CARACTÉRISATION DE COMPOSÉS FeCr ÉLABORÉS PAR MECANOSYNTHÈSE

Mechanical alloying is a dry and high energy milling process to synthesize materials (composites, alloys…), from a mixture of elemental powder or alloying particles. Mechanical alloying can be described as repeated cold welding between particles and fracturing of the particles. In order to study the mechanism of alloy FeCr formation by mechanical alloying as a function of the milling time, we have prepared two compositions (Fe85Cr15, Fe80Cr20) in planetary ball mill (Fritsch Pulverisette 7), from elemental Fe and Cr powders, using steel containers and balls (n = 5, diameter = 12mm). The vial rotation speed was 750 rpm. The ratio of the weight of the balls to the weight of the powder R = 3/35. The milling was interrupted after 1h of milling during 30 mn to avoid the increase in the temperature inside the vial. Obtained powders were characterised by X-ray diffraction, scanning electron microscopy (SEM) and Mössbauer spectrometry. Scanning microscopy micrographs show the progressive changes in morphology of the ball milling particles. At the initial stage of ball milling, the powder particles are flattened and show a wide range of sizes. For longer milling time a considerable refinement is observed. X-ray diffraction patterns shows that after 1h of milling, diffraction lines of bcc-Fe and bcc-Cr are still resolved. While, the Bragg peaks become broader with increasing milling time and forms one peak after 6h of milling leading to a formation of bcc solid solution with a lattice parameter close to a1 =2,8744 Å for the Fe85Cr15 compound and a2 =2,8802 Å for the Fe80Cr20 compound after 24h of milling. The average grain size is about 9 nm for the two compositions. It is about 7 nm after 48h of milling for the Fe80Cr20 compound. The broadening of the absorption lines of the Mössbauer spectra as a function of the milling time can be attributed to the structural disorder due to the inter diffusion of Fe and Cr. This is confirmed by the shift of the hyperfine field distribution (HFD) to the smaller fields. After 24h of milling, the hyperfine field distribution curves of the two compositions which are binomial are consistent with an homogenous disordered solid solution. The formed FeCr alloys have compositions comparable to those of starting constituents such as Fe86,5Cr13,5 and Fe78Cr22. Further milling times in the case of the Fe80Cr20 compound, leading to the demixtion of the solid solution (Fe,Cr). This can be confirmed by the shift of HFD's towards higher fields and by the increase of the average hyperfine field

Élaboration et caractérisation de composés FeCo.

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Propriétés microstructurales et magnétiques d’un alliage nanostructuré à base de FeCo.

Le nanocomposite type Hitperm de composition Fe31Co31Nb8B30 (wt.%) a été préparé par broyage mécanique haute énergie dans un broyeur planétaire. Les variations morphologiques, structurales et microstructurales des poudres broyées ont été suivies par microscopie électronique à balayage et diffraction des rayons X. Le changement du comportement magnétique avec le temps de broyage et la température de traitement a été étudié par spectrométrie Mössbauer, mesures magnétiques et analyse calorimétrique différentielle. Les observations morphologiques ont montré l’existence d’une large distribution de la taille et la forme des particules de poudres broyées avec une augmentation de la taille après 10 h de broyage. Un affinement considérable est achevé pour des temps plus longs. L’affinement Rietveld des spectres de diffraction X a révélé la formation des borures métastables Fe3B et Fe23B6 après 1 h de broyage et la décomposition de la phase Fe23B6 en Fe3B, Fe2B et Fe α après 10 h. Un état stationnaire, caractérisé par une matrice amorphe dans laquelle sont dispersés des nanograins FeCo, est observée à partir de 25 h de broyage. La cinétique d’amorphisation est décrite par deux coefficients d’Avrami, n1 = 1,41 et n2 = 0,34.

