Synthesis and characterization of nanocristalline Fe75

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3^ème Conférence Internationale sur

le Soudage, le CND et l’Industrie des Matériaux et Alliages (IC-WNDT-MI’12) Oran du 26 au 28 Novembre 2012,

http://www.csc.dz/ic-wndt-mi12/index.php 167

Synthesis and characterization of nanocristalline Fe

75

Si

25

Alloy prepared by high energy ball mill

FAGHI L., TRIAA S. ,TAFAT A.,SIAHMED f. AZZAZ M*

1 *Laboratoire des Sciences et Génie des matériaux (LSGM).Faculté de Génie Mécanique Université des Sciences et de la Technologie HB.

BP 32 ElalliaBabezzouar ALGERIE.

Email address : lfaghi@usthb.dz

Mechanical alloying is a powder metallurgy processing technique involving cold welding, fracturing, and rewilding of powder particle in a high energy. It has been used to obtain nanocrystalline alloy. Fe-25wt% Si alloys were synthesized using a planetary ball mill (Retsch PM400). Xray diffraction was used to identify and characterise various phase during the milling process. It is shown that the FeSi solid solution was formed after 4 hours milling. The study state grain size is about 10 nm. Many nanostructures magnetic materials have exhibited excellent soft magnetic properties, which suit so many applications. We used the electromagnetic methods and Xray (like a reference methods), to characterize the variation of structure and their influence.

Keywords: Fe-Si powder; Mechanical alloying; nanomaterials ; X ray Diffraction Introduction

The Fe-Si system alloys being mixed with boron, carbon and other materials are distinguished for to their particular magnetic and mechanical properties. So these alloys are used for the production of information recording equipment and materials[1]. Melted Fe Si alloys have been widely studied due to their soft magnetic character, with a combination of both a low magnetostriction and a high saturation magnetization which make this material an excellent candidate for the transformer nucleus. However, melted alloys with high Si content are very brittle .The high energy ball milling technique allows a wide range of compositions to be mechanically alloyed for producing these alloys as powders [2,3] with the advantage that brittleness can be ignored whatever the silicon content is.Recently, nanocrystalline magnetic materials have been intensively investigated because of their remarkable properties such as saturation magnetization, coercivity, magnetic ordering temperature and hyperfine magnetic field, which significantly differ from those of microcrystalline materials and are sensitive to the structure and microstructure [4]. Especially, mechanical alloying (MA) has been proved to be a competitive method for making nanocrystalline and amorphous alloys [5,6]. With the silicon concentration being up to 10-30 at.% the DO3-type hyperstructure Fe3Si is formed in which Fe atoms occupy the Si sites when the Si concentration is lower than the stoichiometric value, and silicon atoms occupy the Fe sites when the Si concentration is higher than stoichiometric value.In order to analyse the possibility of maintaining this excellent magnetic softness after reducing the particle size we have prepared nanocrystalline powders and we have analysed the influence of particle size on magnetic properties.

Experimental

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http://www.csc.dz/ic-wndt-mi12/index.php 168 Elemental Fe and Si powders of 99.9% purity and particle sizes smaller than 10 µm were separately weighed and mixed to obtain the desired composition. The mechanical alloying process was performed in a high- energy planetary ball mill (Retsch PM400). To prevent oxidation phenomena, the mixed powder was sealed in a cylindrical vial under an argon atmosphere with stainless steel balls. The ball-to-powder weight ratio was 14:1. The vial rotation speed was equal to 260 rpm. Different milling times ranging from 4 to 56 h were used.

To avoid excessive heating during milling, each 15 min of milling was followed by a pause of 10 min under the Aratmosphere. A Malvern laser diffractometer was selected for particle sizing (Malvern Mastersizer S [7]). A dry dispersion unit was used, with a test sample of approximately 1 g, 1.5 bar air pressure and 60%

feed rate.

X-ray diffraction experiments were performed with a Philips X-Pert Pro diffractometer in continuous scanning mode using Cu Kαradiation. The X-ray patterns were analysed using Philips X-Pert Plus software [8]. The grain sizes and the internal strains were obtained using the Williamson-Hall method [9].

