at SciVerse ScienceDirect

Intermetallics 25 (2012) 95e100

Contents lists available

Intermetallics

journal homepage: www.elsevier .com/locate/ intermet

Liquid-immiscibility-induced formation of micron-scale crystalline/amorphouscomposite powder

Y. Yu a,b, Y. Takaku c, M. Nagasako c, C.P. Wang a,b, X.J. Liu a,b,*, R. Kainuma c, K. Ishida c

aDepartment of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, PR ChinabResearch Center of Materials Design and Applications, Xiamen University, Xiamen 361005, PR ChinacDepartment of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan

a r t i c l e i n f o

Article history:Received 28 November 2011Received in revised form6 January 2012Accepted 7 February 2012Available online 29 March 2012

Keywords:A. CompositesB. Glasses, metallicC. Rapid solidification processingF. Electron microscopy, transmissionG. Magnetic applications

* Corresponding author. Department of MaterialCollege of Materials, Xiamen University, Xiamen 3612187888; fax: þ86 592 2187966.

E-mail address: lxj@xmu.edu.cn (X.J. Liu).

0966-9795/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.intermet.2012.02.003

a b s t r a c t

Liquid immiscible alloy systems present a unique opportunity in developing the micron-scale crystalline/amorphous composite powder using the gas atomization method. On the basis of the CALPHADapproach, the compositions of CoeSieBeCu alloys system exhibiting the liquid immiscibility have beendesigned. The produced gas-atomized powders show a Cu-rich crystalline/CoeSieB rich amorphouscomposite microstructure. The above-mentioned powders possess almost the same coercive force as thatof CoeSieB amorphous powders, while their saturation magnetization decreases with increasing Cuconcentration. Such Cu-rich crystalline/CoeSieB rich amorphous composite powders may have appli-cations in the field of heat release of soft magnetic devices and ferrofluids.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Amorphous alloys have been developed in a large number ofalloy systems due to their attractive properties such as superiorelasticity limit, high strength, and good softmagnetic characteristics[1]. However, inmany cases their applicability is limited by the poorplasticity resulting from shear localization and work softening [2].To solve the problem of the limited plasticity of amorphous alloys,a newgroup ofmaterials: amorphousmatrix composites containinga secondary ductile crystalline phase, has been successfullyproduced by different methods such as the ex situ introduction ofcrystalline particles or fibers [3e5], the nanocrystal precipitation[6e9], and the in situ formation of crystalline dendrites [10e12].

Recently, great attention has been paid to develop crystalline/amorphous composites through liquid immiscibility [13e17].Liquid immiscible systems (e.g. AlePb, CueFe and CueCo) havinga positive enthalpy of mixing between the two major elements, arecharacterized by the stable or metastable liquid miscibility gap.When the melt is rapidly cooled into the liquid miscibility gap, theliquid phase separation takes place and the liquid separates into

s Science and Engineering,005, PR China. Tel.: þ86 592

All rights reserved.

two liquids. If two phase-separated liquids have lowand high glass-forming ability (GFA), respectively, then the crystalline/amorphouscomposite could be formed after rapid cooling process such as meltspinning and suction casting. The gas atomization method nor-mally used to fabricate micron-scale crystalline/crystalline liquidimmiscible composite powder under the condition of rapid solidi-fication [18e21], however, has not been reported to produceexpected crystalline/amorphous composite so far. The objective ofthe present work is, to develop the micron-scale crystalline/amorphous composite powder induced by liquid immiscibility.

2. Alloy design

In order to obtain the above-mentioned composite powder, animmiscible alloy system should be firstly selected. We note that theCoeSieBeCu alloy system is a good candidate for this investigation,since it not only includes the CoeSieB system [22] showing highenough GFA but also has an atomic pair with a positive enthalpy ofmixing between Co and Cu elements (see Fig. 1(a)) [23]. However,how to pinpoint the composition of an alloy exhibiting liquidimmiscibility appears to be a scientific challenge. Obtaining suchinformation on the liquid immiscibility exclusively from experi-ments is burdensome and expensive. The calculation of phasediagrams (CALPHAD) technique, which has been recognized to bean important tool to significantly reduce time and cost during the

Co Cu

B

Si

+6

-14

-38 -19

-24 0

Cu / at.%20 40 60 80

Tem

pera

ture

/

600

800

1000

1200

1400

1600

1800

L1 + L2

Liquid

Co2B + Co2Si + (Cu) + εCo

Co2B + Co2Si + (Cu) + αCo

Co2B + (Cu) + αCo

Co2B + (Cu) + εCo

Co2B + Liquid + αCo

Co2B + Co2Si + Liquid + αCo

CuCo75Si10B15

ba

Fig. 1. (a) Relationship of heat of fusion (kJ/mol) among constituent elements in the CoeSieBeCu quaternary system, and (b) the calculated vertical section of the CoeSieBeCuquaternary system for the composition cut between Co75Si10B15 (at.%) and pure Cu.

