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Mixing at micrometric and nanometric scale in mechanically alloyed Fe60Cr40
Physica B 327 (2003) 140–143
Mixing at micrometric and nanometric scale in mechanically alloyed Fe60Cr40 A. Fnidiki*, C. Lemoine, J. Teillet, J.J. Malandain Groupe de Physique des Mat!eriaux UMR CNRS 6634, Universit!e de Rouen, 76821 Mont-Saint-Aignan C!edex, France
Abstract Mechanical alloying (MA) of iron and chromium powder mixtures was performed from 0 to 190 h. SEM, . transmission electron microscopy with energy-dispersive X-ray analyses and Mossbauer spectrometry were used to study the mixing at micrometric and nanometric scale. At the stationary mixing state (E20 h of milling), MA of Fe60Cr40 powder mixtures gives particles of several micrometers consisting of magnetic nanograin cores characteristic of crystalline Fe60Cr40 alloy surrounded by a disordered paramagnetic grain surface (at 300 K), which increases during milling. At 85 h, the nanograins are totally disordered. For a longer time of milling, a de-mixing occurs by partial recrystallization of this disordered structure. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.46.+w; 76.75.+i; 61.43.gt Keywords: Nanomaterials; Mechanical alloying; Kinetics; Grain boundaries
1. Introduction Mechanical alloying (MA) provides an interdispersion of elements through a repeated cold welding and fracture of free powder particles. During MA of an elemental powder mixture, the grain size decreases to a nanometric scale and the elements mix together. Progressively, the concentration gradients disappear and eventually the elements are mixed at the atomic scale. The end product depends on many parameters such as the milling conditions ([1–3] and references therein)
*Corresponding author. Tel.: +33-35-14-67-65; fax: +33-3514-66-52. E-mail address: [email protected] (A. Fnidiki).
and the thermodynamic properties of the milled system. Structural and magnetic properties of Fe100 xCrx obtained by MA, were already described in previous review papers ([4–6] and references therein). The aim of this paper is to determine at both microscopic and nanoscopic scale the properties of mechanically alloyed Fe60Cr40 powder mixtures using SEM and TEM with energy-dispersive X-ray (EDX) analyses and . Mossbauer spectrometry.
2. Experimental MA of a mixture of pure Cr powder (99.0%) and pure Fe powder (99.9%) was carried out in a
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 1 3 - 1
A. Fnidiki et al. / Physica B 327 (2003) 140–143
Fritsch planetary mill (Pulverisette P5), using hardened commercial Fritsch steel vials of 80 cm3 and five balls of diameter 20 mm. The weight of the initial mixed powder sample was about 12 g and the ball to powder weight ratio was 13:1. To minimize oxidation, the vials were always sealed under argon atmosphere. The MA powders were analysed by Jeol FX 2000 transmission electron microscopy (TEM) and by SEM with EDX analyses. The mixing state at the atomic scale . was characterized by 57Fe Mossbauer transmission spectrometry at room temperature, using a 57Co source in a rhodium matrix.
141
3. Structure of the FeCr powder mixture MA of Fe60Cr40 powder mixtures gives particles of several micrometers consisting of magnetic nanograin cores characteristic of the alloy Fe60Cr40 [7,8]. SEM images allow to study the shape and the size of powder particles. The more ductile the alloy is, the more predominant the soldering events are, and the more the powder particles are big. For example, the iron and the chromium are ductile metals, but after 30 h of milling, pure Cr powders are smaller (E0.5 mm) than those of pure Fe
. Fig. 1. SEM images and Mossbauer spectra of a Fe60Cr40 powder mixture obtained after different milling times.
142
A. Fnidiki et al. / Physica B 327 (2003) 140–143
(E1.5 mm), because iron is more ductile than chromium. For the first milling hours of an iron and chromium mixture, a layered structure, constituted of alternate Cr and Fe layers is observed (Fig. 1a), as usual during milling of ductile elements. These layers are gradually destroyed under the repeated shock effect of balls on the powder. Finally, for a long milling time, powder particles are equiaxed and relatively homogeneous in size (Fig. 1b and c). Different analyses have been performed by TEM, for several concentrations, and for different milling times. We will present here only images after 12 h of milling. From the first milling hours, the dark field images show domains with different contrasts, measuring some nanometers. A rotation of the sample changes white zones into black and vice-versa, proving that these domains correspond to adjacent grains with different orientation. Consequently, the particles are constituted of grains measuring some nanometers (o10 nm). The electronic diffraction patterns present concentric rings characteristic for the FeCr coherent domains, confirming thus the polycrystalline character of the powder. When the milling time is prolonged until 12 h (Fig. 2), the size of the grains decreases again. Domains that diffract are better defined and smaller. The crystalline size is estimated to be about 473 nm, which is in rough
agreement with previous X-ray diffraction results for the powder milled at 12 h [8].
