Effect Of Stacking-fault Energy On The Mechanical Properties And Microstructure Of Ultrafine-grain Cu And Cu-Zn Alloys

Bulk ultra-fine grain (UFG, typically d< 500 nm) materials and nanocrystalline (NC, d< 100 nm) material have special physical and chemical properties since the UFG/NC materials contain a high density of interfaces. UFG/NC materials which usually have high strength but disappointingly low ductility have been fabricated by severe plastic deformation (SPD) techniques. So, how to improve the ductility of these materials is one of the hot focuses in material field. Work hardening results from accumulation of crystal defects and relates with evolution of microstructure in material. The main goal of work hardening theory is to explain the stress-strain curve and learn the evolution of microstructure over the process of strain. Therefore, it is necessary to deepen the research on working hardening.In this research, Zn was added into the pure Cu in order to decrease stacking fault energy of pure Cu. The Stacking-fault energy of the pure Cu is about 80 mJ/m2.The Stacking-fault energy (SFE) of Cu-Zn alloys decreases with increasing Zn content (Cu,10, 20and 30 wt% Zn with SFE～80,36,18 and 14mJ/m2 respectively). Three kinds of different componential alloy gained by melting are single phase and substitution solid-solution alloys. The sample was processed by cold-rolling repeated under liquid nitrogen temperature. Grain has been refined to nanometer or micrometer scale. Mechanical properties of these samples were examined by tensile test. The metallographic microstructure of samples was observed with optical microscope. Simultaneously, the microstrain, average grain sizes, dislocation and twin density were examined by using X-ray diffraction (XRD), so as to investigate the influence of stacking fault energy (SFE) on the mechanical properties, microstructure and work hardening behavior. The work hardening rate curve of these samples was computed from the engineering stress-strain curves after compression test. The work hardening rate curve of Cu-30 wt% Zn (low SFE FCC) is composed of four distinct stages during simple compression testing. At the same time, stages of working hardening rate and corresponding evolution of microstructure were investigated.Through tensile test of cold-rolling samples, the yield strength of Cu, Cu-10 wt% Zn, Cu-20 wt% Zn is 410MPa,602MPa and 674MPa, respectively and the elongation to failure is 2.11%,2.63% and 3.48%, respectively. It is obvious that the strength and ductility of samples simultaneously increase with decreasing SFE. XRD analysis results show that grain size of these samples is 112nm,51nm and 46nm, respectively, microstrain is 0.09%,0.12% and 0.19%, respectively and dislocation density is 0.11x1015m-2,0.32x1015m-2 and 0.55×1015m-2, respectively. And twin density is 0.05%,0.25% and 0.74%, respectively. XRD analysis results also show that a reduction in the SFE leads to both a decrease in the mean size and an increase in the dislocation and twin densities. It is evident that the strain hardening rate of the samples has increase with decreasing SFE. The Cu-20 wt% Zn alloy has both higher strength and higher ductility than the copper. Cu-20 wt% Zn alloy has the highest strain hardening rate that account for the best uniform elongation and ductility. The higher strength of the UFG bronze is owed to its smaller grain size, higher twin density, higher dislocation density and solution hardening. This work indicates that simultaneous improvements in the ductility and strength of UFG Cu-Zn alloys can be obtained by decreasing SFE.The curve of work hardening rate in compression test indicates that the work hardening rate increases with decreasing SFE. The work hardening rate curve of Cu-30wt%Zn (low SFE fcc) comprises four distinct stages during simple compression testing.