Materials Letters 60 (2006) 309–312www.elsevier.com/locate/matlet

Crystallization behavior of co-containing FINEMETamorphous alloy melt-spun ribbon

Chengdong Li a,⁎, Xuelei Tian b, Xichen Chen c, A.G. Ilinsky d, Likai Shi a

a National Engineering Research Center for Non-ferrous Metal Composites, General Research Institute for Non-ferrous Metals, Beijing 100088, Chinab Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China

c Institute of Physics, Chinese Academy of Sciences, Beijing 100080, Chinad Institute of Metal Physics, Ukrainian Academy of Sciences, UA-03680 Kiev, Ukraine

Received 17 August 2004; accepted 27 May 2006Available online 20 September 2005

Abstract

The glass forming ability (GFA) of Fe68.5Si13.5B9Cu1Nb3Co5 was investigated, and the crystallization kinetics of amorphous Fe68.5Si13.5-B9Cu1Nb3Co5 melt-spun ribbons was studied by the Differential Scanning Calorimeter (DSC) method in the mode of continuous heating. Theapparent activation energy Ex, Ep1 and Ep2 (356.19±37.11, 288.12±9.63 and 407.82±20.10 kJ/mol, respectively) derived from the Kissingerplots were calculated by the onset and peak temperatures Tx, Tp1, Tp2, which display a strong dependence on the heating rates. The crystallizationbehavior during continuous heating at 10 K/min to 823 K, 923 K and 1023 K were investigated using X-ray diffractometry (XRD) andtransmission electron microscopy (TEM). As a result, the nanocrystalline α-Fe(Si) could be obtained when the temperature was up to 823 K, andno new phase was found due to adding component Co to FINEMET (Fe73.5Si13.5B9Cu1Nb3).© 2005 Elsevier B.V. All rights reserved.

Keywords: Glasses; Crystallization; Kinetics; FINEMET; Cobalt

1. Introduction

In 1988, Yoshizawa et al. successfully developed the Fe–Si–B–Cu–Nb nanocrystalline soft magnetic alloy known asFINEMET by means of annealing its amorphous precursor alloy[1]. Since then, nanocrystallization has become an importantway to synthesize nanocrystalline magnetic materials. The α-Fe(Si) particles with an average diameter of about 10 nm embed inthe amorphous matrix, which provides excellent magneticproperties. Based on the composition of FINEMET, extensivestudies have been made on the effect of Cu, Nb [1–8] and otherelements, as exemplified for Co [9] and Ni [10], on formationand properties of nanocrystalline structure. Comparing to theatom of Fe, atom of Co has the similar radius, electronegative,and outer layer electron configuration. So it will be interesting tostudy the new FINEMET type alloy. However, the glass forming

⁎ Corresponding author. Tel.: +86 10 82241227; fax: +86 10 62355099.E-mail address: lichengdong2000@yahoo.com.cn (C. Li).

0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2005.05.084

ability and the kinetics of crystallization of the amorphousribbons are not considered in literature [9] and others.

In this paper, the glass forming ability of the new FINEMETtype alloy were studied. And the crystallization kinetics wasanalyzed by using the Kissinger method. Meanwhile, thecrystallization of amorphous Fe68.5Si13.5B9Cu1Nb3Co5 alloywas also identified by means of XRD and TEM.

2. Experimental

The master alloy ingot of amorphous Fe68.5Si13.5B9Cu1-Nb3Co5 (at.%) was prepared by arc melting pure metals underargon atmosphere. The purity of all the components used washigher than 99.93%. Amorphous ribbons with a cross sectionof about 0.02 mm×3.00 mm were prepared by melt spinningunder an Ar atmosphere. For melt spinning, the alloy wasremelted in a quartz tube followed by ejecting through anozzle onto a rotating Cu wheel. The X-ray and electrondiffraction reveals a typical amorphous halo in the as-quenched alloy. The nanocrystalline samples were produced

Fig. 1. DSC curves of Fe68.5Si13.5B9Cu1Nb3Co5 melt-spun ribbons at differentheating rates: (a) 5 K/min; (b) 10 K/min; (c) 15 K/min; (d) 20 K/min.

