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Magnetic properties of cementite powder produced by reaction milling
Journal of Alloys and Compounds 474 (2009) 396–400
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Magnetic properties of cementite powder produced by reaction milling D. Chaira a,∗ , B.K. Mishra b , S. Sangal a a b
Department of Materials and Metallurgical Engineering, Indian Institute of Technology Kanpur, Kanpur, India Institute of Minerals and Materials Technology, Bhubaneswar Council of Scientific & Industrial Research, India
a r t i c l e
i n f o
Article history: Received 19 February 2008 Received in revised form 22 June 2008 Accepted 24 June 2008 Available online 3 August 2008 Keywords: Metals and alloys Mechanical alloying Magnetic measurements X-ray diffraction Mossbauer spectroscopy
a b s t r a c t Cementite powder obtained by reaction milling of elemental iron and graphite powder in a dual-drive planetary mill is used to study its magnetic property. The effect of milling and subsequent annealing of the cementite powder on its magnetic property is studied by using a vibrating sample magnetometer and Mossbauer spectroscopy. The coercive force, the specific saturation magnetization, and the Curie temperature are measured. These values depend on the phase composition obtained by milling and subsequent annealing. It has been found that after milling the saturation magnetization decreases and the coercivity increases. With annealing, coercivity initially increases and then decreases with increase in annealing temperature; where as saturation magnetization only increases in the same range. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Cementite is one of the basic components of steel and white cast iron and plays a critical role in steel technology. Historically, cementite has been investigated as a component of steels; it essentially affects the mechanical properties. Nanocrystalline cementite is a very important compound for potential applications in catalysis, gas sensors and in possible reduction of the cost required to produce bulk quantities of metallurgical materials. Coated nanocrystalline iron particles are of special interest as active components of ferrofluids, recording tape, and flexible disk recording media since iron has highest magnetic moment among other ferromagnetic transition metals. A protective layer on the particle’s surface prevents interactions between closely spaced bits and provides oxidation resistance. Carbon, metallic oxides, SiO2 and Cu were previously used as coating layer. Nikitenko et al. [1] synthesized iron nanoparticles by sonochemical method. They showed that sonochemical decomposition of a solution of Fe(CO)5 in diphenylmethane results in a solid product, which is characterized as iron nanoparticles embedded in a polymeric matrix. Its consequent annealing in argon at temperatures up to 1073 K leads to the growth of iron nanoparticles, covered by an iron carbide protective layer. However, some attention has been paid to the magnetic properties of steel. Nonetheless a lot remains to be known about the magnetic behav-
∗ Corresponding author. Tel.: +91 9438370956; fax: +91 6612462999. E-mail address: [email protected] (D. Chaira). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.099
ior of cementite. It is known that iron carbide and nitride particles are excellent magnetic materials, exhibiting high saturation magnetization ( s ) and coercivity (Hc ) and are suitable for high-density magnetic recording. In fact, iron carbide and nitride particles have improved stability than pure iron particles [2,3]. The determination of the temperature at which cementite ceases its ferromagnetic property might prove useful in understanding the effects of thermal treatment on the properties of carbon steel. Reaction milling of elemental iron and graphite in planetary mill is a way of producing cementite powder with fine grain size. It has been established that depending on the carbon concentration, the final product contains ␣-Fe, an amorphous Fe–C mixture, cementite and other iron carbides. Here we investigate the effect of milling and annealing on the magnetic properties of cementite powder. Yelsukov et al. [4] studied magnetic properties of Fe–C alloys with C concentration in the initial powder mixture varying between 5 and 25 at.%. They showed the effect of milling and annealing on saturation magnetization and coercivity. They also determined that coercive force depends on carbide type ((Fe3 C)D or Fe3 C), its amount, and grain size. It was found that finely dispersed Fe–C powder with Hc and s value in the range of 10–300 Oe and 140–220 A m2 /kg respectively could be produced by mechanical alloying (MA) followed by thermal treatment. Xu et al. [5] studied magnetic properties of supersaturated Fe1−x Cx (0 ≤ x ≤ 0.9) solid solution with nanocrystalline structure prepared by mechanical alloying. The nanocrystalline and large grain phases were formed with 0 ≤ x ≤ 0.67 and 0.75 ≤ x ≤ 0.9 respectively. In the large grain phase, magnetization follows the simple magnetic dilution and
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397
Fig. 1. Two unit cells of cementite with (a) prismatic and (b) octahedron environment of the carbon atoms. Large black balls show iron atoms, small white are carbon.
