Structure And Properties Of CrNbNiTiV High Entropy Alloy

High entropy alloys?HEAs?are a new kind of alloys which are made by mixing five or more elements in equal or near equal molar ratio by various preparation methods.Due to the existence of the four major effects,the alloys can have high strength,high hardness,wear resistance,corrosion resistance,high temperature oxidation resistance and thermal stability.CrNbNiTiV high entropy alloy was prepared by vacuum arc melting and annealed.The phase structure,microstructure,composition,hardness,corrosion resistance and high temperature oxidation resistance of the as cast alloy were investigated by X-ray diffraction,scanning electron microscope,energy dispersive spectrometer,microhardness tester,electrochemical workstation and high temperature carbon rod resistance furnace.The effect of annealing temperature on the microstructure,hardness and corrosion resistance of the alloy were studied and analyzed.It is found that the phase composition of as-cast and annealed CrNbNiTiV high entropy alloy is FCC+?-Ti+Ni Ti+Ni Ti₂,and the microstructure is dendrite.After annealing,part of?-Ti is spheroidized,part of?-Ti is coarsened and grows into snowflake gradually.It contributes to the uniform distribution of Ni Ti and Ni Ti₂compounds when annealing temperature is less than or equal to 800?.Ni Ti₂phase precipitates above 900?,and the non-uniformity of distribution increases.The grain size of the alloy is refined after annealing,and the grain size is the smallest at 700?.The average microhardness of CrNbNiTiV high entropy alloy is 797HV of as-cast condition,831HV annealed at 600?,878HV annealed at 700?,825HV annealed at800?and 813HV annealed at 900?.The microhardness of the alloy is improved by annealing treatment,which is affected by the grain size and the distribution uniformity of Ni Ti₂.The finer the grain size is and the more uniform the distribution of Ni Ti₂phase is,the higher the hardness of the alloy is.The alloy shows good thermal stability and softening resistance.According to the potentiodynamic polarization test results,the corrosion current density I_corrof as-cast CrNbNiTiV high entropy alloy in 3.5wt.%Na Cl and 5wt.%HNO₃solutions are 5.61×10^-6A/cm²and 6.01×10^-7A/cm²respectively,and the pitting potentials E_pare 0.550V and 1.278V respectively.The results show that as-cast CrNbNiTiV high entropy alloy has pitting corrosion in both solutions,and corrosion pits appear in FCC phase,which indicates that this phase is preferentially corroded.In3.5wt.%Na Cl solution,due to the more uniform structure and less defects in the annealed alloy,the way of Cl^-diffusion into the material through defectsthe is reduced,which lead to the improvement of corrosion resistance after annealing.The best corrosion resistance is obtained after annealing at 900?;In 5wt.%HNO₃solution,the corrosion resistance of CrNbNiTiV high entropy alloy is reduced by annealing.The reason is that the number of corrosion holes increases after grain refinement,the distribution of Ni Ti and Ni Ti₂phases is more uniform,and the galvanic corrosion is aggravated.The corrosion resistance of CrNbNiTiV high entropy alloy annealed at900?is the closest to that of as-cast alloy.The oxide layer of CrNbNiTiV high entropy alloy was formed by oxidation at600?for 50 h.The outer layer is composed of loose and coarse Ni V₂O₆,and the inner layer is composed of fine and dense Cr Nb O₄and Ti O₂mixed oxides.The alloy showed good oxidation resistance at 600?.During the oxidation process of the alloy at 700?and 800?,due to the formation of volatile V₂O₅with low melting point,a thicker and porous oxide layer is formed.The high temperature oxidation resistance at this temperature is poor.