Etude structurale d’un mélange de poudres Fe57Co21Nb7B15 nanostructurées

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Elaboration et caractérisation d’un composite à base de diatomite

Composites based on phase change material were developed by impregnating paraffin with raw diatomite, calcined diatomite, kaolin, and kaolin/raw diatomite mixture for the thermal energy storage in the buildings. The phase change material has been optimized from paraffin wax (PW) and paraffin oil (PO). Cement was fabricated with different amounts of raw diatomite. Structural, microstructural and thermal properties have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Infrared spectroscopy (FT-IR), differential scanning calorimetry and thermogravimetric analysis(DSC-TGA). Raw diatomite structure consists of three phases: quartz, calcite and amorphous silica.After calcination, the formed phases are α quartz, β quartz, cristobalite and bicalcium silicate. Kaolin is essentially constituted of kaolinite. SEM micrographs show that PCM is well impregnated in the pores of diatomite. FT-IR results reveal the absence of chemical interaction between paraffin and diatomite. The paraffin/calcined diatomite composite has a melting temperature of 28.44°C and a latent heat of about 56.40 J/g. The phase transition temperature and latent heat of the paraffin/raw diatomite composite are 28.78°C and 41.26 J/g, respectively. PW-PO/diatomite composites are good materials for thermal energy storage in buildings because of their thermal reliability. The addition of 5% of raw diatomite to the cement improves its compressive strength

Matériaux magnétocaloriques nanostructurés : corrélation microstructure-comportement magnétique

Les composés type Nanoperm Fe92-xNb8Bx (x = 5, 10, 15 et 20) ont été élaborés par broyage mécanique haute énergie à partir de poudres élémentaires pures dans un broyeur planétaire type Retsch PM 400. Les changements morphologiques, microstructuraux et structuraux ont été suivis, en fonction de la teneur en bore, par microscopie électronique à balayage et diffraction de rayons X en utilisant le programme d’affinement "MAUD" basé sur la méthode de Rietveld. Les propriétés magnétiques, thermiques et magnétocaloriques ont été étudiées par spectrométrie Mössbauer, magnétomètres et analyse calorimétrique différentielle programmée, respectivement. Les morphologies des mélanges de poudres sont différentes les unes des autres et sont dépendantes de la teneur en bore. La distribution des particules de poudre est relativement large avec une grande quantité de fragments fins dont la majorité possède une taille de l’ordre de 1 μm. L’affinement Rietveld des diagrammes de diffraction montrent la formation d’une structure nanocomposite où le Feα et le borure Fe2B sont dispersés dans une matrice type amorphe pour toutes les compositions. La solution solide Fe(Nb) a été seulement observée pour le mélange 5 %B alors que les mélanges 15 et 20 %B ont révélé la présence du borure FeB. La taille moyenne des cristallites des différentes phases obtenues varie entre 7 et 24 nm. Les résultats de la spectrométrie Mössbauer confirment ceux de la diffraction de rayons X. L’augmentation de la teneur en bore conduit à une augmentation de la proportion relative de la phase type amorphe et de la coercivité, Hc, tandis que l’aimantation à saturation, Ms, diminue jusqu’à atteindre 92 uem/g pour le mélange 20%B. Le durcissement magnétique est mis en évidence par l’augmentation importante de Hc pour l'échantillon 20 %B de 0,014 à 0,0595 T en abaissant la température de 400 à 5 K, respectivement. La température de Curie, TC, de la phase amorphe augmente avec l’augmentation de la teneur en bore jusqu'à 15 %B, puis diminue. Le mélange de poudre 20%B présente des caractéristiques magnétocaloriques et le maximum de l’entropie magnétique,|ΔSM| augmente avec le champ magnétique appliqué atteignant une valeur de l’ordre de 1,45 J/kg.K. Les thermogrammes DSC présentent plusieurs pics endothermiques et exothermiques qui s’étalent sur le domaine de températures 50–700°C.

