Journal of NanomaterialsVolume 2011 (2011), Article ID 798257, 5 pagesdoi:10.1155/2011/798257

Research Article

Pulsed Current Activated Consolidation of Nanostructured Fe₃Al and Its Mechanical Property

Tae-Wan Kim,¹ In-Yong Ko,¹ Jung-Mann Doh,² Jin-Kook Yoon,² and In-Jin Shon1,3

¹Division of Advanced Materials Engineering, Research Center for Advanced Materials Development, Engineering College, Chonbuk National University, 664-14 Deokjin-dong 1-ga, Deokjin-gu, Jeonju, Jeonbuk 561-756, Republic of Korea²Advanced Functional Materials Research Center, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Republic of Korea³Department of Hydrogen and Fuel Cells Engineering, Specialized Graduate School, Chonbuk National University, Chonbuk, 561-756, Republic of Korea

Received 18 April 2010; Revised 30 May 2010; Accepted 14 June 2010

Academic Editor: William W. Yu

Copyright © 2011 Tae-Wan Kim et al. Tae-Wan Kim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A nanopowder of was synthesized from 3Fe and Al by high-energy ball milling. A dense nanostructured was consolidated by pulsed current activated sintering method within 2 minutes from mechanically synthesized powders of and horizontally milled powders of 3Fe+Al. The grain size, sintering behavior, and hardness of sintered from horizontally milled 3Fe+Al powders and high-energy ball milled powder were compared.

1. Introduction

Iron aluminide (Fe₃Al) is of interest for structural applications at elevated temperature in hostile environments. This is because it generally possess excellent oxidation and corrosion resistance, relatively lower density and lower material cost than Ni-based alloy [1, 2]. However, its use has been limited by its brittleness at room temperature. This drawback can be improved by grain size reduction. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties [3, 4]. As nanomaterials possess high strength, high hardness, excellent ductility, and toughness, undoubtedly, more attention has been paid for the application of nanomaterials [5, 6].

Conventional methods of processing iron aluminides, including melting and casting have been investigated till now [7, 8]. However, none of these methods yield nanostructures. In recent years, some efficient methods were reported to fabricate the fine grain size materials, such as mechanical alloying, pulsed current activated sintering [9, 10]. The mechanical alloying process, which involves a repeated cold-working, fracture, and welding, makes microstructure refinement and alloy formation. The pulsed current-activated sintering is a rapid consolidation processing method where uniform, dense, and fine grain materials can be obtained by applying pressures and passing pulsed current the compact. And the process, due to spark plasma formed between the powder particles, enhances the distorted energy of the particles and increases the rate of the diffusion between the particles [11–14].

The purpose of this study is to produce nanopowder of Fe₃Al and densify nanocrystalline Fe₃Al compound within two minutes from mixtures of 3Fe+Al powders and mechanically synthesized Fe₃Al, respectively, using this pulsed current activated sintering method and to evaluate its mechanical properties and grain size.

2. Experimental Procedures

Powders of 99.5% Fe (10 m, Alfa, Inc) and 99% pure Al (−200 mesh, Samchun Chemical, Inc) were used as a starting materials. 3Fe and Al powders ratio were mixed by two-type methods. Firstly, the powders were milled in a high-energy ball mill, Pulverisette-5 planetary mill with 250 rpm and for 10 hrs. Tungsten carbide balls (8.5 mm in diameter) were used in a sealed cylindrical stainless steel vial under argon atmosphere. The weight ratio of ball-to-powder was 30 : 1. Secondly, the powders were mixed in polyethylene bottles using zirconia balls with ethanol and the process was performed at a horizontal rotation velocity of 250 rpm for 10 h. The grain sizes of Fe₃Al were calculated using Suryanarayana and Grant Norton’s formula [15] where is the full width at half-maximum (FWHM) of the diffraction peak after instrument correction; and are FWHM caused by small grain size and internal stress, respectively; k is constant (with a value of 0.9); is wavelength of the X-ray radiation; L and are grain size and internal strain, respectively;   is the Bragg angle. The parameters B and follow Cauchy’s form with the relationship: , where B and are FWHM of the broadened Bragg peaks and the standard sample’s Bragg peaks, respectively.

After milling, the mixed powders were placed in a graphite die (outside diameter, 45 mm; inside diameter, 20 mm; height, 40 mm) and then introduced into the pulsed current activated sintering system made by Eltek in South Korea [9]. The four major stages in the synthesis are as follows. The system was evacuated (stage 1). And a uniaxial pressure of 80 MPa was applied (stage 2). A pulsed current (on time: 20 s; off time: 10 s) was then activated and maintained until densification was attained as indicated by a linear gauge measuring the shrinkage of the sample (stage 3). Temperature was measured by a pyrometer focused on the surface of the graphite die. At the end of the process, the sample was cooled to room temperature (stage 4).

The relative densities of the synthesized sample were measured using the Archimedes method. Microstructural information was obtained from product samples which were polished and etched using a solution of H₂SO4 (20 vol.%) and H₂O (80 vol.%) for 35 s at room temperature. Compositional and microstructural analyses of the products were made through X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX). Vickers hardness was measured by performing indentations at load of 20 kg and a dwell time of 15 s on the sintered samples.

