Effect of iron powder content on microstructure and mechanical properties of Al2O3p/high manganese steel composites prepared by casting infiltration

Al2O3p/high-manganese steel-matrix composites were successfully fabricated by gravity casting infiltration, with iron powder added in the preforms to adjust the Al2O3p fraction. The effects of the iron powder content (38, 48, and 55 wt%) on the microstructures and mechanical properties of the composites were investigated. With the increase in the iron powder content in the preform, the Al2O3p fraction decreased (57–38 vol%), while the hardness and compressive strength of the composite gradually increased. The highest compressive strength was 1000.3 MPa (55-wt% iron powder). The highest work hardening rate (55-wt% iron powder) well reflected the synergistic effect between the matrix and reinforcement to prevent dislocation movement. The water glass binder formed thick interface layers between Al2O3p and matrix, which transformed the Al2O3/metal interface bonding from mechanical bonding to metallurgical bonding. A too thick interface layer deteriorated the mechanical properties of the composites.


Introduction
Wear-resistant parts applied in mining, cement, metallurgy, and electric power industries, such as hammer heads, lining plates, grinding balls, grinding rollers, and grinding discs, are strongly worn off and quickly fail by various materials such as sands, ores, soils, and grinding bodies [1][2][3][4]. Traditional metallic wear-resistant materials, such as austenitic high-manganese steel [5], cannot easily simultaneously meet the requirements of high hardness and high toughness. Based on the high strength and toughness of metal materials, ceramicparticle-reinforced metal-matrix composites (MMCs) include high-hardness ceramic particles into the metal materials and exhibit a synergistic coupling effect between the matrix and ceramic particles through interface control to simultaneously meet the requirements of high hardness and high toughness and improve the performances of traditional metal wear-resistant materials. Therefore, MMCs become a new generation of wearresistant materials with high wear performances [6][7][8][9][10].
In recent years, extensive studies have been carried out on the fabrication and wear resistances of Al 2 O 3 particle (Al 2 O 3 p)-reinforced steel-matrix composites (Al 2 O 3 p/steel), owing to the high hardness, wear resistance, and low cost of Al 2 O 3 p and high adaptability to large-scale industrial applications. However, the wettability between Al 2 O 3 and molten iron is very low, which hinders the fabrication of Al 2 O 3 p/steel composites by liquid-phase fabrication technologies such as casting infiltration [11] and leads to poor mechanical properties and wear resistances of the composites due to the poor bonding of the Al 2 O 3 /steel interface [12]. To overcome the low wettability, Wang et al [13] plated Ni on the surface of alumina particles and fabricated Al 2 O 3 p/heat-resistant steel-matrix composites by a negative-pressure infiltration technology. Lu et al [3,14]  Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. fabricated Al 2 O 3 p/40Cr steel-matrix composites by squeeze casting infiltration. Tian et al [15] prepared coarsegrained (1-3 mm) Al 2 O 3 p/high-manganese steel-matrix composites by lost foam casting. The above studies overcame the preparation problems of Al 2 O 3 p/steel composites through particle coating, external force, and coarse Al 2 O 3 particles, and thus have limitations in terms of cost and application. Gravity casting infiltration is a promising method to prepare composite materials. Compared to squeeze casting and vacuum casting, the preparation cost is lower, the process is simpler, and it is easier to prepare large-scale and complex composites. Jiang et al [16] fabricated a honeycomb ZTAp/high-chromium cast iron composite and ZTAp/high-manganese steel composite by gravity casting infiltration combined with powder activation, in which a microscale reactive powder was wrapped around Al 2 O 3 particles in preforms, which reacted with the iron melt and promoted its infiltration into the preforms during casting. ZTAps were coarse particles with sizes of 1-3 mm, which are conducive to the infiltration as well as the honeycomb structure of the ceramic preforms. Jiang et al prepared 1-3 mm coarse-ZTA-reinforced high-chromium cast iron honeycomb composites by gravity casting infiltration. Zhou et al [17] prepared a 150-180 μm fine-particle-reinforced high-manganese steel honeycomb composite by gravity casting infiltration. Thus, there are few reports on the fabrication of fine Al 2 O 3 p-reinforced steel-matrix composites by gravity casting infiltration and mechanical properties of Al 2 O 3 p/steel composites.
In this study, fine microscale Al 2 O 3 ps (average size: 150 μm) uniformly reinforced high-manganese steelmatrix composites were prepared by a powder-activated gravity casting infiltration technology. An iron powder was added into the preforms to adjust the volume fraction of Al 2 O 3 p in the composites. The effects of the iron powder content on the microstructures and mechanical properties of the composites were studied, particularly the structure of the Al 2 O 3 p/steel interface.

