Investigation of the thermal deformation behavior exhibited by oxidation products in Fe-Si alloys

The thermal deformation behavior of oxidation products from Fe-Si alloys with varying Si contents was systematically investigated using a thermal simulation testing machine during compressive deformation at temperatures ranging from 800 to 1100 ℃ . The results indicate that FeO exhibits excellent plastic deformation ability and is the primary component undergoing deformation during the thermal process. The plasticity of the oxide product improves with higher deformation temperature, as does the coordination between the oxide product and matrix, and interface straightness. However, an increase in Si content leads to a decrease in FeO content within the oxide product, resulting in reduced overall plastic deformation ability and reduced deformation coordination of oxidation products with the substrate. At the interface, Fe 2 SiO 4 forms a spinel solid solution composed of Fe 2 SiO 4 with FeO and SiO 2 that possesses a certain plastic deformation ability due to its FeO content. Moreover, the rise in Si content leads to an increased concentration of point defects in FeO near the interface side of the matrix, thereby facilitating dislocation climbing of FeO and resulting in a steady-state increase in plastic deformation. Thus the increase in the concentration of cationic defects in FeO due to the elevated Si content and the generation of Fe 2 SiO 4 solid solution at the interface between the oxidation products and the substrate are the main reasons for the improvement in the �atness of the interface between the oxidation products and the substrate.


Introduction
During the rolling process, the high surface temperature of the steel plate and exposure to air result in a large amount of iron oxide on its surface.Even after undergoing high-pressure water phosphorus removal treatment, the steel still maintains a high temperature and quickly generates a new layer of iron oxide on its surface.The uneven deformation caused by rolling force between the billet and roll destroys the straightness of their interface with substrate, causing iron oxide to peel off from the steel surface and be pressed into it, resulting in defects such as pockmarks, pits, and indentations [1][2][3][4].Therefore, it is imperative to comprehend the heat deformation behavior of steel oxidation products in order to enhance the surface quality of hot-rolled steel.
Current research primarily focuses on the impact of deformation temperature, rate, and other factors on the thermal deformation behavior of iron oxide.Sun et al. [5] investigated the high-temperature thermal deformation behavior of iron oxide through hot rolling experiments and found that it can deform in harmony with the substrate at elevated temperatures.The rolling speed has a minimal effect on its deformation behavior at a given substrate under pressure, while temperature exerts a greater in uence.
The effects of rolling temperature and rolling deformation on the properties of hot rolled mild steel iron oxide were investigated by Han et al. [6] It was found that at high temperatures, the iron oxide exhibits a certain degree of plasticity which is further enhanced with increasing rolling temperature.Suárez et al. [7] investigated the deformability of iron oxide within the temperature range of 650 to 1150 ℃ and found that above 900 ℃, the deformation temperature was more complete with excellent plasticity.Conversely, below 700 ℃, a large number of cracks were produced in the iron oxide resulting in poor plasticity.Hidaka et al. [8] examined the high-temperature mechanical properties of iron oxide generated from pure iron at temperatures ranging from 600 to 1200 ℃.The results indicated that as deformation temperature increased, tensile strength decreased while elongation increased for FeO.In contrast, Fe 3 O 4 exhibited higher tensile strength but lower elongation than FeO; whereas Fe 2 O 3 maintained high tensile strength even at 1200 ℃ with hardly any elongation.Graf et al. [9,10] proposed for the rst time to use three major iron oxides with different oxygen concentrations as pure powders to independently analyze the thermal deformation process and all properties of each oxide during hot rolling, in order to verify the deformation properties of iron oxide.The results indicate a correlation between porosity and ow stress for each oxide, as well as between temperature, strain rate, and ow stress.Fe 2 O 3 and Fe 3 O 4 exhibit a small degree of deformation while FeO has the maximum degree of deformation; furthermore, due to its hardness, Fe 2 O 3 exhibits a maximum ow stress that is equal or even higher than that of the steel matrix at the same temperature.Utsunomiya et al. [11] explored the effect of hot rolling deformation on iron oxide using a thermal processing method with glass coating.The results showed that when the deformation amount was less than 20%, there were no cracks in the surface of experimental specimens, while strip cracks perpendicular to the rolling direction appeared when they exceeded 30%.Moreover, as deformation increased, crack width also increased.