SEM and X-ray microanalysis studies were performed on a Philips XL 30 microscope coupled to an energy dispersive analyser (EDX).

Mössbauer spectra were obtained at room temperature with a Wissel instrument in the constant acceleration mode, using a radioactive ⁵⁷Co source diffused into a rhodium matrix.Metallic iron was used for energy calibration and also as a reference for isotope shift.The Mössbauer spectra were evaluated with the Recoil software using the Voigt-based hyperfine field distribution method (HFD-VB-F) [10].

The magnetic properties of the powders were investigated with a vibrating sample magnetometer (VSM, TOEI INDUSTRY,VSM-5) at an applied magnetic field of up to ±12.5 kOe at room temperature.

3. Results and discussion

For all powders of the present investigation, the powder particle size distribution is not Gaussian and is not bimodal and has a wide particle size range varying between 0.5 and 70µm.

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http://www.csc.dz/ic-wndt-mi12/index.php 169 Fig. 1.Comparison on the particle size distribution of Fe Si 25% at different milling time determined by laser diffraction.

0 1 2 3 4 5 6 7 8

0,1 10

VOLUME(%)

PARTICLE SIZE (µm)

4h 8h 16h 32h 56h

Materia l name

Particle size data average percentile values (µm)

spa n

D10 D50 D90

4H 1,90

1

10.012

34,671

2.9 91

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http://www.csc.dz/ic-wndt-mi12/index.php 170 Table1 The particle size characteristics of Fe Si 25% at different milling time.

Fig. 1show particle size distribution of Fe Si 25% at different milling time determined by laser diffraction.

Respectively, plotted in log-percentage form, i.e., percent volume retained as a function of the logarithm of the powder particle size. The weighted residuals were less than 3.66% and the obscuration levels were between 5 and 20% for all measurements, confirming satisfactory dispersion conditions during measurement [7].

Further, to test for the repeatability or sample measurement stability, the same measurements were repeated after 5, 7 and 10 min [11] and the repeat results differed by a maximum of ±1–2% from the original

measurements. Particle size data average percentile values (µm) at 10, 50 and 90% median values and The

8H 1.66

0

6.602

22.540 3.1 22

16H 1.61

3

5.647 13.193 2.1 49

32H 1.57

6

7.604 26.326 2.8 69

56H 1.25

7

3.80 11.342 2.7

30

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http://www.csc.dz/ic-wndt-mi12/index.php 171 polydispersity of the powder was expressed by the span are listed in Table 1. The average particle size value is about 25µm for the Fe Si 25%after 4 hours milling time, 5.4 µm after 8 hours milling time, 3.51µm after 16 hours milling time, 0.61 µm after 32 hours milling time and 0.42µm after 56hours milling time.

the particle size distributions shown in Fig. 1 result in approximately the same Fe Si 25% evolution at

different milling time, for 4h, 8h and 16h with a final distribution have two distinct populations of particles. It seems that the finest particle population, come from an erosion of the intermediate particles, submitted to a high local shear stress, during a short period of time.

After 32h and 56h milling a final distribution having one population of particles, these fine particles indicate that the homogeneity of powder mixtures is excellent.

.Powders X-ray diffraction of the samples at different milling times gives a clear indication of the change that occurs in the material. Fig. 2 shows the diffractograms of the Fe25%Si alloy for different milling times. All the diffraction peaks have a fairly large line broadening. The line broadening is certainly due to the reduction of the grains size and the increase in the internal strains induced from the fracture and welding processes.

The diffraction peaks in the Fig. 2 at 4h milling time could be well identified as the bcc-Fe(Si) and the DO3- type hyperstructureFe3 Si for the intense peaks, No other phase than Fe3Si and FeSi were detected . at 8h and 16 h milling time ordered B2 (FeSi) superlattice phase , the Fe3 Si for the intense peaks was observed and the reflection peaks (3 1 0) and (1 1 1) corresponding to bcc-Fe(Si)disappear. After 32 hand 56h milling time the diffraction peaks show up the disappearance of the B2 (FeSi)phase.