Elaboration et caractérisation de céramiques ZnO−TiO2

L'orthotitanate de zinc (Zn2TiO4) de structure spinelle inverse a été préparée par la méthode solide en deux étapes. La première étape consiste à broyer mécaniquement un mélange de poudres de ZnO et TiO2 avec un rapport molaire de 2:1 dans un broyeur planétaire à haute énergie sous atmosphère d'argon. Les poudres broyées ont été ensuite calcinées à 900°C pendant 2 heures, puis pressées en pastilles et frittées pendant 4 heures à 1100°C à l'air. La formation des phases, la microstructure, la morphologie de surface et les propriétés optiques ont été étudiées par diffraction des rayons X, spectroscopies Raman, infrarouge à transformée de Fourier (FT-IR), microscopie électronique à balayage couplée à la spectroscopie à rayons X à dispersion d'énergie (EDX), microscopie à force atomique et spectrophotométrie UV/visible. Les résultats des rayons X de la poudre broyée pendant 6 h révèlent la présence de Zn2TiO4 nanocristallin (15,5 %, = 13,2 nm), en plus de l'anatase a-TiO2, ZnO et le rutile r-TiO2. La fraction volumique de la phase Zn2TiO4 augmente avec le temps de broyage. Les pastilles présentent une structure composite dans laquelle une petite quantité de nanograins de rutile r-TiO2 est dispersée dans la matrice de titanate de zinc Zn2TiO4. Les résultats des spectroscopies FT-IR et Raman confirment le caractère biphasique des pastilles. La microstructure de la pastille 18 h est assez propre avec une répartition homogène de la taille des grains par rapport à celle de la pastille 6 h qui présente peu de pores. La rugosité de surface (r.m.s) des pastilles diminue avec l'augmentation du temps de broyage. L'énergie de la bande interdite des pastilles 6 h et 18 h est de l'ordre de 3,22 eV et 3,45 eV, respectivement.

Contribution à l’étude de matériaux magnétiques Fe-Si-B consolidés par SLM

Fe92.4Si3.1B4.5 alloys have been fabricated by selective laser melting using different laser powers (50 to 110 W) and different laser scan rates (0.1-1.5 m/s). X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, atomic force microscopy, 3D profilometer, microdurometer, differential scanning calorimetry (DSC) Mössbauer spectrometry and vibrating sample magnetometry have been used to investigate the structural, microstructural, mechanical, thermal, hyperfine and magnetic properties. The morphology of the produced samples shows generally melted zones, unmelted and interconnected particles and pores. The Rietveld refinement of the XRD pattern of the initial powder reveals the presence of an amorphous phase (64.45%), in addition to α-Fe0.95Si0.05 (11.32 %) and Fe2B boride (24.23 %). A new disordered cubic epsilon’-FeSi B20 structure was obtained for all the prepared samples. The Fe0.95Si0.05 and Fe2B phases are retained from the starting powder. epsilon’-FeSi, Fe0.95Si0.05 and Fe2B phases exhibit nanocrystalline crystallite sizes. The Mössbauer spectrometry confirms the formation of the epsilon’-FeSi. The increase of the saturation magnetization might be related to the depletion of the Si atoms into the Fe0.95Si0.05, while the increase of the coercivity might be related to the structural defects that are induced by the thermal deformation. The DSC curves show the presence of an endothermic peak at 703°C related to the Curie temperature of the Fe(Si) solid solution. The highest values of the microhardness might be due to the formation of a heterogeneous structure with a significant refinement of the crystallite size. The roughness measurements of the samples confirm that the surface smoothness is improved by increasing the energy (high laser power and low scan speed)

Elaboration et caractérisation des propriétés physiques d’alliages magnétocaloriques de type Heusler Ni-Mn-X (X = In, Sn).

