Segregation of Si and Mg at Fe(110)/Al(110) Interface

WANG Hongjin, SU Xuping, WANG Jianhua, SUN Shunping,WANG Bin, JIANG Yong

(1. Jiangsu Key Laboratory of Material Surface Science and Technology, Changzhou University, Changzhou 213164, China; 2. Jiangsu Key Laboratory of Advanced Materials Design and Additive Manufacturing, Jiangsu University of Technology, Changzhou 213001, China; 3.School of Materials Science and Engineering, Central South University, Changsha 410083, China)

Abstract: The interface structure and electronic properties of Fe(110)/Al(110) are investigated by the first-principles plane-wave pseudopotential method. The interface segregation position of Si and Mg is determined, and the effect of Mg and Si on the interface binding of Fe(110)/Al(110) is analyzed by combining the work of separation and charge density. The results show that the Fe(110)/Al(110) interface energy of Fe-Hollow coordination is smaller and the interface structure is more stable. The Fe(110)/Al(110) interface separation surface in the form of Fe-Hollow coordination appears at the sub interface layer on the side of Al (110)near the interface. The interface structure of Mg and Si segregation is similar to that of undoped alloy elements.The calculations also suggest that Mg and Si segregate on the Al (110) side of the interface and occupy the Al lattice on the Al (110) side. The segregation of Mg and Si elements will reduce the interface binding, primarily because the Fe-Si bond and Fe-Mg bond are weaker than Fe-Al bond.

Key words: Fe(110)/Al(110); interface structure; works of separation; first-principles

1 Introduction

Steel and aluminum alloy are most widely used in the structural materials of industry. Steel has high strength and aluminum alloy has light weight and good corrosion resistance. Steel aluminum composite material, which effectively combines the advantages of the two materials, has the advantages of high strength, corrosion resistance, oxidation resistance and light weight[1-3]. Hot-dip aluminizing process is widely used in surface protection of steel. Hot-dip aluminizing(HDA) is to immerse the pretreated steel into a molten Al-based bath at a certain temperature for a certain time. During the process of HDA, atomic interdiffusion and chemical reaction occur between liquid aluminum and solid iron, which results in the formation of Fe-Al intermetallic compound layer. And a layer of aluminum or aluminum alloy will be adhered to the surface of the Fe-Al intermediate layer[4-6]. Hot dip aluminized steel has excellent heat resistance,corrosion resistance, appearance decoration and comprehensive mechanical properties. It is widely used in petrochemical industry, automobile manufacturing,power communication, transportation and construction industry[7,8]. However, due to the great difference in thermophysical properties and low solid solubility between iron and aluminum, Fe atoms and Al atoms are easy to form Fe-Al compound layer composed of FeAl3and Fe2Al5intermetallic compound during the process of hot-dip aluminizing[9-13]. The growth of Fe-Al compound layer with high brittleness and low fracture strength is difficult to control, which will deteriorate the binding strength between steel matrix and aluminum base coating and reduce the machinability of hot-dip aluminum coated steel[14,15]. Therefore, in order to improve the performance of Fe-Al compound layer,the researchers added a certain amount of Si, Mg, Zn,Mn, Cu and Be to the aluminum bath to control the thickness of the alloy layer and improve the corrosion resistance and oxidation resistance of the coating[16-19].

The interface structure and micro morphology between Fe/Al are one of the decisive factors affecting the bonding strength and service performance of the coating system. Therefore, the control of Fe/Al interfacial reaction has become an urgent problem to be solved. It is difficult to characterize the interface structure of Al based coatings. At the same time, the segregation of alloy elements on the interface affects the interface binding strength, which also increases the difficulty of characterization of interface problems.There is still a lack of research on the influence mechanism of alloy elements on interface structure and properties. A better understanding of the addition of beneficial elements of Fe/Al interface bonding is important for its protective performance of the coating and the coating design of HDA. We employ the first principle calculation method[20-23]to build and characterize the Fe/Al interface model at the atomic scale to make up for the deficiency of experimental research.

In this work, we have employed the firstprinciples calculation to deeply study the interface structure, electronic structure and interface strength of Fe(110)/Al(110). At the same time, the influence mechanism of elements Mg and Si on the interface of Fe(110)/Al(110) is revealed, which establishs a theoretical foundation for optimizing the interface structure and regulating the interface properties of Fe/Al coating system.