4. Kinetics of the mechanical alloying The evolution of the chemical mixing between iron and chromium was studied using EDX and . Mossbauer spectrometry. Fig. 3 shows the variation of the ratio s//%at. CrS (averaged on 20 measurements) as a function of the milling time for mixtures of 40% chromium and 60% Fe. The concentration standard deviation s decreases rapidly during the first hours of milling, and tends to 0 for 12 h of milling. This means that before 12 h, the obtained powder is not homogeneous at the micrometric scale of EDX analysis. Nevertheless, SEM analyses are insufficient to study the mixing at atomic scale. To follow this mixing, we . used 57Fe Mossbauer spectrometry. Fig. 1 shows, versus milling time, SEM images of particles of powders as well as the correspond. ing Mossbauer spectra. Three stages can be distinguished: 1. 0–21 h: This stage corresponds to the phase of mixing. A layered structure with alternate iron and chromium layers is observed, that generates interfaces between Fe and Cr. The standard
Fig. 2. (a) Dark field image with a portion of the (1 1 0) FeCr ring, of an Fe60Cr40 particle obtained after 12 h of milling; (b) corresponding electronic diffraction patterns.
A. Fnidiki et al. / Physica B 327 (2003) 140–143
143
8
σ (% at. Cr)
7 6 5 4 3 2 1 0 0
20
40
60
80
100
120
140
160
180
200
t (hour) Fig. 3. Variation of the ratio s//%at. CrS (s is the standard deviation of the chromium concentration) as a function of the milling time for the Fe60Cr40 powder mixture.
deviation concentration s decreases very rapidly, indicating the mixing between iron . and chromium. In addition, Mossbauer spectra present a magnetic contribution attributed to the nanograin cores FeCr and a paramagnetic contribution attributed to the disordered intergranular zone due to oxygen and nitrogen accumulation in the grain boundaries [7]. . 2. 21–85 h: s tends to 0 and the mean Mossbauer hyperfine field of the magnetic contribution remains constant; therefore, a stationary Fe–Cr mixing state is reached. Powder particles enlarge and become equiaxed, reaching their maximal size at 85 h. Phenomena of grains soldering dominate the fractures. The paramagnetic contribution becomes predominant in . Mossbauer spectra, because of the increase of incorporated oxygen and nitrogen, which increases the disordered intergranular part in . particles. At 85 h, the Mossbauer spectrum is only paramagnetic, the whole material is disordered. 3. 85–190 h: s remains weak, but increases slightly. A ferromagnetic contribution appears in M. ossbauer spectra that become totally magnetic
after 190 h of milling. Hyperfine parameters of this magnetic contribution are close to those of pure iron, revealing a de-mixing of the FeCr solid solution in domains rich in iron and domains rich in chromium. This de-mixing occurs by partial recrystallization (under the effect of the milling) of the amorphous type structure, in chromium oxides and nitrides.
References [1] M. Abdellaoui, E. Gaffet, Acta Metall. Mater. 43 (1995) 1087. [2] F. Delogu, M. Monagheddu, G. Mulas, L. Schiffini, G. Cocco, J. Non-Cryst. Solids 232 (1998) 383. [3] A. Iasonna, M. Magini, Acta Mater. 44 (1998) 1109. [4] G. Le Ca.er, T. Ziller, P. Delcroix, J.P. Morniroli, . Mossbauer Spectroscopy in Materials Science, Senec, Slovakia, 7. [5] M. Murugesan, H. Kuwano, IEEE Trans. Magn. 35 (1999) 3499. [6] A. Otmani, B. Bouzabata, A. Djekoun, S. Alleg, Ann. Chim. Sci. Mater. 22 (1997) 201. [7] A. Fnidiki, C. Lemoine, J. Teillet, J. Metastable Nanocryst. Mater. 12 (2002) 37. [8] C. Lemoine, A. Fnidiki, D. Lemarchand, J. Teillet, J. Phys.: Condens. Matter 11 (1999) 8341.
