Effects Of Heat History On Corrosion Resistance Of Al₂CoCuFeNiCr High Entropy Alloy In HNO₃ Solution

As a new metal material with excellent properties such as high hardness,wear resistance,temperature resistance and corrosion resistance,high-entropy alloy（HEAs）has important application prospects in many fields.In this paper,Al₂Co Cr Cu Fe Ni HEA was selected as the research object and it was examinated the following subjects of the effects of superheat treatment,different cooling rates and remelting on its microstructure and corrosion resistance in 0.5mol/L nitric acid solution.And thermal history,phase composition and corrosion principles were discussed.It has revealed the correlation between thermal history and corrosion resistance.The Al₂Co Cr Cu Fe Ni HEA mother alloy ingots were smelted by vacuum arc furnace.The current gear position of the electric arc furnace welding machine was controlled so that the arc heated the mother alloy ingot to different temperatures above the melting point under different heating powers（5.26 KW,6.15 KW,7.28 KW,8.26 KW）.The microstructure and element distribution of the alloy were investigated by X-ray diffractometry and scanning electron microscopy（SEM）.It was found that the alloys with different superheat temperatures were simple FCC+BCC solid solution phases,and no complex phase appeared,and the lattice constant of the solid solution phase in the alloy did not change substantially.As the superheating temperature increased,the microstructure of the alloy became denser and the degree of segregation of the elements was greatly reduced.The corrosion resistance was investigated by full immersion test,polarization curve,cyclic polarization test and scanning electron microscopy.It was found that with the increase of the superheating temperature,the corrosion current density was reduced by more than 1/2,and the range of the passivation zone was also significantly broadened,indicating that the corrosion resistance of the alloy was remarkably enhanced.The Al₂Co Cr Cu Fe Ni HEA with different cooling rates were prepared by copper mold suction casting method with different diameters（2mm,4mm and 6mm diameter）.There were no changes in the phase composition of the alloy at three different cooling rates,and both were simple FCC+BCC solid solutions.As the cooling rate increases,the relative amount of BCC phase in the alloy decreased slightly.In the case of a faster cooling rate,the grain size became finer,the grain boundaries became narrower,the distribution was more uniform,and the degree of element segregation was significantly reduced.With the increase of cooling rate,the corrosion current density of the alloy gradually decreased,the pitting potential increased gradually,the pitting resistance became more excellent,and the passivation film became more and more stable,indicating that the corrosion resistance was obvious improved.Finally,the microstructure and corrosion resistance of Al₂Co Cr Cu Fe Ni HEA of as-cast,remelted and superheated treatment were analyzed and compared.It was found that the alloys of the three states were simple FCC+BCC two-phase solid solution structure,and the lattice constant was slightly reduced after remelting,indicating that the size of the unit cell in the grain was slightly reduced;and it became larger after superheat treatment.The lattice constant demonstrated the growth of the unit cell size in the grain.After remelting,the grains became fine and dense,and the grain boundaries became fine.After superheat treatment,the grains of the alloy grew slightly,the distribution of crystal grains was the densest,the grain boundary was also the narrowest,and the degree of segregation of elements in the dendrite gap was the smallest.After remelting and superheat treatment,the corrosion current density is reduced,and the width of the passivation region is widened,indicating that the corrosion resistance is improved,and the alloy after superheat treatment has the most excellent corrosion resistance.