Enhancing strength and plasticity by pre-introduced indent-notches in Zr36Cu64 metallic glass: A molecular dynamics simulation study

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Abstract

The deformation behavior in Zr36Cu64 metallic glasses with pre-introduced indent-notches has been studied by molecular dynamics simulation at the atomic scale. The indent-notches can trigger the formation of densely-packed clusters composed of solid-like atoms in the indent-notch affected zone. These densely-packed clusters are highly resistant to the nucleation of shear bands. Hence, there is more tendency for the shear bands to nucleate outside the indent-notch affected zone, which enlarges the deformation region and enhances both the strengthening effect and the plastic deformation ability. For indent-notched MGs, when determining the initial yielding level, there is a competition process occurring between the densely-packed clusters leading to the shear band formation outside the indent-notch affected zone and the stress-concentration localizing deformation around the notch roots. When the indent-notch depth is small, the stress-concentration around the notch root plays a dominant role, leading to the shear bands initiating from the notch root, reminiscence of the cut-notches. As the indent-notch depth increases, there are many densely-packed clusters with high resistance to deformation in the indent-notch affected zone, leading to the shear band formation from the interface between the indent-notch affected zone and the matrix. Current research findings provide a feasible means for improving the strength and the plasticity of metallic glasses at room temperature.

Introduction

Brittle fracture of metallic glasses (MGs) at room temperature is a stumbling block as structural materials [1,2]. Introducing notches is a very effective way to prevent the brittle fracture of MGs [[3], [4], [5], [6], [7]]. The effects of notch shape, size and orientation on the deformation of MGs have been extensively studied [[8], [9], [10], [11], [12]], and play a key role in understanding the plasticity of notched MGs. However, contradictory notch effects have been observed due to differences in the location and geometry of particular notches. The weakening, strengthening and even insignificant effects of notches on strength have been observed [[13], [14], [15], [16]]. For example, Gu et al. found that the introduction of notches can enable the failure strength to be reduced by 40 % and change the fracture mode of MGs from shear band to cavitation [17]. Sha et al. used molecular dynamics (MD) simulation to find that symmetrical double-edge notches can cause strengthening by constraining the growth of the plastic zone for nanoscale MGs [18]. Cui et al. found that notches can trigger the occurrence of strengthening and structural ordering along with the recovery of Voronoi volume and Cu-centered full-icosahedra during plastic deformation in Cu64Zr36 MGs [19]. Qu et al. found that notches can improve the plasticity of MGs, but are insensitive to tensile strength [20]. Yang et al. concluded that different inter-void ligament distance can lead to different degrees of strengthening effects for notched MGs [21]. The key factors affecting the strength of notched MGs are complex and diverse, but it is certain that these notches, which are formed by cutting part of the MGs, destroy the integrity of MGs themselves.

Here, we propose a new way of introducing notches without sacrificing the integrity of MGs. Indentation is usually used to study the mechanical properties of MGs [22,23]. During the indentation of MGs, the deformation units are activated firstly in the soft zones near the indenter, and then mature shear bands are formed with further loading [24]. Due to the multi-axial stress state beneath the indenter, multiple shear bands form in a semicircular and radial fashion, allowing sustainable plastic flow in MGs [25]. For the indent-notched MGs, there are numerous fundamental questions to be asked. (1) How would the indent-notches affect the deformation behavior of MGs, i.e., strengthening or weakening, enhancement or embrittlement? (2) How are such effects compared with cut-notches? (3) How would this deformation behavior be related with the atomic-level strain and the amorphous structure induced by the indent-notch?

In this study, using Zr36Cu64 MGs as a model system, we employ MD simulations to perform a systematic study on the effect of indent-notches in order to understand the relationship between indent-notches and deformation behavior at the atomic level. The paper is organized as follows: in Section 2, the methods and details of MD simulations are described. Section 3 presents the results of the enhanced strengthening effect and the plastic deformation ability induced by indent-notches. Section 4 discusses the underlying mechanisms. Section 5 contains the conclusions.

Section snippets

Materials and methods

MD simulation based on the combination of the large-scale atomic/molecular massively parallel simulator (LAMMPS) and the empirical embedded atom method (EAM) potential was performed [26,27]. The initial model Zr36Cu64 consisted of a square box of 5.8 nm with a random distribution of 3600 Zr atoms and 6400 Cu atoms. An NPT (constant number, constant pressure, and constant temperature) ensemble with a Nosé-Hoover thermostat and barostat was adopted [28,29]. It was equilibrated at 2000 K and was

Stress-strain curves

Fig. 2 displays the engineering stress-strain curves of monolithic, cut-notched and indent-notched Zr36Cu64 MGs. To facilitate the comparison across different samples, the engineering stress is calculated by normalizing the force with the initial area of the un-notched ligament. Notably, the ultimate tensile strength of the monolithic, cut-notched and indent-notched Zr36Cu64 MGs is 2.78 GPa, 4 GPa and 4.25 GPa, respectively. For comparison, the notch sensitivity of the ultimate tensile strength

Discussion

As the two notched samples share a similar geometry, the effects of pre-deformation on the local stress/strain states and glassy structure are essential in understanding their different tensile deformation behavior. Fig. 6a shows a snapshot of atoms with local shear strain ηMises > 0.3 at a strain of 7 % for the indent-notched MG, and the indent-notched MG at a strain of 0 % is regarded as the reference state. The red line in Fig. 6a shows that local plastic deformation initiates from the

Conclusions

In summary, MD simulations have been performed to investigate the uniaxial tensile behavior of monolithic, cut-notched and indent-notched Zr36Cu64 MGs, focusing on the effects of indent-notches on improving both the strengthening effect and the plastic deformation ability. The NDR clearly demonstrates that the indent-notches do not sacrifice deformation strain when enhancing the strengthening effect, i.e., the NSR is larger than that of cut-notches. The larger plastic zone size of the

Declarations of Competing Interest

None.

Acknowledgments

This work was supported by the National Natural Science Foundation of China under Grant 51801174; the Program for the Top Young Talents of Higher Learning Institutions of Hebei under Grant BJ2018021; the National Key R&D Program of China under Grant 2018YFA0703602; the Hong Kong Scholars Program under Grant XJ2017049; LL would like to acknowledge the support from the National Science Foundation under grant number CMMI 17-0267.

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      For instance, by altering the direction and positions of notches on both sides of specimens, the inherent shear fracture of BMGs can be delayed to some extent, and even macroscopic tensile plasticity is achieved [15–18,28–30]. During deformation, shear bands are easily initiated and activated near notches, and the influence of notches on shear band propagation plays an essential role in subsequent plastic deformation [15–18,28–30]. According to finite element methods and molecular dynamics simulations, the failure mode may vary from shear banding to necking, which strongly depends on the depth and sharpness of the notches [29,31,32].

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