Mechanism of mechanical alloying phase formation and related magnetic and mechanical properties in the FeSi system

Journal of Alloys and Compounds, 198 (1993) 155-164 J A L C O M 670

155

Mechanism of mechanical alloying phase formation and related magnetic and mechanical properties in the Fe-Si system M. A b d e l l a o u i * Centre d'Etudes de Chimie Mdtallurgique, CNRS, 15 Rue G. Urbain, F-94407 l~try sur Seine Cedex (France)

T. B a r r a d i SUPELEC, F-91192 Gif sur Yvette Cedex (France)

E. G a f f e t * ' * * Centre d'Etudes de Chimie Mdtallurgique, CNRS, 15 Rue G. Urbain, F-94407 ~try sur Seine Cedex (France)

(Received September 7, 1992; in final form February 2, 1993)

Abstract Based on X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) investigations, energy dispersive X-ray (EDX) chemical analysis, thermal analysis (DSC and DTA) and thermomagnetogravimetry (TMG) investigations, the far from equilibrium crystalline to amorphous phase transition induced by mechanical alloying (MA) in the Fe-rich side of the Fe-Si system was studied. Starting from a mixture of Fe and Si powders, MA leads to an expansion (up to 16 wt.% Si) of the A2 crystalline disordered solid solution phase domain. In this composition field an amorphous phase is also detected. For an Si content larger than 16 wt.% Si, a mixture of metastable phases (an amorphous and high temperature phases) and stable phases (low temperature phases) was detected. The crystalline to amorphous phase transition was a polymorphic phase transition which was attributed to instability of the crystalline lattice. The expansion of the A2 phase domain up to 15 wt.% Si was confirmed by Vicker's microhardness measurements, as well as the starting of the ordering reaction A2-B2 above 16 wt.% Si and the existence of the D03 phase at 17 wt.% Si. The influence of the structural state and the grain size on the magnetic properties such as the coercive force and the saturation magnetization was also studied. A high coercive force value of 17.2x 103 A m -a at 1000 Hz frequency and 0.15 T magnetic induction was reported for the 5 wt.% Si composition sample. An increase in ductility and low hysteresis loop were observed for the 10 wt.% Si composition.

1. Introduction

Owing to the combination of a low magnetostriction and a high saturation magnetization, the crystalline alloys of the Fe-Si system are suitable for the construction of transformers. The amorphous alloys allow control of the magnetic anisotropy. Such control was attributed to the high atomic mobility of the amorphous state [1 ]. The high effective quenching rate characteristic of the melt spinning process (107-10 s °C S-1) seems necessary for amorphization of the Fe-Si system alloys (6 wt.%) [2]. This is related to the low glass transition and the low crystallization temperatures which make it very difficult to stabilize the amorphous structure at room temperature. In this sense mechanical alloying seems to be a promising method for producing Fe-Si *Present address: ISITEM, CNRS, La Chantrerie, CP 3023, F-44087 Nantes, Cedex 03, France. **Author to whom correspondence should be addressed.

0925-8388/93/$6.00

amorphous phases which are stable at room temperature [3, 4]. Since adding silicon to electrical steels has become common practice because silicon increases the resistivity with a consequent reduction in magnetic losses, the magnetic properties of the Fe-Si alloys have been the subject of study of many researchers. The magnetic properties such as coercive force, power losses, permeability and saturation magnetization are strongly influenced by the structural state and grain size of specimens. In fact, Narita and Enokisono [5] studied the magnetic properties of an Fe-6.5wt.%Si alloy and concluded that aging at 500 °C which involved D03 order structure was more effective in improving the magnetic properties than treatments at higher temperature which presumably produced B2 ordering. More recently, Degauque et al. [6] concluded that losses were lower in Fe-6.5wt.%Si ribbons aged at 700 °C, which involved B2 ordering, than in those aged at 500 °C.

