Oxidation Behavior Of The Co-Cr-Fe-Ni-Al_x(x=1.0,1.5,2.0) High-entropy Alloys At High Temperatures

The high entropy alloy can apply in many fields as construction material and function material,because of practicality,machinability and so on.Bulk alloy buttons with the compositions of Co-Cr-Fe-Ni-Al_x?x=1.0,1.5,2.0?were produced by vacuum arc melting.Oxidation of three alloys and effect of Al content at oxidation were systematically investigated by means of Optical microscopy,X-ray diffractometry?XRD?,Scanning Electron Microscopy,Electron Probe Micro-analysis and mechanical test,at 800?,900?and 1000?.The results show that three type of alloys tend to form simple body-centered cubic sold solution phase with a little of?phase at casting.The hardness of casting alloys boost with increase of Al in different alloys.The microhardness of Co-Cr-Fe-Ni-Al_2.0.0 alloy is 540HV.At different temperatures,the structure of Al_1.0 and Al_1.5 alloy near the oxide layer gradually transformed BCC phase to FCC phase.XRD showed that the intensity of FCC phase structure is very low in the Al_2.0 alloy.At the 800?,oxide products of three alloys are Al₂O₃phase,and thickness of oxide layer is about 1?m.At 900?,oxidation products of three alloys are Al₂O₃ phase at first period of oxidation.After 100 hours,three alloys show different oxidation results.Al_1.0.0 alloy oxide layer is mainly divided into three layers,Cr₂O₃ spinel phase which is in surface of oxide layer,Al₂O₃ phase which attach to Cr₂O₃ spinel phase,AlN which grow in the area of dissipation.Al_1.5 alloy oxide layer is mainly divided into four layers.The outermost layer is Al₂O₃ phase.Spinel scale which is rich in Cr,Fe,Co is attach to Al₂O₃ phase.The inner layer is razor-thin Al₂O₃ phase which cover on AlN.Al_2.0 alloy form an external Al₂O₃ scale and an internal subscale spinel of Fe,Cr.At 1000?,each alloy oxide scales at 10h are the same as the oxidation at 900?under 100 h.Cr react with O₂ to form gaseous CrO₃,why is the reason of weight loss.Single acicular Al₂O₃ phase remains in alloy with long time oxidation at 1000?.The oxidation of three alloys follow the parabolic kinetics at 800?and 900?.Al_1.0.0 alloy gain weight at first,then lost weight at 1000?.The oxidation curves of Al_1.5.5 and Al_2.0 are satisfied with cyclical model which gain weight at first,secondly lost weight,finally gain weight.At 800?,Al_1.0 alloy belongs to the anti-oxidization,whose average oxidation weight gain rate is 1.438?10^-4 mg²?cm^-4?h^-1.The maximum weight gain and the average weight gain rate are proportional to the Al content in the alloy at 800?.The maximum weight gain and the average weight gain rate are inverse ratio with Al content in the alloy at 900?.The maximum weight gain of Al_1.0 is 2 mg?cm^-2.The maximum weight gain is proportional to the Al content in the alloy at 1000?.The maximum weight gain of Al_2.0 is 9.8 mg?cm^-2.The results of high temperature oxidation kinetics of three alloys are discussed.The oxidation of alloys contains two periods.Firstly,the weight gain of alloy is controlled by interfacial chemical reaction.The rate of average weight gain rate is fast.Finally,the control mode of weight gain changes from chemical reaction to elements diffusion which is contain external diffusion of metal cations and internal diffusion of anions.This paper researched the dynamic oxidation proceed of Co-Cr-Fe-Ni-Al_x?x=1.0,1.5,2.0?high entropy alloys.It is theoretical direction of service environment about high entropy alloys.This paper focuses on the establishment of the oxidation kinetics mechanism which affords references for other high entropy alloys.