Research progress of Fe/Al2O3 interface based on first principles

Fe-Al alloy has excellent oxidation resistance and has a good application prospect in high-temperature environments. Its oxidation resistance mainly comes from the Al2O3 oxide film on the surface, and the Fe/Al2O3 interface is the weakest link between the oxide film and the substrate. With the development of computational materials science, using first-principles calculations to study the Fe/Al2O3 interface has gradually become a research topic of concern. According to the different construction methods of the Fe/Al2O3 interface, the existing theoretical models of Fe/Al2O3 interface structure are divided into three categories: γ-Fe/α-Al2O3 interface model, α-Fe/α-Al2O3 interface model and other Fe/Al2O3 interface models. Their characteristics are compared and analyzed. The latest research results of Fe/Al2O3 interface optimization measures and strengthening mechanisms are listed, and a summary of how alloying elements affect the ability of Fe/Al2O3 interface bonding is provided. The study of strengthening mechanisms can lead to better design of interfaces for new materials and thus improve material properties.


Introduction
Fe-Al alloys not only have the advantages of low density and low cost, but also have excellent corrosion, oxidation and vulcanization resistance [1].They could be used as innovative materials in the aerospace and automotive industries, as well as in energy conversion systems [2].It is a potential new material for the aerospace and automotive industries as well as for energy conversion systems.The resistance to high-temperature oxidation depends largely on the denseness of the oxide coating that covers the surface [3].The study of the Fe/Al 2 O 3 interface in this alloy system has become a hot research topic.As a typical heterogeneous structure [4], the metal's and oxide's physical and chemical properties, such as crystal structure, electronic state, thermal expansion coefficient, etc., are quite different, and it is difficult to form a common lattice or a common lattice between the two [5].Furthermore, the interfacial bonding strength is minimal.In the service environment of the material, delamination at the Fe/Al 2 O 3 interface is most likely to happen, leading to the failure of the material system.Therefore, the study of the Fe/Al 2 O 3 interface is crucial for enhancing the adhesion of oxide film, which is one of the important contents of Fe-Al alloy research.
The binding mechanism at the interface is very complex, and it is the combined result of the effects of electron orbital hybridization, charge transfer, chemical bond polarization, and dispersion [5].Due to the difficulty of sample preparation, testing and characterization, some important structural features are difficult to obtain by existing experimental means.In recent years, computer simulation calculations have also come to the fore.However, finite element simulation is not suitable for microscopic models and cannot study the strengthening mechanism in essence.It can only simulate the mechanical properties, temperature, speed and other parameters of relatively large components in the process of molding or deformation.First-principle calculations based on first principles theory [6] is a technique used in quantum mechanics to examine the electrical structure of systems with many electrons, which can avoid the difficulties encountered in the experiment.The difficulties encountered in interface testing and characterization are avoided by constructing the interface with the structural characteristics of each atom in the system, acting on the electronic behavior according to the principle of interaction between nuclei and electrons and its basic laws of motion, and analyzing the bonding of the interface so as to explain the interface phenomena [7].
This paper summarizes the theoretical models of Fe/Al 2 O 3 interfacial structure by researchers, reviews the current research on the optimization of Fe/Al 2 O 3 interfacial structure based on first principles, lists the research results of some domestic and foreign scholars on the interfacial reinforcement mechanism, and aims to summarize the results of different researchers and condense them into a brief review to help other researchers who would like to use first principle calculations to study the Fe/Al 2 O 3 interface.

Fe/Al 2 O 3 Interface models
According to the different surfaces used in building the interface models, this paper classifies the Fe/Al 2 O 3 interface models in the existing studies into three categories: γ-Fe/α-Al 2 O 3 interface models, α-Fe/α-Al 2 O 3 interface models, and other Fe/Al 2 O 3 interface models.

γ-Fe/α-Al 2 O 3 interface models
Among the available studies, using their perspectives, five scholars have created interface models, which consist of Fe( 111  The models built by the above five scholars are all γ-Fe/α-Al 2 O 3 interface models, and the difference lies in the different starting sites when cutting the crystal surface and the various number of layers that were chosen.However, due to the different observation angles of the rhombic cells, they present a large difference in the pictures.

