Effect of Fe on the Microstructure and Mechanical Properties of Fe/FeAl2O4 Cermet Prepared by Hot Press Sintering

The Fe/FeAl2O4 cermet was prepared with Fe-Fe2O3-Al2O3 powder by a hot press sintering method at 1400 °C. The raw materials for the powder particles were respectively 2 µm (Fe), 0.5 µm (Fe2O3), and 0.5 µm (Al2O3) in diameter, the sintering pressure was 30 MPa, and the holding time was 120 min. The effects of different Fe mass ratios on the microstructure and mechanical properties of Fe/FeAl2O4 cermet were studied. The results showed that a new ceramic phase FeAl2O4 could be formed by an in situ reaction during the hot press sintering. When the Fe mass ratio was increased, the microstructure and mechanical properties of the Fe/FeAl2O4 cermet showed a change law that initially became better and then became worse. The best microstructure and mechanical properties were obtained in the S2 sample, where the mass ratio of Fe-Fe2O3-Al2O3 was 6:1:2. In this Fe mass ratio, the relative density was about 94%, and the Vickers hardness and bending strength were 1.21 GPa and 210.0 MPa, respectively. The reaction mechanism of Fe in the preparation process was the in situ synthesis reaction of FeAl2O4 and the diffusion reaction of Fe to FeAl2O4 grains. The increase of the Fe mass ratio improved the wettability of Fe and FeAl2O4, which increased the diffusion rate of Fe to FeAl2O4 grains, which increased the influence on the structure of FeAl2O4.


Introduction
Iron-based cermet has the high hardness of ceramics, good thermal stability, good wear resistance, corrosion resistance, as well as metals' thermal conductivity and toughness [1][2][3][4]. It has mainly been used in aviation, automotive, and petroleum engineering, as well as engineering in machinery such as brakes or clutches [5][6][7][8][9]. The low raw material cost of iron-based cermet greatly reduces the preparation cost and promotes the wide application of metal-based ceramics. Fe has relatively good wettability with carbides such as TiC, VC, WC, ZrC, and Cr 3 C 2 . Therefore, there have been many studies conducted on iron-based cermet with carbides as reinforcements [10][11][12][13]. However, its high price limits its use in a wide range of applications. In recent years, research on iron-based cermet using inexpensive and widely sourced Fe and Al 2 O 3 as the main raw materials has seen some development.
Bansal [14] studied the interface bonding between Fe and Al 2 O 3 due to the formation of spinel phase FeAl 2 O 4 , which improves the wettability of metal and ceramic phases. Konopka [15] investigated the influence of Fe content on the microstructure and fracture toughness of iron-based cermet, and found that the fracture toughness of Fe/Al 2 O 3 cermet depended on the Fe content and the formation of FeAl 2 O 4 during the sintering process, as well as the formation of FeO around the iron particles. The formation of micro-crack defects between FeAl 2 O 4 and Fe led to passivation of the external stress of the cermet, which led to splits and deflections in the cracks. Other studies have found that the fracture toughness of the cermet improved as the amount of FeAl 2 O 4 was increased [16,17]. The above studies demonstrate that metallic Fe has an important influence on the performance of iron-based cermet. Gupta [18][19][20][21] used Fe and Al 2 O 3 (5-30 wt%) to prepare iron-based cermet by powder metallurgy, and found that the iron-based cermet prepared with 5 wt% Al 2 O 3 and 95 wt% Fe had the lowest total surface wear. Shuai Li [22] prepared iron-based cermet with low-grade bauxite powder and reduced iron powder, and found that with increasing Fe content, the compactness of the cermet increased, the volume density increased, and the water absorption rate and microporosity decreased; compressive strength and bending strength also had a greater impact.