Heusler metallic ribbons of Ni50Mn50-y(Sn, In)y (y = 5, 10, 15 and 20 at.%) and (Ni50Mn40Sn10)100-yFey (y = 1, 2 et 3 at.%) have been prepared by melt spinning from ingots obtained by arc melting by using highly pure Ni, Mn, Sn, In and Fe. Structure, microstructure, thermal and magnetic properties have been investigated by x−ray diffraction, scanning electron microscopy, differential scanning calorimetry and magnetic measurements.According to the e/a ratio, the ribbons exhibit an austenitic state with a granular morphology and/or a martensitic state with a plate-like morphology. The substitution of Mn atoms by Sn or In ones decreases the phase transition temperatures. After the heat treatment, the expansion of the cell volume increases the Mn-Mn interatomic distances and enhances the ferromagnetic ordering. Fe doping strengthens the Fe-Mn exchange interaction that leads to the increase of the Curie temperature TcMart and the saturation magnetization. The critical exponents have been estimated by means of several methods such as the modified Arrott plots, Kouvel-Fisher method and critical isothermal analysis. For Sn15, Sn20, In15, In20, Fe1, Fe2 andFe3 ribbons, the values of the critical exponents beta, gamma, and delta are close to those predicted from the mean field model revealing a dominated long-range order of magnetic interactions. However, the Fe0 ribbons (around TcM) and In20 (around Tc*) exhibit an unconventional behavior that might be related to the presence of strong antiferromagnetic interactions and the minor second phase. Sn15, Sn20 and In20 samples are good candidates for magnetic refrigeration near room temperature.

Elaboration et étude de matériaux magnétiques nanostructurés (Fe1-xMnx)2P

The mechanical alloying process has been used to elaborate the (Fe1⎼xMnx)2P phosphides (with x = 0.15, 0.25, 0.40, and 0.75) in a high energy planetary ball-mill Retsch PM 400/2. Morphological, microstructural, structural, magnetic and thermal properties of the ball milled powders were investigated by scanning electron microscopy, X-ray diffraction by using the MAUD program which is based on the Rietveld method, Mössbauer spectrometry, magnetometers and differential scanning calorimetry. The XRD results reveal the coexistence of the centered cubic Fe(Mn)⎼type, hexagonal Fe2P⎼type and Mn2P⎼type, orthorhombic MnP/FeP⎼type and tetragonal Fe3P-type structures for all samples. The room temperature Mössbauer spectra confirm the formation of the Fe(Mn)⎼type solid solution and non⎼stoichiometric Fe2P⎼, Fe3P⎼ and FeP⎼type phases. The saturation magnetization exhibits a comparable behavior to that of the average hyperfine magnetic field. The saturation magnetization, coercivity, and squarness Mr/Ms ratio values decrease with increasing the temperature from 5 to 400 K. The endothermic and exothermic peaks, that have been observed in the DSC curves, can be related to different phase transitions. The endothermic peak at about 582–589°C can be attributed to the Curie temperature of the Fe(Mn)⎼type solid solution. Several fundamental magnetic parameters have been extracted from the approach to saturation magnetization.

Etude des propriétés physiques des couches nanostructurés à base de Ni obtenues par électrodéposition

Nanostructured Ni‒P coatings were prepared by electrodeposition on a cooper substrate at different applied potential and deposition time. Morphology, structure, microstructure, mechanical, magnetic and electrochemical properties have been studied by scanning electron microscopy, X‒ray diffraction by using a structural refinement program (MAUD) based on the Rietveld method, microduremeter, vibrating sample magnetometer and potentiodynamic analysis. The deposits are dense, regular and uniform. The phosphorus content is about 8.64 ‒12.2 at. %. The Rietveld refinement of the XRD patterns reveals the formation of Ni(P) solid solution with two structures Ni(P)‒1 and Ni(P)‒2 dispersed into an amorphous matrix, or a mixture of Ni2P, Ni(P)‒1 and Ni(P)‒2. The microhardness values of the coatings increased with decreasing the crystallite size of Ni(P)‒2 phase. The coercivity and magnetization are plating condition dependent. The corrosion resistance of deposits increased with increasing the applied potential.