the coercivity Hc are mainly due to the dissolution of carbon in grain boundaries. In the nanocrystalline phase, the alloying effect of carbon is revealed by a distinct reduction in average magnetic moment. Li et al. [6] prepared fine particles of Fe–C alloy by an AC arc discharge between iron and carbon electrodes with different Ar pressures and also studied magnetic properties and Curie temperatures. Schastlivtsev et al. [7] noticed that carbon atoms could occupy four positions between the iron sites in cementite. Two of them (prismatic and octahedron) were repeatedly specified as possible places of the carbon atoms. Two other positions referred to as distorted prismatic and octahedron positions were not discussed earlier. Carbon in the last two positions is very close to iron atoms and these configurations look improbable. The contemporary experimental efforts on this issue could not give unequivocal verification of the carbon positions. Fig. 1 shows two unit cells of cementite with prismatic and octahedron environment of carbon atoms. The unit cell contains two nonequivalent iron (four atoms of Fe and eight atoms of Fe of different types) and one-carbon (four atoms) positions. Arzhnikov and Dobysheva [8] studied the structural peculiarities of cementite and their influence on the magnetic characteristics. They observed that the carbon positions actually depend on the mechanical and thermal treatment. The relevant structural changes manifest themselves in both the mechanical and magnetic properties, which they illustrated by analyzing the variation in the coercive force as a function of the annealing temperature for the plastically deformed samples. Given the above inconclusive understanding about the magnetic properties of cementite particles we decided to take a fresh look. The approach here has been to synthesize cementite powder by reaction milling using elemental iron and graphite powder in a dual-drive planetary mill. The purpose of this paper is to test the cementite powder for its magnetic properties under different annealing conditions.
Table 1 Specifications of the milling systems Mill type
Dual-drive planetary mill
Gyratory shaft length
750 mm
Jar dimensions Length Diameter Jar speed Shaft speed
180 mm 135 mm 475 rpm 225 rpm
Grinding media Type Ball size Ball weight/jar Ball to powder ratio by weight Motor rating
Steel 6 mm 2.5 kg 10 5 and 3 HP
2.2. Materials and methods Milling experiments were carried out with elemental iron and graphite powder of 99% purity as starting materials. Each jar was loaded with 250 g of powder and 2.5 kg of steel balls. A dispersant known as LicowaxTM was added during milling. After few hours of milling, powder was taken out from the mill to ascertain the progress of reaction. In all the experiments, milling was carried out for a period of 40 h to obtain the final sample. In the annealing experiments, the powder milled
2. Experimental 2.1. Dual-drive planetary milling Reaction milling of a mixture of elemental iron and graphite powders were carried out in a specially built dual-drive planetary mill. Details of the dual-drive planetary mill design are available [9]. Specifications of the milling system are provided in Table 1. The dual-drive planetary mill consists of a gyratory shaft and two cylindrical steel jars, both are rotated simultaneously and separately at high speed. The mill developed specifically for the synthesis of nanocarbides has a rotating shaft that sweeps a circle of diameter 750 mm. The two steel jars of 13.5 cm diameter (2500 ml each) rotate about their own axes around the common axis of the main shaft. The planetary mill is powered by two motors. A 5 HP motor works on the main rotating shaft and 3 HP motor drives the jars. The rotating speed of both motors can be varied independently and continuously by a frequency controller. Fig. 2 shows the design of dual-drive planetary mill, which was used for this study.
Fig. 2. Design of dual-drive planetary mill.
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Fig. 3. XRD pattern of powder milled for different periods.
for 40 h were taken in a quartz tube and vacuum-sealed. The powders were then annealed in the temperature range of 300–800 ◦ C for 1 h. A detailed morphological characterization of cementite powder produced by reaction milling in pulverisette mill is reported elsewhere [10–13]. It has been reported in the literatures that more than 100 h is required to produce cementite powder from elemental iron and graphite in pulverisette mill. Whereas in the present study only 40 h is required to produce the same in dual-drive planetary mill. Magnetic measurements of the milled and annealed powders were performed in a vibrating sample magnetometer (VSM) of ADE Technology Inc., USA. The maximum capacity of the applied field is 17.5 kOe. The Curie temperature of the cementite powder was measured in parallel field vibrating sample magnetometer of EG&G PARC Inc., USA. The Mossbauer spectra were recorded at room temperature by employing a standard Mossbauer spectrometer and a 57 Fe source. The velocity scale was calibrated using a standard ␣-Fe foil and the isomer shifts were given relative to the center of the ␣-Fe spectrum.
3. Results and discussion 3.1. XRD result In order to identify different phases of the particle mixtures during the reaction milling process, a small sample of milled product was repeatedly picked up at regular intervals for X-ray diffraction analysis. The XRD patterns of the end product (as milled powder) are shown in Fig. 3 for different intervals of milling time. Several interesting observations are made. First, the graphite peak disappears after about 20 h of milling. This is a general observation in this study, which may be due to amorphization of graphite during milling. Second, the peak positions of iron shifts to a lower angle with increasing milling time, which proves that carbon atoms form a solid solution with iron. Also the iron peaks broaden and their integrated intensity decrease with milling time, which indicate accumulation of lattice strain and reduction of crystal size. Finally, after 40 h of grinding, the X-ray spectra show presence of cementite along with unreacted elemental iron and ferrite. To study the stability of cementite, 40 h milled final powder was vacuum-sealed in a quartz tube and annealed at 600, 700 and 800 ◦ C for 1 h. Fig. 4 shows the XRD patterns of powder milled for 40 h and then annealed at different temperatures for 1 h. Several conclusions can be drawn from the XRD patterns. Firstly, the XRD pattern shows the peaks of cementite at 600 and 700 ◦ C. But at 800 ◦ C no cementite peak is visible. The reason is that cementite decomposes into iron and graphite at that temperature. Cementite is unstable at 800 ◦ C. Secondly, XRD pattern shows only cementite peaks; other iron carbides are not present. Most likely other iron carbides have also undergone a phase transition.