In this study, hot pressing and sintering were used to add a reinforcing Fe 2 O 3 phase to the Fe/Al 2 O 3 system, and FeAl 2 O 4 was formed by the in situ reaction of Fe, Fe 2 O 3 , and Al 2 O 3 to improve the wettability between Fe and the ceramic phase. The metallic Fe phase changed from solid phase to liquid phase during the hot pressing sintering process. This would promote the fluidity of the metallic phase in the ceramic phase and affect the material transfer process, and then further affect the structure and mechanical properties of the Fe/FeAl 2 O 4 cermet. Therefore, determining the law of Fe's influence on the microstructure and mechanical properties of Fe/FeAl 2 O 4 cermet was one of the key links to preparing an Fe/FeAl 2 O 4 cermet with excellent performance. In addition, Fe, Al 2 O 3 , and Fe 2 O 3 are inexpensive and have a wide range of sources. Currently, they are the main components of metallurgical solid waste, such as zinc slag, steel slag, and red mud. This work could provide an important theoretical basis for the comprehensive utilization of metallurgical solid waste [23,24].

Materials and Methods
The raw materials in this experiment were analytically pure Fe powder (2 µm), Fe 2 O 3 powder (500 nm), and Al 2 O 3 powder (500 nm). The raw material ratios of the five sintered samples are shown in Table 1. The ingredients were obtained according to the ratio in Table 1. Absolute ethanol was used as the dispersion medium, and a XQM-2 vertical planetary ball mill was used for ball milling. The ball milling speed was 300 r/min, and ball milling time was 10 h. After ball milling, the samples were placed in a DZF-6050 vacuum drying oven at 120 • C for 24 h, and the vacuum was pumped to 100 Pa during drying. After drying, the mixed powder was passed through a 200-mesh sieve and put into a ZT-40-21Y high-temperature hot press sintering furnace to prepare the Fe/FeAl 2 O 4 at 1400 • C and 30 MPa for 120 min, and the vacuum was pumped to 10 −2 Pa during sintering. The relative density of the prepared samples was measured by the Archimedes principle, the bending strength was measured using the three-point bending method with a CMT4202 universal material testing machine with a crosshead speed of 0.5 mm/min and span of 30 mm, and the Vickers hardness was measured at a loading force of 49.05 N (5 kg) for 10-15 s by Tukon2500 Vickers hardness tester. Phase and composition analysis (XPert PRO MPD, PANalytical, The Netherlands) was carried out with an X-ray diffractometer. The microstructure and element analysis were carried out with SEM and EDS (GeminiSEM 300, Zeiss, Germany), respectively.

Preparation Principle of Fe/FeAl 2 O 4 Cermet
Using the analysis of thermodynamic software FactSage, it was shown that Fe, Fe 2 O 3 , and Al 2 O 3 powder can spontaneously synthesize FeAl 2 O 4 through an in situ reaction under the experimental conditions, in the following Equation (1): Due to the excessive Fe content in the mixed powder of ingredients, Fe 2 O 3 and Al 2 O 3 reacted completely after the in situ reaction and the Fe became redundant. The FeAl 2 O 4 produced by the in-situ reaction combined with the redundant metal Fe, and the Fe/FeAl 2 O 4 cermet was prepared during the process of hot pressing and sintering. The in situ reaction occurred on the three-phase interface of Fe liquid, Fe 2 O 3 , and Al 2 O 3 . This was an interface reaction-driven wetting, according to the free energy change control theory of interface reaction proposed by Aksay [25], as shown in Equation (2): where σ 0 SL is the solid/liquid interface energy before the reaction, A is the interface area, and ∆G r is the free energy change produced by the interface reaction product per unit volume. According to Aksay [25], the decrease of free energy in the interfacial reaction is the main driving force controlling the wetting process. The improvement of wettability is caused by the decrease of free energy. The interfacial reaction is more intense, ∆G r is lower, and the wettability of the system is better. For Reaction (1), with increasing Fe content in the raw material, the in situ reaction for the synthesis FeAl 2 O 4 would be more intense, and the wettability between liquid Fe and FeAl 2 O 4 would be better. With the wetting of Fe and FeAl 2 O 4 , the increased concentration gradient of Fe would increase the diffusion rate of Fe to FeAl 2 O 4 grains. The change law states that Fe accumulates in the FeAl 2 O 4 grains as they grow, which has a greater impact on the structure of FeAl 2 O 4 at the macro level, and this is reflected in changes in microstructure and mechanical properties.
