npj: 金属玻璃中的软区—梯度原子堆垛结构与塑性

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自金属玻璃（非晶合金）发现以来，如何在看似“无序”的原子排布中寻找有序的局域堆垛结构（特别是中程序）是理解其宏观性能的关键。

为此，来自哈尔滨工业大学的苏彦庆教授和美国加州大学伯克利分校的Robert Ritchie教授领导的团队，利用大规模的分子动力学计算模拟，旨在探索不同的非晶合金体系间共同存在的局域原子排布特点。根据体系内每个原子周围不同的配位环境，非晶中所有原子可分为六大类。通过统计不同类别原子间的空间相关性发现，在不同体系的非晶中的纳米尺度区域都存在着一种梯度原子堆垛结构，其局域结构表现为从松散原子堆垛到致密原子堆垛的渐变行为，不同类型原子的性能同样展现出一种梯度的演变。进而非晶合金内部可具体地分割成三类不同的区域：类固态区、过渡区、以及类液态区，每一个区域都拥有各自专属的原子类型。研究证明，不同区域与剪切转变的关联程度同样存在着梯度演变。类液态区，特别是其边界周围的原子（包括类固态和过渡态原子），最易成为剪切转变的形核位置，其增殖行为多优先选择类液态区进行。

因此，根据所发现的中程序原子堆垛特点，非晶中的软区有了明确的结构组成，即类液态原子和它们近邻原子的组合。这一发现有助于定量的比较不同非晶合金中软区的数量，进而更深入的理解不同非晶合金力学行为的差异。据此，研究者对微合金化后部分非晶合金强度和塑性同时改善的现象进行了细致的讨论并给出了合理的解释，为人们设计高强韧性的金属玻璃材料提供了理论指导。

该文近期发表于npj Computational Materials 4: 41 (2018) ，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Nanometer-scale gradient atomic packing structure surrounding soft spots in metallic glasses 

Binbin Wang, Liangshun Luo, Enyu Guo, Yanqing Su, Mingyue Wang, Robert O. Ritchie, Fuyu Dong, Liang Wang, Jingjie Guo & Hengzhi Fu 

The hidden order of atomic packing in amorphous structures and how this may provide the origin of plastic events have long been a goal in the understanding of plastic deformation in metallic glasses. To pursue this issue, we employ here molecular dynamic simulations to create three-dimensional models for a few metallic glasses where, based on the geometrical frustration of the coordination polyhedra, we classify the atoms in the amorphous structure into six distinct species, where “gradient atomic packing structure” exists. The local structure in the amorphous state can display a gradual transition from loose stacking to dense stacking of atoms, followed by a gradient evolution of atomic performance. As such, the amorphous alloy specifically comprises three discernible regions: solid-like, transition, and liquid-like regions, each one possessing different types of atoms. We also demonstrate that the liquid-like atoms correlate most strongly with fertile sites for shear transformation, the transition atoms take second place, whereas the solid-like atoms contribute the least because of their lowest correlation level with the liquid-like atoms. Unlike the “geometrically unfavored motifs” model which fails to consider the role of medium-range order, our model gives a definite structure for the so-called “soft spots”, that is, a combination of liquid-like atoms and their neighbors, in favor of quantifying and comparing their number between different metallic glasses, which can provide a rational explanation for the unique mechanical behavior of metallic glasses.

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