Research On Annealed Microstructure And Corrosion Resistance Of CoCrFeNiM_x High Entropy Alloy Coatings Prepared By Laser Cladding

High-entropy alloys（HEA）,which are also called multi-principal alloys,have higher entropy of mixing and better performance than traditional alloy and have been a research hotspot of new metal materials in recent years.The AISI 1045 steel（45#）is used widely but it would be failed in marine environment such as shipping due to its low hardness and no-abrasion resistance.CoCrFeNiM_x,（M=Al,Mo,Ti,B;x=0.5,1,1.5）HEA coatings was prepared on the surface of 45#by laser cladding and annealed at different temperature of 700 ^oC,900 ^oC,1100 ^oC for 5 h.SEM,EDS,XRD and electrochemistry measuring analysis were applied to study the microstructure,hardness and performance of corrosion resistance of high-entropy alloy coating.The results show that the phase structures of CoCrFeNiM_x HEA coatings were mainly FCC phase and a little BCC phase.The addition of Al and Ti improve the formation of BCC phase.Annealing changed the crystal structure of HEA coating,andσphase appeared in both Co Cr Fe Ni Mo_x and Co Cr Fe Ni Al_x after annealing.The microstructure of Co Cr Fe Ni B_x HEA coating is mainly dendritic structure,and the interdendritic structure is mainly eutectic structure of（Fe,Ni）solid solution and a small amount of M_xB phase.The coating structure of Co Cr Fe Ni Mo_xHEA is mainly dendritic structure and a small part of equiaxed crystal structure.The increase of Mo element content promotes the transformation of some microstructure into equiaxed crystal.The Co Cr Fe Ni Al_x HEA coating is mainly composed of equiaxed crystals and dendrites.The addition of Al element promotes the transition of the coating from the original single-phase FCC structure to the two-phase FCC+BCC structure.The microstructure of Co Cr Fe Ni Ti_x HEA coating is mainly dendritic structure and a small amount of equiaxed crystals.The phase composition is mainly FCC+IMC.Annealing treatment can change the crystal structure of the alloy coating.The Co Cr Fe Ni Mo_x and Co Cr Fe Ni Al_x HEA coatings haveσphase after annealing.The CoCrFeNiM_xHEA coating retains a fine microstructure after annealing.The hardness of CoCrFeNiM_x HEA was apparently higher than 45#steel matrix.The coating hardness was increased with the contents of Al、Mo、Ti、B increasing.Among the Co Cr Fe Ni B_xHEAs,the Co Cr Fe Ni B_1.5 high-entropy alloy coating has the highest hardness of 730 HV after annealing at 700°C.Among the Co Cr Fe Ni Al_x HEAs,the Co Cr Fe Ni Al_0.5 high-entropy alloy coating has the highest hardness of 566 HV after annealing at 900°C.Both Co Cr Fe Ni Mo_x and Co Cr Fe Ni Ti_x HEA coatings have the highest hardness after annealing at 900°C.The Co Cr Fe Ni Mo_1.5 HEA coating has the highest hardness after annealing at 900°C,which is 925 HV;Co Cr Fe Ni Ti_1.5 has the highest hardness,which had reached 1144 HV after annealing under 900^oC,and it was five times more than 45#steel matrix.It shows that the annealing treatment can significantly improve the hardness of the HEA coating.Co Cr Fe Ni Mx high-entropy alloy exhibits good softening resistance to high temperature annealing.The annealed CoCrFeNiM_x HEA had good corrosion resistance in 3.5%Na Cl solution.All the coating had better corrosion resistance than 45#steel.The Co Cr Fe Ni Al_0.5,Co Cr Fe Ni Al_1.0 and Co Cr Fe Ni Mo_1.0 HEA after annealing at 1100 ^oC had the better corrosion resistance in 3.5%Na Cl solution.Co Cr Fe Ni Mo_1.0 had the higher corrosion potential than 304 stainless steel and comparative small corrosion current density;Co Cr Fe Ni Ti_0.5 and Co Cr Fe Ni Ti_1.5 HEA alloy had the highest corrosion potential after annealing at 900 ^oC,which were-0.396 V and-0.398 V respectively.The corrosion current density of Co Cr Fe Ni Ti_0.5 HEA was 5.349×10^-7 A·cm^-2,which was 3 orders lower than 45#steel,and so the alloy had the best performance of corrosion resistance.The corrosion types of high entropy alloy coatings are mainly uniform corrosion and local corrosion.