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Intermetallics 17 (2009) 218–221

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Intermetallics

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

Fabrication and soft-magnetic properties of Fe–B–Nb–Y glassy powdercompacts by spark plasma sintering technique

Sangmin Lee a,*, Hidemi Kato b, Takeshi Kubota b, Akihiro Makino b, Akihisa Inoue b

a Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8577, Japanb Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

a r t i c l e i n f o

Article history:Received 31 May 2008Received in revised form28 July 2008Accepted 28 July 2008Available online 3 January 2009

Keywords:Magnetic intermetallicsMetallic glassesMagnetic propertiesPowder metallurgyMagnetic applications

* Corresponding author.E-mail address: smlee@imr.tohoku.ac.jp (S. Lee).

0966-9795/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.intermet.2008.07.021

a b s t r a c t

Magnetic properties of (Fe0.72B0.24Nb0.04)95.5Y4.5 sample were investigated. The sample was producedfrom glassy powders made by the gas-atomization and consolidation using the spark plasma sintering(SPS) technique. Maximum relative density of 99.5% was achieved in the spark plasma sintered (SPSed)compact due to the viscous flow enhanced by the applied stress even under the glass transitiontemperature. X-ray diffraction pattern of the compact indicates that the glassy structure was maintainedthrough the SPS process. However, the results of differential scanning calorimetry (DSC) showed that theglass transition temperature and crystallization temperature of the SPSed glassy compact shift toa higher and lower temperature, respectively, that is, a smaller DTx. Saturation magnetization of theSPSed glassy compact became 10% higher than that of the initial glassy powder. The Curie point wasenhanced from 522 K for the glassy powder to 548 K for the SPSed glassy compact. Spin-exchangeinteraction is expected to be enhanced by a short-range scale atomic rearrangement caused by the highapplied stress and temperature during the SPS process.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Many amorphous alloys have been developed for structuralapplications using their unique mechanical, physical properties andchemical corrosion resistance [1–3]. Recently, more attention hasbeen paid to improve the soft-magnetic properties of the Fe-basedamorphous alloys owing to their high efficiency on energy trans-form as a core material, since their soft magnetic properties havea direct effect on the efficiency of transformers during stepping upor stepping down [4–15].

Spark plasma sintering (SPS) technique is one of the mostpromising methods to fabricate bulk metallic glasses (BMGs) usingviscous deformation of glassy powders at the supercooled liquidregion. In recent, the number of these kinds of reports hasincreased gradually according to the improvement of the SPStechnique [16–29]. However, most of them are focused on gettingover the barriers of the critical size on amorphous or glassy alloys aswell as on enhancing plasticity with mixing different alloys orphases for the structural application. The SPS technique can bringabout the breakthrough of the critical glass size and compositedesigning which are strongly limited if the conventional castingapproach is taken.

All rights reserved.

In this paper, we are focusing on the consolidation of the gas-atomized ferrous glassy powders densely without devitrificationusing the SPS technique as well as the influence of the SPS processon the soft-magnetic properties. An (Fe0.72B0.24Nb0.04)95.5Y4.5 glassyalloy was selected for the present research, since the alloy wasreported to have excellent soft-magnetic properties, the largestglass forming ability (GFA) and thermal stability at the supercooledliquid region [30].

2. Experimental procedures

(Fe0.72B0.24Nb0.04)95.5Y4.5 ingots were prepared by the arcmelting method with Fe (99.9%), B (99.5%), Nb (99.9%) and Y (99.9%)in a highly purified argon atmosphere after vacuuming thechamber at 2�10�3 Pa. The surface oxide layer of the ingots wasremoved for preventing the heterogeneous nucleation duringsolidification. (Fe0.72B0.24Nb0.04)95.5Y4.5 glassy powder was fabri-cated using the gas-atomizing method with a high-pressure argongas; then sorted by the sieve with 150 mm mesh. Glassy powdersunder 150 mm are compacted in a 12 mm-diameter tungstencarbide die by 2 punch rods with applied stress of 600 MPa fromthe ambient temperature to glass transition temperature of 873 K.Glassy structure and phase transition temperature of both glassypowder and SPSed compact were confirmed by the X-ray diffrac-tometry (XRD) with Cu-Ka radiation from 20 to 80� of 2q and thedifferential scanning calorimetry (DSC) at a heating rate of 0.67 K/s,

Inte

nsit

y (a

rb. u

nit)

Glassy powder

SPSed BMG

Cu-K α

a

S. Lee et al. / Intermetallics 17 (2009) 218–221 219

respectively. The supercooled liquid region, which is defined asa temperature spun between the glass transition temperature (Tg)and the onset temperature of crystallization (Tx), DTx¼ Tx� Tg wasalso determined from the DSC curves. The outer appearance andfracture surface of the SPSed glassy compact were observed withthe scanning electron microscopy (SEM) to investigate a bondingstate between powders after the consolidation. Magnetic proper-ties were measured with a vibrating sample magnetometer (VSM,Digital Measurement Systems, VSM-5) for saturation magnetiza-tion, and with D.C. B-H loop tracer for coercivity. A ring-shapeSPSed glassy compact was prepared and used for comparison of thecoercivity with the cast BMG reported in a literature. [30] Curiepoints were determined using Magneto-Thermo-Gravimeter (MTG)at the same heating rate with the DSC measurement of 0.67 K/s.