Y. Yu et al. / Intermetallics 25 (2012) 95e10096

development of materials, can effectively provide a clear guidancefor the materials design [24e26]. Moreover, it can predict liquidmiscibility gap, calculate liquidus curves, and volume fraction ofliquid phase in the present work. Thus, the thermodynamicdescription for the CoeSieBeCu quaternary system obtained by theCALPHAD approach is of great essence. For the quaternary system,there are six constituent binaries and four constituent ternaries.Using the thermodynamic description for the CoeSieBeCuquaternary system [27] constructed by our group, the isothermaland vertical sections can be calculated by using the CALPHADmethod. For the quaternary system, the complete isothermalsection is a tetragonal volume under a constant pressure, and then,a two-dimensional vertical section can be obtained from cuttingwith constant alloy compositions. In the present work, consideringthat the reported Co75Si10B15 (at.%) alloy exhibits the standardamorphous state [22], a vertical section of the CoeSieBeCuquaternary system for the composition cut between Co75Si10B15and pure Cu has been calculated, and is shown in Fig. 1(b), wherea stable liquid miscibility gap (L1þL2) exists. According to thisvertical section, the liquid immiscible (Co0.75Si0.1B0.15)100 � xCux

(x ¼ 30, 45, 55, 65, 80, at.%) alloys were selected, and the calculatedcompositions of two separated liquids appear to be CoeSieB richand Cu-rich, respectively.

3. Experimental procedure

The master CoeSieBeCu alloys were first melted in a highfrequency induction furnace under an argon atmosphere. Themoltenalloys were heated to about 1600e1650 �C in the crucible, which isover the liquidus temperature of the selected CoeSieBeCu alloys, toensure sufficientmixing of all components. After being held for about10 min for homogenization, the molten alloys were then cast intoa cylindrical cast-iron mold. Then the master alloy ingots were mel-ted again in a quartz nozzle surrounded by an induction coil. Thepowders with 10e250 mm in diameter were prepared usingconventional nitrogen gas atomization, where the pouring temper-ature is about 1600e1650 �C and the gas pressure for atomizing isabout 5MPa. The cooling rate is 103e104 K/s, depending on the size ofthe atomized powder. After standard metallographic preparation,observation of cross-sectional microstructure and mapping ofconstituent elements in each phase of the powder were carried outby electron probe microanalyzer (EPMA) (JXA-8100R, JEOL, Japan).The crystal structure of the constituent phases was co-identified by

X-raydiffraction (XRD) (X’Pert PRO-PANalytical, Philips, Netherlands)and transmissionelectronmicroscopy (TEM) (JEM-2100, JEOL, Japan).Here, it should be noted that, before TEM observation, preparation ofTEM sample for the micron-scale atomized alloy powder, is neitherlike bulk alloys (usually thinned by electropolishing) nor like nano-powder (taken by copper net). In the present work, the micron-scale atomized CoeSieBeCu powders, were firstly embedded inthe resin, and then thinned by focused-ion beam (FIB) instrument(SMI-2050, SEIKO, Japan). The thermal properties of the gas-atomized CoeSieBeCu powders were determined by differentialscanning calorimetry (DSC) (DSC-404C, NETZSCH, Germany) ata heating rate of 20 �C/min using sintered Al2O3 as the referencespecimen. The magnetization curves of the gas-atomizedCoeSieBeCu powders at room temperature were studied usinga vibrating samplemagnetometer (VSM) (TVA-5, TOEIKOGYO, Japan)under a maximum applied field of 16 kOe.

