ON THE AMORPHIZATION OF Fe-Si BY MECHANICAL ALLOYING

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ON THE AMORPHIZATION OF Fe-Si BY MECHANICAL ALLOYING

V. Martín, A. García-Escorial, A. Martín, F. Carmona, F. Cebollada, P.

Adeva, J. González

To cite this version:

V. Martín, A. García-Escorial, A. Martín, F. Carmona, F. Cebollada, et al.. ON THE AMORPHIZA-

TION OF Fe-Si BY MECHANICAL ALLOYING. Journal de Physique Colloques, 1990, 51 (C4),

pp.C4-197-C4-203. �10.1051/jphyscol:1990424�. �jpa-00230784�

COLLOQUE DE PHYSIQUE

Colloque C4, suppl6ment au 11'14, Tome 51, 15 juillet 1990

ON THE AMORPHIZATION OF Fe-Si BY MECHANICAL ALLOYING

V.E. M A R T ~ N , A. GARC~A-ESCORIAL*, A. M A R T ~ N , F. CARMONA, F. CEBOLLADA, P. ADEVA* and J.M. GONZALEZ*'

Instituto de Ciencia de Marteriales, CSIC. Serrano 144, SP-28006 Fadrid, Spain

Centro Nacional de Investigaciones Metalurgicas, CSIC Avda. Gregorio del Amo 8 . SP-28040 Madrid, Spain

"~aboratorio de Magnetism0 Aplicado, RENFE-UCM. Apdo. 155, SP-28230 Las Rozas, Madrid and Instituto de Cienca de Materiales, CSIC, Serrano 144, SP-28006 Madrid, Spain

Un melanqe de poudres de Fe et Si (3:l en proportion atomiaue) a Cte moulu pendant des temps differents a l'aide d'un moulin planetaire a billes. On a realise des etudes de la microstructure'et des proprietes des materiaux ainsi prepares utilisant DRX, MEB, CDB et MEV. On a trouve des evidences de la formation de la solution solide Fe7sSi25 et de

l'amorphisation partialle des materiaux prepares.

ABSTRACT

A mixture of Fe and Si powders ( 3 : 1 in atomic prouortion) has been milled for different periods of time in a planetary ball mill.. Investisations of the microstructure and pro- perties of the materials so prepared were performed usj.na XRD. SEM, DSc and VSM techni- ques. Evidences about the partial formation of the solid solution Fe7sSi25 as well as about the amorphous structure of the as-milled material were found.

1.- INTRODUCTION

Crystalline allovs on the Fe-Si system, usually with low Si content (tvpicallv around 6%

weiaht), have been commonlv used in the fabrication of transformers because they allow to com- bine hish maanetic permeabilitv (achievable by cold rollins induced textures which 0riginat.e masnetic anisotropies) with low masnetostriction values. More qenerally Fe-Si allovs cons- titute the most economic and widespread soft maanetic materials family used in the electrical industry.

Amorphous Fe-Si a1)ovs could potentially associate some of the intrinsic properties of crvsta- lline Fe-Si (such as hish saturation maqnetization and Curie temperature) with the possibility of com~lete control of the structure of masnetic anisotropies. This control should be favoured bv the low values of the "as-~repared" maqnetic anisotropy (characteristic of transition metal-metalloid amorphous allovs due to t.heir lack of lons ranse crystalline order) and by the atomic mobility characteristic of the amor~hous state that allows the induction of anisotro- pies bv maqnetic field andtor stress annealins as it has been extensively studied in tFeCo)- (SiR) ra~idlv quenched amor~hous alloys'.

Maanetlc nro~erties of amornhous Fe-Si have been reported inz-3, where this material was studied in thin-film and powdered forms prepared, res~ectivelv, bv triodc sputterins deposi- tion and S ~ a r k erosion (the material so obtained was only ~ a r t l v amorphous in this case). The hiah effective quenchinu rate characteristic of these techniques (I.07-IOB OC/sec) seems to be necessary to amorphize Fe-Si allovs since, to the authors knowledae, there is no information in the literature about amorphous Fe-Si (with Si contents close to 6% weiqht) orepared by melt suinnina or similar t.echniaues. This fact is prohablv related to low slass transition and crvstallization temperatures of the melt. spun mat.erial which makes very difficult to stabilize the amorphous structure at. room temperature. As it has been established in Ni-Zr system*, mechanicallv alloved amorphous materials exhibit hiqher crvstallization temperatures than melt spr~n materials of similar composjtions. In this sense, mechanical allovinq appears as a