© 1993- Elsevier Sequoia. All rights reserved

156

M. Abdellaoui et al. / Properties of the Fe-Si system

To explain the origin of this difference, Alto6 et al. [7] studied the effect of aging treatments in the temperature range 400-700 °C on the microstructure and magnetic properties of Fe--6.4wt.%Si. Since treatments of 1 h led to a lower value of coercive force at 1025 °C (37 A m -~ instead of 100 A m -1 in the as-cast state), the authors concluded that the significant improvement in magnetic properties is associated with the increase in the grain size and the development of a texture, since both act in the direction of increasing the domain wall mobility. After aging treatments at different temperatures, the authors concluded the following. (1) The existence of 1/41111] antiphase boundaries of B2 showing a marked anisotropy which have a preference for the (100) planes and the existence of oxide particles induce deterioration of the magnetic properties since they lead to reduced mobility of the magnetic domain walls (the 600 °C aging temperature case). (2) Large grain size and more isotropic B2 antiphase boundaries seem to be promising (the 700 °C aging temperature case). The effect of magnetic domain structures and grain size on the coercive force has been studied recently. In fact, Cunha and Johnson [8] used Lorentz microscopy to study the Fe-6.06wt.%Si magnetic domain structures in specimens of small grain size (10/zm or less) and large grain size (200 /~m or more). The authors concluded that a complex domain structure is formed in small grain size specimens. In fact, the interaction structure formed at the grain boundaries is a significant fraction of the total domain structure and is then pinned at the grain boundaries, resulting in a material with relatively high coercive force. Large grain size specimens have an antiparallel domain structure for which the interaction structure is confined to the vicinity of the grain boundaries. Such a structure is only a small fraction of the total domain structure, resulting in a material with low coercive force and low core loss. In addition to the effects of grain size and structural state, it has been reported that the plastic deformation and then the dislocation density affect the magnetic properties. Indeed, Dvorovenko et al. [9] reported the variation of structural, magnetic and mechanical parameters of Fe-l.5wt.%Si alloy during plastic deformation. The authors concluded that the coercive force and the microhardness were governed by the level of microdistortion (illustrated by the full width at halfheight (FWHH) of the X-ray line). Such microdistortion is a result of dislocations created during deformation of the material. This suggests that dislocations are insurmountable obstacles for the domain walls and determine the level of coercive force of the materials.

Since it was reported that there is a drastic reduction in ductility of the Fe-Si alloys for compositions above 4.5 wt.% Si [7, 10] and since the ductility is a primordial condition for alloy rolling, the influences of the structural state on microhardness have been studied. Faudot et al. [11] reported the dependence of microhardness on structural state for the composition Fe-6.5wt.%Si. The authors concluded that the D03 phase hardens the material whereas the B2 phase softens it. The primary purpose of this work was to investigate the effect of mechanical alloying on the Si solubility limit of the A2 disordered supersaturated solid solution phase (up to 16 wt.% Si) and on the amorphous phase formation in the composition range 0~
2. Experimental details 2.1. Milling conditions corresponding to determination of the influence of composition on the stationary endproduct The mechanical alloying (MA) processes corresponding to determination of the influence of composition on the stationary end-product structure were carried out using a classical Fritsch planetary high energy ballmilling machine (Pulverisette P5/2) or a specially designed machine, the so-called G5, which has the same physical characteristics as the Pulverisette P5/2 but allows control by an electronic tachometer of the various effective rotation speeds during the mechanical alloying process. The rotation speeds of the disc and the vials were respectively f~ = 340 rev min-1 and to= 765 rev min- a 10 g of a mixture of pure Fe powder (Prolabo, R.P. Normapur) and pure Si (hyperpure polycrystalline silicon) pieces, with compositions ranging from 0 up to 17 wt.% Si, were introduced into a cylindrical tempered steel container of capacity 45 ml. This procedure was performed in a glove box filled with purified argon. Each container was loaded with five balls (diameter 1.5 cm, mass 14 g). The containers were sealed in a glove box with a Teflon O-ring and the milling thus proceeded in a stationary argon atmosphere. 2.2. Chemical microanalysis (EDX-SEM) In order to evaluate possible container contamination or element depletion which may have occurred by friction of the particles on the balls and on the vials of the container, energy dispersive X-ray (EDX) analyses were carried out using an Si-Li detector and Tracor

M. Abdellaoui et al. / Properties of the Fe-Si system EDX analyser in conjunction with scanning electron microscopy (SEM) (Zeiss DSM 950). A semi-quantitative program with internal references (SQ from Tracor) was used to analyse the EDX-SEM spectra.