The FactSage thermodynamic software was used to draw the phase diagram of the Fe-Fe 2 O 3 -Al 2 O 3 ternary system under these experimental conditions, and the composition design included samples S1, S2, S3, S4, and S5, as shown in Figure 1.      Figure 2 shows the XRD results of the hot press sintering of samples. The results show that the phase composition of each sample was metal phase Fe and FeAl2O4. The above results are consistent with the results of theoretical thermodynamic calculations and indicate that an in situ reaction occurred in the interface between liquid Fe, Fe2O3, and Al2O3 during the hot press sintering process. 10 20        Figure 4 shows the fracture structure of different samples, which could characterize the combination of the metal phase and ceramic phase to a certain extent. The fracture of the Fe/FeAl2O4 cermet was mainly intergranular. With the increase of Fe, the microstructure of samples S1-S5 changed significantly. The bonding effect among the grains in S1 was poor, a large number of grain boundaries were exposed, and relatively more pores appeared. The section of the sample in S2-S4 became dense, the grains grew significantly, and the bonding effect among the grains was significantly improved. In S2-S4, the pores  samples S1-S5 changed significantly. The bonding effect among the grains in S1 was poor, a large number of grain boundaries were exposed, and relatively more pores appeared. The section of the sample in S2-S4 became dense, the grains grew significantly, and the bonding effect among the grains was significantly improved. In S2-S4, the pores were reduced, but slight cracks appeared in S4. The surface in S5 was powdered, the crystal grains were obviously smaller, and more pores appeared. The change rule of the microstructure was consistent with the conclusions of Aksay's interface reaction free energy change control theory [25]. The increase of Fe affected the physical and chemical reactions in the hot press sintering process. On the one hand, it promoted the in situ reaction, reduced the interfacial energy of the solid/liquid surface, and improved the wettability of Fe and FeAl 2 O 4 . On the other hand, it increased the concentration gradient of Fe on the interface between Fe liquid and FeAl 2 O 4 , and increased its diffusion rate. Figure 4 shows the fracture structure of different samples, which could characterize the combination of the metal phase and ceramic phase to a certain extent. The fracture of the Fe/FeAl2O4 cermet was mainly intergranular. With the increase of Fe, the microstructure of samples S1-S5 changed significantly. The bonding effect among the grains in S1 was poor, a large number of grain boundaries were exposed, and relatively more pores appeared. The section of the sample in S2-S4 became dense, the grains grew significantly, and the bonding effect among the grains was significantly improved. In S2-S4, the pores were reduced, but slight cracks appeared in S4. The surface in S5 was powdered, the crystal grains were obviously smaller, and more pores appeared. The change rule of the microstructure was consistent with the conclusions of Aksay's interface reaction free energy change control theory [25]. The increase of Fe affected the physical and chemical reactions in the hot press sintering process. On the one hand, it promoted the in situ reaction, reduced the interfacial energy of the solid/liquid surface, and improved the wettability of Fe and FeAl2O4. On the other hand, it increased the concentration gradient of Fe on the interface between Fe liquid and FeAl2O4, and increased its diffusion rate.  The analysis results of the point scan and the surface scan of the fracture energy spectrum in S2 are shown in Figures 5 and 6. The spot scanning showed that the grain at the spot was FeAl2O4. The surface scanning showed that the bright area was the Fe phase, and The analysis results of the point scan and the surface scan of the fracture energy spectrum in S2 are shown in Figures 5 and 6. The spot scanning showed that the grain at the spot was FeAl 2 O 4 . The surface scanning showed that the bright area was the Fe phase, and the dark area was the FeAl 2 O 4 phase. This result is completely consistent with the XRD results. Fe, Al, and O were distributed in the whole area, and as a result of the combined point scanning, FeAl 2 O 4 existed in the whole area. In the area where FeAl 2 O 4 was evenly distributed, there was a concentrated area of Fe, which indicated that metal Fe diffused into the FeAl 2 O 4 grains during the in situ reaction. The material transferred between FeAl 2 O 4 and Fe indicated that the wetting process followed the reaction-driven wetting mechanism. The increase of Fe liquid increased the reaction driving force, which improved the wettability between the two phases. This agrees with the interface reaction free energy change control proposed by Aksay's [25].

