Microstructure And Properties Of CoCrFeNiAl_x(x=0.1,0.5,1) High Entropy Alloy Induced By Laser Surface Remelting

High-entropy alloys are more and more favored by researchers due to their high strength,fine corrosion resistance,outstanding wear resistance and high temperature oxidation resistance,and have broad application prospects.However,there are some defects such as high internal stress and composition segregation in the preparation process of bulk high entropy alloy,which will inevitably cause adverse effects on the properties of the alloy.Laser surface remelting(LSM)is an advanced energy taking along beam surface modification technology in the past few years.It turns the phase composition and structure of the matrix material under the heat effect of laser beam,and can modify the structure of the sheet layer underneath the surface,so as to achieve the purpose of strengthening the surface properties.In this project,LSM modification technology was used to control the microstructure of CoCrFeNiAl_x(x=0.1,0.5,1)high-entropy alloys with different simple solid solution phases.The evolution of phase structure,micro-morphology and composition of the alloys before and after laser treatment were systematically discussed.The microhardness,wear behavior,corrosion resistance and high temperature oxidation resistance on the surface of the samples before and after laser treatment were measured.After Laser remelting,the XRD results displayed that the samples show a single FCC phase(CoCrFeNiAl_0.1and CoCrFeNiAl_0.5)or a mixture of BCC+B2 phase(CoCrFeNiAl₁).The results of OM and SEM showed that with the increase of Al content,the solidification structure changed from columnar cell to columnar dendrite,and finally to equiaxed dendrite.After LSM treatment,the element composition was closer to the nominal composition and the grain size was refined.A consecutive and compact remelting layer was formed on the surface of the alloy,and as the Al content increases,the thickness of the remelted layer gradually increases.EBSD analysis showed that the structure of CoCrFeNiAl_0.1after LSM treatment was more uniform,and the orientation of crystal grains was more random;The crystal structure of CoCrFeNiAl_0.5was more refined and the grain orientation was(101);the CoCrFeNiAl₁crystal growed preferentially along the(001),(101)and(111)directions after laser treatment.TEM analysis results showed that after Laser surface remelting,dislocations structure formed in CoCrFeNiAl_0.1,which displayed a single-phase solid solution with FCC structure;A large number of high-density dislocations were found in the surface layer of CoCrFeNiAl_0.5,and BCC precipitates were found in the grains.Two regions of interdendritic and dendritic were formed on the surface of the alloy;CoCrFeNiAl₁formed a B2-based BCC structure,and high-density dislocations structure appeared in the modified layer,and spherical nano-dispersed phase B2 was also distributed.LSM treatment improved the microhardness,friction and wear properties of the three high entropy alloys.The CoCrFeNiAl₁alloy exhibited the highest average hardness of 541.5HV after laser treatment.The solid solution strengthening caused by Al atoms,the high-density defect structure,the grain refinement after remelting,and the dispersion strengthening caused by the nano-scale B2 and BCC phases all played an important role in the hardness and wear resistance of the material surface.The corrosion performance test results showed that the CoCrFeNiAl_0.1after laser treatment had the best corrosion resistance.The LSM treatment induced the dense remelted layer structure formed on the surface,the homogenization of the composition,and the protective effect of the precipitated sacrificial anode were all conducive to improving the corrosion resistance of the high-entropy alloy.The results of oxidation resistance test showed that with the increase of Al content after LSM treatment,the parabolic velocity constant K_Pvalue gradually decreased and was lower than that before LSM treatment.CoCrFeNiAl_0.1formed a consecutive and uniform Cr₂O₃oxide layer;The outer Cr₂O₃oxide film and inner Al₂O₃precipitates were formed on the surface of CoCrFeNiAl_0.5;CoCrFeNiAl₁samples formed a uniform and compact single alumina oxide layer over the whole region.The oxide film thickness of the three alloys after100h oxidation was about 5?m?4?m?3?m,respectively.The main reason for the improvement of oxidation resistance after laser remelting was that a dense remelted layer was formed on the surface of the alloy,the elements on the alloy surface were uniformly distributed,and the surface structure of the remelted layer was refined.