2.3. X-ray investigations After continuous milling, a small amount of the mechanically alloyed powder was extracted from the container and glued onto a silica plate for further Xray investigations. X-ray diffraction (XRD) patterns were obtained using a (0-20) Philips diffractometer with Co Ka radiation (A = 0.17889 nm). The acquisition conditions were A(20)=0.1 °, and At~step(20)=40 s. A numerical method - the ABFfit program - was used to analyse the XRD patterns and to obtain the position and full-width at half height (FWHH) of the various peaks. In the ABFfit program [12], the spectrum is modelled by a polynomial background with a maximum degree of two plus a set of simple shaped peaks. The Y value of the pattern is Y(x) = Bkg(x) + ~ Peak,.(x, S,, li, Si, L,)

(1)

i=l

where i is the index of the n possible peaks, x is the Bragg angle in 20, Bkg is a polynomial defining the background, Si is a peak shape parameter allowing the selection of the appropriate function and Ii, Xi, Li represent the intensity, position and F W H H of the ith peak. Bkg(x) = bo + b~(X--Xm) + bz(x --Xm)2

4)= 0.9MB cos0

157 (3)

where ~b, B and 0 are respectively the crystalline grain size (in fingstr6ms), the FWHH and the angular peak position (in 20) of the Gaussian contribution of the crystalline peak. The Bragg expression was used to determine the interreticular distance value corresponding to the diffraction peak position 0: h = 2d sin0

(4)

2.4. Thermal analysis The thermal analysis was carried out using a DSC2c Perkin Elmer apparatus. 60 mg of as-milled powder (Fe, 2.5, 5, 10, 15, and 16 wt.% Si samples) or 50 mg (17 wt.% Si sample) were sealed in a copper capsule and heated from 50 °C up to 725 °C. The heating rate was 40 °C min-l. 2.5. Magnetic characterization 2.5.1. Measurement of the Curie temperature Our thermomagnetism set-up is suitable for following the magnetic evolution of the as-milled sample which hangs from an electromagnetic balance CI 2B type (thermomagnetogravimetry process) between the pole pieces of a permanent hard magnet. If the magnetic gradient OH/OZ is vertical and constant over the entire space occupied by the substance of mass m, the force Fm on the sample acts as the gravitational force. It can thus be measured by weighing methods and is given by the Faraday expression

(2)

Fm=mXH OHIOZ where xm is the abscissa of the centre [0mi,, 0re,x] angular domain of the observed pattern. The functions which describe the peaks have been reparameterized in the ABFfit program as a function of integrated intensity I, the mean position of the peak /z and the F W H H L, and are of three kinds: Gaussian, modified Lorentz or Cauchy. In our work, for the [110], [200] and [211] positions (the A2 crystalline diffraction peaks used to calculate the crystalline grain size and the crystalline lattice parameter), one Gaussian contribution (two Gaussian contributions for the [110] position, a crystalline contribution and an amorphous phase contribution) is not sufficient to determine the best fit. It is shown that a superposition of several Gaussian contributions (the number of Gaussian contributions is a function of milling duration) is required to obtain the best fit. Such a method was previously applied by Gaffet and Harmelin [13], Gaffet [14] and Cocco et al. [15]. The effective diameters of the particles (hereafter referred to as ~b)were calculated from Scherrer's expression:

(5)

with X the mass susceptibility and H the magnetic field. The mass susceptibility X is given by the expression x = M / B [16], where B is the magnetic induction and M is the magnetization. In the ferromagnetic state the magnetization is given by the following expression [16] M = N/xth [/~A(M/kb)T]

(6)

with N the number of atoms per unit volume, /~ the atomic magnetic moment, k b the Boltzmann constant, and T the temperature (the Curie temperature is then Tc = NIx2 A/kb ). The magnetization decreases from M m a x to zero as the temperature increases from T = 0 °C to T= Tc [16].

2.5.2. Hysteresis power losses The measurement of the coercive force, the saturation induction and the power losses were carried out using a wave generator with the possibility of frequency and amplitude control, an amplifier-integrator, and an oscilloscope.

M. Abdellaoui et al. / Properties of the Fe-Si system

158

The coercive force and the remanence values were reported for the following compositions: pure iron, 5, 10, 15 and 16 wt.% Si. For each given Si content, 7.5 g of the as-milled powder was introduced in a circular torus whose physical characteristics were a 41 mm primitive diameter and a 19.6 mm z section. A sinusoidal electrical current with a controlled frequency (produced by the wave generator) was injected in a 600 spiral coil coiled around the circular torus, in order to produce a magnetic field H which induces a magnetic flux. Based on the Lenz law, the variation of the magnetic flux across the torus section induces a potential difference in the bounds of a second coil (1100 spirals). This potential difference is given by the following expression E = - N20~/Ot

(7)

with N2 the number of spirals in the second coil and q~ the magnetic flux for one spiral. The magnetic induction is given by integration of the potential difference (the amplifier-integrator apparatus) and is given by the following expression: e