Effect on Mechanical Properties
The relative density, Vickers hardness, and bending strength of Fe/FeAl 2 O 4 cermet with different proportions were analyzed, as shown in Figure 7. The increased Fe content increased the relative density of the Fe/FeAl 2 O 4 cermet, which remain at about 94% after sample S2. This experimental result is consistent with the SEM result in Figure 4, which shows that the porosity of Fe/FeAl 2 O 4 cermet increased from S1 to S2, and the porosity of  The analysis results of the point scan and the surface scan of the fracture energy spectrum in S2 are shown in Figures 5 and 6. The spot scanning showed that the grain at the spot was FeAl2O4. The surface scanning showed that the bright area was the Fe phase, and the dark area was the FeAl2O4 phase. This result is completely consistent with the XRD results. Fe, Al, and O were distributed in the whole area, and as a result of the combined point scanning, FeAl2O4 existed in the whole area. In the area where FeAl2O4 was evenly distributed, there was a concentrated area of Fe, which indicated that metal Fe diffused into the FeAl2O4 grains during the in situ reaction. The material transferred between FeAl2O4 and Fe indicated that the wetting process followed the reaction-driven wetting mechanism. The increase of Fe liquid increased the reaction driving force, which improved the wettability between the two phases. This agrees with the interface reaction free energy change control proposed by Aksay's [25].

Effect on Mechanical Properties
The relative density, Vickers hardness, and bending strength of Fe/FeAl2O4 cermet with different proportions were analyzed, as shown in Figure 7. The increased Fe content increased the relative density of the Fe/FeAl2O4 cermet, which remain at about 94% after sample S2. This experimental result is consistent with the SEM result in Figure 4, which shows that the porosity of Fe/FeAl2O4 cermet increased from S1 to S2, and the porosity of S2-S5 remained unchanged. The Vickers hardness of Fe/FeAl2O4 cermet first increased and then decreased with the increase in Fe content. The bending strength first increased and then decreased with the increase in Fe content. The Vickers hardness and bending strength reached their maximum values in S2 (1.21 GPa and 210.0 MPa, respectively). The effect of Fe on the microstructure and mechanical properties of Fe/FeAl2O4 cermet was caused by the wetting process between Fe liquid and FeAl2O4. According to Aksay's theory [25], the increase in Fe content effectively improved the wettability of Fe and FeAl2O4, not only improving the bonding ability of Fe and FeAl2O4, but also providing a favorable channel for the diffusion of Fe to FeAl2O4, and promoting the compactness, Vickers hardness, and bending strength of cermet. However, with the further increase in Fe content, the diffusion rate of Fe to FeAl2O4 increased, and Fe continued to accumulate in the growing FeAl2O4 grains, eventually leading to the collapse of the FeAl2O4 structure. Therefore, the increase in Fe was conducive to the improvement of the mechanical properties of the Fe/FeAl2O4 cermet to a certain extent, but too much Fe aggravated the reaction and wetting on the interface between Fe and FeAl2O4, and affected the structure of FeAl2O4. Mac- The influence law of Fe relative to the Fe/FeAl2O4 cermet's structure and mechanical properties is consistent with Aksay's interface reaction free energy change control theory [25]. The reaction mechanism of the Fe/FeAl2O4 cermet prepared by hot press sintering was obtained as shown in Figure 8. As the content of Fe increased, the in situ synthesis The influence law of Fe relative to the Fe/FeAl 2 O 4 cermet's structure and mechanical properties is consistent with Aksay's interface reaction free energy change control theory [25]. The reaction mechanism of the Fe/FeAl 2 O 4 cermet prepared by hot press sintering was obtained as shown in Figure 8. As the content of Fe increased, the in situ synthesis Reaction (1)  Fe/FeAl2O4 cermet.