Oxidation products exhibit distinct physical and chemical properties due to their varying crystal structures and ionic valence states.During the rolling process, oxidation product grains undergo elongation, compression, and rotation, resulting in different crystal orientations and deformation patterns.This signi cantly impacts the deformability of iron oxide during hot rolling.Therefore, numerous researchers have utilized EBSD techniques to investigate the effects of crystal structure, orientation, and weaving on oxide phase deformability [12][13][14].Suárez et al. [15] analyzed by EBSD that the oxide layer formed on ultra-low-carbon steel surfaces is predominantly composed of well-plasticized FeO at temperatures above 900 ℃ with two types of ber weave: cubic or rotating-cubic; when deformation reaches a certain level, ber weave rotates in direction.The deformation behavior of hot rolled high strength steel with three times iron oxide was analyzed by Wang et al.As a frequently utilized alloying element, silicon can signi cantly enhance the corrosion resistance and mechanical properties of steel [18][19][20].During hot rolling, Si exhibits a higher a nity for O than Fe, resulting in the formation of internal oxides (SiO 2 ) and external oxides (Fe 2 SiO 4 ) on the surface of the steel matrix prior to Fe oxide formation [21][22][23].The presence of Si affects high-temperature oxidation behavior due to different crystal structures in oxidation products resulting in varying diffusion abilities of iron ions in Si and Fe oxide products.Yang et al. [24] investigated the high-temperature oxidation behavior of silicon steel and found that in the initial stage of oxidation, a SiO 2 layer rapidly forms on the surface of the steel plate, while Fe oxides react with Fe 2 SiO 4 to form a solid solution.As oxidation progresses, O 2− diffuses inward from the gas phase and reacts with Si within the matrix, resulting in the formation of SiO 2 oxide particles and the gradual development of an internal oxide layer rich in Si within the matrix [25][26][27].Furthermore, when Fe-Si alloys are exposed to temperatures above 1173 ℃, lowmelting solid solution Fe 2 SiO 4 formed by SiO 2 and FeO present in iron oxide will signi cantly affect their oxidation behavior.Speci cally, Suárez et al. [28] found that at such high temperatures, an enriched layer of molten Fe 2 SiO 4 will grow outward through penetration into iron oxide as it acts like a nail to increase adhesion between oxides which affects descaling effects [29,30].He et al. [31,32] studied silicon steel and observed that after melting occurs in Fe 2 SiO 4 its depth of penetration increases with temperature.
However, steel containing Si tends to form a Si-enriched layer on the substrate surface during oxidation, which exists at the iron oxide-substrate interface and greatly affects the thermal deformation behavior of iron oxide during rolling.Currently, the effect of Si on the high-temperature thermal deformation behavior of oxidation products is unclear.Therefore, it is necessary to conduct compression deformation experiments on Fe-Si alloy at 800 ~ 1100 ℃ to clarify the mechanism of high-temperature deformation of oxidation products and the effect mechanism of Si on their deformation behavior.This study aims to provide theoretical guidance for improving surface quality in hot-rolled steel products containing Si.

Experimental materials and methods
For this experiment, Fe-0.2Si and Fe-0.8Si alloys were utilized, with their respective chemical compositions detailed in Table 1.The cylindrical specimens measured 8 mm in diameter and 15 mm in length, featuring two parallel concave grooves at both ends measuring 2.5 mm deep and 5 mm wide at the bottom surface of each groove.To facilitate oxidation product growth during the pre-oxidation stage, the bottom surface of each groove was polished to a roughness of 1.5 µm using a grinder as shown in Fig. 1.All deformations mentioned herein refer solely to those on the bottom surface of each groove and are not related to any deformations occurring elsewhere on the specimens.The compression deformation experiments were conducted using the MMS-300 thermal simulation tester developed by Northeastern University, and the experimental process is illustrated in Fig. 2. The thermocouple was initially soldered at the center of the specimen, followed by placing it in the assembled thermal simulation experiment machine and ensuring a perfect t with the top of the concave groove using a compression hammerhead.The experiment proceeded according to the preset deformation process, and high-temperature compression experiments were conducted using the thermal simulator while maintaining contact between only the bottom surface of the groove and the surface of the hammerhead.After each experiment, the replacement of specimens occurred before repeating compression deformation experiments.The speci c steps are as follows: (1) Pre-oxidation stage: The specimen was heated to a target temperature of 1100 ℃ at a rate of 100 ℃•s − 1 and the furnace chamber was lled with air.The specimen was then held at the target temperature for 5 minutes to generate initial oxidation products.