2 Computational methods

The first principle calculation in this work is carried out by VASP software[24]based on density functional theory. The exchange correlation potential of elements is described in the form of PBE[25]projection plane wave pseudopotential (PAW)[26]. Fe-3p63d64s2，Si-3s23p2，Al-3s23p1and Mg-3s2are valence electrons,and the other electrons in the inner layer are core electrons. The atomic structures of Fe and Al are shown in Fig.1. The lattice sizes of Fe and Al are 2.866 5 and 4.049 5 Å. As a typical BCC structure metal, the surface energy of Fe (110)＜(100)＜(111)[27], and the surface energy of Fe (110) is the lowest and most stable.Considering the lattice size of Fe and Al, the surface of Fe(110) and Al(110) can form a coherent lattice. The Fe(110) surface and Al(110) surface are shown in Fig.2.In order to ensure lattice matching, A-B-A Fe(110)/Al(110) interface coherent model is built. The Fe(110)and Al(110) interface structures adopt symmetrical surfaces. In order to ensure that there is no interaction between atomic layers in thez-axis direction, a vacuum layer with a thickness of 16 Å is added to the interface model. The monkhorst pack[28]method is used to divide thek-space grid to characterize the energy integral. Ak-space grid number of 8 × 8 × 1 is selected. The cutoff point of energy characterization is 400 eV, the force on a single atom is less than 0.1 eV/nm, and the total energy convergence value is 5.0×10-5eV/atom。

Fig.1 Crystal structure of Fe and Al: (a) Fe; (b) Al

Fig.2 Surface and lattice size of Fe(110)/Al(110): (a)Fe; (b)Al

3 Results and discussion

3.1 Interface energy and works of separation of Fe(110)/Al(110)

Fig.3(a) shows the built A-B-A structure of Fe(110)/Al(110) interface model, and Fig.3(b)shows the coordination of Al(110) surface atoms on Fe(110) surface. There are four possible coordination positions of Al atoms on the surface of Fe(110).The structural parameters and interfacial energy of Fe(110)/Al(110) interface are showed in Table 1. T,H, B and O respectively represent top position, empty position, bridge position and other positions. Before optimization, the coordination of Al(110) surface is at the top, hollow, bridge and other positions of Fe(110) surface. After optimization, only two methods are stable. The Al atom on the surface of Al(110) is coordinated at the top of Fe atom (Fe-Top structure) or at the hollow of Fe atom (Fe-Hollow structure). The interface of the other two coordinations are unstable.

Table 1 Structural parameters and interfacial energy of Fe (110) /Al (110) interface

Fig.3 Interface structure of A-B-A type of Fe(110)/Al(110) and coordination of Al(110) surface on Fe(110): (a) Interface structure of Fe(110)/Al(110); (b) Coordination of Al(110)surface on Fe(110) surface

The interface energy can be described by the interface energy equation[29]:

According to Eq.(1), the calculation results of interface energy of Fe(110)/Al(110) are shown in Table 1. The interface energy depends on the coordination of the two phases in the interface structure. The interface energy of the two phases are different due to different coordinations. However, the interface energy values of the two coordinations have little difference.The interface energy of Fe-Hollow coordination is smaller (1.29 J/m2), and the interface energy of Fe-Top coordination is larger (2.09 J/m2). The Fe-Hollow structure has the smallest interface energy, which indicates that this type structure is the most stable. In the actual, Fe(110)/Al(110) interface structure prefers to be combined in the form of Fe-Hollow structure.

The interface strength between Al coating and Fe depends on the interface structure of Fe(110)/Al(110).The interface strength is evaluated by calculating the works of separation(Wsep)[30].Wsepof Fe(110)/Al(110)interface is evaluated by

In the process of Fe/Al interface adhesion,the atomic charges of Fe and Al in the sub interface layer near the interface may accumulate towards the interface, which will weaken the binding effect of the coating. Therefore, it is very important to calculate the work of separation between the sub interface layers near the interface. By comparing the work of separation, the interface separation surface is estimated,and the actual binding strength of the interface is determined.

The interface structure and work of separation of Fe(110)/Al(110) are shown in Fig.4. The red mark in Fig.4 represents the theoretical separation surface.It can be seen from the figure that the coordinations of Fe-Top structure and Fe-Hollow structure are at the sub-interface layer on the side of Al(110) near the interface, and the calculated work of separation is the smallest. The work of separation of Fe-Top structure(6.81 J/m2) is greater than the work of separation of Fe-Hollow structure (4.80 J/m2). Therefore, it is seen that the interface binding of Fe-Top structure is relatively high, and the interface binding of Fe-Hollow structure is poor.

