# Cracking and Toughening Mechanisms in Nanoscale Metallic Multilayer Films: A Brief Review
## 4 Other Fracture Properties
### Radiation Induced Embrittlement
The increase of stress \(\Delta\tau\) due to He bubbles can be acquired by:  
\[\Delta\tau=\tau_{i}(1-\frac{L}{\sqrt{2}h}) \tag{29}\]  
where \(L\) is the average obstacle spacing and \(\tau_{i}\) is the average shear strength of obstacles. As the layer thickness \(h\) is small enough to ensure the interface crossing mechanism dominates in as-deposited  
Figure 19: Schematic illustration of glide dislocation interactions with nanometer-scale bubbles with the spacing of \(l\). Numbers 1–3 show the steps of dislocation movement across the bubbles. \(\phi_{c}\) is the semi-critical angle at which the dislocation breaks away from the pinning points [96]. multilayers (Figure 20b), hardness has no increase after implantation, corresponding to \(h<1\) in Equation (29); the model of CLS dominates in multilayers when the layer thickness \(h\) is large, bubbles inside layers become obstacles which can enhance the hardness, leading to irradiation-induced hardening, which can be described by Equation (29); the dislocation pile-up mechanism dominates in as-deposited multilayers as the layer thickness \(h\) is further increased, thus \(\Delta\tau=\tau_{i}\)[96]. On the other hand, the introduction of abundant defects in materials at nanometer-length scale is one of the available strategies to achieve significantly enhancement of radiation tolerance [136]. Thus, NMMFs with large density of heterophase interfaces are evidently more resistant to radiation damage and have a higher He solubility [128]. The special role of heterogeneous interface in irradiation is often considered as follows. Firstly, NMMF nanostructured composites exhibit order-of-magnitude increases in He concentration. Helium implantation studies have shown that Cu/Nb nanocomposites have a higher He solubility than either Cu or Nb [137; 138; 139]. Secondly, the density of bubbles has been dramatically reduced in NMMFs. Although the He distribution is symmetrical along the interface, Figure 21 shows that obviously different bubble morphologies developed in the adjoining layers of Cu/Nb NMMF [128]. No bubbles were observed progressing into the Nb layer and the majority of bubbles were confined to the Cu side. Thirdly, the interface can effectively restrict the size of bubbles. Through an in-situ study, [PERSON] et al. reported that 50 nm Ag/Ni NMMF has the smallest average defect size among Ag, Ni and their average value [136]. Fourthly, Helium capture ability mainly depends on the density of mismatch dislocation at the interface. Figure 22 shows the excellent agreement between misfit dislocation intersection densities at interfaces and the critical He concentrations per unit area of interface for a number of fcc/bcc NMMFs [140]. It must be pointed out that, except for He irradiation, for embrittlement induced by different radiations, the heterophase interfaces can act as effective sinks for defects from both strong obstacles such as interstitials, interstitial loops and stacking-fault tetrahedra, and relatively weak obstacles such as He bubbles. Therefore, NMMFs possessing excellent mechanical properties and having tunable interface structure and length scale, have stimulated increasingly interest in the research of radiation environment. Figure 20: Schematic illustration of the length scale dependent deformation mechanism in multilayers after He irradiation. (**a**) When \(h\) is much larger than \(l\), the deformation is via CLS with dislocations pinned by nanometer-scale bubbles. (**b**) When \(h\) is much smaller than \(l\), the yield strength is determined by the crossing of single dislocation across interfaces, which contains a number of nanometer-scale He bubbles in the interface plane [96]. Interfaces in NMMFs are good traps for implanted He, at the same time, they are also susceptible to He-induced embrittlement [140]. Thus, understanding the toughening mechanisms transition of NMMFs after irradiation is of great importance.