Fig. 2. Relation between lnΦ and characteristic temperatures Tx and Tpi(scattered dots) and their fitted results (solid lines).

Fig. 3. Kissinger plots of ln(T2 /Φ) versus 1 /T obtained from linear heating DSCscans for Fe68.5Si13.5B9CuNb3Co5 ribbon.

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from amorphous ribbon specimens by non-isothermal heattreatment to various temperatures under an Ar atmosphere.

Thermal parameters such as glass transition temperature Tg,onset crystallization temperature Tx and the peak temperaturesTp1, Tp2 were measured at different heating rates by NetzschDSC404 Differential Scanning Calorimetry (DSC) with anaccuracy of ±0.3 K under a flowing argon atmosphere.Temperature and energy calibrations of the instrument wereperformed using the well-known melting temperature andmelting enthalpies of high-purity indium supplied with theinstrument. The samples with the same mass, 20 mg, were putinto the alumina crucibles every time. And in all cases, aconstant flow of Ar (20 ml/min) was maintained to provide aconstant thermal blanket within the cell of instrument and toeliminate thermal gradients, which ensured the validity ofmeasurement. The crystallization behavior and the non-isothermal crystallization kinetics were studied by these thermalparameters. Structures of the annealed specimens wereexamined by using X-ray diffractometry (XRD, RIDAKU, D/max-rB, CuKα) and transmission electron microscopy (TEM)(Hitachi H-800 Electron Microscopy). Thin foil specimens fortransmission electron microscopy (TEM) were prepared by ionbeam milling method.

3. Results and discussion

The typical DSC spectra obtained from as melt-spun ribbons duringcontinuous heating at different heating rates (5, 10, 15, 20 K/min) are

Table 1Atomic radii and electronegative parameters of the elements of the alloy

Element Atomic number Atomic radius/nm Electronegative

Fe 26 0.126 1.83B 5 0.098 2.04Si 14 0.132 1.90Cu 29 0.128 1.90Nb 41 0.146 1.60Co 27 0.125 1.88

Fig. 4. Typical X-ray diffraction patterns obtained from melt-spun Fe68.5Si13.5-B9Cu1Nb3Co5 ribbons.

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shown in Fig. 1. All the DSC spectra show two exothermic eventscorresponding to the crystallization process.

As shown in Fig. 1, the onset temperature of first crystallizationtemperature Tx and two peak crystallization temperatures Tp1, Tp2 areaffected by the heating rates. It is obvious that thermal parameter Tx,Tp1 and Tp2 shift to the higher temperatures with increasing heating rateΦ, which means the crystallization process has kinetic effect.

There is an approximate linear relation between the characteristictemperatures mentioned above and natural logarithm of heating rate,i.e. the obtained data can be fitted by a linear equation:

T ¼ AT þ BT lnU ð1Þwhere AT and BT are constants, T is absolute temperature, Φ is theheating rate. The fitted results of line for Tx, Tp1 and Tp2 duringcontinuous heating process are shown in Fig. 2.

According to the onset and peak shifts of the linear heating DSCcurves at different heating rates, the apparent activation energy ofcrystallization can be obtained by using the Kissinger plots [11]. TheKissinger equation is as follows:

lnðT2=UÞ ¼ E=RT þ Constant ð2Þwhere Φ is the heating rate, E is the apparent activation energy for theprocess, R is the gas constant and T is a specific absolute temperature

Fig. 5. Typical bright field TEM image and corresponding SADP obtained from m

such as peak temperature Tpi (i=1, 2), which can be measured atselected heating rates Φ. The equation above can also be expressed asfollows:

lnðT2=UÞ ¼ A=T þ B ð3Þwhere A and B are constants. By plotting ln(T2 /Φ) versus 1 /T, astraight line with the slope of A will be obtained, and the apparentactivation energy of the whole reaction is calculated by the product of Aand R. Three Kissinger plots are shown in Fig. 3. The obtainedactivation energies Ex, Ep1 and Ep2 are 356.19±37.11, 288.12±9.63and 407.82±20.10 kJ/mol, respectively. Here, the activation energiesEp1 and Ep2 indicate the participation of different phases in the first andsecond stage during continuous heating for the specimens.

