# Cracking and Toughening Mechanisms in Nanoscale Metallic Multilayer Films: A Brief Review
## 4 Other Fracture Properties
### Radiation Induced Embrittlement  
With nuclear accidents occurring frequently in recent years, the safety and reliability of nuclear reactor operation gathered much attention in the materials community [124]. Helium (He) implanted in nuclear reactor components, both fuels and structural elements, causes changes in their chemical compositions and degrades their mechanical and thermal properties [125; 126; 127]. Thus, the urgent mechanical requirements for structure materials under irradiation environment are needed such as: thermal creep resistance, increased strength, neutron radiation damage resistance [124] and superior corrosion. The mechanical performance of structure materials is still a key factor restricting the service life of the reactor, operation stability and exchange efficiency in the advanced nuclear fusion/fission reactors. To extend the operating limits and lifetimes of nuclear reactors, the ability to prevent degrading material properties by the growth of bubbles and the subsequent formation of blisters is required [128; 129]. Electron and light ion irradiation of crystalline solids leads to the creation of large numbers of vacancies and interstitials that agglomerate to form nanometer-sized bubbles, dislocation loops, stacking-fault tetrahedra and voids [128; 130]. After irradiation, the defects (stacking-fault tetrahedra, cavities or dislocation loops) increase the stress requirement for yield and subsequent plastic flow, leading to irradiation hardening [96]. In conventional metals, radiation hardening is caused by the  
Figure 18: Length scale dependent deformation behavior on the evaluated adhesion energy in NMMFs. Two regimes divided by \(\lambda_{crit}\) represent the \(\lambda\)-independent and \(\lambda\)-dependent ranges, respectively [52]. interaction of dislocations with two types of radiation induced defects: strong obstacles such as stacking-fault tetrahedra, interstitial loops and interstitials, and relatively weak obstacles such as He bubbles [131]. A barrier model is developed to estimate strengthening mechanism due to strong barriers. The increase in yield strength \(\Delta\sigma\) after radiation is expressed by:  
\[\Delta\sigma=M\alpha^{\prime}\mu b\sqrt{N_{d}d_{i}} \tag{25}\]  
where \(M\) is the Taylor factor, \(\alpha^{\prime}\) is a parameter that depends on the average barrier strength, \(N_{d}\) is the defects density across the radiation damage region and \(d_{i}\) is the diameter of interstitial loops. He bubble is treated as a weak obstacle when the bubble size is small enough. The FKH model is applied to estimate the He bubbles induced enhancement of yield strength, \(\Delta\sigma\), by [132]:  
\[\Delta\sigma=\frac{1}{8}M\mu b{d_{b}}{N_{d}}^{2/3} \tag{26}\]  
Orowan model [133; 134; 135] produces another describing of the hardening effect due to the dislocation pinning at point obstacles. For strong obstacles with spacing \(l\), it is assumed that the glide dislocation bows to a semicircle with a radius of \(l/2\). However, weak obstacles are overcome at the applied shear stresses less than which is needed to bend the dislocation to a semicircle, thus the Friedel equation is adopt [133; 134]:  
\[\tau=\frac{\mu b}{2\pi L}\ln(\frac{L}{r_{d}\sqrt{\cos\varphi_{c}}})(\cos \varphi_{c})^{3/2} \tag{27}\]  
\[L=\frac{1}{\sqrt{2N_{d}r_{d}}} \tag{28}\]  
where \(r_{d}\) is the radius of the defect cluster and \(\varphi_{c}\) is half of a critical bow-out angle between lines of a dislocation cutting an obstacle (Figure 19). The radiation-induced hardening in NMMFs was found to decrease with decreasing individual layer thickness. No changes in hardness was measured in multilayers with \(h\) less than 10 nm in Ag/V after implantation [96]. In the case of CLS, a glide dislocation loop confined in a layer with thickness \(h\) also experiences pinning from bubbles in the layers (Figure 20a).