Plastic Deformation Mechanism And Mechanical Behavior Of Gradient Copper And Copper Alloys

Gradient structured materials can effectively solve the strength-ductility trade-off relation in traditional material to a certain extent,which not only shows a superior high strength and high plasticity combination,but also exhibits good surface wear resistance and fatigue resistance,attracting more and more attention.At present,there still exist many challenges and opportunities in the controllable preparation of the gradient structure,the microstructure and properties of the gradient structured materials,the plastic deformation mechanism and the mechanical behavior under deformation.In this paper,the surface mechanical attrition treatment(SMAT)was used on pure copper and Cu-Al alloys with different stacking fault energy(SFE)to obtain the gradient structure,the gradient structure effectively improved the strength of the materials,while its plasticity can be better maintained.The SMAT treatment was carried out on pure copper at different time and temperatures.Increasing SMAT time and lowering temperature will lead to a thicker gradient layer generated.As the temperature reduced,the dynamic recovery is inhibited and the grain refinement is more obvious.The deformation mechanism transformed from dislocation motion into twinning,a large number of twins appears in the low temperature treatment sample,in which the yield strength is much higher.For the gradient structured pure copper obtained by SMAT,when the volume fraction of the gradient layer reaches a certain range,it exhibits a significant extra work hardening,resulting in an obvious strain hardening rate up-turn.This phenomenon can be ascribed to the dislocation movement during deformation.The relative mobile dislocation density first decreases and then tends to increase during deformation.Cu-Al alloys(Cu-2.2wt.%Al,Cu-4.5wt.%Al,Cu-6.9wt.%Al)with different SFE were processed by SMAT at cryogenic temperature to obtain a gradient structure,the Cu-4.5 wt%Al alloy with a medium SFE is more applicable for SMAT process to improve the comprehensive properties.The increase in yield strength is mainly caused by the gradient layer(GS).The thicker GS layer and the smaller grain size will lead to more significant increment in yield strength.The uniform elongation(UE)is primarily controlled by dynamic recovery behavior during deformation.The K2 value,which represents the dislocation annihilation(dynamic recovery)in sample,was obtained from Kocks-Mecking model.The results show that the minimal K2 appears in the Cu-4.5A1 alloy with the medium SFE,indicating a high UE caused by prohibited dislocation annihilation and delayed plastic instability.The low exhaustion of mobile dislocation indicates that the dislocation annihilation during plastic deformation is slow and more mobile dislocation is preserved,which can delay the generation of necking and maintain good ductility.XRD was used to measure the dislocation and twin density of Cu-Al samples before and after tensile test.It was found that the superior strength and plasticity coordination was mainly caused by the change of dislocations and twins during the dynamic deformation process.During the deformation process,the formed high density of twins could keep the accumulated dislocations easily slip along the TBs and effectively lower the exhaustion of mobile dislocation,maintaining a high work hardening capability.In the gradient structured material,the yield strength calculated from the simple rule of mixture based on Hall-Patch formula is much smaller than the actual yield strength of the sample.The high strength of the gradient structured sample is not only contributed by the grain refinement,but also contributed by the back stresses produced by the accumulated dislocations in the graded grain.The Bauschinger effect in the gradient structured material is more obvious.As the strain gradient is larger,more geometrically necessary dislocations must be generated,impeding the generation and proliferation of new dislocations,so resulting in a higher back stress,which contributes to the high yield strength.The synergetic strengthening in gradient structure materials is mainly caused by grain refinement and GND generation.