# Cracking and Toughening Mechanisms in Nanoscale Metallic Multilayer Films: A Brief Review
## 4 Other Fracture Properties
### Plasticity Instability
#### 4.5.2 Shear Banding Behavior  
Shear banding behaviors were studied by many researchers and studies have been carried out to reveal mechanism of inhomogeneous flow in nanocrystalline, amorphous solids and nano-multilayers. The mechanisms of shear bands in NMMFs are attributed to the diminishing strain hardening ability and low strain rate sensitivity as the number of dislocations is limited. The hindered dislocation motion inside small grains is expected to make interfaces more active to accommodate plastic deformation, which leads to interface-related deformation behaviors, such as grain boundary sliding, grain rotation, etc. Experimental results of Au/Cu NMMFs [143] demonstrate that inhomogeneous shear banding becomes prevalent (Figure 24) when the \(h\) of the multilayered composite approach the nanometer range. Additionally, the width of shear bands decreases with the decrease of \(h\) while the maximum interface kink angles and pileup height increases with the decrease of \(h\)[144; 145; 143]. Along with further microscopic observation, two different types of interface morphologies are observed with the variation of \(h\), which is corresponding to two shear-deformation behaviors [146]. On one hand, cooperative layer buckling induced shear banding in multilayers with \(h\) of 25 nm (Figure 25a). On the other hand, direct localized shearing go across a layer interface along a shear plane of the multilayers with \(h\) of 250 nm (Figure 25b). On a nanometer scale, buckling-assisted GB sliding leads to plastic instability of the multilayer, while on a submicron scale or more, dislocation plasticity dominates deformation instability of the multilayer (Figure 26[88]). [PERSON] et al. discovered that the plastic instabilities of Au/Ti multilayers under indentation changed from shear bands to layer buckling induced-delamination when the layer thickness was increased from 25 nm to 250 nm [16].  
Figure 23: Pre-loading (**a**) and post-loading (**b**) scanning electron micrographs of micropillars of the Mg/Ti multilayer with \(h\) = 100 nm. The post-loading image indicates that under compression, the Mg layers (the layers of bright contrast) have been squeezed or extruded out between the mechanically rigid Ti layers [32].  
The plastic deformation transition from plasticity localization (soft phase dominates) to shear banding deformation (co-deformation of both layers) in NMMFs has also been detected in micropillar compression tests. [PERSON] [13] and [PERSON] [101] studied the deformation mode of Cu/Nb nanopillar, and found that, with \(h\) changing from 70 nm to 40 nm to 5 nm, the deformation mode changed from uniform extrusion of the soft layer gradually to a shear banding deformation and finally (\(h\) = 5 nm) to the highly localized shear fracture. The internal characteristics size related deformation process was also detected in Al/Pd NMMFs under pillar compression [147]. [PERSON] and coworkers [99; 61] systematically investigated the effect of internal feature sizes (individual layer thickness \(h\)) and extrinsic  
Figure 26: Schematic illustration of two different deformation mechanisms: (**a**) Au/Cu multilayer with \(h\) = 25 nm, which is corresponding to the elastic buckling-assisted GB sliding; and (**b**) Au/Cu multilayer with \(h\) = 250 nm, which is corresponding to the dislocation plasticity-dominated shearing [88].  
Figure 24: FIB cross-sectional views of the indents in the Au/Cu multilayers with \(h\) of (**a**) 250 nm, (**b**) 100 nm, (**c**) 50 nm, and (**d**) 25 nm [143].  
feature sizes (pillar diameter \(\varphi\)) on the deformation modes of Cu/Zr NMMFs and found that: with the decrease of the \(h\), the deformation mode changed from the dislocation dominated symmetrical slip to shear localization and shear fracture; with the decrease of \(\varphi\), the deformation mode changed from global shear to local shear. Figure 27 shows the deformation mode map (shear localization, shear + extrusion and extrusion) for NMMF.