Several parameters have been used to predict the GFA (glassforming ability) of metallic glass. The most often used parameter is thesupercooled liquid region ΔTx (=Tx−Tg). Calculated by the detailedresults of DSC traces (b) in Fig. 1, ΔTx of Fe68.5Si13.5B9Cu1Nb3Co5alloy is 102.8 K, which means that this new FINEMET type alloy haslarge supercooled liquid region, large GFA and high thermal stabilityagainst crystallization. The values of the Tg /Tm and Tx /Tm, which arethe other two adopted parameters to predict the GFA, are alsocalculated to be as big as 0.51 and 0.59, respectively. Generallyspeaking, the alloy Fe68.5Si13.5B9Cu1Nb3Co5 has large GFA.

In this alloy, the atomic size increases systematically in the orderof BbCobFebCubSibNb (shown in Table 1). Although partialreplacement of Fe by Co does not change the difference range in atomicsize among elements, the mixing energy between Co and other atomsare negative, which is in favor of diffusion of Co atoms. Furthermore,atoms of B, Fe, Si, Nb and Co can also have a tendency forming newcompounds (such as CoB, Co2B, Co3B and NbCo2) or atomic clusterswith two or more kinds of atoms. So the Fe–Si–B–Cu–Nb–Co systemhas a larger degree of confusion, and the higher atomic packing densityof liquid structure leads to bigger viscosity and the supercooled liquidregion. So, atoms rearrangement becomes difficult from short-rangeorder to long-range order during solidification and then the nucleationand growth of nuclei are restrained. This is the right reason for the largeglass forming ability.

To understand the Co effects on the crystallization behaviors,annealing treatments were performed under Ar atmosphere to differenttemperatures (823 K, 923 K and 1023 K). Fig. 4 shows typical XRDtraces obtained from annealed ribbon specimens corresponding todifferent heat treatment conditions respectively. It was shown that thesharp diffraction peaks superimposed on the weak halo pattern,indicating partial crystallization occurred during annealing treatment.

elt-spun Fe68.5Si13.5B9Cu1Nb3Co5 alloy annealed up to 1023 K at 10 K/min.

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With the temperature increased, the halo basically disappeared, whichindicates complete crystallization.

FromXRD data, it can be seen that when the annealing temperature isup to 823K,α-Fe(Si)would precipitate in the amorphousmatrix, which iscorresponding to the first exothermal peak on the DSC traces. And thewider peak of α-Fe(Si) indicates that the size of α-Fe(Si) is in thenanometer order. The TEM image in Fig. 5, α-Fe(Si) particles with anaverage diameter about 10 nm embed in the amorphous matrix, provesthis. In the SADP of Fig. 5, the rings of α-Fe(Si) and Fe2B are shownwhich is in good accordance with the XRD result in Fig. 4. With theannealing temperature increased to higher ones, the new phases such asFe2B, Fe23B6, Fe3.5B and NbB2 are precipitated. The reason for nocompounds was found containing Co was speculated as that the Co wasmostly dissolved in Fe. So the crystallization process is amorphous→α-Fe(Si)+amorphous→α-Fe(Si)+Fe2B+Fe23B6+Fe3.5B+NbB2.

4. Conclusion

The GFA of Fe68.5Si13.5B9Cu1Nb3Co5 and the crystallizationkinetics of amorphous Fe68.5Si13.5B9Cu1Nb3Co5 melt-spunribbons were studied by using the DSC method in the mode ofcontinuous heating. The apparent activation energy Ex, Ep1 andEp2 are 356.19±37.11, 2889.63 and 407.82±20.10 kJ/mol,respectively, which was derived by the Kissinger method. Thecrystallization behavior was investigated using XRD and TEM,which showed that the nanocrystalline structure could be obtained

and no new Co-containing phase was found. The crystalliza-tion process is amorphous→α-Fe(Si)+amorphous→α-Fe(Si)+Fe2B+Fe23B6+Fe3.5B+NbB2.

Acknowledgement

This work was supported by the National Natural ScienceFoundation of China under the grant No. 50271037.

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