Fig. 4. XRD pattern of powder annealed at different temperatures.
3.2. Magnetic properties study To study the magnetic properties of Fe–C alloys and the effect of milling, the powder milled for different periods were tested in the VSM. Fig. 5 shows the magnetization curve at room temperature for different periods of milling. As milling continues, saturation magnetization reduces and after 40 h of milling it reduces from 170 to 80 emu/g. As milling progresses magnetization reduces due to the alloying effect of C into Fe. The decrease in magnetization is attributed to the separation of ␣-Fe and demagnetization effect, with decrease in the particle size. The saturation magnetization of the ultra fine Fe3 C particles prepared by Hirano and Tajima [14] using carbonizing of Fe3 O4 particles by carbon monoxide was reported around 117 emu/g. This may be related to the different structures and compositions of the particles by different methods. Fig. 6 shows magnetization and coercivity as a function of milling time. From the figure it can be observed that saturation magnetization decreases rapidly up to 6 h and then decreases slowly up to 40 h. The rapid decrease in magnetization during the early stage of milling suggests a dramatic decrease in the purity of Fe particles and formation of amorphous graphite. The decrease in
Fig. 5. Magnetization curve of powder milled for different periods at room temperature.
D. Chaira et al. / Journal of Alloys and Compounds 474 (2009) 396–400
399
Fig. 6. Saturation magnetization and coercivity plot of powder milled for different periods.
particle size leads to an increase in surface area to volume ratio, causing large demagnetization affects, which in turn also result in rapid decrease in magnetization value. At the initial milling period, the rate of size reduction is very high. In addition, reduction in particle size lead to increased disordering of the arrangement of atoms on the surface, which may also cause reduction in magnetization. During the period of nanocrystalline phase formation, saturation magnetization continues to decrease while coercivity increases. The reduction in saturation magnetization suggests the decrease of inter atomic distance between Fe and C atoms. Further milling enhances the solubility of C in Fe lattice and this is translated to a monotonic decrease in the magnetization as shown in Fig. 5. The low coercivity (Hc = 130 Oe) after 10 h of milling also indicates very low dissolution of graphite into iron. After 40 h of milling Hc is around 325 Oe; the high coercivity is due to the dissolution of carbon into Fe due to milling. The evidence of formation of lamellar structures at the early milling times agree with the results found by Tanaka et al. [15]. The magnetic properties of any powder change when subjected to heat treatment. To study this effect the milled powder samples were annealed and magnetic measurements of the annealed powder samples were carried out. Fig. 7 shows that saturation magnetization and coercivity as a function of annealing temperature. Initially coercivity increases with increasing temperature and
Fig. 8. Curie temperature measurement for 40 h milled final powder.
reaches a maximum at 500 ◦ C and subsequently decreases with further increase in temperature. This uncommon behavior may be attributed only to the redistribution of the carbon atoms in the Fe3 C system: without such redistribution, annealing should increase the average crystal size, reduce imperfections of the sample, and decrease the coercive force. In contrast saturation magnetization reaches a minimum at 500 ◦ C and then increases with temperature. At 300 ◦ C, saturation magnetization is 60 emu/g and coercivity is 320 Oe. At 800 ◦ C, coercivity and saturation magnetization become numerically equal and is around 100. At 800 ◦ C magnetization has increased drastically and coercivity has reduced sharply. At this temperature cementite intensively decomposes to iron and graphite. So we believe that the decrease in coercivity and increase in magnetization is due to decrease of cementite amount and increase in the iron content in the powder. The temperature at which cementite loses its ferromagnetism is useful to analyze the effects of thermal treatment upon the properties of carbon steels. Fig. 8 shows the change of magnetic moment with temperature. Initially the magnetic moment slightly increases and then it decreases gradually with temperature. At 250 ◦ C the magnetic moment decreases suddenly and its value is less than 1 emu/g. At this temperature, magnetism changes from ferromagnetism to paramagnetism. This is the Curie temperature. The graph shows that the Curie temperature is not a fixed temperature. At 150 ◦ C magnetic moment starts to decrease and at 225 ◦ C it is very close to zero. It may be concluded that the final powder contains more than one phase. The Curie temperature of Fe3 C was reported to be between ∼210 and 215 ◦ C [16]. Smith et al. [17] showed a pronounced drop (about 90%) in the magnetization of cementite between 180 and 250 ◦ C. 3.3. Mossbauer spectra study
Fig. 7. Saturation magnetization and coercivity plot of annealed powders.
