Strain Rate Sensitivity of Cu/Ni and Cu/Nb Nanoscale Multilayers

Different from monolayers of same components, nanoscale multilayers have different mechanical properties owing to their relatively high interfacial density, such as extremely high yield strength, high ductility and outstanding wear resistance. Furthermore, their precise modulation period and unique interfacial structures contribute to investigate the plastic deformation mechanism of metal materials. As the plastic deformation behaviors of nanoscale multilayers were reflected in a thermal activation process, strain rate sensitivity index m can be used to characterize the tendency of material strengthening as the strain rate increases. To investigate the impacts of modulation period and interfacial structures upon strain rate sensitivity of nanoscale multilayers, Cu/Ni nanoscale multilayers with different periods (A=4 nm, 12 nm, 20 nm) were prepared on Si substrate with e-beam evaporation technologies, while Cu/Nb nanoscale multilayers with different periods (A=5 nm, 10 nm, 20 nm) were prepared on Si substrate with magnetron sputtering technologies. Under vacuum conditions, the Cu/Ni nanoscale multilayers of different periods were annealed at 200 and 400 degrees C for 4 h respectively, and the Cu/Nb nanoscale multilayers of different periods were annealed at 200, 400 t and 600 degrees C t for 4 h respectively. Microstructures of Cu/Ni and Cu/Nb nanoscale multilayers were characterized with XRD and TEM. Besides, the hardness of nanoscale multilayers was measured by nano -indentation techniques under different loading strain rates (including 0.005, 0.01, 0.05 and 0.2 s-(1)). The results suggested that strain rate sensitivity was impacted by interfacial structures and grain size. Both increased density of incoherent interfaces and grain size could result in weaker strain rate sensitivity. As the period increases, the density of incoherent interfaces and the grain size of Cu/Ni nanoscale multilayers increased, leading to a decline in the strain rate sensitivity. While for Cu/Nb nano scale multilayers, the density of incoherent interfaces decreased and their grain size was enlarged with longer period, the m value kept unchanged as a result. As the annealing temperature increasing, the strain rate sensitivity of Cu/Ni and Cu/Nb nanoscale multilayers generally tended to decline, which should be ascribed to increased density of incoherent interfaces and grain size in the course of annealing.