Mechanical Properties And Microscopic Deformation Mechanism Of Gradient-structured Cu-Al-Zn Alloys

Gradient structure(GS) materials have received extensive attention in recent years due to their superior strength-ductility combination.In Cu-Al-Zn alloy,Zn element has the function of solid solution strengthening and improving the plasticity of Cu-Al alloy.Therefore,Cu-Al-Zn alloy has good application prospects.In this paper,three kinds of GS Cu-Al-Zn alloy samples with different stacking fault energies(SFEs)were prepared by surface mechanical attrition treatment(SMAT)at cryogenic temperature.The results show that for both the initial annealed sample and the GS samples processed for the same processing time,uniform elongation and ultimate tensile strength increase with decreasing SFE,whereas the yield strength is not sensitive to SFE.In the case of the same SFE,as the SMAT processing time increases,the yield strength and ultimate tensile strength of the GS copper-aluminum-zinc alloy sample increase and the uniform elongation decrease.The combination of low SFE and optimized SMAT processing time is beneficial to obtain an optimized combination of strength and ductility.The Cu-5.5wt.%Al-4.5wt.%Zn alloy having the lowest SFE processed by SMAT for 5 min exhibits an optimized combination of strength and ductility.Its yield strength reached 221.53 MPa,while still maintaining the high ductility of 62.57%.The optimized 5min processing time can sufficiently refine the grains in the GS layer without significantly increasing the thickness of the GS layer.The thickness of the GS layer of the SFE-7-2min and SFE-7-5min samples are both about 250 microns.Also,sufficient grain refinement promotes the better accumulation of geometrically necessary dislocation(GND),thus forming rather effective heterodeformation induced(HDI)stress strengthening and HDI strain hardening to enhance strength while maintaining good ductility.Additionally,during the tensile process,the near-surface layer of the GS copper-aluminum-zinc alloy sample undergoes plastic deformation earlier than the sub-surface layer,so mechanical incompatibility occurs at the interface between the two layers.This needs to be coordinated through GND,and the accumulation of GND also contributes to HDI stress strengthening and HDI strain hardening,thereby improving strength and ductility.Digital image correlation experiments show that the shear band embryo of the gradient structured sample is formed by interconnecting the local high strain point groups in the early stage of tensile.The dispersed shear bands steadily evolve during the uniform elongation of the gradient structured sample,thereby helping to maintain ductility.In the process of uniform elongation,the evolution of the dispersed shear band in the SFE-40-2min sample is more stable than that in the SFE-40-15 min sample.Therefore,the ductility of the SFE-40-2min sample is higher than that of the SFE-40-15 min sample,the former can reach 27.72%,while the latter is only 16.99%.