The 40 h milled powder and annealed powders were characterized by a standard Mossbauer spectrometer and employing a 57 Fe source at room temperature. The velocity scale was calibrated using metallic iron foil. Isomer shifts are referred to ␣-Fe. Table 2 shows the different parameters obtained from Mossbauer spectra. Mossbauer spectra show there are two parts: magnetic and nonmagnetic. The magnetic species formed after 40 h milling, which constitutes 77.5% of the total area, has Mossbauer parameters very similar to those reported for iron carbide, Fe3 C (cementite). Cemen-
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Table 2 Different parameters obtained from Mossbauer spectra Sample
IS (mm/s)
QS (mm/s)
LWD (mm/s)
HMF (kOe)
Area (%)
40 h milled powder
0.7264 0.1499
0.9886 0.0164
1.0919 0.5044
203.3
22.5 77.5
0.8858 0.1556
0.7580 0.0085
0.5703 0.3453
207.5
25.2 74.8
Annealed at 700 ◦ C, 1 h
large linewidths reflect a distribution of magnetic hyperfine fields and/or the existence of either an amorphous state or disordered state. 4. Conclusions It is possible to synthesize cementite powder from elemental iron and graphite by reaction milling in a dual-drive planetary mill. Only 40 h of milling is required in the present study, whereas more than 100 h is required in commercially available mills. The powder sample obtained by reaction milling was magnetically characterized. It was found that the saturation magnetization decreases and coercivity increases with the extent of milling. This observation was mainly attributed to dissolution of carbon into iron and introduction of defects during milling. The powder sample was also annealed and magnetically characterized. The coercivity of the annealed powder increased up to 500 ◦ C due to the movement of the carbon atoms from the octahedron positions to the prismatic ones in order to decrease the total energy. The decrease in the coercive force after annealing above 500 ◦ C was believed to be due to the degree of homogeneity of the crystal state, which increases with annealing. It was also found that cementite loses its ferromagnetism property above 225 ◦ C. References
Fig. 9. Mossbauer spectra analysis of 40 h final milled powder milled and powder annealed at 700 ◦ C for 1 h.
tite powder prepared by reaction milling of iron and graphite, having hyperfine magnetic field of 203.3 kOe and isomer shift of 0.15. Matteazzi and Caer [18] observed magnetic field of 206 kOe with a chemical shift of 0.18 mm/s. Samples of Fe3 C prepared from iron oxide yield internal magnetic fields of 207 kOe and isomer shift of 0.18 mm/s. The non-magnetic part, which is almost 22.5% of the total area, may be clusters of graphite atom diffuse into Fe atoms and other iron carbides which are non-magnetic in nature. Fig. 9 shows the Mossbauer spectra of 40 h milled final powder and powder annealed at 700 ◦ C for 1 h. The Mossbauer spectrum was fitted primarily on the basis of the hyperfine parameters on Fe3 C. This resulted in a fit with the following components of subspectral areas indicated: Fe3 C type (77.5%) and rest ␣-Fe, Fe7 C3 , cluster of graphite in 40 h milled final powder. Whereas 74.8% Fe3 C and rest ␣-Fe, Fe7 C3 , cluster of graphite were present in powder annealed at 700 ◦ C for 1 h. The linewidths of the subspectra associated with Fe3 C phase is 0.50 mm/s for final milled powder and 0.34 mm/s for annealed powder. It was also noted that linewidth decreases after annealing due to increase in crystallinity and decrease in defects and disordering. Campbell et al. [19] observed large linewidth of 1.2–1.5 mm/s for amorphous Fe75 C25 samples milled for 70 h compared with those 0.5 mm/s obtained on milling other Fe1−x Cx composition in the range of 0.20 ≤ x < 0.8 in the same mill. Such
[1] S.I. Nikitenko, Y. Koltypin, V. Markovich, E. Rozenburg, G. Gorodetsky, A. Gedanken, IEEE Transactions on Magnetics 38 (2002) 2592–2594. [2] X. Bi, B. Ganguly, G.P. Huffman, F.E. Huggins, M. Endo, P.C. Eklund, Journal of Material Research 8 (1993) 1666–1674. [3] S. Hirano, S. Tajima, Journal of Material Science 25 (1990) 4457–4461. [4] E.P. Yelsukov, A.I. UI’yanov, A.V. Zagainov, N.B. Arsent’yeva, Journal of Magnetism and Magnetic Materials 258/259 (2003) 513–515. [5] S. Xu, J. Zhang, M. Zhong, Y. Liu, Z. Zhang, H. Chen, Z. He, Journal of Magnetism and Magnetic Materials 292 (2005) 126–134. [6] J. Li, C.B. Liu, Y. Zhao, Y. Lin, Z. Deng, Journal of Magnetism and Magnetic Materials 195 (1999) 470–475. [7] V.M. Schastlivtsev, I.L. Yakovleva, D.A. Mirzaev, K. Yu Okishev, Physics of Metals and Metallography 96 (2003) 313–320. [8] A.K. Arzhnikov, L.V. Dobysheva, Journal of Physics: Condensed Matter 19 (2006) 196214–196223. [9] D. Chaira, B.K. Mishra, S. Sangal, Materials Science & Engineering A 460/461 (2007) 111–120. [10] H. Hidaka, T. Tsuchiyama, S. Takaki, Scripta Materialia 44 (2001) 1503–1506. [11] G.W. Wang, S.J. Campbell, A. Calka, W.A. Kaczmarek, Nanostructured Materials 6 (1995) 389–392. [12] T.N. Rochman, K. Kawamoto, H. Sueyoshi, Y. Nakamura, T. Nishida, Journal of Material Processing Technology 89/90 (1999) 367–372. [13] M. Umemoto, G.Z. Liu, K. Masuyama, J.X. Hao, K. Tsuchiya, Scripta Materialia 44 (2001) 1741–1745. [14] S. Hirano, S. Tajima, Japanese Journal of Applied Physics 29 (1990) 662–668. [15] T. Tanaka, S. Nasu, K.N. Ishihara, P.H. Shingu, Journal of Less Common Metals 171 (1991) 237–247. [16] E.M. Cohn, L.J.E. Hofer, Journal of Chemical Physics 21 (2) (1953) 354–359. [17] S.W.J. Smith, W. White, S.G. Barker, Proceedings of the Physical Society of London 24 (1) (1911) 62–69. [18] P. Matteazzi, G.L. Caer, Journal of American Ceramic Society 74 (6) (1991) 1382–1390. [19] S.J. Campbell, G.M. Wang, A. Calka, W.A. Kaczmarek, Materials Science and Engineering A 75 (1997) 226–228.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Magnetic properties of cementite powder produced by reaction milling D. Chaira a,∗ , B.K. Mishra b , S. Sangal a a b