Fabrication of composites
Al 2 O 3 p and iron powder with mass ratios of 10:6, 10:9, and 10:12 (contents of iron powder of 38%, 48%, and 55%, respectively) were mixed in a ball mill tank at a speed of 40 r min −1 for 2 h. A sodium silicate binder (3 mass%, modulus: 3.0) was added to the mixture and fully stirred. The mixture was filled into a mould and heated to 200°C in a muffle furnace for 2 h to obtain the Al 3 O 2 p preforms.
A high-manganese steel was melted with a medium-frequency induction furnace. The preforms were fixed in the cavity of a sand box. Finally, gravity casting was carried out at 1600±10°C. Al 3 O 2 p/high-manganese steel composites were obtained after the steel melt solidified.
A LJWJ2515H4S water jet cutter was used to cut the composites from the casting ingots, and then machined to compressive test specimens with sizes of 10 mm×10 mm×20 mm and metallographic samples with sizes of 10 mm×10 mm×10 mm. The composites, whose preforms had iron powder contents of 38%, 48%, and 55%, are denoted as S1, S2, and S3, respectively. High-manganese steel samples were also cut from the composite casting ingots without ceramic preforms, denoted as S0. The hardness of the materials were measured with an HR-150 Rockwell hardness tester. The microstructure of the material was observed by a Leica EZ4D optical metallographic microscope and S-3400N scanning electron microscope. The composition was analysed by a Bruker energy spectrometer. The volume fraction of Al 3 O 2 p in the composite was calculated by the Image-Pro Plus software. The compressive properties of the materials was measured with an SHT4305 mechanical testing machine.

Results and discussion
3.1. Optical micrographs Figure 2 shows optical micrographs of the Al 2 O 3 p/high-manganese steel composites and matrix material.   microstructure at a high magnification, in which Al 2 O 3 p well contacts the matrix, without obvious cracks, casting defects, porosities, and shrinkage cavities. The matrix around Al 2 O 3 p is a grey pearlite and white ferrite. These pearlite and ferrite phases could have emerged from the addition of iron powder in the preforms. The dilution of the infiltrated high-manganese steel melt by the iron powder in the preforms during casting reduces the carbon and manganese contents so that the composite cannot achieve a single-phase austenite microstructure, but only forms the pearlite and ferrite structure [18]. Through a calculation with the Image-Pro Plus software, the volume fractions of Al 2 O 3 p (including the interface layer) in figures 2(a)-(c) are determined to be 57%, 48%, and 38%, respectively. Figure 3 shows the effect of the mass ratio of the iron powder in the preforms on the volume fraction of Al 2 O 3 p in the composites. A lower iron powder content led to a higher Al 2 O 3 p fraction in the composite; i.e., the addition of iron powder adjusted the volume fraction of Al 2 O 3 p in the composites. Figure 4 shows the phase analysis results of S2, which are typical for all composites besides the difference in the addition amount of iron powder. In addition to ferrite and Al 2 O 3 , there is FeAl 2 O 4 or MnAl 2 O 4 in the

