# Understanding the Fracture Behaviors of Metallic Glasses--An Overview
## 2 Fracture Behaviors of MGs
### Fatigue Performance of MGs  
Fatigue rupture is one of the main failure modes in engineering material [71]. The fatigue study of MGs can be traced back to the mid-1970 [72; 73]. Due to the size limit, the tests of the early discovered MGs were carried out with ribbon samples. The Pd-Si MG ribbons show a fatigue crack growth threshold as high as \(\Delta K_{\text{th}}=9\) MPa-m\({}^{1/2}\)[74], which is even higher than that of high-strength steel [75]. With the development of bulk metallic glasses, the fatigue properties can be measured by bulk samples under tension, compression, and bending tests. Although the fatigue ratio (the ratio of the fatigue endurance limit to the yielding stress) of most BMGs is near 0.3 [76], the fatigue endurance limit (the maximum stress amplitude to which the material is subjected for \(10^{7}\) cycles without failure) is still high, due to their high yielding strength. Some Co-based and Fe-based MGs can reach a fatigue limit as high as 2 GPa [77]. Table 2 shows the data of the fatigue endurance limit and the fatigue ratio of some reported MGs with a size larger than 2 mm and other commonly used metal materials including steel and Titanium alloy.  
It is noticed that the measured fatigue properties of MGs are sensitive to the test method, test environment, sample quality, sample size, surface condition, and etc. Different results have been reported by different studies [81; 89; 90]. For example, the pour casting fabricated Zr\({}_{55}\)Cu\({}_{30}\)Ni\({}_{5}\)Al\({}_{10}\) MG shows better fatigue properties, as the size and distribution of flaws could be tuned by the fabrication method. Thermal relaxed MGs could show higher fatigue limit, due to the relatively low free volume content [91; 92; 93]. The Poisson's ratio, sample size [94] and the content of some alloying elements [87] also have significant influences on the fatigue behavior.  
Figure 7 shows a typical fractograph of a brittle Mg-Cu-Ag-Y MG after tension fatigue fracture. The fractograph of the fatigue cracked MGs can be divided into the crack initiation site, the crack growth region and the fast fracture region [86; 95]. There can be more than one crack initiation site on one sample. The crack growth region is relative smooth, with many regular fatigue striations perpendicular to the crack growth direction (Figure 7c). The spacing between striations is at micron order or submicron order. The fast fracture region is rough (Figure 7c). Vein patterns and melting features can be observed on the fractograph (Figure 7d), which result from the local heating and viscosity drop during the rapid fracture.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Consituent** & \begin{tabular}{c} **Fatigue Endurance** \\ **Limit \(\sigma_{\text{L}}\) (MPa)** \\ \end{tabular} & \begin{tabular}{c} **Sample** \\ **Size/mm** \\ \end{tabular} & \begin{tabular}{c} **Fatigue Ratio** \\ \end{tabular} & \
\begin{tabular}{c} **Test Mode** \\ \end{tabular} &
\begin{tabular}{c} **Ref.** \\ \end{tabular} \\ \hline Pd\({}_{49}\)Cu\({}_{30}\)Ni\({}_{10}\)P\({}_{20}\) & 340 & \(\varphi 5\times 10\) & 0.2 & bending & [67] \\ Fe\({}_{48}\)Cr\({}_{15}\)Mo\({}_{14}\)Er\({}_{2}\)C\({}_{15}\)B\({}_{6