Effet de l’addition du Nb et du Cu sur les propriétés structurales et hyperfines des poudres et couches Fe-Si-B nanostructurées

Mechanically alloyed Fe73,5Si13,5B9Nb3Cu1 powders were used as feedstock materials for depositing nano/amorphous coatings by high-velocity oxy-fuel (HVOF) thermal spraying process. Morphology, structure, microstructure, hyperfine, magnetic, mechanical, electrochemical and thermal properties have been investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Mössbauer spectrometry (MS), hysteresismeter, microdurometer, scratch tester,potentiodynamic analysis and differential scanning calorimetry (DSC). The coating structure consists of bcc Fe(B)-type, bcc Fe(Si)-type, and amorphous phases. The crystallite sizes of Fe(Si)-type and Fe(B)-type solid solutions increase after spraying to about (72-82) and (48-81) nm, respectively. The Mössbauer spectrometry results reveal the presence of Fe2B boride.The squareness ratio Mr/Ms and the coercivity values of the coatings decrease with increasing the number of passes. The porosity, corrosion resistance and scratch resistance decrease with increasing the coating thickness. The average microhardness values vary from 543.5 Hv100 to 642 Hv100. Several crystallization temperatures are revealed in the DSC scans in the 484-630°C and 629-683°C ranges related to the crystallization of Fe(Si)-type phase and formation of borides, respectively. The effect of Nb and Cu addition has been evidenced by comparing the properties of Fe73,5Si13,5B9Nb3Cu1 powders and coatings with those of Fe75Si15B10 ones. It is observed that the addition of Cu and Nb increases the amorphous relative fraction and the microhardness,enhances the corrosion resistance, decreases the adhesion and increases the crystallization temperatures and activation energy.

Étude numérique des propriétés optiques et dynamiques du réseau des alliages semi-conducteurs.

In this work, we are interested in the study of optical properties, dielectric, elastic and dynamic quaternary system GaxIn1-xAsyP1 -y adapted to different substrates. The method used for this work is the empirical pseudopotential method EPM associated with the virtual crystal approximation (VCA) with and without considering the effect of disorder, combined with the Harrison bond orbital model are used. These methods are simple and give quick and reasonably reliable results. We have study the variation of: refractive index n, elastic parameter C11, C12 and C44, shear modulus, Young modulus, Bulk modulus, force constants α and β and also the phonon frequencies ωLO and ωTO in function of the variation concentration of Arsenic. The numerically calculated values show reasonable agreement with the experiment and values listed in literature. Special attention has been devoted to the effects of compositional disorder and substrate upon the features of interest. It is found that the compositional disorder has an important effect on the studied properties and should be included in the calculations. The substrate effect is also found to change the properties of interest suggesting thus that more diverse opportunities regarding optical elastic and lattice vibration properties may be provided by a proper choice of the substrate and the composition y.

CARACTÉRISATION D’ALLIAGES NANOSTRUCTURÉS À BASE DE Fe-Co PREPARÉS PAR BROYAGE MÉCANIQUE

Nanostructred Fe50Co50 and (Fe50Co50)80Al20 powders were prepared by mechanical alloying in a high energy planetary ball mill (P7) under argon atmosphere. Morphological, structural, microstructural, magnetic and thermal characterizations of the milled powders were investigated by scanning electron microscopy, X-ray diffraction, vibrating sample magnetometer, Mössbauer spectrometry and DSC, respectively. In situ XRD was used to follow the phase formation during annealing. The solid state reaction between Al and Fe50Co50 mixture leads to the formation of a non-homogeneous bcc Fe(Co, Al) solid solution and a B2 Al(CoFe)-type phase having a CsCl structure, after 3 h of milling. The Mössbauer spectrometry results reveal the existence of a paramagnetic Al-rich Fe(Al) environment and magnetic components related to the Fe(Co) and Fe(Al) solid solutions. Further milling leads to the disappearance of the paramagnetic phase and the formation of a Fe(Co, Al) solid solution. The coercivity and saturation magnetization values are of about 68.7 Oe and 124 emu/g, respectively, after 48 h of milling. The DSC scans present exothermic and endothermic peaks related to several processes such as phase transformation, recovery, strain relaxation, order-disorder transition and grain growth. The soft magnetic character of the mechanically alloyed powder is preserved after a heat treatment up to 1000°C.

