Zhang Hao, Zhong Cheng, Douglas Jack F, Wang Xiaodong, Cao Qingping, Zhang Dongxian, Jiang Jian-Zhong
International Center for New-Structured Materials (ICNSM), Zhejiang University and Laboratory of New-Structured Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China.
Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
J Chem Phys. 2015 Apr 28;142(16):164506. doi: 10.1063/1.4918807.
We investigate Cu-Zr liquid alloys using molecular dynamics simulation and well-accepted embedded atom method potentials over a wide range of chemical composition and temperature as model metallic glass-forming (GF) liquids. As with other types of GF materials, the dynamics of these complex liquids are 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 model of relaxation, an extension of the Adam-Gibbs (AG) model, changes in the activation free energy ΔGa with temperature of both the Cu and Zr diffusion coefficients D, and the alpha structural relaxation time τα 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. We also find coexisting clusters of relatively "immobile" atoms that exhibit predominantly icosahedral local packing rather than the low symmetry packing of "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 where the icosahedrally packed regions correspond to crystal grains, and the strings reside in the relatively disordered grain boundary-like regions exterior to these locally 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.
我们使用分子动力学模拟以及广泛接受的嵌入原子方法势，在很宽的化学成分和温度范围内对铜锆液态合金进行了研究，将其作为典型的金属玻璃形成（GF）液体。与其他类型的GF材料一样，这些复杂液体的动力学特征是存在“动态非均匀性”，其表现形式为高迁移率原子的瞬态聚合物团簇，而这些团簇又由呈现线状协同运动的原子团簇组成。根据作为Adam-Gibbs（AG）模型扩展的弛豫弦模型，铜和锆扩散系数D以及α结构弛豫时间τα的活化自由能ΔGa随温度的变化，可以通过平均弦长L的变化得到很好的近似描述。特别地，我们证实弦是AG中抽象的“协同重排区域”的具体体现。我们还发现存在相对“不迁移”原子的共存团簇，这些团簇主要呈现二十面体局部堆积，而非“迁移”原子的低对称性堆积。然后，这两种不同类型的动态非均匀性与不同的流体结构状态相关联。因此，玻璃形成液体类似于多晶材料，其中二十面体堆积区域对应于晶粒，而弦存在于这些局部有序区域外部相对无序的类似晶界的区域中。液体中存在局部化（“不迁移”）和游动（“迁移”）粒子之间的动态平衡，因此动态非均匀性可被视为一种自组装过程。我们还表征了线状原子运动过程中局部原子自由体积的变化，以便更好地理解这些流体激发的起始和传播。