~romisi.nct technique to obtain amorehous Fe-Si stable at room temperature. There is also a This work was srrpport.ed by Spanish CICYT under arants MAT88-0202 and MAT88-0254

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990424

C4-198 COLLOQUE DE PHYSIQUE

second reason makina speciallv interesting the investisation of Fe-Si alloyins by this technique since, even in the case of not achievina sisnificative amorphization, it is expectable that the obtained materizl could present. a nanocrvst.alljne structure which aaain leads to a very soft masnetic behavior as it was recentlv demonstrated in5 where nano- crvstalline Fe-Si-R was obtained bv rapid solidification followed by thermal treatments. In this case it was necessary to use Cu and Nb as additives (both insoluble in a-Fe) to impede the arain arowth typical of the crvstallization of melt spun Fe-Si-R.

A recent work6 reports results on mechanicallv alloyed Fe-Si (6.5% Si weiaht) without a clear evidence of the amor~hicitv of the material despite the very Iona millina times used (up to 900 hours). In our opinion, the c o m p o s i t i o n - i n v e s t i s a t e d in this work, althouqh tvpical of electrical steels, was not very promisins from the point of view of an easy amorphization considerins the equilibrium Fe-Si phase diaaram7. In this work, and trvins to obtain amorphous Fe-Si, we have prepared bv mechanical alloyina8 a material of nominal composition Fe7sSils that was chosen as a compromise between low Si contents tv~ical of electrical a~olications and Si contents close to 50% at. that consjderina the eau] librium phase diasram appears as more easilv amorphizable bv this technisue.

Mechanical alloyins was carried out in a nlanetary hiah energy ball mill (Pritsch Pulverisette 6 ) startina from pure element powders (Fe 99.9%. nominal particle size -325 mesh and Si 99%.

nominal particle size -325 mesh). SEM observations of the startins powders revealed wide distributions of particle size uarticularlv in the case of Si particles (see Fiq. 1). Actual mean size were evaluated as Sum both tor Sj and Fe part.ic1es.

F ~ U . 1 SE1 micrograph of the initial mixture of Fe and Si powders

All the studied samples were prepared from a mixture of Fe and Si powders of nominal compo- sition Fe75Si75 at. These powders were sealed in a cylindrical asate vial under Ar atmosphere (99.996%. dried) together with stainless steel balls (9.5 mm diameter) with a ball-to-powder ratio of 12:l. All the millinq processes were performed usinq millins intensity 8 in a 10 divisions scale and milled material was always manipulated in a slove box under Ar atmosphere.

Samples were characterized after 20, 40, 60, 90, 130 and 173 hours millins and for the sake of comparison a sample of the same nominal composition was prepared by meltins and castins the initial mixture of powders in an induction furnace.

Sample characterization was carried out by X-ray diffraction (usins CuKa radiation), scanning electron microscopv (in a microscope equipped with a EDX unit), thermal analvsis (differential scanninq calorimetry) and maqnet.izatjon measurements (periormed in a vibratinq sample maanetometer equipped with hish temperature furnace).

3.- RESULTS

In Fis. 2 we present X-rays diffractoqrams corresuondinq to the material obtained after diffe- rent millinq times in comuarison with the diffractoqram correspondinq to the initial Fe-Si mixture. No traces of oxides were detected in all the as-milled powders. It was not possible to check by X-ray diffraction the effective formation of Fe-Si alloy because of .the small va- riation of lattice parameters introduced in the crystalline Fe by the solution of Si.

Fiq. 2 X-ray diffractourams of samples milled for different periods of time in comparison with diffractoqram of the initial mixture of Fe and Si powders.

Nevertheless, it is clear that after 20 hours millinu Si peaks disaopear. The disappearance of these peaks can be attributed to the absorption of harder Si particles into the more easily deformable Fe particles. As millinq time increases uu to 60 hours the intensity of the less.

intense peaks decreases (and even qoes to zero) indicatinq a proqressive diminution of qrain size.

0 50 100 150 200

MILLING TIME ( h )

Fiu. 3 Mean qrain size of the as-milled samples as a function of milling time In Fis. 3 we represent the mean urain size as a function of the millinq time as it was eva- luated usinu Scherrer ex~ression:

D = 0.9 X / a e cos e

From the fiuure 3 it is clear the stabilization of the mean qrain size after 20 hours up to 60 hours mjllins time. Further diminution is observable from 90 to 130 hours of millinq.