The variation of the lattice parameter is represented in the Fig. 3.At the first hours of the milled powders, the lattice parameter forB2 (FeSi) superlattice phase as decreases. After 4 h of milling the lattice parameter of bcc-Fe(Si) it increases gradually with increase of the milling time up to 2.837709°A. The expansion of the lattice parameter can be explained as due to the order–disorder transition induced by mechanical alloying. In fact, since the intensity ratio (evaluated from the X-ray data of Fig. 2) of the peak (2 1 1)B2to the peak (1 1 0) decreases Decreased to the disappearance at 32 h of milling, there is a probable transformation of the ordered B2 phase to a Fe3 Si phase. This effect has previously been reported for the FeSi (with 12 at.% of silicon) alloy [12,13].

The grain size of the Fe–25.%Si powder was calculatedfor various ball milling times by the Hall–Williamson equation[14]; the results are shown in Fig. 4. As the ball milling timewas increased the grain size decreased.

In particular, grain sizerapidly decreased until a ball milling time of 32h, at which pointit saturated to about 13 nm.

Fig. 4 shows the values of crystalline grain size and microstrain as a function of milling time. And the increase of the milling time up to 32h decreases the grain size to 15 nm with a high internal micro strain value (0.8 %).

The elemental processes leading to grain size refinement generally include three stages [15,16]. In our case, the variations of the microstrains level are described by only two stages; the deformation is localized in shear

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http://www.csc.dz/ic-wndt-mi12/index.php 172 bands consisting of an array of dislocations with high density.

Fig. 2. X-ray diffraction of Fe94Si6 powders ball milled for different periods of time.

Fig. 3.The variation of the lattice parameter at different milling time for Fe3Siphase.

Then at a certain strain level, these dislocations annihilate and recombined to small angle grain boundaries separating individual grains.

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http://www.csc.dz/ic-wndt-mi12/index.php 173 First, the deformation is localized in shear bands consisting of an array of dislocations with high density. Then at a certain strain level, these dislocations annihilate and recombined to small angle grain boundaries

separating individual grains. Finally, the orientations of the single crystalline grains with respect to their neighboring become completely random.

Fig. 4.Variation of the grain size andthe lattice strain of the Fe25%Si with milling time.

The powder morphology was studied by SEM. Fig. 4shows the morphological evolution of Fe75Si25 alloy as a function of milling time (4, 8, 16, 32 and 56 h). It can be seen that different morphologies are present during the mechanical alloying stages.

The starting Fe powder consisted of coarse Fe particles with a mean particle size of 70 µm. After mechanical milling for 4 h, the mean particle was reduced to 10–20 µm with irregular and isotropic shapes. After 8h, the morphology and mean particle size were similar to those of mechanically milled after 4 h powder with a slightly broader particle size distribution and the structure was much finer, as shown in Fig. 4b. Most particles were found in the range of 5–10 mm.

For 16 h of milling time, the particles change into a flake or platelet shape .As a result of intensive fracture and cold welding as presented in Fig. 4c,and a stage where the platelet shape particles dominate for example for 32 h of milling (see Fig. 4d). Some of these particles present a layered structure formed of superposed Si and Fe layers. By increasing the milling time, the mechanical alloying progress and the refinement of particles size continues.

56 h of milling (Fig. 4e), typical of materials prepared by mechanical alloying for ductile or brittle elements as indicated by Davis et al. [17] Guittoum et al.[18] and Otmani et al. [19]. the majority of particle grains exhibit a round shape with small diameter; however, one can still note the presence of some big ones having a platelet shape.[18]

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http://www.csc.dz/ic-wndt-mi12/index.php 174 Fig. 4. SEM micrographs of mechanically milled :(a) 4h milling time, (b) 8h milling time, (c) 16h milling time, (d) 32h milling time and (e) 56h milling time powders .

a c b

d

e

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http://www.csc.dz/ic-wndt-mi12/index.php 175 The Mössbauer spectroscopy allowed by the intermediary of the atoms probed to obtain information about the formation process of iron-based alloys, and on the other hand, get information concerning the local change of magnetic properties.

