\mathrm{IM}\). Moreover, we find that the average diffusion coefficient D nearly coincides with \(D_\mathrm{Al}\) of the smaller atomic species (Al) and that the ‘hopping time’ associated with D coincides with \(\tau _\mathrm{JG}\) to within numerical uncertainty, both trends being in accord with experimental studies. This indicates that the JG \(\beta \)-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 \(\beta \)-relaxation becomes the prevalent mode of structural relaxation. As an unanticipated aspect of our study, we find that \(\hbox {Al}_{90}\hbox {Sm}_{10}\) 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. On the other hand, the temperature dependence of the DH and dynamical properties, such as the structural relaxation time, can be significantly altered from ‘ordinary’ GF liquids.

Graphic abstract

中文翻译：

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金属异质性，协同运动和Johari-Goldstein $$ \ beta $$β松弛在金属玻璃形成材料中表现出从弱到强的转变

摘要

我们研究佐哈里-戈尔茨坦（JG）\（\测试\）在一个模型中-relaxation过程金属玻璃形成（GF）材料（\（\ hbox中的Al {} _ {90} \ {hbox中的Sm} _ {10} \）），以前通过频率相关的机械测量和致力于平衡特性的模拟研究，通过基于经过验证和优化的原子间电势的分子动力学模拟进行了广泛研究，其主要目的是从动力学上更好地理解这种普遍弛豫过程的性质。异质性（DH）观点。当前相对较低的温度和长时间的仿真显示JG \（\ beta \）松弛时间\（\ tau _ \ mathrm {JG} \）与移动粒子簇的寿命之间存在直接的对应关系\（\ tau _ \ mathrm {M} \），如先前的DH研究中所定义，该关系是\（\ alpha \）松弛时间\（\ tau _ {\ alpha} \ ）和固定粒子簇\（\ tau _ \ mathrm {IM} \）的寿命。此外，我们发现平均扩散系数D几乎与较小原子种类（Al）的\（D_ \ mathrm {Al} \）一致，并且与D相关的“跳跃时间”与\（\ tau _ \ mathrm {JG} \）在数值不确定性范围内，两种趋势均与实验研究一致。这表明JG \（\ beta \）松弛主要由较小的原子组成，并且观察到这种松弛过程与GF材料在低温下GF材料的分子扩散速率之间存在直接关系，其中JG （β）松弛成为结构弛豫的普遍模式。作为我们研究中未曾料到的方面，我们发现\（\ hbox {Al} _ {90} \ hbox {Sm} _ {10} \）表现出易碎至强（FS）的玻璃形成，这在许多其他情况中都可以发现金属的GF液体，但是这一事实并不能极大地改变这种材料中DH的几何性质以及DH与动力学性质之间的关系。另一方面，DH的温度依赖性和动力学特性（例如结构弛豫时间）可以从“普通” GF液体中显着改变。

图形摘要