High-Temperature Oxidation And Corrosion Of High-entropy Alloys

High-entropy alloy, including Al0.5CoCrFeNi, Al0.5CoCrFeNiSi0.2 and Al0.5CoCrFeNiSi0.2, was prepared by the vacuum arc melting method. High temperature oxidation resistance and corrosion resistance of high-entropy alloys were measured. The performance of oxidation and corrosion, and the effect of the element Si and Ti were studied. As a type of alloy, Al0.5CoCrFeNi high-entropy alloy was rolled to obtain different deformation for the research on the influences of deformation on the high temperature corrosion resistance of high entropy alloy Al0.5CoCrFeNi. In order to analyze the oxidation and corrosion micromechanism, the micrographs and microstructures of alloys were characterized by optical microscope (OM), scanning electron microscopy (SEM) coupled with electron diffraction spectrometry (EDS), X-ray diffractometry (XRD).The results show that the oxidation weight gain of three high-entropy alloys was normally less than 800℃ to 900℃, and the oxidation resistance at this temperature is of excellence. The oxidation resistance decreases with the temperature increasing. The surface oxide begins to spall at 1000℃. The oxidation weight loss apperes at 1100℃, but the oxides spallation aggravate. The relation of their high temperature oxidation resistance is expressed as Al0.5CoCrFeNi>Alo.5CoCrFeNiSio.2>Al0.5CoCrFeNiTi0.5 at the same temperature. No obvious oxide appears on the surface of Alo.sCoCrFeNi while it does Al2O3 for Al0.5CoCrFeNiSi0.2-Meanwhile, mixed oxide appears for Al0.5CoCrFeNiTi0.5 at 800℃. While the temperature is more than 800℃, the similar oxide, including Cr2O3, Al2O3, CoCrO4 mixed oxide and A1N nitride, apperars on Al0.5CoCrFeNi and Al0.5CoCrFeNiSi0.2-Furthermore, various oxides containing Ti, except Cr2O3 and Al2O3 appears on Al0.5CoCrFeNiTi0.5. The oxide layer and diffusion layer grows thicker with the temperature increasing.There is great influence for the molecule ratio of cryolite mixed molten salt on the corrosion rate of Al0.5CoCrFeNi high-entropy alloys. And the alloy has the least corrosion rate when the molecule ratio is 1.8. Loose corrosion holes grow on the surface of the alloy. Severe corrosion occures for the high-entropy alloy Al0.5CoCrFeNi in (25wt% NaCl+75wt% Na2SO4) molten salt at 800℃ and 900℃, and the severe corrosion weight loss apperars, meanwhile, the internal oxidation and sufidation may occures. The external layer is grown by NaFeO2, NaNiO2, CoO3 mixed oxides, and the internal corrosion layer is grown by Cr5S6 sulfide.As the rolled deformation increasing, the corrosion weight gain of Al0.5CoCrFeNi high-entropy alloy increases in (25wt%NaCl+75wt%Na2SO4) molten salt at 700℃, and the corrosion resistance decreases. The alloy formed the same corrosion products with different deformation, including Cr2O3, Al2O3, CoCr2O4 Ni2CrO4, (Fe0.86Al0.14)2O3 mixed oxides and CrS sulfide. As the deformation increasing, the delamination of surface oxides aggravates and the cracks generate. Meanwhlie, the corrosion layer turns thicker.