Influence of mechanical alloying time on the properties of Fe3AI intermetallics prepared by spark plasma sintering

Journal of University of Science and Technology Beijing Volume 14, Number 4, August 2007, Page 331

Materials

Influence of mechanical alloying time on the properties of Fe,AI intermetallics prepared by spark plasma sintering Chengchang Jia, Qing He, Jie Meng, and Lina Guo Materials Science and Engineering School, University of Science and Technology Beijing, Beijing 100083, China (Received 2006-08-29)

Abstract: The Fe,Al-based intermetallics were prepared by mechanical alloying and spark plasma sintering (SPS), and the influence of milling time on the properties of materials was investigated. The phase identification was investigated by X-ray, and the surface morphology and fractography were observed by scanning electron microscope (SEM). The mechanical properties such as bending strength, strain, and microhardness were tested. The results show that Fe reacts with A1 completely to form Fe,Al during short SPS processing time. The relative densities of the sintered samples were nearly 100%. The mechanical properties of the sintered samples can be improved along with the milling time. The representative values are the bend strength of 1327 MPa and the microhardness of 434.

Key words: Fe,A1 intemetallics; mechanical properties; mechanical alloying and milling; powder metallurgy

1. Introduction Fe,AI intermetallics have been widely studied among this field because of their low cost, low density, good wear resistance, ease of fabrication, and resistance to oxidation and corrosion. However, there are some inferior properties to be conquered for commercial applications, such as low ductility exhibited at low temperature and limited workability [ 1-31. There are many methods to improve the material properties, such as composites, addition of elements, heat treatment, control of grain size, and so on [4]. Especially, the control of grain size to nanometer scale and the addition of elements will effectively improve the poor properties of Fe,Al intermetallics, because the fine grain size can increase the yield strength and improve the ductility [ 5 ] , and the addition of elements can strengthen the matrix phase and grain boundaries, and may suppress the grain growth. Conventional methods of processing Fe,Al intermetallic, including melting and casting, and traditional powder metallurgy, have been investigated. In recent years, some efficient methods were reported to fabricate the fine-grain-size metals and alloys. These included mechanical alloying and spark plasma sintering (SPS) 16-71. The mechanical alloying process involves a repeated cold-working, fracturing, and welding results in microstructure refinement and alloy formation. Micro structural refinement may easily result in fine Corresponding author: Chengchang Jia, E-mail: jcc @ rnater.ustb,edu.cn

grains of micrometer size particles. The shortcoming of this process is the ease of formation of microdefects that may decrease the properties of the final products and import pollution [8]. The SPS is a rapid solidification processing method; and uniform, dense, and fine-grain materials can be obtained by applying pressures and passing electric pulse current to the compact. The atom migration activity and diffusion rate were enhanced because of the spark plasma between the particles [9- lo]. The addition of elements was another effective method to improve the mechanical properties of Fe,Al intermetallics, and nickel, chromium, boron, and carbon etc. have been used [ 1 1- 131 for this purpose. In this study, the fabrication of Fe,Al by spark plasma sintering (SPS) from elemental and mechanically activated powders was investigated. The purpose of this study is to produce near-full, dense and fine grain-size specimen by mechanical alloying and SPS. Excellent mechanical properties have been attributed to the uniformity of microstructures and fineness of grain sizes.

2. Experimental Three elemental powders of A1 (-200 mesh, 99.5% in purity), Fe (-200 mesh, 99.5% in purity), and B (23 pm) were mixed by the common ball mill for 2 h in the stoichiometric ratio of A1 (30at%) and B (OSat%). Also available online at www.sciencedirect.com

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The mechanical alloying was perfornied for 5 and 10 min by three-dimensional vibratory mill in air. The stainless steel ball and vial were used, and the ball-topowder ratio was 10:1 . Fig. 1 gives a photograph of the milling machine.

perature is a temperature where the maximum displacement of punch takes place in the sintering process. This curve presents the evolution of the SPS synthesis conditions. During the SPS processing, the sample is shrunk and densified under the actions of temperature and pressure. At the same pressure, when the temperature at which the maximum displacement achieved for element powder is about 580°C it is about 500°C for the powder milled for 10 min. It can be seen that the mechanical alloying (high energy ball milling) can effectively accelerate the densification of powder during the succedent sintering process.

Fig. 1. Three-dimension vibration milling machine.

A high-strength graphite mould was used during the SPS process, the size of the inner surface was 020.4 mm and the inner surface was covered with a thin carbolic paper. The mixed powders were compacted into the graphite mould. The graphite mould with the powder was placed inside the SPS reaction chamber, which was then exhausted from ordinary pressure to 15 Pa approx. for S min. The following step was to set the sintering temperature at 1050°C for 5 min and the pressure to SO MPa.