C4-200 COLLOQUE DE PHYSIQUE

In comparison with the djtfractoqram correspondinq to the powders milled durins 60 hours, whlch exhibit a sinqle widened peak centered close to main a-Fe (and FelsSi.95 solid solution) peaks, the results correspondinq to the powders milled durinu 90 and 130 hours show a clear increase of the intensity of this main peak (and two more distinuuishable peaks that were not present in the 60 hours milled sample).

Microstructural observations do not al low direct conf irmation of the vroqressive diminution of the mean srain size with increasins milling time. Polycrvstalline conslomerates were formed with a considerable dispersion in size as it is clear from the picture in Fis. 4 where a cross section of powders milled durins 60 hours is presented. In this picture, dark areas co- rrespond. according to EDX analysis, to be rich in Si. particles (mayhe with the presence of SiO7) and brishter resions were identified as havins a mean composition close to the nominal.

It is also apparent from this analysis that Si diffusion in isolated small Fe particles is difficult since measured local com~osition inside these particles shows an excess of Fe in relation to the nominal com~osition.

Flu. 4 RE1 microsraph of the powders milled durjnu 60 hours. J n the cross section of large conslomerates, dark areas correspond to rich in Si particles. Grey areas were identified as

havinu a mean com~osition Fe75Si~5. Smaller and briahter particles are rich in Fe.

TEMPERATURE ("C 1

Fig. 5 DSc thermosrams of as-milled samples for different millins times.

Differential scannins calorimetry (DSc) resn!ts correspondina to as-prepared samples are shown in Fis. 5. These thermosrams were performed under N.r atmosphere usinq a heatins rate of 40

~ C / m i n . Exothermjc peaks were observed in all the samples at temperatures close to 1 9 5 o C

(with the exception of 173 hours milled sample where the peak was observed at 225 o C ) . These peaks were observed to he jrreversjble after a 600 " C , 30 min. annealins as it is shown in

Fia. 6 where we present thermosrams correspondins to annealed samples. It is worthwhile to mention that the thermosrams corres~ondins to the different samples are very similar after thermal treatment and similar too to the cast sample. Particularlv, a small peak centered around 5 8 0 D C , was observed in all the annealed samples (inset in Fis. 6 ) . This temperature coincides with the Curie temperature of a crystalline solid solut.ion of S j in Fe with a composition Fei s Si7 5

.

160 260 360

TEMPERATURE ("C)

-

3

0, _LI peak 582

I- < 2.5

W I

TEMPERATURE ('C J Fis. 6 DSc reaults obtained in thermally treated samples (dashed lines) in comparison with as-prepared results (continuous lines). Inset on 60 hours mllled sample results shows the peak associated with the ferro-paramaqnetic transitions observed at hish

temperature.

Fis. 7 shows in comparison the diffractosrams corres~ondins to the original, mixture of powders and to the powders milled durinq 6 0 hours before and after the above mentioned thermal t.reat.ment. Asa1.n no traces of oxides were detected i.n the thermally treated material. It is clear from the results the sharpenins of the peaks observed in the treated sample if compared to those apaearinq in the as-prepared sample diffractosram.

Microstructure of the annealed samnles does not show any sisn~ficntlve dltf~rence wlth the as-

milled materlal ( s e e Flq. 8 ) (

1

A S - PREPARED

.

^S1

Q Fe

ORIGINAL MIXTURE

A

A

30 45 M) 75 2 8

Flu. 7 D~ttractoarams of h 0 hours m ~ l l e d sam~lea (as-milled and after 600 ^{D C ,} 3 0 minutes anneal). Diffractoaram of the oriqinal mixture of. oowdern is also shown.

Magnetic characterizat-ion wils carried ont by measurins masnetizatjon curves at room tempera- ture and the thermal variation of the masnetization in the ranqe from room temperature to 7 0 0 oC (under an appjied rnasnotic field of 4.8 X ,105 Am--l).

Results shown in Fiq. 9 corresuond to the tbermomasnetic hehavior measured in 60 and 130 hours milled samples. A small kink is observed at 700 oC (same tcmnerzture as the peal3 observed in DSc measurements) It is also noticeable the stew decrease of the maanetication at auproxima- tely 580 oC in coincidence with the ferro-para transition of the Fe7sSj75 crystalline solid solution9. Anyway. masnetization does not n11l1 at 580 oC indicatinq clearly the presence of phases others than Fe~sSi7s (to oar o~jnion basically a-Fe with small amounts of Si diluted).