2θθ/ degree

20 30 40 50 60 70

700 800 900 1000

SPSed BMG

Powder

T(979 K)

(

(964 K)(870 K )

(873 K)

Tg (881 K)

Heating Rate 0.67 K/s

Ribbon

Temperature, T/ K

Exo

ther

mic

Tx(979 K)

(980 K)

(870 K )

80

b

Fig. 1. (a) XRD and (b) DSC traces obtained from (Fe0.72B0.24Nb0.04)95.5Y4.5 glassypowder and the SPSed BMG.

3. Results and discussion

Fig. 1 shows (a) XRD and (b) DSC results of the (Fe0.72B0.24

Nb0.04)95.5Y4.5 gas-atomized powders and SPSed compact. No sharppeak observed in the XRD patterns of both (Fe0.72B0.24Nb0.04)95.5Y4.5

atomized powder and SPSed compact in Fig. 1(a) indicates that theatomized powders were frozen into glassy state and the glassystructure was maintained during the SPS process. On the other hand,a couple of changes were found in DSC curves after SPS process onthe glassy powders. At around 928 K within the supercooled liquidstate a unique exothermic peak, which has been reported in theprevious research [30], was apparently observed on DSC trace of theglassy powder. The exothermic reaction has been studied by DSC andTEM observations and a considerable origin for this phenomenonwas concluded to the chemical short-range ordering caused by thepositive mixing enthalpy between Y and Nb elements [30].

From the comparison of DSC curves of the powders and theSPSed glassy compact, Tg (¼881 K) and Tx (¼964 K) of the SPSedglassy compact show noticeable increase and decrease againstthose (Tg¼ 873 K, Tx¼ 980 K) of the powders, respectively.Furthermore, the exothermic peak in supercooled liquid region,which was observed in the initial powders, was disappeared in DSCcurve of the SPSed glassy compact although the SPS process had

Fig. 2. SEM images of (a) glassy powders, (b) crushed and cut faces, (c) SPSed face and (d) enlarged crushed face of the SPSed BMG.

Fig. 3. Outer morphology and surface appearance of an SPSed (Fe0.72B0.24Nb0.04)95.5Y4.5

glassy BMG with a thickness 2.5 mm, outer diameter of 12 mm, and an inner diameterof 6 mm.

S. Lee et al. / Intermetallics 17 (2009) 218–221220

been conducted only up to the glass transition temperature.According to Jin et al., the applied shear stress can reduce the glasstransition temperature of a Zr-based bulk metallic glass drasticallywith rate of 40 K/500 MPa [31]. If we apply this relation for thepresent Fe alloy, Tg becomes 825 K (¼873� 48 K) by the appliedstress of 600 MPa. Thus, the SPS process with heating up to 873 Kcan fairly induce the exothermic chemical short-range ordering.

Fig. 2 illustrates SEM images of (a) (Fe0.72B0.24Nb0.04)95.5Y4.5

glassy powders, (b) machined and fracture surfaces, (c) outersurface contact to the tungsten carbide punch and (d) fracturesurface of the SPSed glassy compact. The size distribution of com-pacted powders is ranged from 2–3 to w50 mm and no evidencesfor crystallization are observed such as a large group of powdersdue to their magnetic attraction related to the higher coercivity ofa ferrous crystalline phase, as shown in Fig. 2(a). Furthermore,a clean surface of the powder is one of the characteristics for theiramorphous state. In our study, crystallized powders show a largegroup of powders, namely small ones are attached to a big one, andsome craters on the surfaces are observed due to their shrinkagesfrom amorphous to crystalline phase transition.

As seen in Fig. 2(b)–(d), the outer surface of the SPSed glassycompact is completely different from the machined and fracturesurfaces. No apparent grain boundaries between the powders canbe observed except for defects at the SPSed surface as seen inFig. 2(c). However, the grain boundaries are remained among themas clarified in the fracture surface shown in Fig. 2(d). This indicatesthat the glassy powders are condensed completely at the inside ofthe SPSed compact owing to the enhanced viscous flow, but the thinmetallic oxide layers are remained and isolate each metallic glassypowders [32]. Hence, the electrical resistivity can be increased bythe electrical insulator made of the metallic oxide layers.

As demonstrated in Fig. 3, the ring-shape of SPSed glassycompact, outer diameter of 12 mm, inner diameter of 6 mm and2.5 mm in thickness, was prepared with ultra-sonic machining andwas used for the measurement of coercivity with a D.C. B-H looptracer. The center disk departed from the SPSed ring was used formeasuring a saturation magnetization with a VSM.