(2) Compression deformation stage: The compressive deformation was conducted at temperatures ranging from 800 to 1100 ℃, with a rate of 0.1 s − 1 and different preset deformations (10%, 25%, and 40%).The preset deformation refers to the initial oxidation product region at the bottom of the concave groove.
(3) Cooling stage: Reaching room temperature at a cooling rate of 50 ℃•s − 1 following the completion of compression deformation experiments.
(4) Preparation and analysis of samples: After inlaying, grinding, mechanically polishing, and treating with alcohol etching, the cross-section and phase composition were analyzed using JEOL-JXA-8530F Electron Probe Micro-Analysis (EPMA) with Cu-Kα radiation X-Ray Diffraction (XRD).Additionally, the crystal structure of the oxidized products was characterized using an Electron Backscatter Diffraction (EBSD).

Morphological characteristics of oxidation products in cross-sections following pre-oxidation of Fe-Si alloy
The cross-sectional morphology and phase composition of the oxidation products generated by Fe-Si alloy, following pre-oxidation at 1100 ℃, are depicted in Fig. 3.The oxidation products exhibit a typical layered structure, with a Si-enriched layer adjacent to the substrate, followed by a thick FeO layer in the middle, then a relatively thin Fe 3 O 4 layer, and nally an outermost thin Fe 2 O 3 layer.As FeO is a cationde cient P-type semiconductor oxide with numerous internal cation vacancies and electron holes that facilitate outward diffusion of metal cations, it forms the thickest layer among all oxides generated during high-temperature oxidation.The average thickness of oxidized products for Fe-0.2Si and Fe-0.8Si alloys was approximately 133.82 µm and 108.64 µm respectively; while different Si contents do not signi cantly affect the structural type of oxidation products in Fe-Si alloys, they have more pronounced effects on their oxide proportions due to increased thicknesses of Si-enriched layers at interfaces between matrixes and oxidation products that hinder diffusion of Fe 2+ from matrices to outermost layers.

Morphological characteristics of oxidation products in cross-section under varying deformation rates
The cross-sectional morphology of oxidized Fe-Si alloy products after deformation at 800 ℃ with varying deformation rates is presented in Fig. 4. For preset deformation rates of 10%, 25%, and 40%, the actual deformation rates of oxidized Fe-0.2Si alloy were determined to be 9 oxidation products in Fe-0.8Si alloy, while the proportion of FeO decreases.As a result, the outer layer of oxide experiences more severe aking and cracking, leading to overall worse deformation of iron oxide.
The morphological characteristics of the oxidized products of Fe-Si alloy in cross-section, following deformation at 1100°C with varying deformation rates, is depicted in Fig. 5.The actual deformation rates for Fe-0.2Sialloy oxidation products were 10.51%, 25.27%, and 40.03%, with average thicknesses of the oxidation products measuring 119.75 µm, 100 µm, and 82.04 µm, respectively, while the actual deformation rates for Fe-0.8Sialloy oxidation products were measured at 9.51%, 24.24%, and 39.07% with corresponding thicknesses of oxidized product measuring in at approximately 97.77 µm, 80.24 µm and 66.08 µm after compression deformation.The highest deformation temperature was achieved when reaching a temperature of 1100 ℃; thus, resulting in an improved ability to deform the oxide layer within Fe-Si alloys under similar amounts of deformation compared to lower temperatures such as 800 ℃ (as shown in Fig. 4).This coordinated deformation resulted in fewer cracks and spalling areas produced within the main bearer layer (FeO), indicating that it has higher plasticity abilities when exposed to this speci c temperature range than other temperatures tested previously.Furthermore, the straightness between oxide product interfaces and substrates after undergoing deformation is greatly improved compared to those observed during testing conducted at 800 ℃.