The Mössbauer spectrum of the Fe75Si25 milled his shown in Fig. 6. After 4 h and 8h milling time, respectively, this spectrum can be decomposed into two subspectra corresponding to a sextet due to the ferromagnetic α-Fe (Si) phase and a single line due the paramagnetic B2 phase.This paramagnetic phase seems to increase and can be related to a phase rich in Si. Comparatively in the X ray analysis, this phase is not seen for all diffractogramsdue to its low concentration.

Estimated areas of both magnetic sites show that the atomic concentration of the ordered B2 phase is about 9.5%. The hyperfine field distribution of the Ferromagnetic A2 site represented in Fig. 7 is made of two peaks located at 24.4 and 33.9 Tat 4 h milling time and 23.1, 34.2T at 8 h milling time. The peak at high hyperfine fields can be identified as due to the Fe atoms in the grain bcc phase of about one atom of silicon as a near neighbor [21].

After 16 milling timethe Mössbauer spectrum is a doublet with larger quadrupole. Previous results on the FeSi alloys [22] have revealed a distribution of hyperfine fields with a tail and small peaks at low hyperfine

fields.The mean values of the corresponding fields are approximately 22.6and31.6T

The spectrum of the 32h and 56hsamples was fitted with the previous sextet but with lower area [23], and a HFD . The HFD corresponds to the BCC disordered FeSi alloy. Beyond 32h, only the HFD is considered to describe the hyperfine structure.The HFDs are composed of four main peaks.The mean values of the

corresponding fields are Approximately 31.9,25.1, 19.5 and 13.2T for 32h and 33.8,26.2,20.9 and 10.8Tfor 56 h.These hyperfine fields correspond to solid solution of Si in bcc Fe. The relative areas of the four sextets can be used to obtain the approximate Si composition. The latter is about 19% which can correspond to Fe3Si phase.Our results are not far from thosefoundGuittoum et al [18]and Bensebaa et al [20].

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http://www.csc.dz/ic-wndt-mi12/index.php 176 Fig. 5.Mössbauer spectra of mechanically milled Fe75Si25 at different time and corresponding HFD.

In Fe–Si magnetic alloys, it is known that the critical size of a domain for the evolution of superparamagnetism is about 20 nm [24].

The coercivity value of the Fe–25%Si powders was measured by a VSM at various ball milling times.

Fig. 6 shows the changes in the coercivity force as a function of ball milling time. After 16 h, the coercivity force was reduced, at which point a steady state was achieved at a final force of approximately 38 Oe and corresponded to 56 h of ball milling. This is in contrast to conventional soft magnets, where magnetization does not play a significant role in coercivity.It was assumed that this reduction in coercivity was due to the decrease in grain size[25].

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http://www.csc.dz/ic-wndt-mi12/index.php 177 Coercivity increases with decreasing grain size due to an increase in the volume fraction at the grain boundary, in bulk materials, hindering domain wall motion. However, the coercive force also rapidly decreases with decreasing grain size due to the

Fig. 6.Coercivity of Fe 25%Si powder as a function of ball milling time.

evolution of superparamagnetism when the grain size is reduced tothe nanometer scale (10 nm) [26].

Acknowledgments

The authors thank A.TAFAT for his constant help.

Conclusion

From XRD spectra, we have followed the formation of Fe75Si25 alloy from elemental Fe and Si powder. It is found that after 4 h milling, the FeSi and Fe3Si starts to form; between 8 and 16 h there is a coexistence of the FeSi (B2) phase and Fe3Si; beyond 32 h all the Fe atoms are dissolved and the powder is completely

transformed into the alloy phase (Fe3Si) . Also, the interpretation of the Mössbauer spectra corroborated the fact that Fe75Si25 starts to form after 4 h milling time and that after 32 h there is only the alloy phase. The SEM images taken at different milling time allowed us to follow the morphology of the materials at different stages. The increase in milling time from 0h to 16 h led to a decrease of the lattice parameter

forthe FeSi phase and the grain size and increase strain values and from 32h there is an increase of the lattice

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http://www.csc.dz/ic-wndt-mi12/index.php 178 parameter for Fe3Si phase and stress values and a decrease of the grain size. The coercive field HC decreases with increasing milling time.