=

-

(1/N2S)JE Ot

(8)

with S the circular torus section. The magnetic field is proportional to the electrical current I(t) injected into the primary coil and is given by the following expression: H(t) =NlI(t)/l

(9)

with N1 the number of spirals in the primary coil, I(t) the electrical current, and l the magnetic path length (l = Doze with Dp the primitive diameter of the torus). The hysteresis loop is plotted in an oscilloscope by putting the magnetic field H as the X-axis and the magnetic induction or the magnetic flux as the Y-axis. 2.6. Microhardness characterization Measurement of Vicker's microhardness was carried out using a "Durimet" microhardness apparatus. The microhardness values were measured for the following compositions: pure iron, 5, 10, 15, 16 and 17 wt.% Si. The samples were embedded in the resin and then mechanically polished. Each microhardness value was determined from no fewer than eight measurements.

3. Results and discussion

3.1. Milled end-product structure as a function of the initial composition 3.1.1. O<~Si ( w t . % ) < 1 6 In this composition range, a mixture of the A2 c.c. disordered solid solution phase [17] and an amorphous phase (there is no amorphous phase for as-milled

compositions ranging from pure Fe up to 2 wt.% Si) was observed by analysis of XRD patterns. According to the Fe-Si equilibrium phase diagram [17], it is worth noting that the domain corresponding to the crystalline A2 c.c disordered solid solution has been considerably enlarged - up to 16 wt.% Si (the equilibrium phase diagram value at room temperature is 4.25 wt.% [17, 18]). The position of the diffusion halo, which is related to the presence of the amorphous phase, corresponds to the high intensity [110] crystalline peak position of the A2 phase. Therefore the amorphous phase composition is assumed to correspond to the related crystalline phase. 3.1.2. 17 wt.% Si The ordered D03 structure (Fe2Si type) [5, 17] and an amorphous phase, with a diffuse halo located close to the high intensity D03 crystalline peak position, were detected. For silicon contents larger than 17 wt.%, the MA induced phases are detailed in our previous paper [19]. 3.2. Thermal stability The differential scanning calorimetry (DSC) investigations, respective to the first heating, show two exothermic peaks (the first has a low intensity and was spread over a large temperature domain, the second is located more precisely and has a large intensity) and an endothermic peak for the 15 wt.% Si composition. The DSC investigations respective to the second heating show only the presence of a low intensity endothermic peak. Figure I shows the thermal response as a function of DSC heating temperature for the composition range from zero (pure iron) up to 17 wt.% Si. Based on XRD pattern analysis after DSC investigations, one may propose that during the first heating the low intensity exothermic peak corresponds to strain relaxations which occur at low temperature (room temperature up to 300 °C), whereas the high intensity exothermic peak was attributed to crystallization of the amorphous phase. The endothermic peak in the first heating occurs at a high temperature (the limit of the DSC machine). Therefore interpretation of the phenomenon is difficult, and requires further investigation by differential thermal analysis (DTA). Figure 2 shows the DTA curves for the 15 wt.% Si composition, In contrast to the DSC investigations, there was no endothermic peak, but three successive exothermic peaks can be observed (the first and the third were spread out, the second was located more precisely). One can explain the first peak by strain relaxations, according to refs. 13 and 14, the second peak by amorphous crystallization and the third peak by ag-

M. Abdellaoui et al. / Properties of the Fe-Si system

DSC 40°C min-1 Si wt. % = ~ ~. . . . .

DSC 40°C min-1 Si wt. % =

15 \

d

:. /

/ ,

\ \

2.5

__ : First heating ..... : S e c o n d h e a t i n g __ : F i r s t h e a t i n g ..... : S e c o n d h e a t i n g

~:

P tn'(' Fe I

200 400 600 T E M P E R A T U R E (°C)

200

400

600

T E M P E R A T U R E (°C)

Fig. 1. D S C curves of mechanically alloyed F e - S i p o w d e r s as a function o f composition.

