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Nanometer-scale gradient atomic packing structure surrounding soft spots in metallic glasses (金属玻璃中软区周围的纳米级梯度原子堆垛结构)
发布时间:2018-08-13

Nanometer-scale gradient atomic packing structure surrounding soft spots in metallic glasses (金属玻璃中软区周围的纳米级梯度原子堆垛结构)
Binbin WangLiangshun LuoEnyu GuoYanqing SuMingyue WangRobert O. RitchieFuyu DongLiang WangJingjie Guo & Hengzhi Fu
npj Computational Materials 4:41 (2018)
doi:s41524-018-0097-4
Published online:30 july 2018
Abstract| Full Text | PDF OPEN

摘要:对于金属玻璃(非晶合金)来说,无定形结构中原子堆垛的隐藏序,及其如何提供塑性项的起源,长期以来一直是理解这一材料塑性变形机理的关键所在。为了解决这一问题,我们采用分子动力学计算模拟的研究方法,构建了数种非晶合金的三维模型,并且基于每一原子周围配位多面体的几何阻挫差异,将非晶结构的原子共分为六个不同的种类。对于不同体系的非晶合金,在纳米尺度范围内,都存在着“梯度原子堆垛结构”,也就是说非晶态的局域结构表现为一种从原子松散堆垛到致密堆垛的梯度演变,原子的性能同样表现为一种梯度的变化。据此,非晶合金中存在三个可识别的区域:类固态区、过渡区以及类液态区,每一个区域都具有各自专属类型的原子。此外,我们还证明了类液态原子与剪切转变的关联性最强,过渡态原子次之,而类固态原子对剪切转变贡献最低。与“GUMs”模型不同的是,我们的模型考虑了中程序的作用,进而给出了明确的”软区“结构,即类液态原子和它们近邻原子的组合,这将更有利于定量地比较不同非晶合金中”软区“的数量,为非晶合金独特的变形行为提供合理的解释   

Abstract: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. 

Editorial Summary

Metallic glasses: gradient atomic packing and plasticity (金属玻璃:梯度原子堆垛与塑性) 

梯度原子堆垛结构中的类液态原子可能决定了金属玻璃的塑性。来自中国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,依据原子的堆垛方式,采用分子动力学模拟对不同体系金属玻璃中的原子进行分类研究,从而理解了非晶合金中的中程有序。他们发现局域原子结构,凭借类液态区、过渡区和类固体区,逐渐从松散的原子堆垛转变为致密的原子堆垛,类液态原子和它们近邻原子一起构成“软区”,并与局部不可逆原子重排的萌发密切相关。这种梯度原子堆垛结构模型可以帮助我们更好地理解和设计非晶合金的塑性行为。

Liquid-like atoms in a gradient atomic packing structure might determine plasticity in metallic glasses. A team led by Yanqing Su at the Harbin Institute of Technology in China and Robert Ritchie at the University of California, Berkeley, in the USA, used molecular dynamics simulations to classify atoms in different metallic glasses according to their stacking, thereby accounting for the amorphous medium-range order in metallic glasses. They found that local atomic structures gradually transitioned from a loose to a dense stacking of atoms via liquid-like, transition, and solid-like regions, and that liquid-like atoms and their neighbors were equivalent to ‘soft spots’ and associated with initiation of local irreversible atomic arrangements. Modeling this gradient atomic packing structure may help us better understand and design plasticity in glassy alloys.

 
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