Study On The Surface Nanocrystallization Of (Fe₅₀Mn₃₀Co₁₀Cr₁₀)₉₇C₂Mo And CoCrNi Alloy

High entropy alloy is a simple solid solution structure composed of multi principal elements,and it has excellent comprehensive properties such as lattice distortion,slow diffusion of atoms and high stability of structure,which has become the international research hotspot in recent years.In this paper,the high strength and toughness(Fe₅₀Mn₃₀Co₁₀Cr₁₀)₉₇C₂Mo and Co Cr Ni high entropy alloys are selected as the research objects.After vacuum arc melting and select different rolling heat treatment processes,the sliding friction treatment was carried out so as to study the influence of surface nano treatment on the microstructure and mechanical properties of these two high entropy alloys.At the same time,the microstructure and properties of surface nano treated alloys after annealing are analyzed.Scanning electron microscope?SEM?,electron backscatter diffraction?EBSD?,XRD and transmission electron microscope?TEM?were used for further characterization.The results show that:The rolled(Fe₅₀Mn₃₀Co₁₀Cr₁₀)₉₇C₂Mo alloy has a single FCC structure,with uniform composition distribution and equiaxed crystal.Due to the large amount of twinning and dislocation entanglement in the deformation process,the alloy has certain strength and good plasticity.The thickness of the deformed layer after sliding friction treatment is about 800?m,and the alloy is still a single FCC solid solution.However,the micro amount of carbide,dislocation tangle,twin and new phase of precipitated HCP structure make the alloy strengthened.The inhomogeneity of cross-section structure will be increased by sliding friction,and the maximum value will be reached when the parameters of sliding friction treatment are 400N and 100T respectively.The microhardness of the cold-rolled(Fe₅₀Mn₃₀Co₁₀Cr₁₀)₉₇C₂Mo alloy is the highest at 300?and 400?,which has reaching 555HV.Compared with the cold alloy,HCP precipitates increases after annealing.After annealing,the strength of the surface nanocrystalline and rolled alloy is improved,and a large number of dislocations and twins are produced,which makes the annealed alloy have higher strength and certain plasticity.After cold rolling and annealing at 900?,the microhardness and yield strength of Co Cr Ni alloy decreased gradually,while its shape increased gradually.When the annealing temperature is increased to 1000?,the thermal diffusion speed is accelerated,and the microstructure and properties have no obvious change with the increase of time after the rapid completion of recrystallization.When the annealing temperature continues to increase to 1100?,the thermal diffusion is further accelerated,and after the rapid completion of recrystallization,grain growth also occurs and affects the properties of the material.The microstructure of Co Cr Ni alloy is uniform in rolling state,and the structure after sliding friction treatment is gradient deformation.The surface grain can be refined to nanocrystalline,and the grain size gradually changes to rolling grain with the increase of distance from the surface.The microhardness of the section shows that the gradient deformation layer is about 400?m.The surface nanocrystalline Co Cr Ni alloy has good low-temperature properties and back stress strengthening ability.Its yield strength and shape under liquid nitrogen temperature reach 1050MPa and 42%,and back stress strengthening reaches540MPa.After annealing,the gradient deformation layer of the surface nanocrystalline Co Cr Ni alloy has no obvious change,but the grain size grows slightly.After annealing in severe deformation zone,new phase precipitates,which leads to further improvement of material strength.