DTA

__ : First heating ..... : S e c o n d h e a t i n g

%

© V

10 °C rain-1 I

300

500

159

3.3. Supersaturated solid solution A2 phase and amorphous phase formation Figure 3 shows the crystalline lattice parameter as a function of silicon content for a ball-milling duration of 168 h. From this figure, one can observe that the milling process induces expansion of the crystalline lattice parameter in comparison with the corresponding equilibrium A2 phase, and that the order-disorder reaction which takes place at 4.7 wt.% Si (9 at.%) [20] in the equilibrium state has been translated to 15 wt.% Si in the MA state. The onset of the reaction is shown by a sudden change in slope of the curve. Based on the crystalline to amorphous phase transition induced by ball-milling in pure Ge and Si systems [13, 14], one may propose that refinement of the crystalline A2 grains leads to a lattice expansion as the MA evolves. That is also the explanation of Fig. 3 in which the lattice parameter of the A2 crystalline phase obtained by MA is larger than the corresponding value obtained under equilibrium thermodynamic conditions [20]. Such an expansion of the lattice parameter was explained previously as a way of compensating for the surface energy by the volume energy. Furthermore, the lattice expansion may explain the supersaturated crystalline A2 phase domain. This effect of the lattice expansion on the existence of a supersaturated crystalline phase domain has been previously reported in the MA immiscible Si-Sn and Si-Zn systems [21]. As the milling process evolves, the continuous refinement of the crystalline grain which is accompanied by Si supersaturation is no longer compensated for by lattice expansion. Therefore, as the crystalline phase grains reach a critical value, the surface energy becomes the dominant factor and then the crystalline A2 phase becomes unstable and the amorphous phase is formed.

I

700

T E M P E R A T U R E (°C)

2.871@,~ ,

.

.

Fig. 2. D T A curve o f t h e mechanically alloyed 15 w t . % Si sample.

. . . . . . . 168 hours milling lattice parameter

•

~ 2.862 B-~.~. ~ . ~ ~ E quilibrinm Lattice. P a r a m e t e r

glomeration phenomenon. The difference between the DSC and the DTA responses may be explained by the DSC heating rate (40 °C min-1), which is significant compared with the DTA heating rate (10 °C min-1). Therefore the thermal resistance between the thermal source and the sample, when the agglomeration phenomenon occurs in the DSC investigations, increases leading to effective energy absorption and then to an endothermic peak instead of the exothermic peak. In the second DSC heating, the single low intensity endothermic peak is attributed to the reversible magnetic transition from the ferromagnetic to the paramagnetic states.

2.853 i

°rder-dis°rder~ \ \ ' \ ~ ' ~ transition \,

•

2.844 m

2.835

. 0

.

.

. 5

.

. . 10

.

. . 15

order-d transition . . . . . 2O 25

~, 30

Silicon Content (at %) Fig. 3. Fe(Si) A 2 lattice p a r a m e t e r o f mechanically alloyed p o w d e r as a function o f Si content. T h e lattice p a r a m e t e r c o r r e s p o n d i n g to the thermal equilibrium condition is also given.

160

M. Abdellaoui et al. / Properties of the Fe-Si system

3.4. Microhardness as a function of the as-milled powder structure Figure 4 shows the variation in microhardness versus Si content. To study the dependence of the microhardness on the structural state, the samples were annealed for 24 h at 500 °C. It is shown in Fig. 4 that the as-milled powder microhardness increases slightly up to 15 wt.% Si; however, considering the error in measurement, we assume that the microhardness value remains constant up to 15 wt.% Si, decreases slightly for 16 wt.% Si and then increases strongly for 17 wt.% Si. Since it was shown by Faudot et al. [11] that the B2 phase softens the material and the D03 phase hardens it, the microhardness was assumed to be a well defined characteristic of the structural state of the material. Thus, the microhardness was assumed to be governed by the A2 phase state up to 15 wt.% Si. This determination of constant microhardness values is in agreement with the expansion of the A2 phase domain. The decrease in microhardness in the vicinity of the composition 16 wt.% Si involves the presence of a small amount of B2 phase which softens the material. Since the intensity of the X-ray (100) superlattice line, characteristic of the B2 phase, is very weak compared with the A2 (200) (normal diffraction peak) X-ray line intensity, evidence of the presence of the B2 phase cannot be provided by X-ray characterization. The microhardness increases strongly for 17 wt.% Si, which is related to the presence of the D03 phase which hardens the material. In the annealed state, the microhardness increases as a function of Si content up to 10 wt.%, decreases up to 15 wt.% where it reaches a minimum value (989 Hv) and then increases up to 16 wt.%. The increase and decrease in the hardness values in the annealed

1800l

I

t

1600

1

"-14oo "-'12001 1000': ~¢// [ 600 0.0

;---~ • x.... t 5.0

;

