**5. Deformation behaviors of nanoglass**

sample. The transition in deformation mode from highly localized shear banding to nonlocalized plastic deformation was associated with the competition between the yield strength of the material and the critical stress required for the formation of mature SBs in the loadbearing materials. Zhong et al. extracted the SB part of a shear banding deformed MG, and applied tensile stress on the SB sample [33]. The SB tended to have softened tensile behavior and homogenous deformation, with lower tensile stress compared with MG matrix and no

Basing on the above investigation, the cavitation and propagation of SBs seems have a relation with various factors, such as size scale, cooling rate, temperature, composition, etc. Li et al. took MD simulations to investigate the tensile deformation behavior of MG samples with multimillion atoms [34]. They found small rob samples showed remarkable resistance to formation of SBs, and behaved unusual necking phenomenon. Three factors were concluded as contribution to the deformation mode size effects: surface effect, sample loading geometry, and finite sample size. With a further study on the shear banding behavior of double-notched sample, they indicated a critical dimension size about 10–20 nm was needed for the nucleation of SBs [35]. Gao et al. found the size effects in the deformation of Cu-Zr MG. With the model diameter gradually decreasing, the deformation mode of MG evolved from highly localized SB formation to homogenous deformation, but the stress increased significantly during the tensile process [36]. Zhong et al. also had an interest in the size effects, and they found with decreasing film thickness of MG, a transition from the localized deformation to the nonlocalized deformation indeed occurred [37]. Their further study revealed that the critical thickness for this transition was sensitive to the composition, and it was correlated with the average activation energy of

Cheng et al. demonstrated the effects of cooling rate and composition on localization of deformation [11, 12]: with a lower cooling rate, the MG exhibited higher strength but easier trend to strain localization; the MG with composition of Cu64Zr36 with high proportion of FI clusters, was more resistant to the initiation of flows but increased propensity to strain localization, while Cu40Zr60 was on the opposite side. Zhong et al. utilized this property and created several composites by controlling the layers thickness and numbers of the two MGs, and investigated the deformation behavior of these composites [39]. They found out that MG samples with high layer numbers present obviously nonlocalized deformation behavior, the criterion for the deformation mode change for MGs was suggested as the competition between the elastic energy densities stored and the energy density needed for forming one mature SB in MGs. A further investigation on the annealing effects revealed that the localizing degree of SBs could also be regulated by annealing, with free volume deduction detected during the structural relaxation process [33]. Nanoindentation simulations on binary MG taken by Shi et al. also presented that the SB morphology under indenter had great dependence on indentation rate [40, 41]. At a lower loading rate, SBs showed wing-like morphology and easily propagated to the surface. In the opposite, wedge-like SBs came to formation and penetrated downward to MG matrix at higher loading rate. In our previous MD simulation work on Cu-Zr MG, we also observed more localized SBs under indentation at lower temperature, and more homogeneous deformation morphology at room temperature, which coincided with the brittleness

obvious localization of atomic strain.

40 Metallic Glasses - Properties and Processing

the atomic level plastic deformation events [38].

characteristic of most MGs at low temperature [42].

With highly inhomogeneous deformation dominated by localized SBs, MGs are easily encountered with catastrophically failure. Some pre-deformation process, like cold rolling, leads to a pseudocomposite structure consisting of a softer phase inside pre-induced SBs and a harder phase in the undeformed regions [45, 46]. These microstructural features improve the macroscopic plasticity by promoting the nucleation of secondary SBs and SB branching, as well as by limiting SB propagation due to the intersection of SBs. An alternative method to prevent a major SB development is regulating the volume-interface ratio and density by introducing particle interfaces into the matrix of MG [47]. Such a MG could be produced via cold compaction of glassy nanoparticles, and is therefore called a nanoglass (NG).

According to the definition of nanoglass, Sopu et al. constructed a 3D periodic Cu-Zr nanoglass with an idealized nanostructure consisting of columnar grains with a hexagonal cross section [48]. Applied with tensile stress during a mechanical test simulation, multiple embryonic SBs were formed along the interfaces and eventually started to propagate through the grain interiors in the NGs. Since the elastic energy was released homogeneously in the whole sample, the local energy release was not sufficient to accelerate any SBs; thus, NGs deformed homogeneously in contrast to the MGs, which exhibited localized deformation in one major SB, shown in **Figure 10**. Comparing the strain localization parameters of the NGs and MGs calculated according to Eq. (1), a more homogeneous deformation in the NGs was supported. Moreover, with an annealing treatment to the NGs, the glass particle interfaces seemed have a partial recovery of icosahedral SRO, led to the increase of strain localization.