Propriétés magnétiques et structurales du composé Fe73.5Si13.5Nb3B9Cu1.

The mechanical alloying process has been used to prepare nanocrystalline Fe73.5Si13.5Nb3B9Cu1 alloy from pure elemental powders in a high energy planetary ball-mill Retsch PM 400/2. Morphological, microstructural, structural, and magnetic characterizations of the milled powders were investigated by scanning electron microscopy, X-ray diffraction using the MAUD program which is based on the Rietveld method, hyteresismeter and differential scanning calorimetry. the crystallite size reduction to the nanometer scale is accompanied by an increase in the atomic level strain. The reaction between Nb and B leads to the formation of a bcc Nb(B) solid solution with a lattice parameter close to 0.3412 nm after 5 h of milling. The complete mixing of the elemental powders gives rise to the formation of a highly disordered Fe(Si)-type and Fe(B)-type solid solutions as well as Fe2B boride after 150 h of milling. The coercivity and magnetization saturation values are of about 62 Oe and 14 emu/g, respectively, after 150 h of milling. Several magnetic transition temperatures are revealed in the DSC scans of the ball-milled powders in the temperature ranges of 500-560°C and 599-606°C, related to the Fe(B) and Fe(Si)-type phases, respectively.

Propriétés magnétiques et stabilité thermique des poudres nanostructurées Fe57Co21Nb7B15

Le broyage mécanique haute énergie a été utilisé pour élaborer l’alliage nanostructuré Fe57Co21Nb7B15 (wt.%) à partir de poudres élémentaires de Fe, Co, Nb et B dans un broyeur planétaire type Fritsch P 7 sous atmosphère d’argon. Les changements hyperfins, magnétiques et thermiques des poudres broyées ont été étudiés par spectrométrie Mössbauer, magnétomètre à échantillon vibrant, analyse calorimétrique différentielle et diffraction des rayons X in situ. Les résultats de la spectrométrie Mössbauer ont révélé la formation de la solution solide Fe(B) au début du broyage (1 h) et l’augmentation de la proportion volumique du fer non mélangé suite à la migration des atomes de Co, Nb et B vers la matrice désordonnée, après 12 h de broyage. La formation d’une phase amorphe magnétique majoritaire (67,5%), dans laquelle sont dispersés des nanograins type FeCo, Fe2B et Fe3B, est obtenue après 48 h de broyage. L’état stationnaire est caractérisé par un champ coercitif de 59 Oe, une aimantation à saturation de 111 emu/g et un champ hyperfin moyen de 19,85 T. La cinétique d’amorphisation à l’état solide est décrite par un coefficient d’Avrami n <1. Mots clés : Nanomatériaux ; Amorphisation ; Broyage mécanique ; Alliages Fe-Co-Nb-B ; Spectrométrie Mössbauer ; Mesures magnétiques.