The cross-sectional microscopic morphology of Fe-Si alloy oxidation products at 900 and 1100 ℃ after a 10% deformation rate is shown in Fig. 6.By comparing the cross-sectional morphology of iron oxide at different deformation temperatures, it can be observed that as the temperature increases, the degree of fragmentation decreases gradually, while the crack size and number decrease gradually, and interface straightness between iron oxide and steel matrix improves continuously.With an increase in Si content, structural ratios of Fe 2 O 3 , Fe 3 O 4 , and FeO in the iron oxide skin change resulting in lower actual deformation rates for the iron oxide skin of Fe-0.8Si alloy compared to those for Fe-0.2Sialloy; however, due to Si oxidation products at the interface between oxidation products and substrate enrichment layer becoming thicker, interface atness improves.
Figure 7 illustrates the average thickness and actual deformation rate curves of oxidized Fe-Si alloy products under varying deformation temperatures and rates, revealing a decrease in coordination between oxide products and matrix with increasing deformation rate.At identical Si content and deformation temperature, the coordination between substrate and oxidation product gradually declines as the deformation amount increases.When compressed at 1100 ℃, oxidation products exhibit better plasticity than at lower temperatures, approaching a nearly equal ratio to the matrix.Under constant deformation volume and temperature, higher Si content leads to decreased coordination due to increased proportions of high-valent oxides (Fe 3 O 4 and Fe 2 O 3 ) relative to low-valent oxides (FeO), which bear most of the plasticity capacity.
The lengths of the oxidation product-matrix interface and the maximum depth of indentation of Fe-Si alloys after different deformation temperatures and deformation amounts are shown in Fig. 8. Comparing Fig. 8 (a) (c), it is evident that increasing deformation amount deteriorates the straightness of oxide product-substrate interface, as indicated by an increase in length of oxide product-substrate interface and maximum depression depth at same deformation temperature and Si content.Conversely, comparing Fig. 8 (b) (d), increasing deformation temperature improves the straightness of the oxide product-substrate interface, as evidenced by a decrease in the length of the oxide product-substrate interface and maximum depression depth under the same deformation volume and Si content.
Furthermore, increasing Si content reduces both the length of the oxidation product-substrate interface and maximum depression depth under conditions of the same deformation temperature/rate, indicating an effective reduction in the depth of the oxidation product pressed into the substrate during the deformation process.

Mechanism of the in uence of elemental Si on the deformation coordination of oxidation products
The thicknesses of different oxidation products of Fe-Si alloys, subjected to compressive deformation at 800 and 1100 ℃ with varying degrees of deformation, are presented in Fig. 9.It is evident from the Figure that when the temperature for deformation ranges between 800 and 1100 ℃, the thickness of FeO in the oxidation products of Fe-Si alloy undergoes maximum reduction while that of Fe 3 O 4 experiences a slight decrease; however, there is almost no change observed in the thickness of Fe 2 O 3 .This implies that among all oxidation products, FeO exhibits greater plasticity than others whereas Fe 3 O 4 has some degree of plasticity and nally, there is negligible plasticity associated with Fe 2 O 3 .Furthermore, it can be inferred from Fig. 9 that the overall reduction in thickness for oxidation products derived from an alloy containing 0.8% Si (Fe-0.8Si) is less as compared to those obtained using an alloy containing only 0.2% Si (Fe-0.2Si).This suggests that alteration in structural ratio due to an increase in Si content affects the overall deformability capacity exhibited by these oxide layers.
The cross-sectional morphology and elemental distribution of the oxidized products of Fe-Si alloy, following deformation at 800 ℃ to strains of both 10% and 40%, are presented in Fig. 10.The distribution of Fe and O elements clearly re ects the structure of the oxidation products.Compared to Fig. 3, which shows the original oxidation products without deformation, it is evident that deformation mainly occurs in the FeO layer, indicating that FeO is primarily responsible for bearing deformation in these products due to its superior plasticity at high temperatures.Conversely, layers such as Fe 3 O 4 and Fe 2 O 3 are less plastic and more prone to cracking or even rupture during compression processes.At temperatures below 800 ℃ for planar compression deformation, intermittent ruptures can occur within oxide layers leading to broken or peeling oxidation products [7,33,34].However, when highly plastic FeO dominates above this temperature threshold in oxide layers subjected to planar compression or hotrolling deformations there is little change in strength allowing them to resist large amounts of deformation without fracture or shedding.
