**1. Introduction**

Metallic glasses (MGs), also named amorphous alloys for their disordered atomic structure, behave excellent mechanical behaviors, including high elastic modulus, high yield stress, and extreme high hardness, compared with traditional crystalline alloys. Considering their excellent mechanical performance in micro-nano scale, MGs have a great application potential in ultra-precision systems. The research work on MGs has been proceeding for over 50 years

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

both experimentally and theoretically. Meanwhile, the corresponding simulation methods with aids of computer science have also been developed to provide assistance for the experimental research from the theoretical aspect. The most used simulation methods for the MGs are finite element method (FEM) and molecular dynamics (MD) method. FEM constructs a constitutive model of MG at the macroscopic scale on time and space, which is coincident with experimental works. For this advantage, it is usually employed to investigate the mechanical properties and shear bands (SBs) formation in the bulk MG systems [1–4]. Although a MG is usually regarded as isotropic and homogeneous from the macroscopic scale, a homogeneous constitutive model is not sufficient for the generation and localization of SBs in a typical brittle MG system. For the improvement, two different SB formation theories, free volume theory [1, 2] and shear transition zone (STZ) theory [3, 4] were employed in constitutive model to provide nucleation possibilities in the MG model, and they are proved quiet effective when depicting the formation process of SBs. However, these FEM simulations are basing on a fuzzy description of flow events according to the two immature theories, but neglect intrinsic dynamics of the flow events from atomic aspect. Furthermore, the theoretical frames and constitutive model for the research of MGs have to build on accurate description of their atomic structure. Since, the lack of effective experimental tools on characterizing the atomic structure of MGs, resolving the intrinsic structure only from experimental methods is unreasonable. Recently, MD methods are developed to offer possibility for the investigation on MGs from atomic level. By simulating the atoms movements and tracking atoms trajectories, MD simulation can calculate the basic thermal information of a MG ensemble, including temperature, potential energy, as well as some derived information such as atom shear stress, atom shear strain, and structure evolution and so on. Thus, MD simulation method is an effective tool when investigating the intrinsic structure of MG, and its connection with mechanical properties, as well as the elastic to plastic deformation transition.

The first model of atomic structure in MG was developed by Bernal, and it was described as a dense random packed structure of equal-sized spheres with the absence of medium-range and long-range order [5]. Bernal used the active hard balls to describe a liquid-like amorphous system, and analyzed the radial distribution function of the model, and found to have a good consistence with that of liquid system. This model provided a possibility of modeling and characterization based on the computer science. The further development of amorphous structure modeling has been based on Bernal's work, and more developed characterization methods have been raised from the aspects of statistic and topology with the aids of MD simulations. The most used characterization methods of MGs with MD simulation are presented as the following in this section, including pair distribution function, Honeycutt-Andersen

Mechanical Properties and Deformation Behaviors of Metallic Glasses Investigated…

Pair distribution function (PDF) is the most classical and important statistical analysis method in amorphous systems; it is widely used in the characterization of liquids and amorphous materials. It is a pair correlation representing the probability of finding atoms, and described as a function of distance *r* from an average center atom. In a monatomic system, PDF is

> <sup>4</sup>*<sup>π</sup> <sup>r</sup>* <sup>2</sup> *<sup>ρ</sup><sup>N</sup>* ∑ *i*=1 *N* ∑ *j*=1,*j*≠*i N*

*δ*(*r* −|*r*

), indicating the generally formation and stability of the glass

¯*ij*| is the interatomic distance between atom *i* and atom *j*, and *ρ* is the number density of atoms in the system with *N* atoms. According to the PDF curve of a MG, the short-to-medium range order information can be manifested by the peak position, peak width, and relative intensity, etc. Conventionally, the nearest-neighbor shell atoms contribute to the first peak, which represents short range order (SRO) in the MG. A further distance up to 1–2 nm contributes the medium-range order (MRO). With distance *r* going larger, PDF gradually converges to unity, which means the atoms are randomly distributed, represents the long-range disorder. The structure evolution from liquid to glass can be shown from the PDF curves during the cooling procedure. **Figure 1** displays the PDF curves of a binary Cu64Zr36 MG in different temperature regions during its preparation process. It can be clearly seen that with decreasing temperature during quenching process, the second peak of total PDF gradually splits and becomes more pronounced, which indicates the formation of the glass phase and enhancement of SRO. Furthermore, the appearing temperature of the split is usually relevant to the

PDF is concise and clear for the characterization of amorphous materials, especially when detecting the amorphous phase transferred from a crystalline system; thus, PDF method is widely used to verify the effective MG models in MD simulations [6]. However, this method is only established on a statistic basis, without considering the exactly certain structure of MGs. It cannot provide the specific topology description of a certain amorphous system, thus

has a limitation when further revealing the atomic structure geometry of MGs.

¯*ij*|), (1)

http://dx.doi.org/10.5772/intechopen.76830

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analysis, and Voronoi tessellation method.

*<sup>g</sup>*(*r*) <sup>=</sup> \_\_\_\_\_\_ <sup>1</sup>

**2.1. Pair distribution function**

glass transition temperature (Tg

defined as:

where |*<sup>r</sup>*

structure.

In this chapter, we mainly take a review on the former research work on the connections between structure and properties in MGs. We began with the current understanding in the structure of MGs and then discuss their proper connection with some mechanical behaviors. Afterwards, more specific discussion on the SBs formation and development will be given based on various mechanics of MD simulations. Finally, based on the limitation of MGs in their applications, we discussed several multi-component materials derived from cast MGs, and have an outlook on the development trend of MG preparation and application.
