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.
Highlights
Compared to their crystalline counterparts, metallic glasses (MGs) are vitrified solids in the metastable state.[1,2,3] The atomic structure of such amorphous matter and its relevance to mechanical behavior is a fundamental and intriguing problem
The current study focuses on the hidden order in amorphous structures, on the length-scale of a couple of nanometers (MRO) and beyond, with the rationale of defining the structural feature of “soft spots”
According to the different disclination density of each Kasper polyhedra, the relevant ones for MGs are divided into five groups,[5,17] which are listed in Table 1 for coordination numbers (CNs) ranging from 8 to 17
Summary
Compared to their crystalline counterparts, metallic glasses (MGs) are vitrified solids in the metastable state.[1,2,3] The atomic structure of such amorphous matter and its relevance to mechanical behavior is a fundamental and intriguing problem. As reflected in the central materials science paradigm that “structure determines properties”, it is important to understand any hidden order in seemingly disordered glassy alloys in order to establish a causal link between such local structure and macroscopic properties. Such an objective, which is universally viable for MGs, has yet to be successfully achieved in condensed matter physics.[4,5]. Despite a lack of long-range order, MGs do possess short-range (SRO) and medium-range order (MRO) that have been extensively characterized by experiments[6,7,8,9,10] and atomic simulations.[5,11,12]. The extended order of atomic packing, on the length-scale of several nanometers and beyond, has become a topic of modeling interest, but with many aspects still uncertain.[5,17,18]
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