The room temperature breaking stress of FeO, Fe 3 O 4 , and Fe 2 O 3 is approximately 0.4 MPa, 40 MPa, and 10 MPa respectively.Consequently, the low breaking stress at room temperature results in brittle behavior of oxidation products formed on steel surfaces.Even small deformation loads can cause detachment and breakage of surface oxides when hot rolled steel is deformed at room temperature.However, unlike at room temperature conditions, oxides exhibit plastic behavior under high temperature conditions [8].This is because the polycrystalline organization of oxidation products in steel determines their deformation, which requires coordination and cooperation among adjacent grains with different orientations as well as overcoming obstacles posed by grain boundaries.Polycrystalline plastic deformation relies on the availability of ve or more independent slip systems to meet the coordination requirements of each grain during deformation, a factor directly in uenced by the crystal structure type.  is spinel, which belongs to the face-centered cubic system, is considered the most likely slip plane for dislocations.Therefore, the primary slip system of Fe 3 O 4 is believed to be , and its plastic deformation results from dislocation slipping along the slip plane.The main slip system for FeO dislocation slip is , but at high temperatures, it shifts to .Crystals with a cubic structure (e.g., Fe 3 O 4 and FeO) are more susceptible to dislocation slipping than rhombic hexahedral crystals (e.g., γ-Fe 2 O 3 ) due to their geometry that limits the number of available dislocation slip systems.This can explain why the plastic deformation of α-Fe 2 O 3 observed in this paper was di cult.Moreover, compared with α-Fe 2 O 3 (a = 0.50 nm, c = 1.37 nm) and Fe 3 O 4 (0.84 nm), FeO (0.43 nm lattice constant) exhibits higher plastic deformability because decreasing Peierls-Nabarro stress point lattice constants tend to increase dislocation mobility [8].
When the deformation temperature ranges from 800 to 1100 ℃, Fe 3 O 4 mainly deforms through dislocation slip, while FeO undergoes both dislocation slip and climbing (dislocation creep).At high temperatures, dislocation climbing dominates and determines the plastic deformation ability of FeO.The diffusion of point defects is considered crucial for dislocation climbing since it contributes to the in ow of point defects into dislocations.The diffusion coe cients at 1000 ℃ are 9×10 − 8 cm 2 •s − 1 for FeO, 2×10 − 9 cm 2 •s − 1 for Fe 3 O 4 , and 2×10 − 15 cm 2 •s − 1 for Fe 2 O 3 .This indicates that rapid point defect ow can promote the dislocation climbing of FeO due to its signi cantly larger diffusion coe cient than other oxides.On the other hand, due to a smaller diffusion coe cient in comparison with that of FeO, point defect ow has much less effect on promoting dislocation climbing in Fe 3 O 4 ; thus its plastic ow is mainly supported by dislocation slip.Additionally, as a common metal cation-de cient nonstoichiometric compound, cation vacancies inside the crystal structure contribute to dislocation climbing in FeO.
Increasing concentration levels of cation defects can improve its steady-state plastic deformation capacity [35].Above 800 ℃ deformation temperature range further enhances the steady-state plastic deformation capacity of FeO due to the accelerated diffusion rate of Fe 2+ and ow of point defects facilitating dislocation climb.