Mixing at micrometric and nanometric scale in mechanically alloyed Fe60Cr40 A. Fnidiki*, C. Lemoine, J. Teillet, J.J. Malandain Groupe de Physique des Mat!eriaux UMR CNRS 6634, Universit!e de Rouen, 76821 Mont-Saint-Aignan C!edex, France
Abstract Mechanical alloying (MA) of iron and chromium powder mixtures was performed from 0 to 190 h. SEM, . transmission electron microscopy with energy-dispersive X-ray analyses and Mossbauer spectrometry were used to study the mixing at micrometric and nanometric scale. At the stationary mixing state (E20 h of milling), MA of Fe60Cr40 powder mixtures gives particles of several micrometers consisting of magnetic nanograin cores characteristic of crystalline Fe60Cr40 alloy surrounded by a disordered paramagnetic grain surface (at 300 K), which increases during milling. At 85 h, the nanograins are totally disordered. For a longer time of milling, a de-mixing occurs by partial recrystallization of this disordered structure. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.46.+w; 76.75.+i; 61.43.gt Keywords: Nanomaterials; Mechanical alloying; Kinetics; Grain boundaries
1. Introduction Mechanical alloying (MA) provides an interdispersion of elements through a repeated cold welding and fracture of free powder particles. During MA of an elemental powder mixture, the grain size decreases to a nanometric scale and the elements mix together. Progressively, the concentration gradients disappear and eventually the elements are mixed at the atomic scale. The end product depends on many parameters such as the milling conditions ([1–3] and references therein)
*Corresponding author. Tel.: +33-35-14-67-65; fax: +33-3514-66-52. E-mail address: [email protected] (A. Fnidiki).
and the thermodynamic properties of the milled system. Structural and magnetic properties of Fe100 xCrx obtained by MA, were already described in previous review papers ([4–6] and references therein). The aim of this paper is to determine at both microscopic and nanoscopic scale the properties of mechanically alloyed Fe60Cr40 powder mixtures using SEM and TEM with energy-dispersive X-ray (EDX) analyses and . Mossbauer spectrometry.
2. Experimental MA of a mixture of pure Cr powder (99.0%) and pure Fe powder (99.9%) was carried out in a
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 1 3 - 1
A. Fnidiki et al. / Physica B 327 (2003) 140–143
Fritsch planetary mill (Pulverisette P5), using hardened commercial Fritsch steel vials of 80 cm3 and five balls of diameter 20 mm. The weight of the initial mixed powder sample was about 12 g and the ball to powder weight ratio was 13:1. To minimize oxidation, the vials were always sealed under argon atmosphere. The MA powders were analysed by Jeol FX 2000 transmission electron microscopy (TEM) and by SEM with EDX analyses. The mixing state at the atomic scale . was characterized by 57Fe Mossbauer transmission spectrometry at room temperature, using a 57Co source in a rhodium matrix.
141
3. Structure of the FeCr powder mixture MA of Fe60Cr40 powder mixtures gives particles of several micrometers consisting of magnetic nanograin cores characteristic of the alloy Fe60Cr40 [7,8]. SEM images allow to study the shape and the size of powder particles. The more ductile the alloy is, the more predominant the soldering events are, and the more the powder particles are big. For example, the iron and the chromium are ductile metals, but after 30 h of milling, pure Cr powders are smaller (E0.5 mm) than those of pure Fe
. Fig. 1. SEM images and Mossbauer spectra of a Fe60Cr40 powder mixture obtained after different milling times.
142
A. Fnidiki et al. / Physica B 327 (2003) 140–143
(E1.5 mm), because iron is more ductile than chromium. For the first milling hours of an iron and chromium mixture, a layered structure, constituted of alternate Cr and Fe layers is observed (Fig. 1a), as usual during milling of ductile elements. These layers are gradually destroyed under the repeated shock effect of balls on the powder. Finally, for a long milling time, powder particles are equiaxed and relatively homogeneous in size (Fig. 1b and c). Different analyses have been performed by TEM, for several concentrations, and for different milling times. We will present here only images after 12 h of milling. From the first milling hours, the dark field images show domains with different contrasts, measuring some nanometers. A rotation of the sample changes white zones into black and vice-versa, proving that these domains correspond to adjacent grains with different orientation. Consequently, the particles are constituted of grains measuring some nanometers (o10 nm). The electronic diffraction patterns present concentric rings characteristic for the FeCr coherent domains, confirming thus the polycrystalline character of the powder. When the milling time is prolonged until 12 h (Fig. 2), the size of the grains decreases again. Domains that diffract are better defined and smaller. The crystalline size is estimated to be about 473 nm, which is in rough
agreement with previous X-ray diffraction results for the powder milled at 12 h [8].
