Research Of Preparation And Constitutive Behavior Of Bimodal Nanocrystalline Cu-Ag Materials

Nanocrystalline(NC) materials have attracted great interest in recent years owing to their excellent mechanical and special physical properties. Generally speaking, NC materials possess high strength and low ductility compared with their coarse-grained conventional counterparts.The deficiency of low ductility and the associated loss of toughness in some materials stop their engineering applications. Therefore, various strategies have been proposed to improve the ductility of NC materials for achieving a better balance of strength and ductility. One efficient approach is to develop a bimodal grain size distribution in which NC grains provide high strength, whereas coarse grains can enhance ductility. Although there have been numerous experimental studies that have provided insight into the constitutive behavior of BNC materials, it scarcely builds explicit and universally theoretical models to take quantitative analysis.In this paper, the constitutive behavior of bimodal nanocrystalline(BNC) Cu-Ag materials was studied. BNC Cu-Ag alloys were prepared by using high temperature and high pressure sintering method. SEM and XRD analysis were used to characterize the microstructure of the sintered samples. The tensile deformation behavior was studied by uniaxial tensile test. A new theoretical model is proposed to describe the mechanical properties of BNC materials. The effect of grain size distribution on the constitutive behavior of BNC materials was discussed. During the plastic deformation, effects of nano-cracks and dislocation emission from crack tips on the constitutive behavior of BNC materials were also analyzed. Failure behavior of BNC materials has been studied according to the combination of Taylor strength theory and the Johnson-Cook model.Failure strain has been analyzed quantitatively. The present study reports on the effects of strain rate and temperature on the mechanical properties of BNC materials.The results show that:(1)Bimodal-structured alloy exhibited high tensile strengths(460MPa) and a large plastic strain to failure(26%) simultaneously, as well as good strain hardening behavior.(2) The strength of the material increases with the decreasing of the nanocrystalline(NC) grain size. When the volume fraction of coarse grains remains unchanged, the strength of BNC materials is not highly sensitive to the change in coarse grain size. The strength is dependent on the volume fraction of coarse grains, and it decreases with increasing the coarse grain(CG) content.(3) The ductility decreaseswith decreasing the NC grain size, and it increases with increasing the CG content, while the strength decreases with increasing the CG content.(4) The nano-cracks in BNC materials make a positive effect on the strain hardening, but no tangible influence on yield stress.(5) The theoretical predictions are compared with the experimental data, and the results are in good agreement with the experimental results. The theoretical model can be capable of describing the constitutive behavior and fracture behavior of BNC materials.