Length scale dependent ductility and fracture behavior of Cu/Nb nanostructured metallic multilayers

By using uniaxial tensile test combining the in situ electrical resistance change method, the influence of modulation period with a wide range spanning from 10 to 250 nm on the ductility and fracture toughness of Cu/Nb nanostructured metallic multilayers on polyimide substrate was measured. The microstructural analysis revealed that the modulation structure of Cu/Nb metallic multilayers was clear and no intermixing between Cu and Nb was been found by using line scanning analysis. The experimental results indicated that both ductility and fracture toughness of the multilayer film exhibited a nonmonotonic change with decreasing modulation period, and reached maximum values at a critical modulation period of about 50 nm. This was attributed to the competing effect between the size of the microcracks initiated in the Nb layer and the role of the Cu layer in blocking crack propagation. This competing effect was qualitatively assessed on basis of fracture mechanics.