Length Scale Effects On Microstructures And Mechanical Properties Of Metallic Multilayers

The gain growth in metallic multilayers is confined between layers because of the periodical modulated structure and is affected by heterointerface. Uaually, metallic multilayers exhibit special mechanical properties compared with the tranditional bulk and alloys. The mechanical properties of multilayers are found relating with the characteristic cale of structures in many studies. The microstructures in-plane and interfacial structures both are varied with the length scale of multilayers, leading to strong length scale effects of properties. Therefore, it is important to study the length scale effect and develop the therotical model between microstructures and mechanical properties. It will provide an effective method for controlling the properties and achieving the optimum design.In present work, a series of nanostructured Cu/Ta, Ag/Cu and Ag/Nb multilayers with different modulation period were prepared by dc magnetron sputtering on Si substrates. The microstructures of the multilayers were characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope. The mechanical properties including hardness, elastic modulus and creep behavior of the multilayers were tested by nanoindentation. The length scale effcts on microstructures and mechanical properties of the multilayers were investigated, and the corresponding mechanisms were discussed. The main conclusions are as follows:1. The Cu/Ta multilayers exihibit obvious length scale effect on hardness at different individual layer thickness (h). When h is in the range of 10～100 nm, the hardness increases with decreasing h and reaches a maximum of 6.13 GPa at 10 nm. The hardness values follow the Hall-Petch relationship, indicating the strengthening of dislocations pile-up. However, when h decreases from 10 nm to 5 nm, a sharp softening can be observed which is resulted from the absence of (3-Ta in Ta layers. After that, the hardness becomes stable with further decreasing h and the strengthening comes from the interface barrier to dislocation transmission. The elastic modulus first increases and reaches a maximum of 131 GPa at 10 nm, then decreases gradually with decreasing h. The softening in Cu/Ta multilayers due to the pahse transformation of Ta layers has never been reported in metallic multilayers before.2. The good combination of high strength and hige conductivity can be obtained in Ag/Cu multilayers. The hardness of Ag/Cu multilayers shows non-monotonic with length scale. The hardness increases with the decreasing h in the range of 5～20 nm and reach a maximum of 3.86 GPa at 5 nm. A softening at h= 3 nm is observed which can be ascribed to the formation of superlattice. Furthermore, an abnormal strengthening occurs at h= 50 nm which can be ascribed to the tortuous interface structures and the misorientation of grains. The elastic modulus of Ag/Cu multilayers decreases with the decreasing h, without showing the strengthening effect of elastic modulus. The decrease of elastic modulus is related with the expansion of interplanar spacing due to the mismatch at interfaces.The electrical resistivity of Ag/Cu multilayers almost keeps unchanged when h≥ 10 and then increases rapidly with decreasing h. The electron scattering at interfaces and grain boundaries accounts for the increase of resistivity. When h= 10 nm, the existence of texture is found to make a greatly reduction in the amount of high angle grain boundaries and hence reduce the electron scattering at the grain boundaries, which contributes to the conductivity. A facile model is developed to evaluate the comprehensive property of strength and conductivity in Ag/Cu multilayers, which is confirmed to be available. The proposal of this model provides a new idea to solve the similar problems.3. The microstructures and mechanical properties of Ag/Nb multilayers are very sensitive to the length scale. With the decreasing h, the cryatal strtuctures are transformed in the order from polycrystalline to texture and then to superlattice. Furthermore, amorphous layers form at the interface when h= 20 and 50 nm. The hardness of Ag/Nb multilayers increases with the decreasing h and shows a sharply rising trend. The hardness is 3.53 GPa when h= 50 nm. While h decreases to 1 nm, the hardness increases to 6.79 GPa and enhances 92.4% compared to that of h= 50 nm. The coherency stess strengthening contributes to the high hardness at small scales.The elastic modulus of Ag/Nb multilayers increases with the decreasing h, showing a strengthening effect on elastic modulus due to the compression of interplanar spacing. However, there is an abnormal decrease in elastic modulus resulted from the presence of amorphous layer.4. The creep behaviors of Cu/Ta, Ag/Cu and Ag/Nb multiyaers all exhibit obvious length scale effects at room temperature. The creep stress exponents of them increase with the decreasing h. It is inferred that the creep processes in Cu/Ta, Ag/Cu and Ag/Nb multiyaers are dominated by the dislocation climb mechanism. The sites of dislocation climb shift from the grain boundaries to the heterogeneous interface with the decreasing h and the coherency relationship contributes to the high stress exponent in the several nanometers scale. In addition, the presence of amorphous layer at interface in Ag/Nb multilayers can impede the process of creep deformation.