4. Results and discussion

Similar to our reported liquid immiscible gas-atomized powders[18e21], the composite microstructure of the liquid immiscible(Co0.75Si0.1B0.15)100 � xCux (x¼ 30, 45, 55, 65, 80, at.%) alloy powderswas also observed, as typically shown in Fig. 2. In the back-scatteredelectron (BSE) image of Fig. 2(a), the inner region of the(Co0.75Si0.1B0.15)45Cu55 (at.%) alloy powders exhibits a gray contrastwith small white circular globules, whereas the outer region of thepowder shows a white contrast with small spherical droplets,indicating the occurrence of liquid phase separation and formationof composite microstructure. In order to investigate the distribu-tion of constituent elements in the above compositemicrostructurein detail, the mapping analysis of EPMAwas performed. The resultsare shown in Fig. 2(b)e(e). The inner region in gray contrastcontains Co, Si, and B, whereas the outer region in white contrastmainly contains Cu. The measured compositions of the CoeSieBrich and Cu-rich phases are Co72.1Si9.2B15.1Cu3.6 (at.%) andCo5.6Si1.8B0.1Cu92.5 (at.%), respectively. The liquid phase separationof CoeSieB rich liquid and Cu-rich liquid predicted by CALPHADmethod was finally confirmed by EPMA analysis.

In order to identify the crystal structure of constituent CoeSieBrich and Cu-rich phases, the XRDmeasurement of the micron-scalegas-atomized (Co0.75Si0.1B0.15)45Cu55 (at.%) powders, was carriedout. Fig. 3(a) shows the XRD pattern of the (Co0.75Si0.1B0.15)45Cu55(at.%) powders, where sharp diffraction peaks corresponding to the

Fig. 2. Mapping analysis of the cross-sectional microstructure of the (Co0.75Si0.1B0.15)45Cu55 (at.%) gas-atomized powders: (a) BSE image, (b) Co image, (c) Si image, (d) B image, and(e) Cu image.

Y. Yu et al. / Intermetallics 25 (2012) 95e100 97

fcc Cu-rich crystalline phase and a broad peak at about 45� can beobviously seen. The peak position of the broad peak is similar tothat for the reported Co75Si10B15 (at.%) amorphous alloy [1], indi-cating that the CoeSieB rich phase shows an amorphous state. In

35 45 55 6

2θ / de

Inte

nstit

y

42 43 44

a

400 500 600 700 80

Tempera

Hea

ting

flow

exo

b

Fig. 3. (a) XRD patterns and (b) DSC heating curve of the

order to confirm the formation of an amorphous CoeSieB richphase in the gas-atomized (Co0.75Si0.1B0.15)45Cu55 (at.%) powders,the DSC measurement was performed, and the obtained heatingcurve is shown in Fig. 3(b), where a broad exothermic peak can be

5 75 85 95

gree

45 46

fcc (Cu-rich)

amorphous (Co-Si-B-rich)

0 900 1000 1100 1200

ture /

gas-atomized (Co0.75Si0.1B0.15)45Cu55 (at.%) powders.

Y. Yu et al. / Intermetallics 25 (2012) 95e10098

clearly seen. Combined with the XRD analysis, the existence of suchan exothermic peak can be attributed to the formation of anamorphous CoeSieB rich phase in the gas-atomized(Co0.75Si0.1B0.15)45Cu55 (at.%) powders.

In order to in situ identify the crystal structure ofconstituent CoeSieB rich and Cu-rich phases, the micron-scale(Co0.75Si0.1B0.15)45Cu55 (at.%) alloy powders, first embedded inresin then thinned by FIB, were investigated by TEM. The TEMbright field (BF) micrograph (see Fig. 4(a)) and correspondingmagnified image of Fig. 4(a) inside red dashed lines (see Fig. 4(b)),clearly show the presence of two different phases with white andgray contrasts. Here, the white and gray contrasts, typicallymarked in “A” and “B” in Fig. 4(b), are the Cu-rich and the CoeSieBrich phases, respectively. Fig. 4(c) shows the selected areadiffraction pattern (SADP) of region “A” in Fig. 4(b) taken along the[100] zone axis, confirming the formation of the Cu-rich crystallinephase with the fcc structure. The SADP of region “B” in Fig. 4(b)(see Fig. 4(d)) shows the amorphous characteristics of the CoeSieB