Fig.4 Interface structure and work of separation of Fe(110)/Al(110):(a) Fe-Top; (b) Fe-Hollow

3.2 Electronic structure of Fe(110)/Al(110)interface

In order to study the characteristics of Fe(110)/Al(110) interface, we analyze the charge gain and loss of the interface, which is helpful to better understand the characteristics of Fe(110)/Al(110) interface. Fig.5 shows the charge density at the Fe(110)/Al(110)interface. From the charge density in Fig.5, it can be seen that the Fe(110)/Al(110) surface atoms have a strong orbital hybridization effect, and Fe atoms form a strong covalent bond with Al atoms at the interface.This result has been proved by the formation of a variety of intermetallic compounds between Fe and Al.

Fig.5 Interface charge density of Fe(110)/Al(110): (a) Fe-Top; (b)Fe-Hollow

Fig.6 represents the deformation charge density at the Fe(110)/Al(110) interface. It can be seen from the deformation charge density in Fig.6 that the subinterface layer on the Al(110) side near the interface in the coordination of Fe-Top and Fe-Hollow structure decrease obviously. It indicates that the binding strength at the sub-interface layer is weak, and the coating will drop from the sub-interface layer on the side of Al (110). This conclusion is consistent with the calculation results of work of separation above.

Fig.6 Deformation charge density of Fe(110)/Al(110) interface: (a)Fe-Top; (b) Fe-Hollow

3.3 Segregation of Mg and Si of Fe(110)/Al(110) interface

Combined with the above conclusions, it can be found that the Fe(110)/Al(110) interface structure prefers to the coordination of Fe-Hollow structure.Therefore, the subsequent works on the effect of alloy elements on the interface elements of Fe(110)/Al(110) is carried out under the Fe-Hollow structure coordination. Therefore, in this work, the effect of alloy elements on the interface elements of Fe (110)/Al(110) will be studied in the coordination of Fe-Hollow structure.

A 2×2×1 super cell is established, when calculating the effect of alloying elements on the interface elements of Fe(110)/Al(110). The interface segregation degree is based on the number of atoms per layer on the Fe(110) surface. The interface structure is doped with single Mg atoms and Si atoms to evaluate the element effect, and the interface segregation degree is 1/4ML.

In order to further study the influence of alloying element segregation on the binding strength of Fe(110)/Al(110) interface, the work of separation and charge density of segregated Si and Mg atoms were calculated.The segregation structure and work of separation of Si and Mg atoms at Fe(110)/Al(110) interface are shown in Fig.7. The value marked in red in Fig.7 represents the minimum work of separation, so it is most likely to cause separation here. It can be seen that when Si atoms segregate at the interface, the theoretical separation surface changes from the sub-interface layer on the Al(110) side near the interface to the Fe(110)/Al(110) interface layer, and the bonding strength of the interface also decreases significantly, which is 2.09 J/m2. When Mg atoms segregate to the interface, the theoretical separation surface is still a sub-interface layer close to Al(110), but the binding strength of the interface is extremely reduced, only 2.09 J/m2. In general, the segregation of Si and Mg elements will affect the interface binding, which should be related to the influence of the segregation of Si and Mg atoms on the binding mode of interface atoms.

Fig.7 Segregation structures and works of separation of Mg and Si at Fe(110)/Al(110) interface: (a) Si; (b) Mg

The charge density of the segregation structures of Si and Mg at Fe(110)/Al(110) interface is shown in Fig.8. It can be seen from Fig.8 that the charge density at the interface of segregated Si and Mg atoms has many similarities with the charge density at the interface without segregation. However, with the segregation of Si and Mg atoms at the interface, the charge of the sub interface layer on the Fe(110) side near the interface increases significantly. As a result,there is obvious charge weakening in the interface layer and the sub interface layer on the side of Al(110),which will be harmful to the binding of the interface layer. Fig.9 represents the deformation charge densities at Fe(110)/Al(110) interface of Mg and Si segregation.In Fig. 9, the Fe-Si bond and Fe-Mg bond formed by the segregation of Si and Mg at the interface are significantly weaker than the Fe-Al bond. The new bond formation causes the charge accumulation to the side near the interface Fe (110), which affects the interface bonding strength.

Fig.8 Charge density of segregation structures of Si and Mg at Fe(110)/Al(110) interface: (a) Si; (b) Mg

4 Conclusions

a) The Fe(110)/Al(110) interface structure of Fe-Hollow coordination is more stable with smaller energy. The calculations also show that the separation surface in the form of Fe-Hollow coordination appears at the sub-interface layer on the Al (110) side.

b) The interface structure of Mg and Si segregation is similar to that of undoped alloy elements. Mg and Si elements will segregate on the Fe/Al interface and occupy the Al lattice on the Al (110)side.

c) The Fe-Si bond and Fe-Mg bond formed by the segregation of Si and Mg at the interface are significantly weaker than the Fe-Al bond. The segregation of Mg and Si elements will reduce the interface binding.

Conflict of interest

All authors declare that there are no competing interests.