Deformation and failure of shocked bulk Cu-Nb nanolaminates

The deformation and failure of bulk Cu-Nb nanocomposites with a nominal layer thickness of 135 nm was investigated under planar shock loading. It was observed that little substructural evolution was evident after shock compression to a peak stress of 7 GPa, while specimens were fully spalled after loading to 7 GPa under free surface conditions. In these fully spalled specimens, the characteristics of ductile failure that formed on the fracture surface were dependent upon the processing route of the nanocomposite. Specifically, process-induced grain-shape differences due to dissimilar rolling passes are linked with differences in the failure response. In addition, incipient failure was also observed. Numerous nanovoids, 20 nm or less in size, nucleated and aligned in a row in the middle of Cu layers. Due to the reflection of the shock wave at the Cu-Nb interfaces, incipient voids tend to nucleate within the Cu phase, which has a higher impedance and lower spall strength than Nb. This occurs rather than nucleation along the Cu-Nb interfaces or in the Nb phase. This finding contradicts the general thinking of failure starting from interfaces, and indicates that the Cu-Nb interfaces are stable under dynamic loading. It is postulated that numerous voids nucleate in the Cu layers under shock loading, then lead to failure through their growth and coalescence.