Étude de couches amorphes et nanocristallines à caractère magnétique

The aim of this work is to study the amorphous and nanocrystalline magnetic Fe75Si15B10 coatings prepared by HVOF thermal spraying from nanostructured powders elaborated by mechanical alloying in a high energy planetary ball mill type Retsch PM400/2. Morphological, magnetic, mechanical, structural and microstructural changes of the milled powders and coatings were followed by scanning electron microscopy, hysteresismeter, microdurometer and X-ray diffraction by using a structural refinement program (MAUD) based on the Rietveld method. The obtained product after 150 h of milling of the Fe75Si15B10 powders is a mixture of 74%Fe(Si, B) solid solution and 26%Fe2B boride with crystallites size of about 13 and 6 nm, respectively. The coercivity and saturation magnetization values are 55 Oe and 6.7 emu/g, respectively. The Fe75Si15B10 coatings are partially amorphous. The Rietveld refinement of the XRD patterns reveals the presence of Fe(Si,B) nanograins dispersed into an amorphous matrix (52-58%). Microhardess increases with increasing coatings thickness while the porosity decreases. The coercivity decreases with increasing coatings thickness whereas the saturation magnetization, Ms, increases. Key words : Nanomaterials ; Coatings ; Fe75Si15B10 alloy ; Mechanical alloying ; HVOF; Amorphous; XRD ; SEM ; Magnetic properties

Caractérisation du composé nanostructuré Fe.6P.1.7C

Le composé nanostructuré Fe-6P-1.7C a été préparé par broyage mécanique haute énergie dans un broyeur planétaire type Retsch PM 400/2. Les variations morphologiques, structurales et microstructurales des poudres broyées ont été suivies par microscopie électronique à balayage, diffraction des rayons X et spectrométrie Mössbauer. La stabilité thermique et la microdureté ont été étudiées par calorimétrie différentielle programmée et microdureté Vickers. Les observations morphologiques montrent l'existence d'une large distribution de la taille et la forme des particules de poudres. L'affinement Rietveld des diffractogrammes X et l'analyse des spectres Mössbauer révèlent la formation, après 12 h de broyage, des phosphures Fe3P et Fe2P, le carbure métastable Fe3C et la solution solide désordonnée Fe(P, C). Le broyage prolongé conduit à une mixture de Fe3P;, Fe3C et Fe(P, C). La cinétique de formation du mélange est décrite par un coefficient d'Avrami n=1,7. Le recuit de la poudre broyée pendant 50 h jusqu’à 220°C conduit à la disparition du phosphure tétragonal Fe3P et la formation de la solution solide désordonnée Fe(P, C). Par contre, celui réalisé jusqu’à 450°C favorise la formation des oxydes Fe2O3 et Fe3O4. Le taux de porosité et la microdureté Vickers des poudres compactées à froid augmentent avec la réduction de la taille des grains.

Propriétés microstructurales et magnétiques d’un alliage nanostructuré à base de FeCo

The Fe31Co31Nb8B30 (wt.%) nanocomposite was prepared by mechanical alloying in a planetary ball mill. Morphological, structural and microstructural variations were followed by scanning electron microscopy and X-ray diffraction.Changes in the magnetic behavior as a function of milling time and annealing temperature have been studied by Mössbauer spectrometry, magnetic measurements and differential scanning calorimetry. Morphological observations have shown the existence of a broad distribution of size and shape of the powder particles with an increase in size after 10 h of milling. An important refinement is achieved on further milling time. The Rietveld refinement of the X-ray diffraction patterns reveals the formation of metastable Fe3B and Fe23B6 borides after 1 h of milling and the decomposition of Fe23B6 into Fe3B, Fe2B and α-Fe after 10 h. A stationary state,charcterized by an amorphous matrix in which FeCo nanograins are embedded, is observed after 25 h of milling. The amorphization kinetics is described by two Avrami parameters, n1 =1.41 and n2 = 0.34. The paramagnetic behavior of the amorphous phase, as evidenced by Mössbauer spectrometry, is due to the presence of non magnetic elements (Nb and B). After 125 h of milling, an amorphous matrix (~95%), where FeCo and Fe2B nanograins are dispersed, is obtained. The reduction of the magnetic moment leads to the decrease of the average hyperfine magnetic field and the saturation magnetization. The excess enthalpy, due to the high density of defects, is released at temperature below 573 K. Crystallization and growth of crystal domains are the dominating processes at high temperatures. Curie temperatures which are close to the ambiant temperature reflect the compositional non-homogeneity of the paramagnetic amorphous matrix.