Adibi et al. constructed a more random distributed NG by involving Poisson-Voronoi tessellation method, using the cast Cu-Zr MG structure as a source of material for the glassy grains

constructed as sandwich architecture, and with a NG surface coated on the surface, MGs could be protected from localized SBs. They further conducted tensile load along the vertical direction to the laminates [52]. The change in the loading direction caused the differences not only in the location of SB initiation but also the critical distance between NG layers for failure mode transition. The MG-NG nanolaminate structure with NG layers closely packed and interfaces oriented parallel to the loading direction was identified as the most effective heterostructure. Adibi et al. constructed another NG-MG nanolaminates by combining layers of MG and NG [53], and further investigated mechanical properties of these nanopillar-shaped nanolaminates with tensile loading simulations. Compared with pure MG and NG, the MG-MG nanolaminate exhibited delayed SB formation and diffused shear banding failure, the NG-MG nanolaminate showed exceptional plasticity to a strain of 0.15 prior to a necking-type failure. These works suggest the MG composites constructed by NG and MG laminates can offer promise for creating structures that combine outstanding

Mechanical Properties and Deformation Behaviors of Metallic Glasses Investigated…

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**Figure 11.** Schematic of the process to generate a NG film sample with Adibi's method [49].

Throughout the review, we enumerated pronounced work on the structure analysis, the connection of structure with the mechanical properties, shear bands initiation and development. With the application of structural analysis methods, the existence of SRO and MRO in MGs is confirmed. The SRO types and proportions vary with MG compositions and preparation process. Several SRO motifs with high five-fold symmetry structure, as well as their interconnected networks, are found to be great contributing factors to the solid-like characteristics of MG, which means they are closely correlated with mechanical properties and deformation patterns of MGs. The intrinsic dynamics of shear banding events were investigated from the mechanics simulations, which provided convincingly supports for the shear banding theories. Furthermore, we also demonstrated recent MD simulations works on mechanical behaviors and deformation properties of several designed nanoglass. They behaved ductile deformation

ability and might be a promising derived material from casted MGs.

strength and ductility.

**6. Conclusion and outlook**

**Figure 10.** Local atomic shear strain distributions for MG, inhomogeneous NG and homogeneous NG at tensile of 8 and 16% [48].

(shown in **Figure 11**). Three specimens were selected with average glassy grain sizes from 5 to 15 nm to investigate deformation mode transition. The deformation modes of the NGs generally transferred from localized shear banding to homogeneous plastic flow with the grain sizes decreasing, and the fine-grained even became superplastically deformed during tensile testing [49]. The effects of composition and grain size were investigated more specifically by involving Voronoi polyhedral analysis method [50]. They found that the mechanical behavior of NGs was regulated by both the grain boundary thickness and the fraction of atoms at interfaces intrinsically. The mechanical behavior of NGs had a composition dependence similar to their parent MG, while the intrinsic deformation behavior only depended on the grain size, and not affected by the composition.

Sha et al. constructed a sandwich architecture composed of NG and MG [51]. The constructed composite had a higher strength than pure NG, and larger plasticity than MG. The improving of plasticity was contributed by the glass-glass interfaces in NG layers and a compressive residual stress in the MG layer. They indicated a strong ductile MG could be Mechanical Properties and Deformation Behaviors of Metallic Glasses Investigated… http://dx.doi.org/10.5772/intechopen.76830 43

**Figure 11.** Schematic of the process to generate a NG film sample with Adibi's method [49].

constructed as sandwich architecture, and with a NG surface coated on the surface, MGs could be protected from localized SBs. They further conducted tensile load along the vertical direction to the laminates [52]. The change in the loading direction caused the differences not only in the location of SB initiation but also the critical distance between NG layers for failure mode transition. The MG-NG nanolaminate structure with NG layers closely packed and interfaces oriented parallel to the loading direction was identified as the most effective heterostructure. Adibi et al. constructed another NG-MG nanolaminates by combining layers of MG and NG [53], and further investigated mechanical properties of these nanopillar-shaped nanolaminates with tensile loading simulations. Compared with pure MG and NG, the MG-MG nanolaminate exhibited delayed SB formation and diffused shear banding failure, the NG-MG nanolaminate showed exceptional plasticity to a strain of 0.15 prior to a necking-type failure. These works suggest the MG composites constructed by NG and MG laminates can offer promise for creating structures that combine outstanding strength and ductility.
