Revista Mexicana de Fısica S58 (2) 69–72 DICIEMBRE 2012

Magnetic and structural properties of nanostructured FeAl alloysproduced by mechanical alloying

H. Bustos Rodrıgueza, D. Oyola Lozanoa, and G.A. Perez Alcazarb1Department of Physics, University of Tolima, A.A. 546, Ibague, Colombia,

e-mail: hbustos@ut.edu.co2Department of Physics, University of Valle,

A.A. 25360, Cali, Colombia.

Recibido el 25 de junio de 2010; aceptado el 12 de octubre de 2010

In this work we report the study of the structural and magnetic properties obtained by Mossbauer spectrometry (MS) and X-ray diffraction(XRD), of mechanically alloyed Fe1−xAlx using the planetary ball mill Fritsch Pulverisette 7. Alloys with 0.05≤ x ≤ 0.5 were milled during24 hours. The obtained XRD patterns present two phases in all the composition range: theα-Fe and the BCC-FeAl. The average crystallitesize of theα-Fe phase increases from 21 forx=0.05 up to 37 nm forx=0.25 and then decreases up to 22 nm forx=0.5, while the behaviorof the average crystallite size for the BCC-FeAl phase decreases from 19 nm forx=0.05 up to 7 nm forx=0.5. The results of the Mossbauerstudy reveal that samples present theα-Fe and BCC-FeAl phases, which contains ferromagnetic (in all samples) and paramagnetic sites (forx ≥ 0.40).

Keywords: Mechanical alloying; X-Ray Diffraction; FeAl alloys; Mossbauer Spectrometry.

En este trabajo reportamos las propiedades estructurales y magneticas obtenidas por espectrometrıa Mossbauer (MS) y difraccion de rayos X(XRD), de muestras de Fe1−xAlx aleadas mecanicamente usando un molino planetario de alta energıa marca Fritsch Pulverisette 7. Alea-ciones con 0.05≤ x ≤ 0.5 fueron molidas durante 24 horas. Los patrones de difraccion de rayos x presentan dos fases en todas las muestras:la α-Fe y la fase FeAl-BCC. El tamano promedio de cristalito de la faseα-Fe aumenta desde 21 parax=0.05 hasta 37 nm parax=0.25y luegodecrece hasta 22 nm parax=0.5, mientras el comportamiento del tamano promedio de cristalito para la fase FeAl-BCC decrece desde 19 nmparax=0.05 hasta 7 nm parax=0.5. Los resultados del estudio Mossbauer revelan que las muestras presentan las faseα-Fe y FeAl-BCC, lascuales contienen sitios ferromagneticos (en todas las muestras) y paramagneticos parax ≥ 0.40.

Descriptores: Aleacion mecanica; difraccion de rayos X; aleaciones FeAl; espectrometrıa Mossbauer.

PACS: 61.05.cp; 75.50.-y; 76.80.+y; 71.20.Be; 71.20.Eh; 71.20.Gj.

1. Introduction

Mechanical alloying (MA) is an important method for boththe preparation of materials which normally cannot be pro-duced by conventional techniques and the investigation offundamental physical properties. The Fe–Al system is of in-terest because the potential commercial application of thesealloys as structural and/or magnetic applications. In recentyears several studies have been published about mechanicallyalloyed FeAl alloys [1–3], investigating the relation betweenthe enhancements of the ferromagnetic state with the disorderof the samples for different compositions and milling times.

2. Experimental

Iron and aluminum powders of 99.9% purity and particlesizes< 100 µm were mixed in the stoichiometries given byFe1−xAlx with x=0.05, 0.15, 0.25, 0.35, 0.40, 0.45 and 0.50in order to be milled in a high energy Fritsch Pulverisette 7planetary ball mill at 280 rpm in an argon atmosphere. Itwas used hardened stainless steel vials of 50 ml of volumeand balls of the same material with 10 mm of diameter. Aball mass to powder mass ratio of 20:1 and a milling time of24 hours were used. The obtained powders were measuredin a transmission Mossbauer spectrometer using a radioac-

tive Co-57/Rh source. The spectra were fitted by using theMOSFIT program [4]. A foil ofα-Fe was used as calibra-tion sample. The XRD analysis to establish the structure,the lattice parameter and the mean crystallite size were per-formed at room temperature for all samples using a PhilipsPW1710 diffractometer with the Cu-Kα radiation and thepatterns were refined by using the Maud program [5].

FIGURE 1. X ray diffraction Fex−1Alx milled during 24 hours.

70 H. BUSTOS RODRIGUEZ, D. OYOLA LOZANO, AND G.A. PEREZ ALCAZAR

FIGURE 2. Dependence of the mean crystallite size with the % Al.

FIGURE 3. Dependence of the volume fraction with % Al.

FIGURE 4. Mossbauer spectra of Fex−1Alx milled during 24hours.

FIGURE 5. Dependence of the isomer shift, quadrupolar splitting(QS) and hyperfine field medium (MHF) with Al concentration.Hyperfine field.

Rev. Mex. Fis. S58 (2) (2012) 69–72

MAGNETIC AND STRUCTURAL PROPERTIES OF NANOSTRUCTURED FEAL ALLOYS PRODUCED BY MECHANICAL ALLOYING 71

TABLE I. Mossbauer Parameters of samples of the Fe1−xAlx system milled during 24 hours. Hyperfine field (HF) values are in Tesla, theisomer shift (IS), line width (Γ) and the quadrupolar shift (QS) are in mm/s.