The cross-sectional morphology of the oxidized products of Fe-0.2Si alloy after deformation was further analyzed using the electron backscatter diffraction (EBSD) technique.The IPF maps in Fig. 12 (a) (b) (c) reveal that the grain orientation of the oxidation products exhibits no discernible distribution of merit orientation, with different colors indicating distinct crystal orientations.From outer to inner regions, the structure comprises thin Fe 2 O 3 ne grains, continuous tight small-sized Fe 3 O 4 columnar crystals, and large-sized FeO columnar crystals near the substrate side.Moreover, a Si-enriched layer is present at the interface between oxidation products and substrate which appears black in Fig. 12 due to unidenti able Fe 2 SiO 4 .With the increasing incidence of depression, the number of FeO grains in the oxidation products of Fe-Si alloy after deformation continues to increase while grain size gradually decreases, indicating dynamic recrystallization during deformation.To analyze the degree and distribution of recrystallization in these oxidation products, software was utilized resulting in Fig. 12 (d) (e) (f).The blue areas represent fully recrystallized tissue, yellow represents incompletely recrystallized tissue, and red represents deformed tissue with some level of recrystallization.As shown in the Figure, different degrees of recrystallization tissues are formed with an increase in deformation rate indicating that dynamic recrystallization does occur during deformation.Furthermore, as the deformation rate increases so too does the degree of deformation which is re ected by an increased content of blue area representing complete recrystallization within oxidation products.
4.2 Mechanism of the effect of Si on the interfacial straightness of oxidation products The cross-sectional morphology and XRD analysis results of the Si-enriched layer at the interface of Fe-0.2Si alloy after pre-oxidation at 1100 ℃ are presented in Fig. 13, while  higher O content and lower metal cation concentration.Therefore, an increase in Si content results in a proportional rise in the concentration of cationic defects within the resultant material.
The cross-section morphology and Fe element distribution of the oxidation products of Fe-Si alloys with different Si contents after oxidation at 1100 ℃ without compression deformation are shown in Fig. 15.
From the content of Fe in the oxidation products, it can be seen that the content of Fe in FeO near the substrate side of Fe-0.8Si alloy is lower than that of Fe-0.2Si, which can be observed more intuitively in the local magni cation diagram, which is consistent with the results of the equilibrium phase diagram of Fe-O with different Si contents shown in Fig. 14.Compared with Fe-0.2Si,FeO has a lower concentration of iron ions, which means that it has higher cationic defects and the increase in the concentration of point defects promotes the dislocation migration of FeO, which improves its steady-state plastic deformation ability; moreover, at the interface between the oxidation product and the matrix, the solid solution formed by Fe 2 SiO 4 and FeO can be deformed along with the oxidation product and the matrix under high-temperature conditions.Therefore, the increase in the concentration of cationic defects in FeO due to the elevated Si content and the solid solution generated at the interface between the oxidation product and the matrix are the main reasons for the improvement of the straightness of the interface between the oxidation product and the matrix.

Conclusions
In this paper, we investigated the thermal deformation behavior of Fe-Si alloy oxidation products and systematically analyzed the effects of deformation temperature, amount, and Si element on their high temperature deformation ability.The main conclusions are as follows: (1) Through investigation of the thermal deformation behavior of Fe-Si alloy oxidation products, it has been discovered that these products exhibit favorable plastic deformation ability above 800 ℃ and can be plastically deformed in conjunction with the matrix.Within the experimental temperature range, higher temperatures correspond to greater plasticity of oxidation products and improved coordination during deformation.Increasing Si content results in a reduction of overall plastic deformation ability for oxidation products, thereby diminishing their capacity for coordinated deformation.
(2) The plastic deformation ability of oxidation products is determined by their crystal structure.FeO exhibits superior plastic deformation ability due to its joint mechanism of dislocation slip and climbing, while the deformation mechanism of Fe 3 O 4 is dominated by dislocation slip, resulting in a second-best deformation ability.Conversely, Fe 2 O 3 displays negligible plastic deformation ability.Therefore, during high temperature deformation processes, FeO bears the primary responsibility for plastic deformation.
(3) The elevated Si content enhances the interfacial straightness between the oxidation products and substrate.At the interface of the Fe-Si alloy oxidation product and matrix, a solid solution composed of Fe 2 SiO 4 with FeO and SiO 2 exists in a spinel structure.Due to the presence of FeO, it exhibits certain plasticity under high temperature conditions and can deform along with the oxidation product and matrix while maintaining a gap from them, effectively relieving compressive stress caused by deformation.Moreover, increased Si content leads to a higher concentration of point defects in FeO which promotes dislocation climbing and improves its steady-state plastic deformation ability, thereby enhancing atness at the interface between the oxidation product and substrate.

