Gradient Nanograined Structures Construction And Mechanical Properties Of Co-Cr-Fe-Mn-Ni High-entropy Alloy

Conventional metal materials often use one or two elements as the alloy matrix,and other small amounts of elements are added to prepare a uniform bulk material.In this paper,the design of the alloy system and the structure construction method of material are different.The Co-Cr-Fe-Mn-Ni high-entropy alloy with equal atomic ratio is selected as the research object,and the surface layer is prepared by surface mechanical compaction treatment?SMGT?with different passes.A nanocrystalline gradient material.The high entropy effect of the high-entropy alloy and the uneven distribution of the grain structure of the gradient material in space make it uniquely different from the traditional single-principal homogeneous block material.Gradient nano-materials with continuous grain size distribution from 12?m to 16nm and grain size distribution were prepared by surface mechanical rolling treatment.Gradient materials with different deformation depths were prepared by different processes.The structural evolution of Co-Cr-Fe-Mn-Ni at different strains was systematically studied.The grain size refinement mechanism was summarized.The grain structure exhibited different microstructure characteristics under different strain deformation,including dislocation control zone and deformation twin layer structure.,ultra-fine grained layered structure,secondary twin structure,equiaxed nanocrystalline structure.By modifying the processing pass of SMGT,the deformation layer with continuous grain size from 16nm to 12?m was prepared.The gradient material is formed together with the uniform bulk coarse-grained material whose core is not deformed.It is found that as the processing pass increases,the gradient material can obtain a deeper deformation layer.By analyzing the microstructure at different depths of gradient material,the deformed layer of the gradient material can be divided into a dislocation control zone,a mechanically controlled fine grain region,and an ultrafine grained structure region,nano-layered structure regions and equiaxed nanocrystalline regions.The grain refinement mechanism of Co-Cr-Fe-Mn-Ni high-entropy alloy was studied,including dislocation slip under low strain to induce grain refinement,and mechanical twinning induced lamellar structure under high strain.The two-dimensional thinning of the structure,the secondary twinning breaks the layered structure into equiaxed nanocrystals.The effect of annealing at different temperatures on the microstructure and hardness properties of uniform coarse-grained Co-Cr-Fe-Mn-Ni high-entropy alloy and gradient structure Co-Cr-Fe-Mn-Ni high-entropy alloy was investigated by heat treatment.The hardness of the nanocrystalline Co-Cr-Fe-Mn-Ni high-entropy alloy was increased from4953 MPa to 6711 MPa and 6399 MPa by annealing the gradient materials at 450°C for 1h and 500°C for 1 h.Annealing will cause phase decomposition of the nanocrystalline Co-Cr-Fe-Mn-Ni high-entropy alloy,and a Cr-rich phase with nano-body-centered cubic?BCC?structure and a Mn-Ni-rich phase will be formed.The hardness change of the gradient material high-entropy alloy after annealing is restricted by the hardening action of the nano-precipitated phase and the softening effect of grain growth.The hardness and tensile properties of the gradient-structured Co-Cr-Fe-Mn-Ni high-entropy alloy were tested.By matching the grain size and hardness data,the high entropy of nanocrystalline Co-Cr-Fe-Mn-Ni was obtained for the first time.The Hall-Petch coefficient of the alloy is 284.31 MPa·?m^-1/2.By analyzing the stress-strain curve of the gradient material,it is found that the gradient nanostructure can significantly improve the yield strength of the material and keep the plasticity of the material at a high level.SMGT 12-pass treatment material can increase the yield strength of high-entropy alloy from 362 MPa to 582 MPa.The sample treated in 9 passes has the best yield strength?539 MPa?and The best uniform elongation?23%?is the ideal gradient nanostructured high entropy alloy.The nanocrystalline and core coarse crystals of the gradient structure can coordinate with each other,which greatly reduces the surface roughening of the material during the stretching process and coordinates the plastic deformation.