4. Kinetics of the mechanical alloying The evolution of the chemical mixing between iron and chromium was studied using EDX and . Mossbauer spectrometry. Fig. 3 shows the variation of the ratio s//%at. CrS (averaged on 20 measurements) as a function of the milling time for mixtures of 40% chromium and 60% Fe. The concentration standard deviation s decreases rapidly during the first hours of milling, and tends to 0 for 12 h of milling. This means that before 12 h, the obtained powder is not homogeneous at the micrometric scale of EDX analysis. Nevertheless, SEM analyses are insufficient to study the mixing at atomic scale. To follow this mixing, we . used 57Fe Mossbauer spectrometry. Fig. 1 shows, versus milling time, SEM images of particles of powders as well as the correspond. ing Mossbauer spectra. Three stages can be distinguished: 1. 0–21 h: This stage corresponds to the phase of mixing. A layered structure with alternate iron and chromium layers is observed, that generates interfaces between Fe and Cr. The standard
Fig. 2. (a) Dark field image with a portion of the (1 1 0) FeCr ring, of an Fe60Cr40 particle obtained after 12 h of milling; (b) corresponding electronic diffraction patterns.
A. Fnidiki et al. / Physica B 327 (2003) 140–143
143
8
σ (% at. Cr)
7 6 5 4 3 2 1 0 0
20
40
60
80
100
120
140
160
180
200
t (hour) Fig. 3. Variation of the ratio s//%at. CrS (s is the standard deviation of the chromium concentration) as a function of the milling time for the Fe60Cr40 powder mixture.
deviation concentration s decreases very rapidly, indicating the mixing between iron . and chromium. In addition, Mossbauer spectra present a magnetic contribution attributed to the nanograin cores FeCr and a paramagnetic contribution attributed to the disordered intergranular zone due to oxygen and nitrogen accumulation in the grain boundaries [7]. . 2. 21–85 h: s tends to 0 and the mean Mossbauer hyperfine field of the magnetic contribution remains constant; therefore, a stationary Fe–Cr mixing state is reached. Powder particles enlarge and become equiaxed, reaching their maximal size at 85 h. Phenomena of grains soldering dominate the fractures. The paramagnetic contribution becomes predominant in . Mossbauer spectra, because of the increase of incorporated oxygen and nitrogen, which increases the disordered intergranular part in . particles. At 85 h, the Mossbauer spectrum is only paramagnetic, the whole material is disordered. 3. 85–190 h: s remains weak, but increases slightly. A ferromagnetic contribution appears in M. ossbauer spectra that become totally magnetic
after 190 h of milling. Hyperfine parameters of this magnetic contribution are close to those of pure iron, revealing a de-mixing of the FeCr solid solution in domains rich in iron and domains rich in chromium. This de-mixing occurs by partial recrystallization (under the effect of the milling) of the amorphous type structure, in chromium oxides and nitrides.
References [1] M. Abdellaoui, E. Gaffet, Acta Metall. Mater. 43 (1995) 1087. [2] F. Delogu, M. Monagheddu, G. Mulas, L. Schiffini, G. Cocco, J. Non-Cryst. Solids 232 (1998) 383. [3] A. Iasonna, M. Magini, Acta Mater. 44 (1998) 1109. [4] G. Le Ca.er, T. Ziller, P. Delcroix, J.P. Morniroli, . Mossbauer Spectroscopy in Materials Science, Senec, Slovakia, 7. [5] M. Murugesan, H. Kuwano, IEEE Trans. Magn. 35 (1999) 3499. [6] A. Otmani, B. Bouzabata, A. Djekoun, S. Alleg, Ann. Chim. Sci. Mater. 22 (1997) 201. [7] A. Fnidiki, C. Lemoine, J. Teillet, J. Metastable Nanocryst. Mater. 12 (2002) 37. [8] C. Lemoine, A. Fnidiki, D. Lemarchand, J. Teillet, J. Phys.: Condens. Matter 11 (1999) 8341.