Caractérisation de l’alliage nanostructuré Fe-8P élaboré par broyage haute énergie

Nanostructured Fe-8P powders were prepared by mechanical alloying. The particles morphology, phase identification, alloying process at the atomic level and thermal stability of the powders were studied by scanning electron microscopy, X-rays diffraction, Mössbauer spectrometry, magnetic measurements and differential scanning calorimetry. The Rietveld refinement of the X-rays diffraction patterns and the Mössbauer spectra analysis show that the FexP (1 < x < 2) and Fe2P phosphide phases are the main product after 3 h of milling. A mixture of Fe3P, FeP, a-Fe1 and a-Fe2 nanophases is obtained after 21 h of milling. The a-Fe1 and a-Fe2 phases have the same lattice parameter, different grain sizes, different hyperfine parameters and different Curie temperatures. The heat treatment of the nanostructured powders, obtained after 21 h of milling, confirms their thermal stability and leads to the formation of FeP and FexP (1 < x < 2) phosphides as well as to the homogenisation of the a-Fe(P) solid solution.

Mécanismes et cinétiques d'amorphisation par broyage mécanique haute énergie de l'alliage Fe77Cr4P8C11.

Les mécanismes et

Mécanismes de formation des composés FeCo et Fe-Co-Mo obtenus par broyage mécanique haute énergie

The present work concerns the formation mechanism study of the FeCo, Fe-48Co-2Mo, and Fe-44Co-6Mo compounds. In order to enhance our knowledgements on the formation process of the ternary Fe-Co-Mo alloys, at solid state and ambient temperature, we have prepared nanostructured powders of pure Fe and equiatomic FeCo compound in which Mo is added and where Fe concentration is constant. X-ray diffraction results show that the mechanical alloying of pure Fe reduces the crystallite size to the nanometer scale (12 nm), increases the microstrains (1.2 %) and the lattice parameter (0.2874 nm), after 40 h of milling. Structural analysis deduced from the Rietveld refinement of X-ray diffraction patterns of FeCo and Fe-Co-Mo compounds milled for 24 h shows the formation of bcc solid solution with a single phase (a = 0.2861 nm) for the FeCo compound and bcc FeCoMo solid solution where unmixed nanocrystalline Mo is embedded for the Fe-Co-Mo compounds. The lattice parameter of the FeCoMo solid solution is Mo content dependent. Microstructural analysis has revealed the nanocrystalline character of the milled powders. The average crystallite sizes are of about 10-13 nm and the microstrains values are close to those of metals and metallic alloys (1-1.6 %). The kinetic dissolution of Mo in the FeCo matrix is described by low and unusually values of the Avrami parameter (n<1). This can be explained by the diffusion at the interface and dislocations segregations. This result is in good agreement with the mechanical alloying process. For the pure iron, the Mössbauer spectrometry has revealed the coexistence of three sites with hyperfine magnetic fields B1=33.1 T, B2=34.2 T, and B3= 31.2 T attributed, respectively, to a perfect locally defect free lattice region, to the crystalline lattice distortion, and to the defect sites. The Mössbauer spectrometry study of the FeCo and Fe-Co-Mo compounds has shown that the reaction between Fe and Co begins after 3 h of milling. The FeCo solid solution formation, after 12 h of milling, in both FeCo and Fe-Co-Mo compounds is due to the low and negative mixing enthalpy of Fe and Co. The substitution of Co atoms by Mo ones is clearly evidenced by the appearance of more and less resolved satellites, on the Mössbauer spectra, of which the intensity increases with the Mo content and the milling time. The fitting results of the Mössbauer spectra with the hyperfine field distribution by zone have revealed the coexistence of three environments types: Fe-rich FeCo-1, Co-rich FeCo-2, and CoMo-rich FeCo3. Key words: Nanomaterials; Fe-Co-Mo alloys; Mechanical alloying; X-ray diffraction; Rietveld method; Mössbauer spectrometry.