Microstructure and mechanical properties of nanoscale Cu/Ni multilayers

The microstructure, hardness and creep behavior of Cu/Ni multilayers were investigated by X-ray diffraction, transmission electron microscopy and nanoindentation. The results show that the multilayers with a good modulation structure formed coherent superlattice at low periodicity. The strength increases as the periodicity falls from 17.5 nm to 7.0 nm, which can be well described by a CLS model. After that, the strength reaches a peak value of 2.53 GPa at the smallest periodicity of 3.5 nm. This value approximately equals to the theoretical peak strength predicted by a coherent stresses model. The stress exponent extracted from nanoindentation creep test varies from 2.20 to 6.87, indicating a dislocation glide-climb creep mechanism. The variation in the stress exponent with decreasing periodicity is discussed and ascribed to a change of the dominating rate from climb-controlled to eventually glide-controlled events.