Hv limit (M A) Hvmedium(M A) Hv limit (24H-500°C) Hv medium(24H-500°C) 10.0

15.0

20.0

Si content (wt%) Fig. 4. Microhardness as a function of Si content corresponding to the as-mechanically-alloyed state and to the annealed state (24 h at 500 °C).

state were found to be strong compared with the asmilled state. X-ray diffraction investigations after annealing revealed the existence of a single A2 phase in pure iron and 5 wt.% Si samples, the existence of a mixture of A2, B2 and D03 phases in the 10 wt.% Si sample, the existence of a mixture of B2 and D03 phases in the 15 wt.% Si sample and finally the existence of only the D03 phase in the 16 wt.% Si sample. Based on these X-ray investigations, we assume that the increase in the microhardness up to 10 wt.% Si can be attributed to the existence of the A2 phase and the growth of the D03 phase which hardens the material as the sample composition approaches 10 wt.% Si. For Si contents greater than 10 wt.%, the growth of the B2 phase leads to a decrease in the microhardness which reaches a minimum at 15 wt.% Si. Then the microhardness strongly increases up to 16 wt.% Si where only the D03 phase exists. The microhardness values in our work were found to be slightly large compared with those reported in ref. 11 (the microhardness values in ref. 11 range from 260 Hv for the B2 phase to 440 Hv for a mixture of B2 and D03 phases). Such an increase of the microhardness is related to the microdistortions induced by dislocations created during deformation of the material. This is reported by Dvorovenko et al. [9]. 3.5. Magnetic characterization 3.5.1. Dependence of the Curie temperature on the as-milled structural state Garcia Escorial et al. [1] report on the magnetic properties of as-milled Fe-Si alloy samples - 5, 25, 37.5, and 50 at.% Si - using a vibrating sample magnetometer (VSM) equipped with a high temperature furnace. The Curie temperature was evaluated by determining the tangent to the o(T) curve (specific magnetization) at the temperature corresponding to the maximum slope. In ref. 1 it was shown that the amorphous Curie temperature differs from that of all the crystalline phases. It was also shown [1] that the measurements obtained when cooling down the samples revealed the irreversibility of the thermomagnetic behaviour, i.e. a monotonic increase and the absence of a Curie transition near the amorphous Curie temperature (350 °C for the Fe75Si25 at.% sample). Taking into account that the amorphous Curie temperature (in transition metal-metalloid systems) is lower, usually by a factor close to 0.8, than that corresponding to crystalline material with the same composition [22], the amorphous phase composition was evaluated. In our work, we report the dependence of Curie temperature on the initial Si content for nominal corn-

M. Abdellaoui et al. I Properties of the Fe-Si system

positions ranging from 0 (pure iron) up to 30 wt.% Si. Figure 5 shows the thermomagnetogravimetry (TMG) responses for both the heating and cooling processes. For the compositions ranging from 0 up to 16 wt.% Si (pure Fe not included), the responses obtained when cooling down the samples revealed reversibility of the thermomagnetic behaviour (for the as-milled pure Fe, the magnetic transition occurred in two steps). In Fig. 1, for the second DSC heating, the presence of a low intensity endothermic peak was attributed to the magnetic transition. Taking into account from this figure that the amorphous crystallization temperature is lower than the magnetic transition temperature, one may propose that the samples start to crystallize before reaching the thermal cancellation of the amorphous phase magnetization. Therefore, such an effect obscures the measurement of the amorphous Curie temperature. Therefore, as is shown in Fig. 5, there is no significant magnetic response corresponding to the amorphous phase for the compositions ranging from 0 up to 16 wt.% Si. For the Fe nanocrystalline phase, it has been observed that Ms (the saturation magnetization) decreases with increasing density of grain boundaries [23] and that the magnetization decreases in as-milled pure Fe powders with decreasing microcrystallite size [24]. Thus, one may propose that the two-step magnetic transition in the as-milled pure Fe is related to the existence of two domains with two different densities of grain boundaries. For the compositions ranging from 17 up to 40 wt.% Si, the TMG responses were not symmetrical and the heating Curie temperature was different from the cooling temperature. The DSC heating Curie temperatures were similar to those reported by Talbot [25], whereas the first TMG heating and the first TMG cooling Curie 80O

i

'~

'