Department of Materials and Metallurgical Engineering, Indian Institute of Technology Kanpur, Kanpur, India Institute of Minerals and Materials Technology, Bhubaneswar Council of Scientific & Industrial Research, India
a r t i c l e
i n f o
Article history: Received 19 February 2008 Received in revised form 22 June 2008 Accepted 24 June 2008 Available online 3 August 2008 Keywords: Metals and alloys Mechanical alloying Magnetic measurements X-ray diffraction Mossbauer spectroscopy
a b s t r a c t Cementite powder obtained by reaction milling of elemental iron and graphite powder in a dual-drive planetary mill is used to study its magnetic property. The effect of milling and subsequent annealing of the cementite powder on its magnetic property is studied by using a vibrating sample magnetometer and Mossbauer spectroscopy. The coercive force, the specific saturation magnetization, and the Curie temperature are measured. These values depend on the phase composition obtained by milling and subsequent annealing. It has been found that after milling the saturation magnetization decreases and the coercivity increases. With annealing, coercivity initially increases and then decreases with increase in annealing temperature; where as saturation magnetization only increases in the same range. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Cementite is one of the basic components of steel and white cast iron and plays a critical role in steel technology. Historically, cementite has been investigated as a component of steels; it essentially affects the mechanical properties. Nanocrystalline cementite is a very important compound for potential applications in catalysis, gas sensors and in possible reduction of the cost required to produce bulk quantities of metallurgical materials. Coated nanocrystalline iron particles are of special interest as active components of ferrofluids, recording tape, and flexible disk recording media since iron has highest magnetic moment among other ferromagnetic transition metals. A protective layer on the particle’s surface prevents interactions between closely spaced bits and provides oxidation resistance. Carbon, metallic oxides, SiO2 and Cu were previously used as coating layer. Nikitenko et al. [1] synthesized iron nanoparticles by sonochemical method. They showed that sonochemical decomposition of a solution of Fe(CO)5 in diphenylmethane results in a solid product, which is characterized as iron nanoparticles embedded in a polymeric matrix. Its consequent annealing in argon at temperatures up to 1073 K leads to the growth of iron nanoparticles, covered by an iron carbide protective layer. However, some attention has been paid to the magnetic properties of steel. Nonetheless a lot remains to be known about the magnetic behav-
∗ Corresponding author. Tel.: +91 9438370956; fax: +91 6612462999. E-mail address: [email protected] (D. Chaira). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.099
ior of cementite. It is known that iron carbide and nitride particles are excellent magnetic materials, exhibiting high saturation magnetization ( s ) and coercivity (Hc ) and are suitable for high-density magnetic recording. In fact, iron carbide and nitride particles have improved stability than pure iron particles [2,3]. The determination of the temperature at which cementite ceases its ferromagnetic property might prove useful in understanding the effects of thermal treatment on the properties of carbon steel. Reaction milling of elemental iron and graphite in planetary mill is a way of producing cementite powder with fine grain size. It has been established that depending on the carbon concentration, the final product contains ␣-Fe, an amorphous Fe–C mixture, cementite and other iron carbides. Here we investigate the effect of milling and annealing on the magnetic properties of cementite powder. Yelsukov et al. [4] studied magnetic properties of Fe–C alloys with C concentration in the initial powder mixture varying between 5 and 25 at.%. They showed the effect of milling and annealing on saturation magnetization and coercivity. They also determined that coercive force depends on carbide type ((Fe3 C)D or Fe3 C), its amount, and grain size. It was found that finely dispersed Fe–C powder with Hc and s value in the range of 10–300 Oe and 140–220 A m2 /kg respectively could be produced by mechanical alloying (MA) followed by thermal treatment. Xu et al. [5] studied magnetic properties of supersaturated Fe1−x Cx (0 ≤ x ≤ 0.9) solid solution with nanocrystalline structure prepared by mechanical alloying. The nanocrystalline and large grain phases were formed with 0 ≤ x ≤ 0.67 and 0.75 ≤ x ≤ 0.9 respectively. In the large grain phase, magnetization follows the simple magnetic dilution and
D. Chaira et al. / Journal of Alloys and Compounds 474 (2009) 396–400
397
Fig. 1. Two unit cells of cementite with (a) prismatic and (b) octahedron environment of the carbon atoms. Large black balls show iron atoms, small white are carbon.
