Applied Materials Today
Volume 21, December 2020, 100828
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An atomic-level perspective of shear band formation and interaction in monolithic metallic glasses

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Abstract

Understanding the relationship between nanoscale structural heterogeneities or elastic fluctuations and strain localization in monolithic metallic glasses remains a long-standing underlying issue. Here, an atomic-level investigation of the correlation between elastic and structural heterogeneities and the mechanisms of shear banding in CuZr metallic glass is conducted using molecular dynamics simulations. The shear band formation and propagation processes and the intersection mechanism of multiple shear bands are evaluated by means of local entropy-based structural identification and von Mises stress calculation. The shear band follows the path of lower order and high entropy while shear deflection and branching occur when approaching regions of low entropy. The local von Mises stress calculation allows predictions on the shear band direction and the propensity for activation and propagation prior to yielding and sheds light on shear band branching and multiplication processes.

Introduction

Enhanced plasticity in metallic glasses (MGs) is ascribed to the formation of many shear bands with small shear offsets [1]. This can be achieved in composite MGs where crystalline heterogeneities promote shear band nucleation and multiplication [2], [3], [4], [5]. Furthermore, the interaction with crystalline inclusions blocks the propagation of shear bands and imposes a confinement on the shear offsets [6], [7]. A high density of shear bands, and consequently large plastic strain, can also be achieved in monolithic MGs with higher Poisson’s ratio [8], [9]. Although they have no microstructure, here atomic-scale structural heterogeneities and elastic fluctuations control the strain distribution and the processes of shear band formation, propagation and multiplication [10], [11], [12], [13]. The evolution and intersection of a large population of shear bands blocks their propagation and stimulates the initiation of even more shear bands, delaying in this way the onset of catastrophic failure [14]. Moreover, the interaction of shear bands changes their dynamics and morphologies, increases the yield stress and promotes work-hardening during deformation [15], [16], [17], [18]. Although shear band interactions have been widely used to explain an increased strain to failure and deformation-induced hardening in MGs [15], the major challenge is to understand and control the shear band blocking mechanism.

While over the last decades substantial progress has been made in post-mortem investigations of the structure of shear bands only few analyses of the shear band dynamics have been reported owing to the spatial and temporal scales of the deformation processes [19], [20]. To date, few methods have been proposed to investigate shear band dynamics and blocking mechanisms [16], [17], [19], [21], [22], [23]. Nanoindentation [17], atomic force microscopy [22], [23] and high-resolution transmission electron microscopy [16] studies have provided a consistent picture of shear band dynamics but could not explain the complex and mutual interaction process and what happens at the shear band intersection points as understanding what drives the changes in strain localization behavior related to the phenomena of shear band propagation and intersection requires systematic atomic-level investigations.

Computer simulations provided insight into the atomic-scale structure and deformation processes in MGs [5], [11], [24], [25], [26]. An important goal of atomistic modeling was to define environmental descriptors to identify and characterize the atomic scale heterogeneities and provide a detailed structure-property relationship for metallic glasses [27], [28]. Here, we derive a local fingerprint based on an approximate expression for the entropy using atomistic simulations, highlighting the strength of the correlation between structural heterogeneities and strain localization in monolithic glasses. The local entropy shows a high predictive power and allows for gradual representation of the local structure without any prior information on a reference configuration. Special emphasis is also given to the von Mises stress that allows us to characterize elastic fluctuations and predict the spatial and temporal evolution of the atomic strain prior shear band propagation and particularly at the shear band intersection point.

Section snippets

Materials and methods

To overcome experimental technical difficulties associated with the observation of the fast processes of shear band propagation and interaction large-scale molecular dynamics (MD) simulations were carried out using the LAMMPS software [29]. The Cu64Zr36 metallic glass has been simulated using the interatomic potential developed by Mendelev et al. [30]. The starting liquid structure was created by randomly distributing 968,000 atoms in a box of 111.0  ×  55.5  ×  2.5 nm with periodic boundary

Influence of structural heterogeneity on shear band dynamics

Although the structural disorder of MGs entails the absence of structural features such as grain and phase boundaries, dislocations and stacking faults, they possess a high degree of short-range order (SRO) and even medium-range order (MRO) [38], [39]. The most commonly found SRO clusters in MGs have a preference to develop five-fold symmetry. These locally favored structure motifs are accompanied by more unfavorable polyhedra, which are needed to fill the spaces between the more ordered

Conclusions

In summary, the correlation between elastic and structural heterogeneities and the atomic-level mechanisms of strain localization in monolithic metallic glasses was investigated. By using a novel local entropy-based structural identification method we found that the shear band follows the path of lower order and high entropy. Moreover, when the shear band approaches higher ordered regions of low entropy, shear deflection and branching occur promoting shear band multiplication. While the local

CRediT authorship contribution statement

D. Şopu: Conceptualization, Formal analysis, Supervision, Writing - original draft, Writing - review & editing. F. Moitzi: Data curation, Formal analysis, Writing - review & editing. N. Mousseau: Formal analysis, Writing - review & editing. J. Eckert: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

D. Ş. acknowledges the financial support by the Deutsche Forschungsgemeinschaft (DFG) through Grant No. SO 1518/1-1. Additional support was provided by the European Research Council under the ERC Advanced Grant INTELHYB (grant ERC-2013-ADG-340025). The authors are grateful for the computing time granted by the Lichtenberg high performance computer of Technische Universität Darmstadt and the high performance cluster of Montanuniversität Leoben. The authors acknowledge Dr. A. Stukowski for

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