~ ~

161

temperatures for the compositions ranging from 17 up to 30 wt.% Si, were lower. It is worth noting that a second heating followed by cooling, for the above composition range, increases the Curie temperature. This fact is related to the increase in magnetization as a consequence of the decrease in the density of grain boundaries and the increase in the grain size as the heating runs are performed consecutively. In the as-milled 30 wt.% Si sample, the Curie temperature is much lower, being the result of the presence of the FesSi3 (To = 120 °C [1]) and the Fe2Si (Tc=270 °C [1]) phases [19]. 3.5.2. Coercive force as a function of Si content The coercive forces were recorded for different frequencies at a given constant magnetic induction. For each composition sample, the induction value corresponds to the saturation induction at the high recorded frequency. Figure 6 shows the variation in coercive force as a function of frequency for the pure Fe, 5, 10, 15 and 16 wt.% Si composition samples. From Fig. 6, one may observe that the samples were magnetically hard. The coercive force was found to increase with increasing frequency. With the exception of the 5 wt.% Si sample, the coercive force recorded at 1.5 × 103 Hz frequency ranges from 2.3 × 103 A m-1 for 15 wt.% Si to 4.7)<10 3 A m -1 for 16 wt.% Si. For the initial composition 5 wt.% Si, the coercive force drastically increases as the frequency increases. It reaches a value of 17.2x 103 A m-1 for a frequency o f 103 Hz. Then, it is clear that the magnetic properties of ultrafine crystallites are different from those corresponding to the bulk state.

I~ 20000

700 //

~600

15000 /

500

//

///

• ....

400

J. Talbot

~-~ •~ 10000 •

I1~

//' /

"~ ~ 200 100 0

() TMG second h e a t i n g ~/'~ TMG second cooling ...... I. . . . . . . . . . . . . . 5 10 15 20 25

5000

-•

/"

]

i

•

Fe (B=0.203T)

+ • • •

5wt% (B=0 148T) 10 wt% (B=0.167T) 15 wt%(B=0.148T) 16 wt%(B=0.148T) i

~

*

, 30

35

Si content (wF/D Fig. 5. Curie t e m p e r a t u r e as a function of Si content. T h e Curie t e m p e r a t u r e s were d e t e r m i n e d by D S C and T M G . T h e reference values of J. Talbot are f r o m ref. 25.

0.0

500.0

1000.0 1500.0 2ooo.o 25oo.o frequency tI~)

Fig. 6. Coercive force of the mechanically alloyed powder as a function of frequency.

M. Abdellaoui et al. / Properties of the Fe-Si system

162

Indeed, it was reported by Tazaki et al. [10], that the ultrafine Fe particles (UFP) were magnetically hard, with a coercivity as high as 1.1 × 103 Oe (87.5 x 103 A m-l). This value is a few orders of magnitude higher than that of bulk Fe. The magnetic properties of UFP of Fe prepared by the gas evaporation technique were reported recently [26]. At a temperature of 10 K, the coercive force was found to increase with decreasing particle diameter and a coercive force of 1.05 x 103 Oe (88.6× 103 A m -1) was reported [26]. It was also found that the value of the effective anisotropy K for the Fe particles was an order of magnitude higher than that of bulk Fe. Since the shape anisotropy was negligible, the authors assumed that the main contribution to anisotropy energy could be from magnetocrystalline anisotropy and surface anisotropy. Figure 7 shows the grain size distribution, the proportion of amorphous phase and the crystalline grain size as a function of the Si content for ball-milling duration of 168 h. From this figure, one may observe that the amount of amorphous phase increases as the Si content increases, and that for the 5 wt.% Si sample, the grain size family histogram (corresponding to wider XRD peaks) is wider than those corresponding to the other compositions. The 10, 15 and 16 wt.% Si samples have the same grain size family histogram, and pure Fe has a small grain size and homogeneous morphology. Since the X R D pattern analysis and the microhardness characterization show an expansion of the A2 phase up to 15 wt.% Si and a limit of the disorder-order reaction A2---)(B2, D03) for the 16 wt.% Si content, the effect of the order-disorder reaction on the magnetic properties is assumed to be small compared with the effects of grain size and microdistortions. The coercive force seems to be synchronous with the lattice microdistortions and then the grain size value.

i I 0

0.0

10.0 15.0 5.0 Silicon c o n t e n t (wt%)

16.0

Fig. 7. Grain size distribution and amorphous content of samples mechanically alloyed for 168 h, as a function of Si content.