Interface analyses of the composites
In this study, a thick reaction interface layer is formed around Al 2 O 3 p, which has a significant impact on the bonding of Al 2 O 3 and steel matrix. Therefore, it is necessary to thoroughly study the interface layer and its combination with Al 2 O 3 p and steel. Figure 5 shows the results of energy-dispersive spectroscopy (EDS) scanning of the interface of S3. Figure 5(a) shows that the black particles are Al 2 O 3 p, the white part is the matrix, and the grey part is the interface layer. The interface layer is mainly composed of amorphous substances [19], such as Na 2 SiO 3 , transformed from the water glass binder, so that it cannot be detected by the XRD pattern in figure 4. Further, the element composition variations between the interface layer, Al 2 O 3 , and steel matrix are thoroughly analysed through EDS line scanning, as shown in figure 6. The composition of the interface layer (area (1)+(2)) is mainly Si, Mn, O, and Al, which is consistent with that in figure 5. The comparison of the contents of Fe and Mn in the interface layer shows that the generated phase in figure 4 is MnAl 2 O 4 , rather than FeAl 2 O 4 , because the Fe content is considerably lower than the Mn content.
Two diffusion regions are formed between the interface layer and steel matrix and interface layer and Al 2 O 3 . Area (2) is the main region of the interface layer, in which the contents of Si, Mn, O, and Al remain stable. Area (1) is the diffusion region between the interface layer and steel matrix, with a thickness of approximately 2 μm. In area (1), the content of Fe increases largely from the low content in the interface layer to the high content in the steel, while the content of Si decreases largely from the high content in the interface layer to the low content in the steel. The contents of Mn, O, and Al change simultaneously with that of Si. This reflects the gradual diffusion of elements between the interface layer and steel matrix. Notably, the source of Mn element is the highmanganese steel matrix. As a result of its diffusion, the content of Mn element in the interface layer is twice that in the steel matrix. This further shows that Mn reacts with the binder to form MnAl 2 O 4 in the interface layer, so that its Mn content is higher than the solid solution value in the steel, as Mn is a strongly oxidising element [20].
Area (3) is the diffusion region between Al 2 O 3 and interface layer, with a thickness of 4 μm, where the Si content decreases largely from the high content of the interface layer to the low content of Al 2 O 3 (almost 0). The diffusion ability of Mn is so high that it continues to diffuse into Al 2 O 3 across the interface layer. In the combined area (3), the total depth of Mn element diffusing into Al 2 O 3 is approximately 12 μm, considerably larger than the diffusion depth (4 μm) of Si in Al 2 O 3 , which also demonstrates that Mn has a high ability to react with Al 2 O 3 .
There is a diffusion zone between the interface layer and steel matrix and between the interface layer and Al 2 O 3 , which indicates a reactive interface between the interface layer and steel matrix. Table 1 shows the mechanical properties of the tested materials. The hardness of the high-manganese steel is 43.7 HRC as the as-cast structure contains a large amount of carbides ( figure 2(e)). With the increase in the iron powder content in the preform, the hardness of the composites gradually increases from 41.1 to 50.1 HRC. (1) With the increase in the iron powder content, the fraction of Al 2 O 3 p in the composites decreases from 57 to 38 vol%, while the hardness unexpectedly increases. (2) In addition, unexpectedly, the hardness of the composite containing 57-vol% Al 2 O 3 is 2.63 HRC lower than that of the matrix steel. According to the mixing law of composites [21], the hardness of the composite should increase with the volume fraction of hard reinforcements and be higher than those of the matrix alloys. Therefore, the results of this experiment are not consistent with the theoretical prediction. Figure 7 shows the stress-strain curves of the Al 2 O 3 p/high-manganese steel composites and matrix steel under a unidirectional compression. Figure 7 and table 1 show that, with the increase in the iron powder content in the preform, the compressive strength of the composite increases. This trend is the same as that of the hardness. In addition, compared to the matrix steel, the strengths and compressive strains of the composites are considerably lower, while the elastic moduli of the composites are considerably higher according to the slopes of the curves of  the composites. Considering the considerable increases in the hardness and moduli of the composites compared to the matrix, the composites could be used to improve the wear resistance of the high-manganese steel under a low impact energy. The compressive strength of the Al 2 O 3 p/high-manganese steel composite with 55-wt% iron powder is 1000.3 MPa, higher than 918.86 MPa for the reported honeycomb ZTAp/high-manganese steel-matrix composite (80-100-mesh ZTAp, porosity of 74.8% in the preform) [22]. The honeycomb composite is an architecture composite, and thus should have a higher strength than those of ceramic-particle uniformly distributed composites. Therefore, the composites prepared in this study have excellent strengths. Figure 8 shows the work hardening rate (θ) and effective flow stress (σ-σ y ) curve. The work hardening rate characterises the change rate of the flow stress with the strain (θ=dσ/dε). The effective flow stress (σ-σ y ) is the effective stress for plastic deformation, i.e., the stress obtained after subtracting the yield stress from the total stress [23]. Figure 8(a) shows that it well reflects the work hardening ability of the high-manganese steel. Because of the high dislocation density in the austenitic high-manganese steel during deformation, a large number of dislocations form a high-density dislocation region, which hinders the dislocation movement, produces a strengthening effect, and results in work hardening of the high-manganese steel [24]. Figure 8(b) shows that the Al 2 O 3 p/high-manganese steel composite has a high work hardening ability as that of the high-manganese steel matrix. The highest work hardening rate and largest effective flow stress are obtained for the composite with the 55-wt% iron powder. The parabolic curve of the Al 2 O 3 p/high-manganese steel composite with the 55-wt% iron powder with two peaks and valleys can better reflect the synergistic action between the reinforcement and matrix to prevent dislocation movement [25]. These types of matrix and reinforcement have a good synergistic effect on the ability to block dislocations, which results in the best compressive properties. According to the analysis in figure 3, the composite with the smallest interface layer has the best work hardening rate. Thus, a thinner interface layer yields a better comprehensive performance. We believe that the thickness of the interface layer will be one of the crucial factors of composites of interest in material research in the future. Figure 9 shows the fracture morphologies of S1 and S3 under the same magnification. As shown in figures 9(a) and (b), the fracture surface of S1 is mainly composed of the interface layer and Al 2 O 3 , with a low amount of steel, which shows that the fracture occurs preferentially in the interface layers. There are two parallel cracks extending in the thick interface layer. The longer crack passes through Al 2 O 3 p and Al 2 O 3 p/steel interface, which indicates cracking at the Al 2 O 3 p/steel interface. A few small dimples are observed on the metal facture surface, which indicates a ductile failure of the matrix. A cascade fracture is observed in one Al 2 O 3 p, which indicates a cleavage fracture [26]. These characteristics show that the fractures of the composites are mixed; brittle fracture is the main mode. Figures 9(c) and (d) show that the characteristics of the fracture morphology of S3 are similar to those of S1.   3.4.4. Effect of the composite microstructure on the mechanical behaviour The above mechanical property analyses show that, with the increase in the iron powder content in the preform, the fraction of Al 2 O 3 in the composite decreases, while the hardness and compressive strength of the composite increase. The hardness of the composite with a high Al 2 O 3 volume fraction (57 vol%) (38-wt% iron powder) is lower than that of the matrix. These results are contrary to the prediction by the composite mixing law.