As seen in Table 1, magnetization curve shows that the glassypowder has a saturation magnetization, Js of 89 emu/g, which isapproximately as same as 87 emu/g of cast BMG. On the other hand,98 emu/g for the SPSed glassy compact is 10% higher than that ofthe initial glassy powder. This could be explained by the short-range ordering mentioned above. What it means, high stress of600 MPa up to 873 K during the SPS process enhances the atomicrearrangement in the glassy structure to be altered forwardincreasing a spin-exchange interaction by reducing free volumes. Itis well known that the excess free volume is removed by the suit-able heat treatment in the amorphous or glassy alloys [33].Therefore, the SPS process can have a similar affection as the heattreatment on the free volume. A short-range scale atomic rear-rangement caused by the high stress and temperature during theSPS process is suggested to increase the saturation magnetizationwith suppression of nucleation and growth of crystalline phases.

Curie points of each alloy were determined using magneto-thermo-gravimeter (MTG), a kind of the magnetic balancemeasurement method. The results of MTG, DSC, VSM and D.C. B-Hloop tracer are tabulated in Table 1. The Curie point increases from

Table 1Thermal properties and soft-magnetic properties of (Fe0.72B0.24Nb0.04)95.5Y4.5 glassy powcrystallization temperature, DTx: temperature spun of the supercooled liquid state (¼Tx

Composition Tc (K) Tg (K) T

SPSed BMG (ring-shape) 548 881 9Glassy powder 522 873 9Melt spun ribbon 870 9Cast BMG (ring-shape) 871 9

522 K of the glassy powder to 548 K of the SPSed glassy compact.Thus, the spin-exchange interaction is expected to be enhanced bya short-range scale atomic rearrangement caused by the highapplied stress and temperature during the SPS process. Theenhanced Curie point in the SPSed glassy compact can support theassumption on the enhanced spin-exchange interaction.

The coercivities were measured using D.C. B-H analyzer with theSPSed glassy compact and those of both the glassy ribbon and castBMG ring were profiled from the previous research for comparison,as shown in Table 1 [30]. The coercivity of 14 A/m for the SPSedglassy compact is approximately 17 times larger than 0.8 A/m of thecast BMG. In this case, the metallic oxide layer could be the mostreasonable origin of the increased coercivity compared to the castBMG. A contamination has a significant adverse effect on thecoercivity by a stress formed around the contamination elements.With the similar point of view, a large and broadly distributedstress between the metallic oxide layer and powder inside can fixmagnetic domains and make it difficult to move along with theapplied magnetic field [34]. Consequently, saturation magnetiza-tion and coercivity of the glassy powder are affected by the gas-atomizing process and the subsequent spark plasma sinteringprocess. This indicates that we can optimize the soft-magneticproperties of the glassy powder compact by controlling the glassypowder fabrication procedures and consolidation conditions.

4. Conclusion

(Fe0.72B0.24Nb0.04)95.5Y4.5 glassy sample was produced by thegas-atomization plus consolidation using the spark plasma sinter-ing technique. The effect of the gas-atomization and subsequentSPS consolidation on the soft-magnetic properties was investi-gated. The results can be summarized as follows.

(1) Maximum relative density of 99.5% is achieved in a glassycompact using spark plasma sintering technique at a tempera-ture near the glass transition temperature and at the appliedstress of 600 MPa.

der and the SPSed BMG, where Tc: Curie point, Tg: glass transition temperature, Tx:� Tg), Hc: coercivity, Js: saturation magnetization.

x (K) DTx (K) Hc (A/m) Js (emu/g)

64 83 14 9880 107 8979 109 0.2–0.582 111 0.8–1.0 87

S. Lee et al. / Intermetallics 17 (2009) 218–221 221

(2) Ring-shape SPSed glassy compact with an outer diameter of12 mm, inner diameter of 6 mm, and 2.5 mm in thickness, wasfabricated for measurement of the soft-magnetic propertieswithout demagnetization.

(3) The saturation magnetization of the SPSed glassy compact is98 emu/g, which is 10% higher than the value of 89 emu/g forthe initial glassy powder. This is considered to be resulted fromthe chemical short-range ordering closely related to a spin-exchange interaction.

(4) The coercivity of the SPSed glassy compact is 14 A/m, which isapproximately 17 times higher than the value of 0.8 A/m of thecast BMG. This result may be caused by the contaminationeffect on the magnetic domain between the metallic oxidelayer and powder inside.

(5) The soft-magnetic properties can be improved by controllingprocessing conditions for the powder fabrication and subsequentconsolidation.

Acknowledgement

The authors are indebted to Prof. K.V. Rao, Mr. Ansar Masoodat KTH for MTG measurement and thank Prof. H. Kimura, Mr.A. Okubo, Mr. Kanomata, Mr. I. Nagano at Institute for MaterialsResearch (IMR), Tohoku University in carrying out the experiments.This work was supported by a Grant-in-Aid for Science Research onPriority Areas, ‘‘Materials Science of Bulk Metallic Glasses’’, fromthe Ministry of Education, Science, Sports and Culture of Japan.

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