**4. Shear bands in MGs**

This part moves on to a view of larger scale on the shear bands and their connection with the deformation of MGs, which is believed as a breakthrough of atom scale deformation in simulations with experimental phenomenon in macro scale. MGs have a similar performance in experiments with other amorphous materials such as silica glasses and polymers [25, 26], with blunt tip applied on the surface, typical slip line patterns will be shown in the cross surface, and the slip lines are regard caused by shear stress, thus called shear bands.

The formation and spreading of highly localized SBs is widely believed responsible for the failure of brittle MGs. But the ductile MGs could show homogeneous deformation without localization of SBs under the same mechanical loading. To describe the plastic flow and reveal the deformation transition mechanism, the initiation of SBs from a shear transformation is a fundamental problem for the starting. One of the most popular explanations is STZ theory raised by Argon [27]. According to the theory, STZs are the clusters composed of tens to hundreds of atoms, and the basic elements undergoing the plastic flow in MG. When a shear stress is applied on the MG, the atoms in the cluster have relative movements, and deformation takes place inside the cluster. The distortion of STZ is an activation process, and needs a certain activation energy and volume, to transfer a STZ from a high energy state to a lower one.

The propagation of a SB from a collection of STZs is next problem to be concerned. Ogata et al. performed shear deformation on Cu-Zr bulk MG with MD simulations [28], and observed the nucleation and localization of SBs during shear deformation. The nucleation of SB was taken place from a local atomic rearrangement like STZ, and this local deformation induced releasing of elastic strain energy in the shear plane, and activated the generation of more STZs. Then, the STZs interacted with each other and formed a SB in the shear direction. Furthermore, the generation of SB enhanced the elastic strain energy of the surrounding materials in the inplane directions, made the spreading of SB more localized.

Shimizu et al. described the propagating and developing of SB as a growth process of embryonic shear band (ESB) [29, 30]. An ESB appeared at the concentrator of a group of activated STZs. When the far-field shear stress exceeds the glue traction with temperature rising induced by frictional heating, and the length of the ESB increased to a critical length over 100 nm, the ESB would become maturing to a localized SB. An aged-rejuvenation-glue-liquid (ARGL) SB model was used for the description of the shear front, and four zones were defined as aged glass, rejuvenated glass, glue, and liquid (illustrated in **Figure 9**). The temperature distributes from room temperature at aged glass end to over the glass transition temperature at the liquid end. Their model was supported by the temperature rising phenomenon, which can reach a maximum over 1000 K, can also be verified from the research work by Lewandowski et al. [31]. They planted a fusible tin coating to observe the temperature rise during bend-test on MG, and observed melting in the SBs formation area, thus deduced that local temperature rise in the bands can reach a few thousand kelvin.

For a further understanding on SBs, the mechanical properties of SBs were investigated by Zhou et al. by testing samples with pre-existing SBs with tensile MD simulations [32]. It was demonstrated that pre-deformation lowered material strength and triggered enhanced strain fluctuation before sample yielding, leading to highly dispersed plastic shearing in the entire

**Figure 9.** The aged-rejuvenation-glue-liquid (ARGL) shear band model [29].

icosahedra participating in the connection scheme of each patterns were calculated. It was found the icosahedra with volume shared type had a lower potential energy and stable structure, Cu65Zr35 had larger fractions of ICOIs with close connection of ICOIs compared to Cu50Zr50, and together with more dense atomic packing. With addition of shear simulations, Cu65Zr35 was found more resistant to applied load and showed higher elastic modulus and yield stress. The highly connected MROs constituted a compact icosahedral network over an extended range, which was resistant to stress-induced shear transformations under applied load. The breaking of ICOI networks during shear deformation are shown in **Figure 8**. This work presented the connectivity of MROs and showed its significant influence on the mechanical properties of MG.

**Figure 8.** The breaking of ICOI networks in the during shear deformation [24].

This part moves on to a view of larger scale on the shear bands and their connection with the deformation of MGs, which is believed as a breakthrough of atom scale deformation in simulations with experimental phenomenon in macro scale. MGs have a similar performance in experiments with other amorphous materials such as silica glasses and polymers [25, 26], with blunt tip applied on the surface, typical slip line patterns will be shown in the cross

The formation and spreading of highly localized SBs is widely believed responsible for the failure of brittle MGs. But the ductile MGs could show homogeneous deformation without

surface, and the slip lines are regard caused by shear stress, thus called shear bands.

**4. Shear bands in MGs**

38 Metallic Glasses - Properties and Processing

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 obvious localization of atomic strain.

The spreading of SBs can also be triggered or suppressed by cyclic loads, and it is relevant to soften or harden behaviors of MGs. Sha et al. performed MD simulations of tensioncompression fatigue on Cu50Zr50 MG [43]. They observed the initiation and propagation of a major SB throughout the MG sample under cyclic loading. The cycling loads accelerated the accumulation of STZs, and triggered the spreading of SB once the aggregates of STZ reached the critical size for shear banding. Meanwhile, the fully formation of the SB was accompanied with stress drops, and this indicated the soften behavior of MG under uniform axial loads. While Deng et al. observed hardening behavior of Cu-Zr MG under cyclic indentation loads in MD simulations [44]. They indicated the post plastic deformation induced by the earlier indentations suppressed the SB formation by locally stiffening some portions of the original shear banding path, and then a higher load was needed for a secondary path of shear banding yield. Our works demonstrated this hardening behavior could be more severe under higher indentation loads or temperature [42]. In fact, these factors would induce higher plasticity in the deforming region, which formed more complex shear banding morphology and prevented localized SBs formation. These works performed that softening or hardening phenomenon could happen in MGs, due to SBs spreading or suppression under different

Mechanical Properties and Deformation Behaviors of Metallic Glasses Investigated…

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

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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

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

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

cold compaction of glassy nanoparticles, and is therefore called a nanoglass (NG).

a partial recovery of icosahedral SRO, led to the increase of strain localization.

types of cyclic loads.

**5. Deformation behaviors of nanoglass**

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 the atomic level plastic deformation events [38].

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 characteristic of most MGs at low temperature [42].

The spreading of SBs can also be triggered or suppressed by cyclic loads, and it is relevant to soften or harden behaviors of MGs. Sha et al. performed MD simulations of tensioncompression fatigue on Cu50Zr50 MG [43]. They observed the initiation and propagation of a major SB throughout the MG sample under cyclic loading. The cycling loads accelerated the accumulation of STZs, and triggered the spreading of SB once the aggregates of STZ reached the critical size for shear banding. Meanwhile, the fully formation of the SB was accompanied with stress drops, and this indicated the soften behavior of MG under uniform axial loads. While Deng et al. observed hardening behavior of Cu-Zr MG under cyclic indentation loads in MD simulations [44]. They indicated the post plastic deformation induced by the earlier indentations suppressed the SB formation by locally stiffening some portions of the original shear banding path, and then a higher load was needed for a secondary path of shear banding yield. Our works demonstrated this hardening behavior could be more severe under higher indentation loads or temperature [42]. In fact, these factors would induce higher plasticity in the deforming region, which formed more complex shear banding morphology and prevented localized SBs formation. These works performed that softening or hardening phenomenon could happen in MGs, due to SBs spreading or suppression under different types of cyclic loads.