It exhibits the highest value for the 5 wt.% Si composition and remains of the same order of magnitude for the other compositions. We therefore assume that a complex magnetic domain structure is formed in the small grain size samples. In fact, the interaction structure formed at the grain boundaries is a significant fraction of the total domain structure and is then pinned in the grain boundaries, resulting in relatively high coercive force materials. So, as the 5 wt.% Si alloy exhibits the wider grain size family histogram, compared with the other composition samples, it will have the highest value of coercive force. This is in agreement with the results of Cuhna and Johnson [8]. In other words, since we assume that the high microhardness values are characteristic of the high dislocation densities created during plastic deformation induced by ball-milling, we suggest that the dislocations are insurmountable obstacles for magnetic domain walls and contribute to the increase in coercive force of the material. This fact was reported by Dvorovenko et al. [9]. The interactions of Block walls with groups and clusters of defects in an Fe-6wt.%Si high strained material was recently reported [27]. The authors assumed that under a high degree of deformation, the dislocations form, together with vacancies and interstitials, a new structural component which is very similar to the interfacial phase (IP) in nanocrystalline materials and may occupy a significant atomic fraction as M6ssbauer effect investigations have shown. It was also shown that the IP content increases with increasing deformation. Then, the increase in coercive force with increasing deformation corresponds to the increase in density of defects which behave as individual defects. In other words, the Block walls will interact with each individual defect. The authors reported a decrease in coercivity for very high deformation. This was interpreted as a cleaning out of the dispersed defects in the crystalline phase and ordering of the defects, mainly dislocations, in the IP. This means that the distances between individual dislocations decrease and the Block walls will interact with whole configurations which could cause the decrease in He. This notion of IP agrees well with the magnetic behaviour of our MA powders. The influence of the frequency on the power losses and then on the coercive force and remanence was reported by Degauque [28]. It was shown that to have a constant magnetic induction when the frequency increases, it is necessary to increase the magnetic field amplitude. Such an increase in the magnetic field amplitude can induce motion of some 180° Block walls, which were blocked at low frequencies under the effect of the local magnetic field He*. Then the increase in frequency leads to an increase in the number of Block wails and in some cases to the nucleation of new magnetic domains. This effect leads to the increase in

M. Abdellaoui et al. / Properties of the Fe-Si system 1.5 10-1

Jr

@ [~ tl •

1.2 101

¢

~ 9.0 10-2

J

Fe (B=0.203T) 5 wt%(B=0.148T) 10 wt% (B=0.167T) 15 wt% (B=0.148T) 16 wt% (B=0.148T)

3.0 10-2 ~

-J'J~ /d

0.0 1001 0.0

2

1000.0

The authors wish to thank Dr. M. Harmelin (DSC) and F. Faudot (DTA) for fruitful discussions, and the workshop staff for technical assistance (CECM/CNRS).

:

/

500.0

Acknowledgments

• ~-:

crohardness value and medium coercive force. Explanation of the high values of coercive force requires more study of the magnetic domain structure by Lorentz microscopy.

J

6.010 -2 /

163

1500.0

t 2000.0

2500.0

frequency (I-I_z) Fig. 8. R e m a n e n c e force of the mechanically alloyed powder as a function of frequency.

coercive force. This is what is observed in our experiments, for each given concentration, when increasing the magnetic field frequency from 100 Hz to 1500 Hz. Figure 8 shows the variation in remanence as a function of frequency for the above-mentioned Si content samples. This figure shows an increase in remanence as the frequency increases. The remanence values are characteristic of the resistance of the material to demagnetization phenomenon. The variation in remanence was found to be the same as that of the coercive force. These remanence values are characteristic of hard materials which manifest high resistance to the demagnetization process.

4. Conclusion Based on XRD pattern investigations and microhardness measurements, the expansion of the A2 phase domain up to 15 wt.% Si and the starting of the disorder-order reaction from A2 to B2 and D03 phases at 16 wt.% Si were reported. Based on DSC investigations, the crystalline to amorphous phase transition induced by ball-milling was confirmed. The influence of the amorphous phase crystallization temperature on the Curie temperature was also studied. The coercive force was found to be strongly coupled with microdistortion of the crystalline lattice and the grain size. A coercive force of 17.2x103 A m -1 at 1000 Hz frequency and 0.148 T magnetic induction was reported for the 5 wt.% Si composition. The coercive forces, recorded at 1500 Hz frequency and 0.148 T induction, for the other composition samples, range from 2.3 x 103 A m -~ for 15 wt.% Si to 4.7x 103 m m -~ for 16 wt.% Si. Particularly interesting magnetic and mechanical properties were reported for 10 wt.% Si: a low mi-

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