The Mechanical Behavior Of Heterogeneous Cu And Cu-Zn Alloy

Here two kinds of metals were chosen for modals to investigate the strain hardening mechanism of heterogeneous Nanostructure（NS）materials.Firstly,after severe plasticity deformation（SPD）,grain size of coarse grain（CG）had been reduced to nanoscale.Based on the grain refinement and subsequent partial recrystallized,heterogeneous NS were generated.Optimizing the heat treatment process parameters,a better match of strength and ductility was achieved.Cyclic loading-unloading-reloading（LUR）tests were performed to understand why the heterogeneous structure can cause strengthening and ductility retention,and how Bauschinger effect and back stress hardening lead to the promotion of strain hardening.On microstructure change before and after tensile deformation,the distribution of grain size was bringing out.（1）Cu: After applying 2 passes ECAP of CG Cu samples to generate shear deformation refining grains,it was found that the yield stress was raised from 46 MPa to about 400 MPa.When introducing partial recrystallized grain,the heterogeneous structure maintained the tensile strength of about 330 MPa and regained uniform elongation of 5%.LUR tests were performed to calculate the back stress and back stress hardening rate.For comparing the hardening rate of flow stress with back stress hardening rate,it is shown that back stress take up more than half of the flow stress and the back stress increase right after yielding of the heterogeneous structure sample.The conclusion is that back stress strain hardening has significant contribution to the global strain hardening.Cyclic stress relaxation tests had been carrying out to analyze the mobile dislocation density and activation volume of CG and heterogeneous NS,which showed that the mobile dislocation density in heterogeneous structures was decreasing along with relaxation strain.Meanwhile,strain hardening of forest dislocation in heterogeneous structure decrease along with the tensile strain.The DIC full strain measurement proved that heterogeneous structure could suppress the strain concentration to maintain the capability of deformation.（2）Cu-30%Zn: The deformation mechanism of Cu-30%Zn alloy is different from the pure Cu as the low stacking fault energy of 14mJ/m2.After large strain of cold rolling,yield stress came up to 750 MPa,comparing with only about 100 MPa of CG sample.A series of heat treatment process of partial recrystallized was conducted to coming up with a good match of strength and ductility,and it is found that after 93% rolling reduction,annealing temperature of 300oC is the best choice.The LUR test also show that back stress play an important role in strain hardening.EBSD revealed that twinning is the main deformation mechanism but not dislocation slip.Moreover,tensile test under 77 K brings out a fact that both the heterogeneous structure and CG achieved a synergy of strength and ductility,which is been reported that the mechanism of promotion is the inhibition of dislocation annihilation under 77 K.In addition,the high strain rate tensile test showed that no matter heterogeneous structure or CG,the yield stress gets improved while the uniform elongation decreases with tensile strain rate increases from quasi-static to 10_-1s_-1,100s_-1.