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.I nHII ufMrltt1tNJltlr"u N/t",...""""II¡".. Muring cooling, IWOCurie temperaturesare detected in both caes, Tbe first one is silualcd al IlxlUI 520°C and lhe: second is measuredal 350-38ooC. These lwo paramagnelic-fcrromagnclie ransilions are:assoc:ialedwith lhe: pbase lransfonnations from \he arnorphou.~Slale induc.:d during he Ihc:rmallrealment. These lwo phaseshaye:already llecn Sludic:din our previous invesligalil>n .101and have:been n:cognisc:das fc:c-(Fe:,Co)uO.(crystallisalion Ie:mperalurefound al 680°C) und '"lB (e:r)'stallisalionIe:mperaluremeasuredal aboul 850°C). These valu.:s are:in good IIgrcl.'lI1cnl ,'llh Ihose laund in líle:ralure(11] and c:xplain why \he samplc:is paramagne:liclar lhe Ic:mp:r.llurc ¡,il!herlhan 52ooCwhc:nthis samplc:is hc:atedunder vacuum. The:selwo difTerenl experiments show thal Ihe TMO signal!5obtained are ver)' sensilive tI> Ihe ,"pc:rime:ntalcondition. While: lwo new ferromagne:licphaseshave e:rystallisedin bolh cases(IccIld'oluD. and C020), a third transformation al \he tempel'8lun: of 55OOChas occurTCdwilh hcaling carric:dout in air, In lhe lalter, an oxidalion laye:ris well visible and e:ausesa ehanaeun Ihe l11agnetie propc:l1yofthe alloy. In arder lo unde:rstandthe:role:ofthis oxidalion layer. Ihe struelure .""..ncialedwith each lransformalion has been inve:sligatc:dby using XRD and TEM. \/ru.:lurul

inve.fliRUliun

DifTerenl healina treatments orIbe sample wilbin Ihe inlereslina lemperalure rangc (Iu 600. 6SII. mil and 700"C; heating rete: 100000min) have been done using a lubular fumace in oir and unc.lcr

a.:uum. Thc lint structurel lransformalions induced by e8Ch\hermal treatment in both casesW\:/'\.' Ihenuhservedand compared by using TEM and XRD melhods. l' igure 2 shows lhe XRD pattems obtained after healina Ibe sample under vacuo",. l/p lu MII"('. lile sample is slilllllOOrphOUSas is shown on \he difTractograrnsQ and h wherc a broad halo is unIr \ isible. As previously rnentioned, \be ribbon begins lo Clystallise al 680°C and al 7()(r\' IdiO"raclograme) and funhermore al 7S00c (pattem d). all Ihe peaks corresponding lo Ihe ~r)slallisalion oflbe fcc-(Fe,Co)n& struclure (Pearsonsymbol cF116. CroC. type) are ddectcd. \

un""

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v.cuum

In .11 H..ling

,.,.: 1O"Cllllin '"

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f .

1 ig. l. TMG sign.ls obl1lined rrom the .morphous 'lamplc: (dolline:

in .ir: salid line: under YlICuum)

Fig. 2. XRD pattems from Ihe simple differenl healing undcr vac:uum

IOer

.

-

454

ISMANAM-2000

The eryslalliscd strueturc obtaincd after hcating thc sample 10 700°C was &Iso obscrvcd by 11 \1

and thc micrographs are prcscntcd in the figure 3. Thc brisht-field TEM image (micrograph,.

I

rc:veals the presencc: of sub mieronie erystaJliscd grains homogencously nucleatcd and rando."I. distributcd through the residual arnorphous matrix (diametcr ofabout JOOrun). At Ibis tempcralUl\ Ihe: itlloy is not fully erystalliscd and the halo eharactcristie: of the amorphous pha.e is observe\! "" Ihe selcctcd area diflTaction pattem (SADP on micrograph b) taken from the arnorphous mah.. The erystalliscd grains grow with a caulif1ower-like shape in thc rcmaining amorphous matrix ;,,~: ¡he fe:e atomie planes are elearly visible on the hish rcsolution TEM image (mierograph e). Th" planes appcar 10 be continuous whatcver thc growth dircction mcaning that cach ¡raiD is a siu;.:l. e:rystal. Thc structure is confirmcd to be a face-centred-cubie by the selcctcd arca difTracll"1 pallem (mierograph ti) showing thc corrcsponding difTractcd spots.

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.,Fil. 3. TEN bnght-ficld imaac (a) from !he samplc hcalCd It 1OO"CuncIcr VICUUITI(hcatinl ntc; I O"C/nllll, Ind the rclltcd diffiactíon plncm takcn from thc Imorphous matrix (b). Hiah-rcsolution imaac from o.k fcc-(Fc,COhAanín

(e) Ind ¡ts rclltcd diffi'action pattem (d)

The TEM investigations have revealed that the dimensions of the fcc grains ll'C not nanometll\ in selle as il ulually the case with similar soft magnetic alloyalike FinemetTM(Fe-Si-Nb-B-Cul ",

~

455

ISMANAM-2000

I ne IIBI CrySJaIllSllUlln ,~Jablishcd as follows:

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oy 1ICIIUn¡ me reU\..O4OI'I~)o

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amorphous -+

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XRD analysesafter Ihermal trealmenls carried out in air by usi"g a lubular fumace are pn.'Senlcd 111ligun: 4. As shown with !he TMO experirnents (Fig. 1.). ,he crystallisation begins al the Icmpcralureof SSO"C. The dilTractogram u on figure 4 peñormed on the samplc heal~:dal 600°(: ,11,,\\S Ihe prcscnee01'crysJalliscd phases while for the same Ih,rmal Ircatment. the sJlmph:WIIS Illund fully amorphousby healing the sample under vacuum. This difTractogrampresentstwo main pcllks IllKI hy belting al higher lemperalure (650 and then 700"C. difTractogramsh and ,.). it is pllssible 10 idenlify lirstly the fec-(Fe,Co)uB6 compound and !secondly a body-ccnlrcd cubic structUJe.n:cognisedas the bcc-Fe phue. Other very small peab are detectedand are Ihose due lo Ihe rormation of I thin oxide layer well visible in brown on the surfau of the sample. Al 750°('. Ihe rihbon is fully cryslallised and the main peak is relaled lo Ihle bcc-Fe phasc. which I,,:cllmes pn:d,'mirwnl. Tbc magnetic sisnal observcd by TMO at SSOOCis then connect..-dwith the: IryslalliSillion of lhe ferromagnetic bcc.Fe. which hls a Curie tcmperalurc or 950"