Preparation And First-principles Calculation Of Fe-Al Intermetallic Compound Coating On Stainless Steel Surfac

0Cr21Al6Nb ferritic stainless steel has high resistivity and good high-temperature oxidation resistance,making it an important electric heating alloy material widely used in the manufacturing of various industrial furnaces and household electrical heating components.Fe-Al intermetallic compounds have advantages such as high hardness,high corrosion resistance,and oxidation resistance.Introducing Fe-Al intermetallic compound coating on the surface of Fe-Cr-Al electric heating alloys can further enhance the material’s high-temperature oxidation resistance and corrosion resistance.This article successfully prepared Fe-Al intermetallic compound coating on the surface of0Cr21Al6Nb ferritic stainless steel by using the method of pack cementation.Taking the typical Fe-Al intermetallic compounds in the coating as the research object,the first principle calculation method based on density functional theory is used to model and calculate these Fe-Al intermetallic compounds,to explore the stability,corrosion resistance,bonding properties,brittleness,toughness,and elastic anisotropy of Fe-Al intermetallic compounds in the coating.Firstly,scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,X-ray diffraction（XRD）,microhardness tester,etc.were used to characterize the microstructure and properties of the samples before and after aluminizing.Then,the formation energy,binding energy,density of states,population,electronic work function,elastic constant,elastic modulus,B/G value,Poisson’s ratio and elastic anisotropy of Fe-Al intermetallic compounds in the coating were studied by first principle calculation method.The main research content of this article is as follows:（1）The aluminized coating mainly consists of Fe₂Al₅ phase and FeAl₃ phase,and Fe₂Al₅ phase mainly exists inside the coating,while FeAl₃ phase only exists on the surface and has a low phase content.During the aluminizing process,Fe₂Al₅ will be preferentially formed,and as the coating thickness increases,Fe₂Al₅ will also be more easily formed,while FeAl₃ only exists on the coating surface and is formed by the reaction between Fe₂Al₅ and the active aluminum atoms that diffuse to the coating.The aluminizing temperature and time have no significant effect on the type of phase composition of the coating.（2）Raising the temperature will promote the diffusion of Al atoms,which will lead to an increase in the thickness of the aluminized coating.When the aluminizing time is 4hours,the growth kinetics equation of the coating under different temperature conditions is ln（h）=-12932.84/T+17.10.As time goes on,the coating thickness will gradually increase,but the diffusion coefficient of aluminum atoms will decrease.At an aluminizing temperature of 700℃,the growth kinetics equation of the coating under different aluminizing time conditions is h=21.93t1/2+10.12.The microhardness of the coating does not differ significantly under different temperature and time,at around 960Hv,which is much higher than the substrate.（3）The calculation results of formation energy and binding energy show that these four Fe-Al intermetallic compounds,including Fe₃Al,FeAl,Fe₂Al₅ and FeAl₃,are thermodynamically stable.Among them,Fe₂Al₅ is the most stable and easy to be formed,FeAl₃ is the least stable,and Fe₃Al is the least easy to be formed.The results of density of states and population indicate that the bonding properties of these four Fe-Al intermetallic compounds are mainly metallic bond and covalent bond,and the contribution of density of states near the Fermi level is mainly from the 3d orbitals of Fe and the 3p orbitals of Al.By calculating the electronic work function,it can be obtained that the corrosion tendency order of four Fe-Al intermetallic compounds is FeAl₃>Fe₂Al₅>FeAl>Fe₃Al.（4）By calculating the elastic constants,it can be found that all four Fe-Al intermetallic compounds meet their respective mechanical stability criteria.The calculation results of elastic modulus,B/G value,and Poisson’s ratio（v）indicate that the bulk modulus of Fe₃Al is the highest,while the shear modulus and Young’s modulus of Fe A are the highest.The toughness of the four Fe-Al intermetallic compounds is in the order of Fe₃Al>FeAl>Fe₂Al₅>FeAl₃.The calculation results of anisotropy indices A^U,A_B,A_G,and shear anisotropy factors A₁,A₂,A₃ indicate that the anisotropy of the four Fe-Al intermetallic compounds is ranked in descending order:Fe₃Al>FeAl>Fe₂Al₅>FeAl₃.（5）Under high pressure conditions,the bulk modulus B,shear modulus G,Young’s modulus E,B/G values,and Poisson’s ratio v of the four Fe-Al intermetallic compounds all increase with increasing pressure.Among them,FeAl,Fe₂Al₅,and FeAl₃ will transition from brittle phase to ductile phase at pressures of 15GPa,17.50GPa,and19.17GPa,respectively.Overall,applying pressure increases the anisotropy of Fe₃Al and FeAl₃,but decreases the anisotropy of FeAl and Fe₂Al₅.