the coercivity Hc are mainly due to the dissolution of carbon in grain boundaries. In the nanocrystalline phase, the alloying effect of carbon is revealed by a distinct reduction in average magnetic moment. Li et al. [6] prepared fine particles of Fe–C alloy by an AC arc discharge between iron and carbon electrodes with different Ar pressures and also studied magnetic properties and Curie temperatures. Schastlivtsev et al. [7] noticed that carbon atoms could occupy four positions between the iron sites in cementite. Two of them (prismatic and octahedron) were repeatedly specified as possible places of the carbon atoms. Two other positions referred to as distorted prismatic and octahedron positions were not discussed earlier. Carbon in the last two positions is very close to iron atoms and these configurations look improbable. The contemporary experimental efforts on this issue could not give unequivocal verification of the carbon positions. Fig. 1 shows two unit cells of cementite with prismatic and octahedron environment of carbon atoms. The unit cell contains two nonequivalent iron (four atoms of Fe and eight atoms of Fe of different types) and one-carbon (four atoms) positions. Arzhnikov and Dobysheva [8] studied the structural peculiarities of cementite and their influence on the magnetic characteristics. They observed that the carbon positions actually depend on the mechanical and thermal treatment. The relevant structural changes manifest themselves in both the mechanical and magnetic properties, which they illustrated by analyzing the variation in the coercive force as a function of the annealing temperature for the plastically deformed samples. Given the above inconclusive understanding about the magnetic properties of cementite particles we decided to take a fresh look. The approach here has been to synthesize cementite powder by reaction milling using elemental iron and graphite powder in a dual-drive planetary mill. The purpose of this paper is to test the cementite powder for its magnetic properties under different annealing conditions.
Table 1 Specifications of the milling systems Mill type
Dual-drive planetary mill
Gyratory shaft length
750 mm
Jar dimensions Length Diameter Jar speed Shaft speed
180 mm 135 mm 475 rpm 225 rpm
Grinding media Type Ball size Ball weight/jar Ball to powder ratio by weight Motor rating
Steel 6 mm 2.5 kg 10 5 and 3 HP
2.2. Materials and methods Milling experiments were carried out with elemental iron and graphite powder of 99% purity as starting materials. Each jar was loaded with 250 g of powder and 2.5 kg of steel balls. A dispersant known as LicowaxTM was added during milling. After few hours of milling, powder was taken out from the mill to ascertain the progress of reaction. In all the experiments, milling was carried out for a period of 40 h to obtain the final sample. In the annealing experiments, the powder milled
2. Experimental 2.1. Dual-drive planetary milling Reaction milling of a mixture of elemental iron and graphite powders were carried out in a specially built dual-drive planetary mill. Details of the dual-drive planetary mill design are available [9]. Specifications of the milling system are provided in Table 1. The dual-drive planetary mill consists of a gyratory shaft and two cylindrical steel jars, both are rotated simultaneously and separately at high speed. The mill developed specifically for the synthesis of nanocarbides has a rotating shaft that sweeps a circle of diameter 750 mm. The two steel jars of 13.5 cm diameter (2500 ml each) rotate about their own axes around the common axis of the main shaft. The planetary mill is powered by two motors. A 5 HP motor works on the main rotating shaft and 3 HP motor drives the jars. The rotating speed of both motors can be varied independently and continuously by a frequency controller. Fig. 2 shows the design of dual-drive planetary mill, which was used for this study.
Fig. 2. Design of dual-drive planetary mill.
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D. Chaira et al. / Journal of Alloys and Compounds 474 (2009) 396–400
Fig. 3. XRD pattern of powder milled for different periods.
for 40 h were taken in a quartz tube and vacuum-sealed. The powders were then annealed in the temperature range of 300–800 ◦ C for 1 h. A detailed morphological characterization of cementite powder produced by reaction milling in pulverisette mill is reported elsewhere [10–13]. It has been reported in the literatures that more than 100 h is required to produce cementite powder from elemental iron and graphite in pulverisette mill. Whereas in the present study only 40 h is required to produce the same in dual-drive planetary mill. Magnetic measurements of the milled and annealed powders were performed in a vibrating sample magnetometer (VSM) of ADE Technology Inc., USA. The maximum capacity of the applied field is 17.5 kOe. The Curie temperature of the cementite powder was measured in parallel field vibrating sample magnetometer of EG&G PARC Inc., USA. The Mossbauer spectra were recorded at room temperature by employing a standard Mossbauer spectrometer and a 57 Fe source. The velocity scale was calibrated using a standard ␣-Fe foil and the isomer shifts were given relative to the center of the ␣-Fe spectrum.