Fig. 4. (a) TEM bright field image of the gas-atomized (Co0.75Si0.1B0.15)45Cu55 (at.%) powder, (the Cu-rich phase being a crystalline phase with the fcc structure, and (d) SADP of region “Bof the references to colour in this figure legend, the reader is referred to the web version o

rich phase. Thus, it can be concluded that, the micron-scale Cu-richcrystalline/CoeSieB rich amorphous composite powders inducedby liquid immiscibility have been successfully developed. To thebest of our knowledge, the amorphous composite powders havealready been studied in the recent years, and produced by variousmethods except for the gas atomization method. This is the orig-inal report on the formation of micron-scale gas-atomized alloypowders with the crystalline/amorphous microstructure utilizedby liquid phase separation. During the gas atomization process, theliquid phase separation of the CoeSieB rich liquid and the Cu-richliquid occurs first, which is followed by the aggregation of bothliquids, resulting in the formation of composite microstructure[18,19,21]. The formation of the CoeSieB amorphous phase indi-cates that the CoeSieB liquid maintains its liquid structure at theglass transition temperature during the cooling process, resultingin the maintenance of the liquid state enough to the formationof crystalline/amorphous microstructure in the gas-atomizedpowders.

b) magnified image of (a) inside red dashed lines, (c) SADP of region “A” in (b), showing” in (b), showing the CoeSieB rich phase being an amorphous state. (For interpretationf this article.)

M (

emu/

g)

H (kOe)61404-8-21- 8 12-16

0

-20

-40

-60

-80

20

40

60

80

x=0

x=30x=45x=55x=65x=80

x=0

x=30x=45x=55x=65x=80

a

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

Ms

(em

u/g)

x / at.%

0

0.125

0.250

0.375

0.500

0.625

0.750

0.875

1.000

Vol

ume

frac

tion

of C

u-ri

ch p

hase

b

Fig. 5. (a) Magnetization curves of the gas-atomized (Co0.75Si0.1B0.15)100 � xCux (at.%) powders at room temperature with a maximum applied field of 16 kOe, and (b) relationship ofboth saturation magnetization and volume fraction of Cu-rich phase via Cu concentration (x/at.%) of above-mentioned powders.

Y. Yu et al. / Intermetallics 25 (2012) 95e100 99

It is well known that the CoeSieB amorphous alloys are high-quality and low-cost soft magnetic materials [1]. For the micron-scale gas-atomized (Co0.75Si0.1B0.15)100 � xCux (x ¼ 30, 45, 55, 65,80, at.%) alloy powders with crystalline/amorphous microstructure,the effect of Cu concentration on the magnetic property ofCoeSieBeCu composite powder is still not clear. Here, theCo75Si10B15 (at.%) amorphous powders were produced forcomparison. The magnetization curves of the gas-atomized(Co0.75Si0.1B0.15)100 � xCux (x ¼ 0, 30, 45, 55, 65, 80, at.%) powdersmeasured by VSM at room temperature with a maximum appliedfield of 16 kOe, are shown in Fig. 5(a), where two key points can beclearly seen: (1) the coercive force of powders, depending on theorigin of soft magnetic CoeSieB rich amorphous phase, is almostsame; (2) the saturation magnetization of above-mentionedpowders, depending on the amount of soft magnetic CoeSieBrich amorphous phase, significantly decreases with increasingnon-magnetic Cu concentration. Considering the high thermalconductivity and excellent corrosion resistance of Cu, such micron-scale Cu-rich crystalline/CoeSieB rich amorphous compositepowders, may be effective for the use of heat release of softmagnetic devices and ferrofluids. In order to design Cu-richcrystalline/CoeSieB rich amorphous composite powders forpotential applications, the relationship of saturation magnetizationand volume fraction of non-magnetic Cu-rich crystalline phase viaCu concentration of Co0.75Si0.1B0.15)100 � xCux (at.%) powders ispresented in Fig. 5(b). Here, it should be noted that, the curve ofsaturation magnetization via Cu concentration was drawn on thebasis of key experimental data points, while the complete curve ofthe volume fraction of Cu-rich phase via Cu concentration wasdirectly calculated under the framework of Thermo-Calc softwareon the basis of reported equation [28]. As a simple example of alloydesign, if the saturation magnetization and volume fraction of Cu-rich phase are designed to be 30e40 emu/g and 0.3e0.4, respec-tively, the optimal range of Cu concentration should be controlledin the range of 37e40 at.%.

5. Conclusions

In summary, on the basis of the CALPHAD approach andobtained thermodynamic description for the CoeSieBeCuquaternary system, the (Co0.75Si0.1B0.15)100 � xCux (x ¼ 30, 45, 55,65, 80, at.%) alloys exhibiting liquid immiscibility has been firstlypinpointed. Secondly, the designed CoeSieBeCu powders have

been produced by gas atomization and experimentally confirmedby TEM to appear the Cu-rich crystalline/CoeSieB rich amorphouscomposite microstructure. Lastly, the coercive force of the above-mentioned composite powders is almost the same as that of theCo75Si10B15 (at.%) amorphous powder, while their saturationmagnetization decreases with increasing Cu concentration. Thepresent results for the micron-scale Cu-rich crystalline/CoeSieBrich amorphous composite powder are promising and demon-strate the possibility of developing gas-atomized crystalline/amorphous composite powder in other multi-component systemsusing liquid immiscibility.