The influence law of Fe relative to the Fe/FeAl2O4 cermet's structure and mechanical properties is consistent with Aksay's interface reaction free energy change control theory [25]. The reaction mechanism of the Fe/FeAl2O4 cermet prepared by hot press sintering was obtained as shown in Figure 8. As the content of Fe increased, the in situ synthesis Reaction (1) was intensified, so that the wettability between liquid Fe and FeAl2O4 was improved. After breaking the wetting barrier between Fe and FeAl2O4, with the increase of Fe, the diffusion rate of Fe to FeAl2O4 grains increased, and it accumulated as the FeAl2O4 grains grew. The continuous accumulation of Fe in the FeAl2O4 grains increased the impact on the FeAl2O4 structure, eventually leading to the collapse of the FeAl2O4 structure. Therefore, increasing Fe was conducive to improving the wettability between Fe and FeAl2O4, however, too much Fe affected the structure of FeAl2O4 and resulted in poor mechanical properties. In this study, the optimum Fe:Fe2O3:Al2O3 ratio was 6:1:2.

Conclusions
In this paper, Fe/FeAl2O4 cermet in different Fe phases was prepared by hot press sintering, and the following conclusions were obtained.
(1) With the increase of Fe, the diffusion of Fe into FeAl2O4 grains occurred; the density of Fe/FeAl2O4 cermet increased, the grains of FeAl2O4 continued to grow, and the bonding ability of Fe and FeAl2O4 increased. However, the ratio of Fe:Fe2O3:Al2O3 was 12:1:2 and 15:1:2, the bonding ability of Fe and FeAl2O4 was decreased, cracks and evenly distributed powdering appeared on the cermet.

Conclusions
In this paper, Fe/FeAl 2 O 4 cermet in different Fe phases was prepared by hot press sintering, and the following conclusions were obtained.
(1) With the increase of Fe, the diffusion of Fe into FeAl 2 O 4 grains occurred; the density of Fe/FeAl 2 O 4 cermet increased, the grains of FeAl 2 O 4 continued to grow, and the bonding ability of Fe and FeAl 2 O 4 increased. However, the ratio of Fe:Fe 2 O 3 :Al 2 O 3 was 12:1:2 and 15:1:2, the bonding ability of Fe and FeAl 2 O 4 was decreased, cracks and evenly distributed powdering appeared on the cermet.
(2) With the increase of Fe, the relative density of Fe/FeAl 2 O 4 cermet first increased and then remained stable. The Vickers hardness and bending strength first increased and then decreased. The relative density of the cermet was maintained at about 94%, with a Fe:Fe 2 O 3 :Al 2 O 3 ratio of 6:1:2. The Vickers hardness and bending strength reached a maximum of 1.21 GPa and 210.0 MPa, respectively.
(3) The reaction mechanism of Fe/FeAl 2 O 4 cermet prepared by hot press sintering contained an in situ reaction synthesis to FeAl 2 O 4 and a Fe diffusion reaction to FeAl 2 O 4 grains. An increase of Fe improved the wettability between Fe and FeAl 2 O 4 , and increased the diffusion rate in the growing FeAl 2 O 4 grains. The Fe continued to accumulate and strengthen in the FeAl 2 O 4 grains. The effect on the structure of FeAl 2 O 4 was macroscopically expressed as the phenomenon that the mechanical properties of Fe/FeAl 2 O 4 cermet improved initially, and then became worse.