Declarations Figures
Schematic [16]  using the EDSD technique.The Fe 3 O 4 in the oxide layer formed columnar crystals, while spherical particles were observed near the matrix.With increasing deformation rate, the Fe 2 O 3 layer gradually embedded into the surface of Fe 3 O 4 layer.Grains with lower Schmidt factor stored higher strain energy as indicated by slip system in direction.Yu et al.[17] studied the organization and evolution of deformed oxide layers in hot-rolled microalloyed steels.They discovered that Fe 2 O 3 and FeO have a strong and weak ber weave parallel to the oxide growth direction in the deformed oxide layers.Additionally, Fe 2 O 3 forms a basal plane along a direction parallel to the Fe 3 O 4 grain surface, while the Taylor factor of oxides signi cantly affects grain deformation.
Fe 2 O 3 belongs to the corundum-type rhombic crystal system and exhibits different crystal structures at varying temperatures.Below 200 ℃, it exists as γ-Fe 2 O 3 with a cubic structure, while above 200 ℃, γ-Fe 2 O 3 transforms into α-Fe 2 O 3 with a rhombic hexahedral structure.Therefore, Fe 2 O 3 formed through high temperature oxidation above 800 ℃ is exclusively α-Fe 2 O 3 .The lattice constants of α-Fe 2 O 3 are a = 0.5 nm and c = 1.37 nm.

Figure 11 (
Figure 11 (a) illustrates the crystal structure of Fe 2 O 3 in which O 2− occupies the densely packed hexagonal lattice and Fe 3+ occupies interstitial positions with only three slip systems.As depicted in Fig. 11 (b), Fe 3 O 4 possesses an anti-spinel cubic structure with a lattice constant of a = b = c = 0.84 nm.The tetrahedral interstitial sites are occupied by Fe 3+ ions at 1/8th occupancy, while half of the octahedral gap positions are occupied by the remaining Fe 3+ and Fe 2+ ions, resulting in twelve slip systems during plastic deformation.In contrast, Fig. 11 (c) shows that FeO exhibits a face-centered cubic structure

Figure 14 illustrates
Figure 14 illustrates the local region of Fe-O equilibrium phase diagrams for Fe-Si alloys with varying Si contents and temperatures.The red and green dashed lines indicate the range of O mass fraction in the FeO single-phase region for Fe-0.2Si and Fe-0.8Si alloys, respectively.The generated FeO in oxidation products of Fe-Si alloys is located within this region, as con rmed by phase diagrams showing that preeutectic Fe 3 O 4 will form during cooling processes.Comparison of the single-phase region of the Figure with its rightward shift upon increasing Si content suggests the formation of P-type semiconductors with diagram and physical diagram of a specimen Page 16/27

Figure 15 Distribution
Figure 15

Table 1
.13%, 22.5%, and 37.26% respectively, with corresponding average thicknesses of oxidation products measuring at 121.6 µm, 103.7 µm, and 83.95 µm.The actual deformation rates of the oxidation products of Fe-0.8Si alloy, in contrast, were measured at 8.09%, 20.59%, and 35.36% respectively, while the average thicknesses of these oxidation products were categorized as 98.76 µm, 85.18 µm, and 69.13 µm.The deformation pattern of iron oxide in both alloys tended to be similar as the amount of deformation increased under a certain temperature condition.Increasing amounts of deformation led to an increase in broken iron oxide degree, crack size and number which resulted in pressing the oxidation products into the substrate interior causing unevenness at their interface.The straightness continuously deteriorated.With a certain amount of deformation, an increase in Si content leads to an increase in the proportion of Fe 3 O 4 and Fe 2 O 3 Table2shows the EDS analysis results.A signi cant amount of spinel solid solution was observed in the Si-enriched layer at the interface, and quantitative analysis of phases using EDS and XRD revealed that this spinel was a Fe 2 SiO 4 phase, with substantial amounts of FeO and SiO 2 also present.This discovery validates that the Fe 2 SiO 4 present in iron oxide of experimental steel is indeed a solid solution of Fe 2 SiO 4 spinel, consisting of FeO and SiO 2 , forming a new solid solution with good deformation ability due to its combination with the deformable substrate; furthermore, it has some gap between itself and substrate which can effectively relieve compression stress caused by deformation.