Study On Oxidation And Corrosion Characteristics Of High Entropy Alloys At High Temperature

In this paper, AlxCoyCrzFeNi high entropy alloys were prepared by vacuum arc furnace system (x, y, z was defined as element mole number, (x=0.5,1,1.5;y=1,1.5,2;z=1,1.5,2). High-temperature oxidation kinetics of the high entropy alloys heated at the temperatures of 800 ℃～1100℃ was measured. The high temperature oxidation resistance was evaluated according to HB5258-2000 "Determination of oxidation resistance of steel and superalloys". The microstructure of oxide layer in surface and cross-section were observed. Thus, the mechanism of the oxidation of high entropy alloys and the effects of alloy element content on the oxidation performance were studied. Meanwhile high temperature corrosion kinetic curves in (25wt%NaCl+75wt%Na2SO4) molten salt at 700 ℃1000℃ were investigated using weight method and then the corrosion mechanism was studied. The results show that:(1)The high temperature oxidation performance of AlxCoyCrzFeNi high entropy alloys heated at 800 ℃,900 ℃,1000℃ possessed better oxidation resistance. During high temperature oxidation of 1100℃, weight loss is serious, and then resistance to high temperature oxidation went worse.(2) Al content of AlxCoCrFeNi high entropy alloys had a greater influence on high temperature oxidation of the alloys, optimal AlCoCrFeNi of high temperature oxidation resistance. Cr content of Al0.5CoCrzFeNi high entropy alloys had a greater influence on high temperature oxidation of the alloy also, but after Al element, optimal Alo.5CoCr1.5FeNi of high temperature oxidation resistance. Co content of Alo.sCoyCrFeNi high entropy alloys had little influence on high temperature oxidation of the alloys.(3) The oxidation mechanism of AlxCoyCrzFeNi high entropy alloys is shown below: interface reaction is predominant at the initial stage of oxidation; with the isothermal oxidation, diffusion process will gradually become the control factors in the subsequent oxidation. Due to elements Al activity is greater than elements Cr, the elements Al priority in oxidation, he elements Cr occurred outside the oxidation. Oxidation products of a certain amount of NiCr2O4, CoCr2O4, Fe (Cr,Al) 2O4, etc. Transition region in nitriding reaction, generate AlN particles. The increase of oxidation temperature, alloy outer oxide layer, inner oxide layer and transition layer thickening. When the oxidation layer is damaged, AlN will be oxidized, inducing to geometric shapes of AlN cavity, so that the high entropy alloys lose their resistance to high temperature oxidation performance.(4) The AlxCoCrFeNi high entropy alloys in high temperature molten salt corrosion incubation period is mainly oxidized and generates a variety of surface oxide, such as Al2O3, Cr2O3, Fe2O3, etc., having certain negative effects on corrosion. When the molten salt corrosion of high temperature continued to a certain time, NaCl reacted with the surface oxide to generate low melting point and strong volatile chloride such as AlCl3, CrCl2, CrO2Cl2, etc. Also, NaCl increased the stress of surface oxide and reduced the adhesion between oxide and substrate, thus leading to crack appear in the oxidation layer. In order to reduce the high temperature corrosion resistance of high entropy alloys. Na2SO4 will decompose into S at high temperature environment, resulting into a change in pH value, alkaline dissolution of surface oxides, and thus internal oxidation and internal curing of the matrix, which deteriorated corrosion resistance to high entropy alloys.