Influence Of Deformation And Annealing Treatment On Mechanical Properties Of Cu And Cu-Zn Alloys

Cu and Cu alloys have been applied in various areas for their superior properties. It is also the key area in the research field in Yunnan province, which wins the reputation of the kingdom of non-ferrous metals. To improve the strength and ductility has been a long-lasting problem for scientists. Severe plastic deformation methods (SPD) are effective in improving strength. In this work, Cu、Cu-10wt.%Zn、 Cu-20wt.%Zn and Cu-30wt.%Zn, whose stacking fault energies are78mJ/m2,35mJ/m2,18mJ/m2, and l4mJ/m2respectively, were studied to investigate the influence of different processing methods upon mechanical properties, such as room-temperature rolling, liquid nitrogen rolling, liquid nitrogen forging, high pressure torsion (HPT) and Split Hopkinson Pressure Bar (SHPB) deformation. Parameters such as deformation temperature, strain rate, and stacking fault energy also play important roles during the deformation process, and have great impact on mechanical properties. In order to investigate the influence of deformation parameters upon mechanical properties, experiments are designed accordingly:Cu-10wt.%Zn, Cu-20wt.%Zn and Cu-30wt.%Zn were rolled both at room temperature and liquid nitrogen; Cu, Cu-10wt.%Zn, Cu-20wt.%Zn were conducted both room-temperature rolling and SHPB. The main methods to investigate the mechanical properties and micro structure were microhardness and tensile testing, XRD and TEM.According to the experiments, it is found that deformation temperature plays a vital role. Both the strength and ductility are higher when rolled in liquid nitrogen. The lower the deformation temperature is, the more easily to form deformation twins. It can also enhance the ability to store dislocation and improve the work hardening rate. Though improving strain rate plays an equal role as lowering deformation temperature in theory, it is not wise to improve the strain rate as high as possible. The higher strain rate is, the more easily local strain deformation happens, which can bring in shear bands. Moreover, stacking fault energy (SFE) is also an intrinsic parameter for materials. Lowering SFE can promote the formation of twins and stacking faults, which is beneficial for grain refinement. A lower SFE indicates a wider stacking fault, which makes it easy for dislocation storage and improves work hardening rate. However, combined with other parameters, there is an optimal SFE. For example, in liquid-nitrogen rolling systems, Cu-20wt.%Zn rolled in liquid nitrogen has the optimal mechanical properties rather than Cu-30wt.%Zn rolled in the same way which has the lowest SFE.Furthermore, annealing treatment has attracted great attention as a special strengthening mechanism. The samples used for annealing treatment were Cu-10wt.%Zn, Cu-20wt.%Zn, Cu-30wt.%Zn by rolling. On one aspect, annealing is good for the formation of annealing twins so that mechanical properties can be enhanced. On another aspect, microstrain and other defects can be eliminated during the annealing process. Noteworthy, annealing hardening can be achieved even when the dislocation density is lower. In the experiment, we found that the optimal annealing temperature is200℃for Cu-20wt.%Zn rolled in liquid nitrogen when both strength and ductility can be enhanced simultaneously, while for Cu-30wt.%Zn rolled at room temperature, the optimal annealing temperature is150℃when strength is enhanced with a slight decrease of ductility. It can be concluded that a proper design of experiment and process can be of great use to improve mechanical properties.