Effect Of La And Ce On The High Temperature Oxidation Resistance Of Fe-Cr-Al Alloy Used For Exhaust Gas Catalyst Support

Fe-Cr-Al alloy as an ideal material for the carrier of exhaust gas purifier because of its low production cost,good high-temperature corrosion resistance,and low thermal expansion coefficient.However,in the extreme environment such as thermal cycle and thermal fatigue,the oxide film is prone to crack and spalling,which leads to its short service life and seriously restricts the application of Fe-Cr-Al alloy.In this paper,rare earth lanthanum and cerium are used to improve the high temperature oxidation resistance of Fe25Cr5Al alloy.On the basis of experiments and first-principles calculations,the oxidation mechanism of Fe25Cr5Al aluminum alloy at high temperature and the action mechanism of rare earths were studied.Thermodynamic calculation and microscopic analysis show that there are large quantity Al N inclusions with large size in Fe25Cr5Al alloy.By observing the surface and cross-section morphology of oxide film,it is found that these Al N have great influence on the integrity of oxide film and the adhesion of oxide film/substrate.With the addition of different contents of rare earth La and Ce,the number,size and type of inclusions decrease significantly with the increase of rare earth content,which eliminates the negative influence of inclusions on the oxide film of Fe25Cr5Al alloy to some extent.At the same time,rare earth La and Ce combine with elements such as S and O in steel to form RE-O-S compound,which reduces the adverse effects of impurity elements such as S on adhesion of oxide film/substrate to a certain extent.Fe25Cr5Al alloys with different contents of RE were oxidized in the air for 1 h,20 h,and 300 h,respectively,and the structure and grain size data were obtained by light microscope and Image-Pro analysis.The results show that the grains of Fe25Cr5Al alloy grow with the increase of oxidation time.The oxidation weight gain shows that the oxidation weight gain of Fe-Cr-Al alloy increases sharply with the increase of grain size.In addition,with the increase of rare earth content,the original grain size of Fe25Cr5Al alloy becomes significantly smaller,and the grain size grows more smaller and the microstructure becomes more uniform after oxidation.XRD and SEM analysis of the composition and morphology of oxide film after oxidation for different time shows that the oxidation mechanism of Fe25Cr5Al alloy is that O^2-diffuses into the matrix from air,while Al³⁺diffuses out from the matrix,and the oxidation reaction proceeds at the interface between oxide film and matrix.Due to the formation of new oxide and the release of growth stress,the oxide film grows transversely and is wavy.Corrugated oxide films are prone to crack or even peel off during thermal cycling,and cannot form protective oxide films well.After the addition of rare earth,on the one hand,it changed the diffusion mode of ions,inhibited the external diffusion of metal ions,and hindered the internal diffusion of oxygen ions after forming a dense oxide film.On the other hand,it plays a pinning role at the interface between oxide film and substrate,and rare earth forms a large number of dispersed rare earth-rich phases in Fe25Cr5Al.During oxidation,these rare earth-rich phases are preferentially oxidized to form rare earth oxides,which serve as fast channels for ion transmission,so that the oxide film grows around rare earth oxides,thus forming a"pin"-shaped oxide strip,which closely connects the oxide film with the substrate and significantly improves the high-temperature oxidation resistance of Fe25Cr5Al alloy.The symmetric tilting grain boundary model of bcc Fe?3[110](112)was constructed by using the heavy site lattice model theory(CSL),and the calculation was performed after replacing the doped La and Ce atoms in the grain boundary and the grain boundary region.The segregation energy results show that La and Ce atoms are more stable in the grain boundary region,and tend to segregate in the grain boundary in steel.Rare earth compounds are formed after the rare earths are concentrated on the grain boundaries,which is beneficial to fix the grain boundaries,making it more difficult for Fe25Cr5Al alloys to grow grains after high temperature oxidation,and improving the high temperature oxidation resistance of Fe25Cr5Al alloys.