Compression fracture morphologies of the composites
According to the analyses of the microstructures and fracture surfaces, the contradiction between the experimental results and mixing law is attributed to the thick interface layers. The interface layers consist of a silicate amorphous material, MnAl 2 O 4 , and other fine crystal phases. The amorphous phases are hard and brittle. They have two effects on the composites. The first is to transform the Al 2 O 3 /metal interface bonding from mechanical bonding to metallurgical bonding ( figure 6), which has a favourable influence on the mechanical properties of the composites. Second, the too thick interface layer is conducive to crack initiation and propagation ( figure 8), which results in a serious deterioration of the mechanical properties of the composites. This occurs because of the existence and cracking effect of the thick interface layer in the composite, and thus the strengthening effect of Al 2 O 3 p on the composite cannot be fully manifested. The increase in the iron powder content in the preform is conducive to a uniform dispersion of the binder in the preform and melting, floating, and removal of water glass during the subsequent infiltration to reduce the thickness of the amorphous interface layer and improve the mechanical properties of the composite. Therefore, it is important to reduce the thickness of the interface layer in future studies.

Conclusions
(1)Al 2 O 3 p/high-manganese steel composites were successfully fabricated by gravity casting infiltration, with water glass as an adhesive and iron powder as an adjusting agent for the Al 2 O 3 p fraction.
(2)With the increase in the iron powder content in the preform from 33 to 55 wt%, the Al 2 O 3 p fraction decreased from 57 to 38 vol%, while the hardness and compressive strength of the composite gradually increased. When the content of iron powder was 55 wt% and the Al 2 O 3 p fraction was 38 vol%, the compressive strength of the Al 2 O 3 p/high-manganese steel composite reached 1000.3 MPa and the work hardening rate (θ) and effective flow stress (σ-σ y ) curve could better reflect the synergistic action between the reinforcement and matrix to prevent dislocation movement.
(3)The water glass binder formed thick interface layers between Al 2 O 3 p and steel in the composites, which mainly consisted of amorphous materials and MnAl 2 O 4 crystal phase. The interface layer formed diffusion regions with the matrix and Al 2 O 3 p, and thus changed the Al 2 O 3 /metal interface bonding from mechanical bonding to metallurgical bonding.
(4)The cracks in the Al 2 O 3 p/high-manganese steel matrix composites mainly occurred in the amorphous interface layer. A too thick interface layer deteriorated the mechanical properties of the composites.