Study On Mechanical Properties Of Copper-aluminum Alloy Treated By Low Temperature Surface Mechanical Grinding

Traditional metal materials often have the characteristics of high strength and poor plasticity or superior plasticity and low strength.They usually come at the expense of a certain aspect,and this"trade-off"relationship severely limits the application of materials.Howecer,the grain size gradient structure materials can greatly improve the"trade-off"relationship that exists in traditional metal materials.In this study,the surface mechanical attrition treatment(SMAT)technology was used to treat Cu-Al alloy samples with different stacking fault energy(SFE)at cryogenic temperature(liquid nitrogen environment)to obtain gradient structure Cu-Al alloy material with a gradient change in grain size from the treatment surface layer to the core.Macroscopic researches such as micro-hardness testing,quasi-static tensile testing,and loading-unloading-reloading(LUR)testing have shown that the surface layer microhardness,yield strength(YS),ultimate tensile strength(UTS)and Hetero-Deformation Induced(HDI)stresses increase with increasing in SMAT-ed time/reducing SFE,and the uniform elongation(UE)increases with reducing in SMAT-ed time/SFE.In addition,it was found that the SMAT-2 min Cu-6.9%Al(SFE-5 m J/m~2)alloy sample showed the best strong plastic matching,and the YS increases by 73%and the UE decreases by only 3%compared with the annealed coarse-grained samples.The combination of high strength and superior ductility of the sample can be mainly attributed to the existence of a gradient structure(GS)layer,the generation of geometrically necessary dislocations(GND),and the introduction of twins.Severe plastic deformation(SPD)introduces a large number of dislocations and other defects to cause obvious grain refinement.Moreover,due to the non-uniformity,multi-scale and multi-stage of grain size gradient structure,the fracture shows a combination of ductile fracture and brittle fracture.The appearance of a large number of twins indicate that the dynamic recovery of dislocations in low-SFE materials is easily suppressed at cryogenic temperature,and the plastic deformation mode dominated by dislocations gradually transforms into twin deformation.During plastic deformation,dislocations interact with twin boundaries to absorb a large amount of plastic strain leads to the simultaneous improvement in the strength and ductility of the materials.The grains of the sub-surface layer in the sample first deformed and generated a large number of GND accumulation,which will help to increas HDI stress and HDI strain hardening resulting in synergistic strengthening to obtain improvement in the strength and ductility of materials simultaneously.The Digital Image Correlation(DIC)method was used to study the?_y and?_x strain distribution on the surface layers of Cu-4.5%Al-2 min sample,and the shear bands were observed on the SMAT-treated surface layers,which helps delay the necking of the material.