Other Fe/Al 2 O 3 interface models
In addition to the above interface model combining a certain number of layers of Fe atoms on the surface and a certain number of layers of Al 2 O 3 atoms, some experts and scholars have also proposed new schemes.Yang et al. [15] studied the REAlO 3 (100)/Fe(100) interface.Instead of using the way of establishing the interface model first and then replacing the atoms therein, they directly used the doped element atoms to establish the crystal cell, and then used it as the basis to create the interface model, as shown in Figure 3(a).The advantage of this model is that it breaks through the limitation of mismatch theory, which is no longer limited to the Fe(111) and Fe(110) crystal faces, and the Fe(100) face is also included in the study, which provides new possibilities for the study of Al 2 O 3 interface.The disadvantage is that dopant atoms have already been added to the cell when designing the cell, and the position and number of dopant atoms have been fixed, so that it is not possible to replace any atoms to optimize the interface, just as in the above interface model.In brief, the γ-Fe/α-Al 2 O 3 interface model targets the austenitic steel system, which has the best high-temperature resistance, on the other hand, the α-Fe/α-Al 2 O 3 interface model targets the ferrite steel system, which is relatively inexpensive and has a cost advantage.Besides, the REAlO 3 (100)/Fe(100) interface model and the Fe/amorphous Na 2 SiO 3 /Al 2 O 3 interface model broaden the research ideas of the Fe/Al 2 O 3 interface.The above interface models are the researchers' design concepts about the Fe/Al 2 O 3 interface, and the study of their properties can provide an explanation for the experimental observations.

Fe/Al 2 O 3 Interface Optimization
In order to improve the material properties, the more common method is to add alloying elements to the Fe/Al 2 O 3 interface.By doping the interface with alloying elements, the interface's free energy was reduced, hence increasing the interface's bond strength.

Ti, Mg, and Mo's effects on Fe/Al 2 O 3 interface bonding
Ti can be segregated at the interface with ease, there is a significant increase in the strength and stability of the interfacial bonding [18].Electronic structure research reveals that while the creation of Ti-Fe and Ti-O chemical bonds increases the work of adhesion, the 3d, 3p, and 4s coupling of Ti permits more electrons to participate in the interfacial interactions [11].
The binding strength at the Fe/Al 2 O 3 contact can be further strengthened by doping with Ti and Mg.Atoms of Mg and Ti seem to act as interfacial binders for improvement, as evidenced by the ease with which atoms of Ti and Mg segregate to the interface and then subsequently combine with atoms of Al or O to form new compounds [18].
The interfaces doped with Ti and Mo elements have obvious atomic segregation behaviors, showing a significant doping segregation effect.Comparative analysis of the heat of precipitation of the interfacial model after doping reveals that Compared to Cr-doped surfaces, the stability of Ti and Mo-doped interfaces is superior [8].

Ni and Cr's effects on Fe/Al 2 O 3 interface bonding
There is little change in the atomic structure of the Cr-doped Fe/Al 2 O 3 interface [8].After the Cr atom replaces the Fe atom, the segregation of the Cr atom at the interface weakens the interaction between Fe and O atom, and weakens the bonding force at the interface [9].In a high-temperature environment, Cr atoms displace Al atoms at the Fe/Al 2 O 3 interface, forming the multilayer oxide (Al x Cr 1-x ) 2 O 3 .The interfacial binding force is reduced, and the binding energy is increased, when Cr atoms enter the system [13].However, Cr atoms diffuse from the alloy matrix to the oxide atom, causing the binding energy to decrease quickly [19].
The doping of Ni and Cr has no effect on increasing the bond strength at the Fe/Al 2 O 3 interface [18].Ni has no effect on enhancing the bonding strength at the interface, and Ni hardly enhances the interfacial interactions and even leads to the deterioration of the interface [11].

