# Understanding the Fracture Behaviors of Metallic Glasses--An Overview
## 2 Fracture Behaviors of MGs
### \"Super Ductile\" Fracture of MGs  
In the last decade, some Zr-based and Pd-based MGs with extremely large fracture toughness have been reported. The measured J-integral toughness of the Zr-based and Pd-based MGs can reach 150 MPa-m\({}^{1/2}\) and more than 220 MPa-m\({}^{1/2}\), respectively [6; 50]. The Zr-based MG can form plentiful multiple shear bands during fracture. The Pd-based MGs can sustain multiple shear bands formation and sliding, without shear band delamination. Figure 4 shows the appearance of another Pd\({}_{77.5}\)Cu\({}_{6}\)Si\({}_{16.5}\) MG sample with a size of size is 2.2 \(\times\) 3.8 \(\times\) 25 mm\({}^{3}\) during 3-point bending [51]. The sample can be severely deformed without fracture (Figure 4a). Plentiful shear offsets can be observed on the side surface of the sample (Figure 4b). The fractograph shows abundant strip features perpendicular to the crack extension direction (Figure 4d,f), which are formed through multiple shear banding. Such a \"Super ductile\" fracture behavior is inspiring, however, it is only observed in small size samples. The reasons for this fracture behavior and high toughness can be ascribed to:  
1. The Pd-based MGs have better deformability than other MG systems and can more easily form multiple shear bands in different loading conditions.
2. The size of the samples used in these studies is small (the thickness and ligament width are not more than 3 mm). Since a smaller sliding distance is needed to release the same stress in a smaller sample, it will be less easy for small samples to reach the critical condition of shear band delamination, i.e. small sample show higher shear band stability. Therefore, sustainable multiple shear banding can be achieved.
3. The value of J-integral toughness depends on the energy absorption during crack extension, rather than the crack-tip stress intensity [22; 52]. Due to the massive energy dissipation via the multiple shear band movements, a high J-integral toughness can be obtained.  
Besides this review, another recent report also indicates that with decreasing sample size, the fracture mode of the Pd\({}_{77.5}\)Cu\({}_{6}\)Si\({}_{16.5}\) MG transits from multiple shear banding-shear band  
Figure 3: Fractograph of a brittle Mg\({}_{54}\)Cu\({}_{28}\)Ag\({}^{\prime}\)Y\({}_{11}\) MG with many periodic nanoscale wavy corrugations. (**a**) The overall appearance of the conchoidal fractograph. (**b**) Enlarged image of wavy corrugations on the conchoidal fractograph. (**c**) Enlarged image of nanoscale wavy corrugations at the edge of the conchoidal fractograph.  
delamination to sustainable multiple shear banding [43]. The fracture toughness increases significantly when the transition occurs. Similar transition can also be observed in other MG systems. Figure 5 shows the fractograph of a pre-notched Zr41.2 Ti13.8 Cu12.5 Ni10 Be22.5 (Vitreloy-1) MG rods after 3-point bending. A long sustainable multiple shear banding can be achieved when the sample size decreases down to \(\varphi\)2 mm. A large region of smooth stripes (shear offsets) can be observed on the fractograph (Figure 5a-e). A long serration process can be observed on the loading curve (Figure 5f). In the previous mechanical studies of MGs, a correlation of better properties and smaller sample size is widely observed [42]. This law is also applicable in the fracture performance.