3. Results and discussion 3.1. XRD result In order to identify different phases of the particle mixtures during the reaction milling process, a small sample of milled product was repeatedly picked up at regular intervals for X-ray diffraction analysis. The XRD patterns of the end product (as milled powder) are shown in Fig. 3 for different intervals of milling time. Several interesting observations are made. First, the graphite peak disappears after about 20 h of milling. This is a general observation in this study, which may be due to amorphization of graphite during milling. Second, the peak positions of iron shifts to a lower angle with increasing milling time, which proves that carbon atoms form a solid solution with iron. Also the iron peaks broaden and their integrated intensity decrease with milling time, which indicate accumulation of lattice strain and reduction of crystal size. Finally, after 40 h of grinding, the X-ray spectra show presence of cementite along with unreacted elemental iron and ferrite. To study the stability of cementite, 40 h milled final powder was vacuum-sealed in a quartz tube and annealed at 600, 700 and 800 ◦ C for 1 h. Fig. 4 shows the XRD patterns of powder milled for 40 h and then annealed at different temperatures for 1 h. Several conclusions can be drawn from the XRD patterns. Firstly, the XRD pattern shows the peaks of cementite at 600 and 700 ◦ C. But at 800 ◦ C no cementite peak is visible. The reason is that cementite decomposes into iron and graphite at that temperature. Cementite is unstable at 800 ◦ C. Secondly, XRD pattern shows only cementite peaks; other iron carbides are not present. Most likely other iron carbides have also undergone a phase transition.
Fig. 4. XRD pattern of powder annealed at different temperatures.
3.2. Magnetic properties study To study the magnetic properties of Fe–C alloys and the effect of milling, the powder milled for different periods were tested in the VSM. Fig. 5 shows the magnetization curve at room temperature for different periods of milling. As milling continues, saturation magnetization reduces and after 40 h of milling it reduces from 170 to 80 emu/g. As milling progresses magnetization reduces due to the alloying effect of C into Fe. The decrease in magnetization is attributed to the separation of ␣-Fe and demagnetization effect, with decrease in the particle size. The saturation magnetization of the ultra fine Fe3 C particles prepared by Hirano and Tajima [14] using carbonizing of Fe3 O4 particles by carbon monoxide was reported around 117 emu/g. This may be related to the different structures and compositions of the particles by different methods. Fig. 6 shows magnetization and coercivity as a function of milling time. From the figure it can be observed that saturation magnetization decreases rapidly up to 6 h and then decreases slowly up to 40 h. The rapid decrease in magnetization during the early stage of milling suggests a dramatic decrease in the purity of Fe particles and formation of amorphous graphite. The decrease in
Fig. 5. Magnetization curve of powder milled for different periods at room temperature.
D. Chaira et al. / Journal of Alloys and Compounds 474 (2009) 396–400
399
Fig. 6. Saturation magnetization and coercivity plot of powder milled for different periods.
particle size leads to an increase in surface area to volume ratio, causing large demagnetization affects, which in turn also result in rapid decrease in magnetization value. At the initial milling period, the rate of size reduction is very high. In addition, reduction in particle size lead to increased disordering of the arrangement of atoms on the surface, which may also cause reduction in magnetization. During the period of nanocrystalline phase formation, saturation magnetization continues to decrease while coercivity increases. The reduction in saturation magnetization suggests the decrease of inter atomic distance between Fe and C atoms. Further milling enhances the solubility of C in Fe lattice and this is translated to a monotonic decrease in the magnetization as shown in Fig. 5. The low coercivity (Hc = 130 Oe) after 10 h of milling also indicates very low dissolution of graphite into iron. After 40 h of milling Hc is around 325 Oe; the high coercivity is due to the dissolution of carbon into Fe due to milling. The evidence of formation of lamellar structures at the early milling times agree with the results found by Tanaka et al. [15]. The magnetic properties of any powder change when subjected to heat treatment. To study this effect the milled powder samples were annealed and magnetic measurements of the annealed powder samples were carried out. Fig. 7 shows that saturation magnetization and coercivity as a function of annealing temperature. Initially coercivity increases with increasing temperature and
Fig. 8. Curie temperature measurement for 40 h milled final powder.
