Study On Mechanical Properties Of Copper Alloy With Surface Mechanical Grinding

In recent years, much attention have been paid to the gradient structure materials due to their perfect mechanical properties. In this paper, Cu-Al and Cu-Zn alloys were processed by surface mechanical attrition treatment (SMAT) to obtain the nano-structured surface layer, and the relationship between plastic deformation mechanism, microstructure and mechanical properties have been deeply investigated.During the SMAT process, high speed flying spherical steel balls impact the sample surface at random direction. As a result, the surface layer underwent the highest strain and strain rate, with the depth increase, strain and strain rate accordingly decrease. The microstructure of the SMAT processed sample can be divided into nanostructured layer, refined structure layer, deformed coarse-grained layer and strain-free coarse-grained matrix along the depth. In this work, room temperature (RT) and low nitrogen temperature (LN) were selected to study the influence of temperature on SMAT process, it was shown that dislocation recovery and recrystallization has been largely suppressed during the LN condition. During the RT process, temperature increase in the sample surface due to the repeated impact of the steel balls. Cu-4.5wt.%A1 alloy with a thickness of 3mm were processed at LN condition for 5min,30min,60min, respectively, it has demonstrated that with the time increase, grain size were gradually decreased and dislocation density, twin density gradually increased. To investigate the effect of volume fraction of gradient-structured layer on mechanical properties, Cu-4.5wt.%A1 alloy were processed at LN condition for 5min with a thickness of 1mm,2mm,3mm,4mm, respectively. It has shown that the yield strength of the global sample was much higher than using the role of mixture (ROM), which prove the presence of synergetic strengthening by the gradient structured sample, it can be explained that the mismatch between the gradient layer (GS) and coarse-grained layer (CG) during the tensile test, which caused the geometrically necessary dislocations tangles in the GS/CG interface, as a result, extraordinary strain hardening was generated. Cu-Al, Cu-Zn alloys with different stacking fault energy (SFE) were processed at LN condition for 5min, it has shown that with the SFE decreased, yield strength and ultimate tensile strength increased, XRD diffraction analysis demonstrate that dislocation density and twin density increased with the SFE decreased. And the average grain size decreased. Moreover, the dominant plastic deformation mechanism will transform from dislocation slip to deformation twin as the twin stress decrease with the SFE decrease. Cu-4.5wt.%A1, Cu-20wt.%Zn exhibit the excellent uniform elongation, with suggest that exist a suitable SEF, the work hard rate was the highest in this SFE stage.Optical observation suggest that out layer transformed to flow structure due to he surface layer undergoes the highest strain and strain rate. EDS analysis showed that Fe element was detected in the surface layer after the SMAT process. AFM observation shown that the sample surface morphology was not flat after the SMAT process. SEM fracture surface shown that two different failure patterns were easily distinguished, which indicate there exist a brittle-ductile fracture manner.