Acknowledgments

This work is supported by the National Natural Science Foun-dation of China (Grant Nos. 51031003 and 51171159), the Ministryof Education of China (Grant No. 707037) and the Ministry ofScience and Technology of China (Grant Nos. 2009DFA52170 and2009AA03Z101). In addition, one of the authors Mr. Yan YU wouldlike to thank the support from the Fundamental Research Funds forthe Central Universities (Grant No. 201112G013) and CSC China.

References

[1] Miller M, Liaw P. Bulk metallic glasses-an overview. New York: Spring; 2008.[2] Wang WH, Dong C, Shek CH. Mater Sci Eng R 2004;44:45e89.[3] Conner RD, Dandliker RB, Johnson WL. Acta Mater 1998;46:6089e102.[4] Choi-Yim H, Conner RD, Szuecs F, Johnson WL. Acta Mater 2002;50:2737e45.[5] Pan DG, Zhang HF, Wang AM, Hu ZQ. Appl Phys Lett 2006;89:261904-1e3.[6] Tsai AP, Chandrasekhar N, Chattopadhyay K. Appl Phys Lett 1999;75:1527e8.[7] Inoue A, Shen BL, Koshibo H, Kato H, Yavari AR. Nat Mater 2003;2:661e3.[8] Prima F, Tomut M, Stone I, Cantor B, Janickonic D, Vlasak G, et al. Mater Sci Eng

A 2004;375:772e5.[9] Wilde G, Peter B, Rosner H, Weissmuller J. J Alloys Compd 2007;434-435:

286e9.[10] Hays CC, Kim CP, Johnson WL. Phys Rev Lett 2000;84:2901e4.[11] Wang GY, Liaw PK, Peter A, Freels M, Peter WH, Buchanan RA, et al. Inter-

metallics 2006;14:1091e7.[12] Das J, Ettingshausen F, Eckert J. Scr Mater 2008;58:631e4.[13] Oh JC, Ohkubo T, Kim YC, Fleury E, Hono K. Scr Mater 2005;53:165e9.[14] Kim KB, Das J, Baier F, Tang MB, Wang WH, Eckert J. Appl Phys Lett 2006;88:

051911-1e3.[15] He J, Li HQ, Zhao JZ, Dai CL. Appl Phys Lett 2008;93:131907-1e3.[16] Ziewiec K, Kedzierski Z. J Alloys Compd 2009;480:306e10.[17] Chang HJ, Yook W, Park ES, Kyeong JS, Kim DH. Acta Mater 2010;58:2483e91.[18] Wang CP, Liu XJ, Ohnuma I, Kainuma R, Ishida K. Science 2002;297:990e3.[19] Wang CP, Liu XJ, Shi RP, Shen C, Wang Y, Ohnuma I, et al. Appl Phys Lett 2007;

91:141904-1e3.

Y. Yu et al. / Intermetallics 25 (2012) 95e100100

[20] Ohnuma I, Saegusa T, Takaku Y, Wang CP, Liu XJ, Kainuma R, et al. J ElectronMater 2009;38:2e9.

[21] Shi RP, Wang Y, Wang CP, Liu XJ. Appl Phys Lett 2011;98:204106-1e3.[22] Inoue A,Masumoto T, KikuchiM,Minemura T. Sci Rep RITU A 1979;27:126e46.[23] Takeuchi A, Inoue A. Mater Trans 2005;46:2817e29.[24] Kaufman L, Bernstein H. Computer calculation of phase diagram. New York:

Academic Press; 1970.

[25] Saunders N, Miodownik AP. CALPHAD (Calculation of phase Diagrams)ea comprehensive guide. Oxford, UK: Pergamon Press; 1998.

[26] Lukas H, Fries SG, Sundman B. Computional thermodynamicsethe calphadmethod. Cambridge, UK: Cambridge University Press; 2007.

[27] Yu Y, Wang CP, Liu XJ, Kainuma R, Ishida K. (unpublished work).[28] Wang CP, Liu XJ, Takaku Y, Ohnuma I, Kainuma R, Ishida K. Metall Mater Trans

2004;35:1243e53.