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Fig. 2. XRD patterns of the samples. 600 I

Phase analyses were made by X-ray powder diffraction (XRD) and the densities were determined by Archimede's method. The bend strength was measured by the three point bending method. The microhardness tests were carried out at the load of 0.49 N and the results were calculated from five dissimilar points. Scanning electron microscope (SEM) was used to analyze the surface morphology and the fractography features.

3. Results and discussion The XRD patterns of the elemental and milled powders, and the sintered sample are shown in Fig. 2 . These diffraction patterns show the evolution of the elemental mixture powders with the prolongation of milling time. It can be seen that XRD peaks are weakened with the extension of milling time. indicating that the grain refinement and a high density of defects are caused by large local strains in the powder particles. As the milling time is increased, A1 peaks are noted to decrease in intensity. especially the ( 1 1 1 ) peak of A]. This may be a factor that an iron-based solid solution was formed. Fig. 3 shows the relation between the characteristic temperature and milling time. The characteristic tem-

Milling time / min

Fig. 3. Relationship between the characteristic temperature and milling time during the SPS process.

Fig. 2 also shows the XRD patterns of the sample by SPS process with the powder milled for 10 min. It can be seen that Fe,Al and Fe,AlC,, are formed during the SPS. The forming of carbide is attributed to the graphite C diffusing into the samples at high temperature which comes from the mould and carbolic paper. The test results of relative density of the samples are presented in Fig 4. The relative densities of all the samples (with element, and 5- and 10-min milled powders) are higher than 98%. The density of the sample with 5-rnin milled powder (99.85%) is higher

C.C. Jia et al., Influence of mechanical alloying time on the properties of Fe,AI intermetallics prepared by

than that of the sample with elemental powder. This is because the former has a finer grain size and a higher atom-migration activity in comparison with the latter. These phenomena show that the SPS process makes sintering easier for milled powders than for elemental powders. 100

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than 13.4% (from 1170 to 1327 MPa), and the microhardness is higher than 28.8% (from 267 to 344 MPa). The mechanical alloying process has little influence on the ductility of the samples.

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Fig. 4. Relationship between the relative density of sintered samples and milling time.

The mechanical properties of the samples such as bend strength and Vickers microhardness are shown in Figs. 5-6. The bend strength and microhardness are increased with the extension of milling time. The mechanically activated powders have a higher surface energy and a finer grain size than the elemental powders. After SPS processing, the samples with milled powders have finer grain size and higher cohesion of the grain boundary than the samples with elemental powders that conduces to excellent mechanical properties. The bend strength of the samples is higher than those of the casting Fe3Al and Fe,Al-Al,O, produced by hot pressing at 1300°C. It is reported that the bend strength of casting Fe,A1 was 896 MPa and that of the Fe,Al-Al,03 compounds vary from 184 to 471 MPa [15]. Comparing the bend strengths and the microhardness of the samples with element and milled powders, it can be seen that the bending strength is higher

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Fig. 6. Relationship between the microhardness of sintered samples and milling time.

The other reason for the improvement of bending strength and microhardness may be the formation of Fe,AlC,,,. This can be explained by Fig. 2. The surface morphology observed by SEM in Fig. 7 exhibits few pores in the samples.

Fig. 7. SEM micrographs of the sintered samples: (a) with element powder; (b) with milled powder.

The fractographies of the samples observed by SEM are shown in Fig. 8. A mixed mode of brittle fracture and gliding fracture is observed, the obvious

cleavage and dimple characters are in the photograms. The forming of Fe,AIC,,, enhances the ductility of Fe,Al intermetallic and increases the gliding fracture

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area.

4. Conclusions FejAl intermetallics have been fabricated by mechanical alloying and SPS process. The mechanical

alloying enhanced the atom migration activity and refined the particle size. The consolidation process by SPS can produce the uniform, fine grain size, and nearly dense materials. The results are summarized as follows.

Fig. 8. Fracture surfaces of the bending samples observed by SEM: (a) with element powder; (b) with milled powder. ( I ) During the mechanical alloying process, some atoms of A1 dissolve in the lattice of Fe without the appearance of iron aluminides.

(2) The Fe,Al-based intennetallic alloys with high density and mechanical properties are made by mechanical alloying and spark plasma sintering under 50 MPa for 5 min at 1050°C.

( 3 ) The high-energy ball milling and SPS process can enhance the mechanical properties of the final samples. Comparing with the element powders, the bending strength of milled powder is 13.4% higher, the microhardness is 28.89 higher.

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