Effects of Mn addition on the devitrification kinetics, microstructure and hyperfine inter-
action parameters of FeCoMnNbB alloys were investigated. As-cast and nanocrystalline samples
were studied by different experimental techniques and results were correlated. Mn alloying pro-
voked a significant lowering of the Curie temperature of the amorphous phase. No big changes
were observed in the microstructure of the nanocrystalline alloys with the Mn additi

Benaini, H.

Blázquez Gámez, Javier Sebastián

Conde Amiano, Alejandro

Conde Amiano, Clara Francisca

Ochin, Patrick

idUS. Depósito de Investigación Universidad de Sevilla

549Influence of Mn content on the microstructure and hyperfine interaction parameters of...© 2008 Advanced Study Center Co. Ltd.Rev.Adv.Mater.Sci. 18(2008) 549-552Corresponding author: A. Conde, e-mail: conde@us.esINFLUENCE  OF  Mn  CONTENT  ON  THEMICROSTRUCTURE  AND  HYPERFINE  INTERACTIONPARAMETERS  OF  AMORPHOUS  ANDNANOCRYSTALLINE  HITPERM-TYPE  FeCoMnNbBALLOYS								 !"# $ %	&	'(')*	+,*-$&.&$'/	'		0#1& 2(%	Received: March 29, 2008Abstract. Effects of Mn addition on the devitrification kinetics, microstructure and hyperfine inter-action parameters of FeCoMnNbB alloys were investigated. As-cast and nanocrystalline sampleswere studied by different experimental techniques and results were correlated. Mn alloying pro-voked a significant lowering of the Curie temperature of the amorphous phase. No big changeswere observed in the microstructure of the nanocrystalline alloys with the Mn addition.1. INTRODUCTIONPartial substitution of Co for Fe in Fe-basednanocrystalline alloys (α-Fe nanocrystals embed-ded in an amorphous matrix) yielded the develop-ment of HITPERM alloys [1], which extended thegood soft magnetic properties of these systemsup to higher temperatures due to the increase ofthe Curie temperature of the amorphous matrix,TCam. A higher TCam increases the maximum tem-perature at which the magnetic coupling betweennanocrystals effectively average out themagnetocrystalline anisotropy [2]. Compositionaltailoring of HITPERM alloys by addition of otherelements (Mn, Ge) to optimize their physical prop-erties has been carried out in some recent works[3,4]. Previous work on Mn effect on FINEMET al-loys can be found in the literature [5]. In this work,the effect of Mn addition on theFe60-XCo18MnXNb6B16 (X = 2,4) alloy series is stud-ied for amorphous and nanocrystalline samples.2. EXPERIMENTALAmorphous ribbons (30 µm thick and 10 mm wide)were prepared by planar flow casting method. Dif-ferential scanning calorimetry (DSC), thermomag-netic gravimetry (TMG), X-ray diffraction (XRD),transmission electron microscopy (TEM) and 57FeMössbauer spectroscopy (MS) were used in thisstudy. The MS spectra were fitted with NORMOSprogram [6].3. RESULTS AND DISCUSSIONFig. 1 shows the nanocrystalline microstructuredeveloped during primary crystallization. The analy-sis of XRD patterns shows no significant differ-ences between the two studied alloys: amorphousplus α-Fe(Co,Mn) phase with a grain size ~12 nmand crystalline volume fraction ~60%. TEM imagesshow similar microstructures of nanocrystals withirregular shape, ~15 nm in size, embedded in a550 H. Benaini, J.S. Blázquez, C.F. Conde, A. Conde and P. OchinFig. 1. Bright field TEM images and XRD patterns of nanocrystalline samples (heated up to 935K).residual amorphous matrix, in agreement with XRDresults. The onset temperatures of the DSC exo-thermic peak, 780 and 787K for 2 and 4 at.