Research On The Interfacial Structure And Mechanical Properties Of Some Nanoscale Metallic Multilayers

Recently, the tendency of new functional materials and devices to being miniature, integrated and laminated makes the mechanical behavior of those materials in nano scale a key scientific issue for the development of the multilayers and devices. In the dissertation, the fcc/fcc system of Cu/Ni and Cu/Co multilayers and fcc/bcc system of Cu/Nb and Ag/Fe multilayers were prepared via electron beam evaporation deposition in high cacuum and the relationship between mechanical properties and microstructure, specially for the detailed structure of the interfaces, are discussed, and the plastic deformation mechanism in different length-scale regime is explored.The results show that for fcc/fcc Cu/Ni and Cu/Co metallic multilayers with fully coherent interfaces, the peak strength equals to coherent stress. It verifies in experiment that the peak strength that can be achieved in fcc/fcc superlattice is mainly determined by conherent stress. The peak strength of the fcc/bcc Cu/Nb metallic multilayers is 3.27GPa, which is consistent with the theoretical value predicted by atomic modeling based on dislocation transmission through interfaces. Meanwhile, varying loading strain rate hardness measurement confirms that the ultrahigh strength of nanoscale Cu/Nb multilayers is partly due to large strain strengthening.The strength (or hardness) of all the investigated multilayers increases with decreasing periodicity. For Cu/Ni, Cu/Co and Cu/Nb multilayers, the plastic deformation follows the confined layer slip model at large periodicity, while it changes to the mechanism of dislocation transmission through interfaces at small periodicity. For Ag/Fe multilayers, the variation in hardness with decreasing periodicity obeys the Hall-Petch-like relationship.There is modulus enhancement of all the investigated multilayers compared with the rule of mixing value. For Cu/Nb multilayers, solid solution of Cu in Nb interfaces caused by asymmetrical growth dynamics leads to 38% modulus enhancement. For fcc/fcc superlattice of Cu/Ni and Cu/Co multilayers, the modulus enhancement is related to compressive interface stress in semi-coherent interfaces or coherent stress in coherent interfaces. While for fcc/bcc Ag/Fe multilayers, the slight modulus enhancement at small periodicity is ascribed to compressive interface stress. The creep process of all the investigated multilayers is dominated by dislocation glide-climb mechanism. For fcc/bcc Cu/Nb and Ag/Fe multilayers, the incoherent interfaces can provide effective climb diffusion paths and thus the creep resistance decreases with decreasing periodicity. On the other hand, the formation of coherent interfaces is disadvantageous to the dislocation climb process and creep resistance of Cu/Ni and Cu/Co multilayers increases with decreasing periodicity. A dislocation model based on dislocation generation and annihilation at semi-coherent interfaces is presented to predict the steady-state deformation of Cu/Ni and Cu/Co multilayers with large periodicity and model predictons agree well with experimental observation.