Research On Mechanical Properties Of Low Temperature Surface Mechanical Grinding Of Copper-germanium Alloy

Materials with gradient structure can effectively solve the relationship between strength and plasticity to a certain extent,and keep the plasticity of materials better when the strength have been improved.In addition,due to the uniqueness of the gradient structure,it also has good surface wear resistance,fatigue resistance and so on.So it has received a lot of attention in recent years.However,in the preparation of gradient structure materials,research on the mechanical properties,microstructure and mechanical behavior of gradient structure materials is not systematic enough,and there are still many problems.In this paper,surface mechanical attrition treatment(SMAT)method was used to prepare gradient structure(GS)specimens in Cu-Ge alloys with different stacking faults(SFEs).Three different Cu-Ge alloy samples(Cu-0.1 at.%Ge?Cu-5.7 at.%Ge and Cu-9.0at.%Ge)with gradient structure were obtained by SMAT at 77K.The experimental analysis shows that the existence of the gradient structure greatly improves the strength and preserves the plasticity to a considerable extent.Transmission electron microscopy(TEM)analysis showed that the surface of the Cu-0.1at.%Ge alloy with high stacking fault energy54 mJ/m~2)had lots of dislocation tangles(DTs)and dislocation cells(DCs),and no twins have been observed after surface mechanical attrition treatment.A large number of dense twins structures were observed in Cu-5.7at.%Ge alloy with low stacking fault energy(15mJ/m~2).While a small number of twins and a large number of stacking faults were found in Cu-9.0 at.%Ge alloy with very low dislocation energy(8 mJ/m~2).The tensile test for the SMAT processed samples shows that the Cu-5.7 at.%Ge alloy shows the best strength-plasticity matching,the yield strength of which is more than three times that of the annealed sample and it is the most obvious improvement in all alloys,meanwhile,the plasticity is well preserved.The yield strength of the Cu-9.0 at.%Ge alloy is only about twice that of the annealed specimen,but it maintains more than 90%of the plasticity.While the yield strength of Cu-0.1 at.%Ge alloy samples increased obviously,the degree of plasticity decreased relatively higher than others.The analysis shows that the rise of yield strength is mainly contributed by the GS layer.Bauschinger's experiment shows that the back stress of the samples after SMAT is higher than annealed,and the back stress in the Cu-5.7 at.%Ge alloy is the highest.The back stress is mainly generated by geometrically necessary dislocations(GNDs),EBSD data shows that the surface GNDs density of Cu-5.7 at.%Ge alloy is the highest,which coincides with Bauschinger's result.The uniform elongation is influenced by the dynamic recovery in the process of deformation and the structure such as twins and stacking faults.Stress relaxation experiments shows,The change of mobile dislocation densities in Cu-9.0 at.%Ge alloy sample after SMAT is basically the same as that in annealed sample,it means the rate of dislocation annihilation is relatively consistent with that of annealed sample.Therefor these samples retain the most of plasticity.