% ofMn, respectively, are higher than that of the Mnfree alloy (761K) [7]. The second DSC exothermicpeak completes crystallization with the formationof boride phases. The kinetic parameters of thenanocrystallization process obtained from TMGisotherms are summarized in Table 1. The valuesdescribe a very slow kinetics characterized by anAvrami index ~1 and an activation energy ~4 eV,typical of nanocrystallization processes. TMG wasalso used to measure TCam (599 and 569K for 2and 4 at.% of Mn, respectively). A strong reductionof TCam was found as the Mn content increases andwith respect to the Mn free alloy (647K) [7].Mn (at.%) n (initial)       n (final)  Q (eV)       2     0.91           0.28 3.8 ± 0.3       4     0.98           0.35 4.4 ± 0.3Table 1. Kinetic parameters: n Avrami exponent(at 773K), Q activation energy from the thermal de-pendence of induction time.551Influence of Mn content on the microstructure and hyperfine interaction parameters of...Fig. 2. Mössbauer spectra and probability distribution of hyperfine magnetic field contributions of thenanocrystalline samples.MS spectra of as-cast samples (not presentedhere) show sextets with broadened and overlappedlines typical for amorphous alloys. They were fit-ted using a distribution of hyperfine magnetic fields(HFD). A bimodal HFD is observed with two over-lapped peaks, centred at ~21 T and ~12 T, respec-tively. The former peak is ascribed to Fe withoutNb as near neighbours and the latter one to Fewith Nb in their vicinity. For nanocrystalline samples(Fig. 2) the MS spectra were fitted using 4 valuesof hyperfine magnetic field, HF, (α-Fe,Co crystal-lites) and two HFD (amorphous phase and inter-face region). There is no difference between theaverage HF of the crystalline phase of both stud-ied alloys (35.4 T), although the isomer shift, IS,for the alloy with 2 at.% Mn content is larger thanfor the alloy with 4 Mn at.% (IS=0.045 and IS=0.032mm/s, respectively, relative to α-Fe foil at 300K).The large value of the average HF is due to thepresence of Co inside the α-Fe crystals. Althoughthe presence of Mn inside the crystals can not beunambiguously established, the effect of Mn on theglobal system is evidenced as a reduction of <HF>as the Mn content increases: for as-cast samplesfrom 20.0 to 18.9 T and for nanocrystalline samples,from 26.8 to 25.7 T, for 2 and 4 at.% of Mn, re-spectively. For the nanocrystalline alloys studied,the fraction of Fe atoms located in crystalline siteswas larger in the case of 2 at.% Mn alloy (60 %)than in the case of 4 at.% Mn (54 %). This differ-ence could indicate that the Mn content in the α-Fephase were larger for the 4 at.% Mn than for the 2at.% Mn alloy, taking into account that from XRDthe crystalline volume fraction is almost the samefor both nanocrystalline alloys.4. CONCLUSIONSIn conclusion, small Mn addition does not affectsignificantly the nanocrystalline microstructure ofFeCoNbB alloys. The presence of Mn inside thenanocrystals can not be unambiguously assertedbut its effect on the global system is clearly de-tected as a reduction of <HF> and TCam.552 H. Benaini, J.S. Blázquez, C.F. Conde, A. Conde and P. OchinACKNOWLEDGEMENTSWork supported by the MEC of Spanish Govern-ment and EU FEDER (Project MAT2007-65227)and the Junta de AndalucÌa (Project P06-FQM-01823). JSB is grateful to Junta de Andalucia for aresearch contract.REFERENCES34/	5%67	8'9(	::	,000.&03&46/		%;<=>'=%2?'@	::	,00".1"$314	%6	%	(	::	 ,&  #.-$3#4A<=>'=%*(	6	::,&  ".&#3"4BC=%AD=8	=E%;*%	=F%G8	;<=>'=::		!" !	,&  "."-3!4*6	%A8'	/(::	,0$1.!-"3-4A<=>'=%		6	::	#,&  .$-

Influence of Mn content on the microstructure and hyperfine interaction parameters of amorphous and nanocrystalline Hitperm-type FeCoMnNbB alloys.

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Influence of Mn content on the microstructure and hyperfine interaction parameters of amorphous and nanocrystalline Hitperm-type FeCoMnNbB alloys.

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