reaches a maximum at 500 ◦ C and subsequently decreases with further increase in temperature. This uncommon behavior may be attributed only to the redistribution of the carbon atoms in the Fe3 C system: without such redistribution, annealing should increase the average crystal size, reduce imperfections of the sample, and decrease the coercive force. In contrast saturation magnetization reaches a minimum at 500 ◦ C and then increases with temperature. At 300 ◦ C, saturation magnetization is 60 emu/g and coercivity is 320 Oe. At 800 ◦ C, coercivity and saturation magnetization become numerically equal and is around 100. At 800 ◦ C magnetization has increased drastically and coercivity has reduced sharply. At this temperature cementite intensively decomposes to iron and graphite. So we believe that the decrease in coercivity and increase in magnetization is due to decrease of cementite amount and increase in the iron content in the powder. The temperature at which cementite loses its ferromagnetism is useful to analyze the effects of thermal treatment upon the properties of carbon steels. Fig. 8 shows the change of magnetic moment with temperature. Initially the magnetic moment slightly increases and then it decreases gradually with temperature. At 250 ◦ C the magnetic moment decreases suddenly and its value is less than 1 emu/g. At this temperature, magnetism changes from ferromagnetism to paramagnetism. This is the Curie temperature. The graph shows that the Curie temperature is not a fixed temperature. At 150 ◦ C magnetic moment starts to decrease and at 225 ◦ C it is very close to zero. It may be concluded that the final powder contains more than one phase. The Curie temperature of Fe3 C was reported to be between ∼210 and 215 ◦ C [16]. Smith et al. [17] showed a pronounced drop (about 90%) in the magnetization of cementite between 180 and 250 ◦ C. 3.3. Mossbauer spectra study
Fig. 7. Saturation magnetization and coercivity plot of annealed powders.
The 40 h milled powder and annealed powders were characterized by a standard Mossbauer spectrometer and employing a 57 Fe source at room temperature. The velocity scale was calibrated using metallic iron foil. Isomer shifts are referred to ␣-Fe. Table 2 shows the different parameters obtained from Mossbauer spectra. Mossbauer spectra show there are two parts: magnetic and nonmagnetic. The magnetic species formed after 40 h milling, which constitutes 77.5% of the total area, has Mossbauer parameters very similar to those reported for iron carbide, Fe3 C (cementite). Cemen-
400
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Table 2 Different parameters obtained from Mossbauer spectra Sample
IS (mm/s)
QS (mm/s)
LWD (mm/s)
HMF (kOe)
Area (%)
40 h milled powder
0.7264 0.1499
0.9886 0.0164
1.0919 0.5044
203.3
22.5 77.5
0.8858 0.1556
0.7580 0.0085
0.5703 0.3453
207.5
25.2 74.8
Annealed at 700 ◦ C, 1 h
large linewidths reflect a distribution of magnetic hyperfine fields and/or the existence of either an amorphous state or disordered state. 4. Conclusions It is possible to synthesize cementite powder from elemental iron and graphite by reaction milling in a dual-drive planetary mill. Only 40 h of milling is required in the present study, whereas more than 100 h is required in commercially available mills. The powder sample obtained by reaction milling was magnetically characterized. It was found that the saturation magnetization decreases and coercivity increases with the extent of milling. This observation was mainly attributed to dissolution of carbon into iron and introduction of defects during milling. The powder sample was also annealed and magnetically characterized. The coercivity of the annealed powder increased up to 500 ◦ C due to the movement of the carbon atoms from the octahedron positions to the prismatic ones in order to decrease the total energy. The decrease in the coercive force after annealing above 500 ◦ C was believed to be due to the degree of homogeneity of the crystal state, which increases with annealing. It was also found that cementite loses its ferromagnetism property above 225 ◦ C. References
Fig. 9. Mossbauer spectra analysis of 40 h final milled powder milled and powder annealed at 700 ◦ C for 1 h.
tite powder prepared by reaction milling of iron and graphite, having hyperfine magnetic field of 203.3 kOe and isomer shift of 0.15. Matteazzi and Caer [18] observed magnetic field of 206 kOe with a chemical shift of 0.18 mm/s. Samples of Fe3 C prepared from iron oxide yield internal magnetic fields of 207 kOe and isomer shift of 0.18 mm/s. The non-magnetic part, which is almost 22.5% of the total area, may be clusters of graphite atom diffuse into Fe atoms and other iron carbides which are non-magnetic in nature. Fig. 9 shows the Mossbauer spectra of 40 h milled final powder and powder annealed at 700 ◦ C for 1 h. The Mossbauer spectrum was fitted primarily on the basis of the hyperfine parameters on Fe3 C. This resulted in a fit with the following components of subspectral areas indicated: Fe3 C type (77.5%) and rest ␣-Fe, Fe7 C3 , cluster of graphite in 40 h milled final powder. Whereas 74.8% Fe3 C and rest ␣-Fe, Fe7 C3 , cluster of graphite were present in powder annealed at 700 ◦ C for 1 h. The linewidths of the subspectra associated with Fe3 C phase is 0.50 mm/s for final milled powder and 0.34 mm/s for annealed powder. It was also noted that linewidth decreases after annealing due to increase in crystallinity and decrease in defects and disordering. Campbell et al. [19] observed large linewidth of 1.2–1.5 mm/s for amorphous Fe75 C25 samples milled for 70 h compared with those 0.5 mm/s obtained on milling other Fe1−x Cx composition in the range of 0.20 ≤ x < 0.8 in the same mill. Such
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