Structural evolution of the mechanically alloyed Fe62

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ALGERIAN JOURNAL OF ADVANCED MATERIALS 4, 173(2008)

ISSN 1111-625X/06/$18 173 @2008 Algerian Materials Research Society

Structural evolution of the mechanically alloyed Fe

62

Nb

8

B

30

powder mixtures

A. Hamouda^1,*, S. Alleg¹, S. Azzaza¹, R. Bensalem¹ and J. J. Sunõl²

1 Laboratoire de Magnétisme et de Spectroscopie des Solides (LM2S), Département de Physique, Faculté des Sciences, Université de Annaba

B. P. 12 (23000) Annaba-Algérie

2 Dep. Fisica, Universitat de Girona, Campus Montilivi, Girona 17017 Spain

*E-mail : hamouda_assia@yahoo.fr

Abstract: Fe₆₂Nb₈B₃₀ amorphous alloy was prepared by mechanical alloying in a planetary ball mill from pure elemental powders. Structural properties of the milled powders were investigated by X-ray diffraction. The mixing of Nb and B leads to the formation of a bcc Nb(B) solid solution after 1 h of milling. A highly disordered Fe(Nb, B) structure in addition to a small amount of the un-reacted α-Fe and the bcc Nb(B) solid solution are obtained after 25 h of milling. The amorphous state is reached on further milling (up to 125 h).

1. Introduction

Fe-based amorphous alloys which exhibit excellent soft magnetic properties are used in commercial applications such as power conversion, conditioning and generation [1]. These alloys are obtained by different methods. Mechanical alloying (MA) process has become a popular method to prepare numerous materials such as amorphous and nanocrystalline alloys [2, 3]. The solid state amorphization occurs by the destabilization of the crystalline structure through the accumulation of structural defects such as vacancies, dislocations and grain boundaries. The increase of the Nb content in the Fe-based amorphous alloys plays a role in the thermal stability, as does B in the generation of a strong magnetic coupling, of the amorphous phase [4]. In the present work, the structural evolution and, therefore, the solid state amorphization process of the ball milled Fe₆₂Nb₈B₃₀ powder mixtures were studied by X-ray diffraction.

2. Experimental

A mixture of elemental pure crystalline Fe, Nb and amorphous B powders with a purity of 99.9% was milled in a planetary ball mill Retsch PM 400, under an argon atmosphere, with a powder-to-ball-weight ratio of about 8:1 and a rotation speed of 350 rpm.

The structural evolution of the milled powders as well as the phase identification have been followed by X-ray diffraction (XRD) measurements using a Brucker D8 Advance diffractometer with Cu Kα radiation (λ_Cu = 0.154056 nm). A numerical procedure based on the Rietveld method combined with a Fourier analysis through the MAUD program [5], was used for the refinement of the XRD patterns.

3. Results and discussion

XRD patterns of the milled powders for various times are shown in Fig.1. As the milling process progresses one observes a continuous decrease in the peak

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A. Hamouda et al. ALGERIAN JOURNAL OF ADVANCED MATERIALS 4, 173(2008)

heights of the crystalline phases and the emergence of extremely broadened peaks related to the formation of a highly disordered structure. Typical broad diffuse maximum characteristic for the short range order of the amorphous state is obtained after 125 h of milling.

Fig. 1: XRD patterns of the milled Fe₆₂Nb₈B₃₀ powder mixtures for different milling times.

The XRD pattern of the un-milled powder shows the Bragg peaks of the elemental bcc α-Fe and bcc Nb with lattice parameters of a_Fe = 0.2867 nm and a_Nb = 0.3303 nm, respectively. The B is not detected by the DRX, probably due to the amorphous state. The appearance, after 1 h of milling, of new peaks on the lower angle side of Nb ones are attributed to the formation of the bcc Nb(B) solid solution (Fig. 2) through the fast diffusion of B atoms into the Nb matrix. The lattice parameter of the bcc Nb(B) solid solution, a_Nb(B) = 0.3429 nm, deviates from the equilibrium value of the perfect crystal lattice. The relative deviation reaches as much as ∆a = 4 %. It has been reported that the ball milling of Nb and amorphous B leads to the splitting of Nb peaks and the formation of the Nb(B) solid solution [6], because of the very small atomic radius of B (rB = 0.087 nm) in comparison to that of Nb (r_Nb= 0.142 nm) on the one hand and the large negative heat of mixing,

∆H_Nb/B = -39 kJ/mol, between B and Nb on the other hand.

Fig. 2: Rietveld refinement for the XRD pattern of the Fe₆₂Nb₈B₃₀ mixture after 1 h of milling.

After 25 h of milling, the crystalline peaks of α-Fe and Nb(B) solid solution are superimposed to a diffuse halo corresponding to the non-coherent structural domains as those characteristics of grain boundaries which exhibit a disordered like amorphous structure [7, 8]. Those domains are attributed to the disordered Fe(Nb, B) structure (Fig.

3). The continuous deformation of the powder particles which favours the reaction kinetics by increasing the chemical diffusivity of B and Nb into the α-Fe matrix leads to the transformation of the grain boundary in an amorphous intergranular layer by the increase of thickness of this layer. It has been reported that, in the case of a lower B content (9, 14 and 20 %), the ball milling of the ternary Fe-Nb-B system gives rise to the formation of a supersaturated solid solution [9]. Both B and Nb have a very small solubility in Fe (<1%). The solid solubility limit increases with ball milling but does not exceed 4%

for B and 5% for Nb in Fe [10]. Those features suggest that the high B content (30%) enhances the amorphization process after segregation of B on the

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α-Fe grain boundaries. The completely amorphous structure is achieved after 125 h of milling.

Fig. 3: Rietveld refinement for XRD pattern of the Fe62Nb8B30 mixture milled for 25 h.

The detailed analysis of the main Bragg peak of the α-Fe is shown in Fig. 4. The slight shift of the peak and, consequently, the increase of the lattice parameter of Fe to about 0.2872 nm are due to the lattice distortion.

Fig. 4: Detailed analysis of the main peak of the α-Fe.

After instrumental broadening corrections and separation of grain size effects from internal strain effects, one can follow the evolution of the average coherently diffracting domain size (grain size), <L>, and the average atomic level strain (microstrain),

<σ²>^1/2, as a function of milling time (Fig. 5). The

reduction of the α-Fe grains down to the nanometer scale (~10 nm) is due to the large plastic deformation, repeated fracturing and cold welding during the milling process. High density of dislocations is also generated with milling, leading to an increase in internal strain of the powder particles to about 0.35%.

Fig.5: Evolution of the average grain size, (<L> ± 1 nm), and the internal strain, (<σ²>^1/2± 0.09 %), of Fe during the milling process.

4. Conclusion

Solid state amorphization reaction by high energy ball milling of the Fe62Nb8B30 powder mixture has been followed by XRD. Initial steps in the alloying process are driven by the accumulation of the structural defects, the reduction of grain size down to nanometer scale and the formation of the bcc Nb(B) solid solution. The total mixing of the elemental powders leads to formation of an amorphous like structure after 125 h of milling.

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44 45

0 300 600

25 h 10 h 1 h 0 h

intensity (a.u)

2 Theta (degrees)

0 10 20

0 200 400 600 800 1000

Milling time (h)

<L> (nm)

0,0 0,1 0,2 0,3 0,4 0,5

<σ 2> 1/2(%)

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