3. Results and Discussion

XRD patterns of milled powder is shown in Figure 1. Fe₃Al was not synthesized during the horizontal rotation ball milling in ethanol, but synthesized during high-energy ball milling. The average grain size of Fe₃Al measured by Suryanarayana and Norton’s formula [15] was 5 nm. Figure 2 shows FE-SEM image of Fe₃Al powder synthesized by high-energy ball milling and X-ray mapping of Fe and Al. From the X-ray mapping, Fe and Al were uniformly distributed and the particles of milled Fe₃Al shown in Figure 2 are bigger. I think the reason in milling processing, iron and aluminum powders were wrapped with each other by server plastic deformation, resulting in grain refinement and mechanical alloying.

Figure 1: XRD patterns of milled powder: (a) horizontal milled 3Fe and Al powder, (b) high-energy ball milled Fe₃Al powder.

(a)

(b)

(c)

Figure 2: FE-SEM image of Fe₃Al and X-ray mapping of Fe and Al in high-energy ball milled powder.

The changes in shrinkage displacement and temperature of the surface of the graphite die with heating time during the processing of Fe-Al system are shown in Figure 3. As the pulsed current was applied, the shrinkage displacement of Fe₃Al continuously increased with temperature up to about C, but the shrinkage of 3Fe+Al gradually increased with temperature up to (A) point and then abruptly increased. When the reactant mixture of 3Fe and Al was heated under 80 MPa pressure to (A) point, no reaction took place as judged by subsequent XRD analysis. X-ray diffraction result, shown in Figure 4(a), exhibits only peaks pertaining to the Fe and Al. However, when the temperature was raised to C, the starting powders (3Fe+Al) reacted producing highly dense products (Fe₃Al), shown in Figure 4(b). The abrupt increase in the shrinkage displacement at the ignition temperature is due to the increase in density as a result of molar volume change associated with the formation of Fe₃Al from 3Fe and Al reactant and the consolidation of the product.

Figure 3: Variations of temperature and shrinkage displacement with heating time during pulsed current activated sintering of Fe-Al system: (a) high-energy ball milled Fe₃Al powder, (b) horizontal milled 3Fe and Al powder.

Figure 4: XRD patterns of the Fe-Al system: (a) sample heated to (A) point (b) heated to C from horizontal milled 3Fe and Al powder, (c) heated to C high-energy ball milled Fe₃Al powder.

Figure 5 shows FE-SEM images of Fe₃Al sintered from (a) horizontally milled Fe+Al powders and (b) high-energy ball milled Fe₃Al powder. The grain size of Fe₃Al sintered from high-energy ball milled Fe₃Al is smaller than that of Fe₃Al sintered from horizontally milled 3Fe+Al powders. The grain size of Fe₃Al sintered from high-energy ball milled Fe₃Al and from horizontally milled 3Fe+Al powders are about 40 nm and 300 nm, respectively. And the corresponding relative density of Fe₃Al are 97% and 96%, respectively. The grain size of Fe₃Al sintered from horizontally milled 3Fe+Al powders is higher than that of Fe₃Al sintered from high-energy ball milled Fe₃Al because Fe₃Al can have grain growth due to exothermic energy when Fe₃Al synthesized from 3Fe and Al during the sintering.

(a)

(b)

Figure 5: FE-SEM images of Fe₃Al sintered at C: (a) horizontal milled 3Fe and Al powder, (b) high-energy ball milled Fe₃Al powder.

The average grain size of the sintered Fe₃Al is not greatly larger than that of the initial powder, indicating the absence of great grain growth during sintering. This retention of the grain size is attributed to the high-heating rate and the relatively short-term exposure of the powders to the high temperature. The role of the current (resistive or inductive) in sintering and or synthesis has been focus of several attempts aimed at providing an explanation to the observed enhancement of sintering and the improved characteristics of the products. The role played by the current has been variously interpreted, the effect being explained in terms of fast heating rate due to Joule heating, the presence of plasma in pores separating powder particles [16], and the intrinsic contribution of the current to mass transport [17–19].

Vickers hardness measurements were made on polished sections of the Fe₃Al using a 20  load and 15 s dwell time. The calculated hardness values of Fe₃Al sintered from (a) horizontally milled 3Fe+Al powders and (b) high energy ball milled Fe₃Al powder were 317 kg/mm² and 458 kg/mm², respectively. This value represents an average of five measurements. Cracks were not observed to propagate from the indentation corner. So, fracture toughness cannot be calculated from crack length. The hardness of Fe₃Al sintered from high-energy ball milled Fe₃Al is higher than that of Fe₃Al sintered from horizontally milled 3Fe+Al powders due to refinement of grain and high density. Zhang and Liu investigated Fe₃Al prepared by vacuum induction melting followed by hot spinning forging. The hardness of Fe₃Al was about 270 Kg/mm² [20]. The hardness of this study is higher than that of Zhang’s work due to refinement of grain size.

4. Conclusions

Using the pulsed current activated sintering method, the densification of nanostructured Fe₃Al was accomplished from mechanically synthesized powder of Fe₃Al and milled 3Fe+Al powders. Nearly full density of Fe₃Al can be achieved within duration of 2 minutes for the applied pressure of 80 MPa and the pulsed current. The average grain sizes of Fe₃Al prepared by pulsed current activated sintering method from horizontally milled 3Fe+Al powders and high-energy ball milled Fe₃Al powder were about 300 nm and 40 nm, respectively. The calculated hardness values of Fe₃Al sintered from horizontally milled 3Fe+Al powders and high-energy ball milled Fe₃Al powder were 317 kg/mm² and 458 kg/mm², respectively.

Acknowledgments

This work was supported by New & Renewable Energy R&D program (2009T00100316) under the Ministry of Knowledge Economy, Republic of Korea.

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