REFERENCES

[1] T. Shinjo, I. Nakamura and NR.Shikazono,J. Phys. Jpn Ploughshare., 18 (1963) 787.

[2] A.Garcıà Escorial, et al. , Mater. Sci. Eng. A 134 (1991) 1394.

[3] C.G Kuhrt,et al.,IEEETrans . Magn . 29 (1993) 2667.

[4]Kuhrt, L.Schultz, J. Appl. Phys. 71 (1992) 1896.

[5] G. Herzer, IEEE Trans. Magn. 26 (1990) 1397.

[6] J.S. Benjamin, Metall. Trans. 1 (1970) 2943.

[7] Malvern Instruments Ltd., Mastersizer 2000 – Unified System for Particle Sizing, Enigma Business Park, Grovewood Road, Malvern, Worcs, UK, 2003

[8] X-Pert Plus software – Program for Crystallography and Rietveld Analysis, Philips Analytical,1999.

[9] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22

[10] K. Lagarek and D. Rancourt, Recoil Software, University of Ottawa, 1998.

[11] Ward-Smith RS, Gummery N, Rawle AF. Validation of wet and dry laser diffraction particle characterization methods. In: 4th World Congress on Particle Technology in Sydney, Australia. 2002.

[12] N. Stevulova, A. Buchal, P. Petrovic, K. Tkacova, V. Sepelak, J. Magn.Magn. Mater. 203 (1999) 190–

192.

[13] Z. Bensebaa, B.Bouzabata, A. OtmaniJournal of Alloys and Compounds, Volume 469, Issues 1-2, 5 February 2009, Pages 24-27

« Study of nanocrystallineFeSi alloys prepared by mechanical alloying » [14] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, 1978.

[15] H.J. Fecht, Nanostructured Material l6 (1995) 33.

[16] Z. Bensebaa et al. / Physics Procedia 2 (2009) 649–654

[17] R.M. Davis, B. McDermont, C.C. Koch, Met. Trans. 19A (1988) 28.

[18] A. Guittoum et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 1385–1392“X-ray diffraction, microstructure, Mössbauer and magnetization studies of nanostructured Fe50Ni50 alloy prepared by mechanical alloying”

[19] A. Otmani, B. Bouzabata, A. Djekoun, S. Alleg, Ann. Chim. Sci.Mater. 22 (1997) 201

[20]Z. Bensebaa et al. / Journal of Alloys and Compounds 469 (2009) 24–27 ”Study of nanocrystallineFeSi alloys prepared by mechanical alloying”

[21] E. Japa, K. Kroo, Phys. Stat. Sol. (b) 96 (1979) K65.

[22] M. Abdellaoui, C. Djega-Mariadassou, E. Gaffet, J. AlloysCompd. 259 (1997) 241–248 [23]R.R. Rodrıguez etal./MicroelectronicsJournal39(2008)1311–1313

[24] T. Zhou, J. Zhang, J. Xu, Z. Yu, G. Gu, D.Wang, H. Huang, Y. Du, J.Wang,Y. Jiang, J. Magn.

Magn.Mater.164 (1996) 219.

[25]Se-Hoon Kim, Young Jung Lee, Baek-Hee Lee,Kyu Hwan Lee, K. Narasimhan, Young Do Kim,J. Alloys Compd.424 (2006) 204–208“Characteristics of nanostructured Fe–33 at.%Si alloy powdersproduced by high- energy ball milling”

[26] B.H. Lee, S.S. Hong, K.H. Lee, Y.D. Kim, J. Alloys Compd. 385 (2004)264.

.