MICROSTRUCTURAL ANALYSIS AND PHASE TRANSFORMATION IN NANOSTRUCTURED Fe POWDERS

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MICROSTRUCTURAL ANALYSIS AND PHASE TRANSFORMATION IN NANOSTRUCTURED Fe POWDERS

S. Azzaza¹, S. Alleg¹, H. Moumeni²and A. R. Nemamcha².

1 Laboratoire de Magnétisme et Spectroscopie des Solides (LM2S), Département de Physique, Faculté des Sciences, Université de Annaba B. P. 12 (23000) Annaba - Algérie.

2 Université 8 mai 1945 Guelma- Algérie.

azzazasonia@yahoo.fr

Abstract :

One of the most efficient approaches for preparing nanostructured materials is mechanical milling. It has frequently been applied in synthesizing nanocrystalline metals, alloys and intermetallics [1– 3]. The MA process which introduces severe plastic deformation into powder particles can refine the grain size down to the nanometer scale for most metals and alloys. The obtained nanostructured powders consist thus of particles composed of nanometer size crystalline grains linked to each other through GBs. The structure of GBs, which has been controversially debated, behave as heterogeneous and partially disordered systems with a significant fraction of atoms residing in defect environments (GBs, surfaces, etc.).

Nanostructured Fe powders were obtained by ball milling in a high-energy planetary ball mill P7 under argon atmosphere using hardened steel containers and balls. The rotation speed was 400 rpm and the ball-to-powder weight ratio was 20:1. Microstructural evolution during the milling process was followed by X-ray diffraction (XRD). The X-ray diffraction patterns were analysed by using the Maud program which is based on the Rietveld method [4]. Differential scanning calorimetry (DSC) measurements were carried out using a universal Genessus 6000 differential scanning calorimeter within the temperature range 293–1273 K at a continuous heating rate of 10 K min⁻¹.

With increasing milling time, the α-Fe diffraction peaks become broader and their intensities decrease gradually (Fig.1), due to the decrease of crystallite size down to nanometer scale, the increase of the internal level strains and the dislocations density (Fig.2). The lattice distortion is evidenced by the increase of the lattice parameter. Above12 h of milling, the grain size remain constant while the GB enthalpy decreases. This behaviour can be explained by the fact that further milling results in a GBs relaxation which causes variations in the thermal and

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structural properties. The paramagnetic nanostructured bcc α-Fe domain is extended by about 48 K at the expense of both the magnetic bcc α-Fe and nonmagnetic fcc γ -Fe as compared to coarse grained pure α-Fe.

Fig.1: XRD patterns of the Fe powders versus milling time. Only the most intense reflection (110) is shown.

0 10 20 30 40

0,0 0,5 1,0 1,5

ρ

D

(1 0

16

/m

2

)

M illing tim e (h)

Fig. 2: Evolution of dislocations density versus milling time.

Keywords : Nanomaterials; Iron; Microstructure; X-ray diffraction; DSC.

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