Composition IS (mm/s) Γ(mm/s)¤ QS (mm/s) HF(T) Area(%)

Fe0.95Al0.05Sextet -0.031 0.310 -0.028 33.0 69

HMFD -0.023 -0.026 29.4 31

Fe0.85Al0.15Sextet 0.010 0.304 -0.032 33.0 45

HMFD 0.035 -0.023 27.3 55

Fe0.75Al0.25Sextet 0.001 0.260 -0.005 33.2 34

HMFD 0.013 -0.012 26.3 66

Fe0.65Al0.35Sextet 0.035 0.284 -0.028 33.1 29

HMFD 0.055 -0.011 22.4 71

Fe0.6Al0.4

Doublet 0.255 0.600 0.470 19

Sextet 0.004 0.310 0.000 32.9 33

HMFD 0.113 0.085 20.3 48

Fe0.55Al0.45

Doublet 0.240 0.44 0.473 35

Sextet 0.012 0.300 -0.043 33.2 36

HMFD 0.076 0.150 19.8 29

Fe0.5Al0.5

Doublet 0.229 0.542 0.484 45

Sextet 0.008 0.328 -0.007 33.0 41

HMFD 0.126 0.255 19,1 14

3. Results and discussion

XRD patterns of representative samples withx=0.05, 0.35and 0.50 and milled during 24 hours, are shown in Fig. 1. Therefinement of the XRD patterns showed, in all the samples,the presence of two BCC phases, one with a lattice parameternearly 2.87A (very similar to that of theα-Fe) and the otherwith a lattice parameter nearly constant of 2.91A (of the or-der of that reported for Fe-Al alloys). The amount of thissecond phase increases as the a1uminum content increases.

The mean crystallite size of the first BCC phase increasesfrom 21 nm up to 37 nm when x change from 0.05 up to 0.25and then decreases up to 21 nm forx=0.50. The behavior ofthe mean crystallite size of the second BCC phase decreasesfrom 19 nm forx=0.05 up to 7 nm forx=0.50, as observedin Fig. 2. The lower and continuous decreasing value of thecrystallite size of the second phase, evidence the fragile char-acter induced by the increase of the Al content. The obtainedcrystallite sizes shows that the samples present a nanometriccharacter.

The volume fraction increase with the increase of the alu-minum concentration for BCC-FeAl phase while for BCC-Fephase decrease, as is shown in Fig. 3.

Figure 4 shows the room temperature MS of samples withx = 0.05, 0.35 and 0.50, respectively, milled during 24 hours.All the spectra were fitted with a sextet and a hyperfine mag-netic field distribution (HMFD). For samples withx ≥0.40 itwas necessary to add a doublet in order to obtain the best fit.The sextet with Mossbauer parameters given by: Isomer Shift(IS) near 0.0 mm/s, quadrupolar shift (QS) near 0.0 mm/s

and hyperfine field (HF) near 33.0 Tesla (see Fig. 5), whichcan be attributed to theα-Fe and corresponds to the firstBCC phase detected by XRD. The HMFD, whose MHF de-creases with the increase of the aluminum concentration (seeFig. 5c), which corresponds to the second BCC phase de-tected by XRD, can be associated to a BCC-FeAl disorderedalloy.

The decreasing MHF is due to the increase of its alu-minum content in such way that when x≥ 0.40 some ofits iron sites are so rich in aluminum that they behave asparamagnetic explaining in this way the necessity to add aparamagnetic doublet. This doublet presents the Mossbauerparameters IS≈ 0.25 mm/s and QS≈ 0.48 mm/s andΓ ≈ 0.59 mm/s. The high IS value of this doublet is due thebig aluminum content which decrease thes electron densityin the iron nucleus [8].

The HMFD and the big line width of this doublet evi-dence the disordered character of this BCC-FeAl phase, dis-order which was induced by the milling process. In Table Ithe obtained hyperfine parameters are shown. It can be notedin this table that the area of the HMFD decreases, that ofthe doublet increases, as is shown in Fig. 6, and the IS ofthe HMFD increases as the aluminum content increases, asis shown in Fig. 5a. Our results are in according with pre-vious reports [1,6,7] which showed that ordered alloys withx > 0.30 are paramagnetic but when they become disordered(by cool work, heat treatment or MA) the ferromagnetic be-havior appears or is enhanced. Current samples are ferro-magnetic up to 50 at. % Al as a consequence of the millingprocess.

Rev. Mex. Fis. S58 (2) (2012) 69–72

72 H. BUSTOS RODRIGUEZ, D. OYOLA LOZANO, AND G.A. PEREZ ALCAZAR

FIGURE 6. Dependence of the spectral area with Al concentration.

4. Conclusions

Mechanical alloying has been used to prepare Fex−1Alxnanostructured samples, withx=0.05, 0.15, 0.25, 0.35, 0.40,0.45 and 0.50 and milled during 24 hours. The obtained sam-ples with these milling conditions are not totally consolidateddue that some of the originalα-Fe powder appears and is notalloyed with aluminum. A BCC-FeAl disordered and ferro-magnetic phase was obtained for all aluminum contents andpresents a decreasing ferromagnetic behavior (the HMFD).The paramagnetic sites appear for x≥0.40.

5. Acknowledgements

The authors would like to thank to the Central Committee ofResearch of the University of Tolima, for the financial sup-port given.

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