The multiphase nature of the samples is also apparent from the comparison of the room tem- perature masnetization values obtained under an applied magnetic field of 7 . 2 ^X 105 Am-' with

C@-202 COLLOQUE DE PHYSIQUE

those corres~ondinu to a-Fe and FersSi7s: which are siunificatively different even considering the possible differences in the magnetization process.

Fis. 8 BE1 microqravh of 60 hours milled h00 UC , 30 min. annealed sample. No siqnificative differences with the as-milled material were observed.

200 Nu WO

Temperature K ) Temperature (C)

Fis. 9 Maqnetizatlon vs temperature of 60 and 130 hours milled samples. These measurements were performed under an applied maanetic field of 4.8 105 Am-'.

4.- DISCUSSION

As i t was vreviouslv mentioned our X-rav results are nejther conclusive about the effective allovins of Fe and Si. nor about its amoruhization althoush thev seem to be com~atible with both facts.

The Dresence of the solid solntjon jn the as-prenared material seems to he suusested bv EDX analvsis. Zones showino a local com~osition similar to the nominal comoosition were observed and we propose to be oriqinated throuah the interdiffusion of Fe and Si favoured hy deforma- tion and solderinq of small Fe particles with imbibed S]. particles. Anyway, it is also clear that isolated articles do not ~articipate in this Process which is related to both the obser- ved excess of a-Fe and the presence of rich in Si areas inside large conslomerates.

Part~al amorphicity of the as-prepared particles js not com~letely clear but seams to he related to the irreversible exothermic peaks observed at 195 O C on DSc thermosrams. As it was mentjoned a cl ear variation of the X-rav di ffractosrams were observed in thermal l

v

treated samples in comuarison with the untreated material susoesti.na the ackj.vation of a crystalliza- tion PTOCPSS. An anomaly in thermomasnetic hehavior, difficult to be attributed to any trans-

formation present in the equilibrium phase diasram, was also observed. So, we propose that the small masnetlzation diminution observed could be related to a crystallization process.

Aft.er thermal treat.ments, DSc and thermomasnetic results allow us to conclude the formation_of a crvstalline solid solution of Si in Fe as it was evidenced bv the coincident observation of a ferro-paramasneti.~ transition at 580 0C (correspondins to a composition close to Curie temperature of crvstallj.ne Fei~Si2s). Formation of this solid solution is only accomplished in a part of the original mixture of .vowders since clearly non-zero masnetization values were measured at temperatures hisher than 580 oC. An excess of a-Fe is then clearly detectable (the Curje temperature of a-Fe js 770 DC).

The proposed presence of amorphous Fe-Si in the as-milled material has to be confirmed bv Mossbaiier and TEM studies that we are currently carrvins out.

5.- REFERENCES

1 O.V.' Nielsen, A. Hernando, V. Madurqa and J.M. Gonzglez. J. Maqn. Magn. Mater. 46341-349 (1985)

2 J.M. Alameda, J.M. Gdnzilez, F. L6pex and J.L. Vicent. J. Magn. Maqn. Mater. 38105 (1983) S. Gir6n. F. Briones and J.L. Vicent Phil. Maq. S 6 4 4 9 (1987)

3 M. Eokinozo and K. Narita. J.A.P. 231020 (1984)

4 L. Schultz in "Procee6inqs of the 6th International Conference on Rapidly Quenched Metals", Montreal 1987 IMater. Sci. Ens. 9 3 1 5 (1988)l

5 G. Herzer, Paper A03, Diqest INTERMAG Conference (1989) Washinuton U.S.A.

6 K. Narita and H. Suaimoto in "Proceedinss of SMM 9" El Escorial 1989 (Spain)

7 0. Kubascheswsk~ in "Fe Rinarv Phase Diasrams" S~rinser Verlaa (1982) pp. 136-139

8

.

^Sch~lltz in "Proceedinas of MRS Europe Meeting on Amorphous Metals and non-Equilibrium Processins" Ed. M. von Allmen. Editions de Phvsisue (Les Ulisl 1984 D. 135

9 R.M. Boxorth in "Ferromasnetjsm" Van Nostrand Co.Tnc. (1953) pp. 79