RE's effects on Fe/Al 2 O 3 interface bonding
The interfacial bonding is strengthened through the doping of rare earth elements (RE) such as La, Ce, Y, and Hf., which not only directly participate in the chemical bonding at the Fe/Al 2 O 3 interface, but also effectively nails the impurity S in the Fe matrix and inhibits the harmful bias of S atoms toward Weaken [11] No Change [18] Weaken [9,13] No Change [8,18] Intensify [18] Intensify [8] Intensify [20][21] Table 1 summarizes and enumerates how elements affect the bonding capacity of the Fe/Al 2 O 3 interface.From this, it can be seen that although a small amount of research has been done on Fe/Al 2 O 3 interface using first-principle calculations, they have demonstrated the great potential of first-principles calculations in the study of Fe/Al 2 O 3 interfaces.

Conclusion
This paper reviews the current progress of research on the Fe/Al 2 O 3 interface using first-principle calculations.It mainly summarizes the interface models designed by researchers for the Fe/Al 2 O 3 interface and classifies them into three categories.Among them, the first two types of models are more basic, but the research on their models has not been fully explored and needs to be further improved; the third type of model has great potential to broaden the ideas and open up new directions for the gradually solidified research lines.Furthermore, this paper summarizes the research results on Fe/Al 2 O 3 interfacial strengthening in the conducted studies, and summarizes the effects of elements on the interfacial bonding ability of Fe/Al 2 O 3 interfaces.The effect of elements on material properties can also be observed from macroscopic experiments, but the exploration of the mechanism requires the help of first-principle calculations, which can be grasped to better design new materials with superior properties.
) surface and Al 2 O 3 (0001) surface in distinct ways.Li et al. [8] established an interface model Following the selection of the low-exponential crystal surface's lowest surface energy structure, which consists of 4 layers of Fe surface with 18 layers of Al 2 O 3 surface, and in the interface's vertical direction, a 15 Å vacuum layer was applied, as shown in Figure 1(a).Liu et al. [9] determined the interface according to the densest row of surfaces and the direction of dense rows for face-centered cubic stacking and dense rows for hexagonal stacking, by combining 4 layers of Fe surfaces with 9 layers as Al 2 O 3 surfaces, as shown in Figure 1(b).Wang et al. [10] combined 5 layers of Fe surfaces and 15 layers of Al 2 O 3 surfaces with a layer of 15 Å vacuum, as depicted in Figure 1(c).Xie et al. [11] considered the effect of stoichiometric ratio on interfacial bonding and created an interfacial model consisting of 15 layers of Al 2 O 3 surfaces and 7 layers of Fe surfaces as depicted in Figure 1(d).Li et al. [12] determined the optimum number of surface layers by calculating the surface energy convergence and created the model as shown in Figure 1(e).

2. 2 .
α-Fe/α-Al 2 O 3 interface models Wang [13] established the Fe/Al 2 O 3 interface based on the two-dimensional mismatch theory by choosing the O-terminal Al 2 O 3 surface, which is steadier than the single-Al-terminal and double-Alterminal interfaces, as shown in Figure 2(a).Tang et al. [14] modeled the interface considering that the Fe(110) and Al 2 O 3 (0001) facets are densely packed facets with relatively low surface energies among the various crystalline facets.After balancing the amount of calculation and the accuracy of the calculation results, 5 layers of Fe surface and 5 layers of Al 2 O 3 surface were selected, the average value of the layer spacing of Fe and Al 2 O 3 phases was taken as the interfacial spacing, and a 10 Å vacuum layer was chosen to establish the model, as depicted in Figure 2(b).

Figure 2 .
Figure 2. Fe(110)/Al 2 O 3 (0001) interface models.The models developed by these two researchers are α-Fe(110)/α-Al 2 O 3 (0001) interface models, which are different from the above γ-Fe(111)/α-Al 2 O 3 (0001) interface model in that γ-Fe is austenitic and the cell has a face-centered cubic structure, whereas α-Fe is ferrite and the cell has a bodycentered cubic structure.Due to the difference in the atomic arrangement of the two cell types, α-Fe and γ-Fe need to be selected with different crystal faces to match Al 2 O 3 (0001).

Table 1 .
[21]strength of covalent bonds was quantified by population analysis, and the Hf-O and Y-O population values were greater than the Fe-O bond population values.This indicates that the rare earth elements reinforce the bonding at the Fe/Al 2 O 3 interface[21].Effects of elements on the interfacial bonding ability of Fe/Al 2 O 3.