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Materials Science and Engineering A 447 (2007) 319–323

Self-assembled bulk metallic glass composite preparedby electro discharge bonding technique

J.S. Park a,b, J. Cho b, D.H. Kim a, N.S. Lee c, K.B. Kim c, W.H. Lee c,∗a Center for Noncrystalline Materials, Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Republic of Koreab Korea Institute of Industrial Technology, Materials & Parts Team, Gwangju Research Center, Gwangju 506-824, Republic of Korea

c Faculty of Nano Technology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea

Received 27 July 2006; received in revised form 2 October 2006; accepted 12 October 2006

bstract

The bulk metallic glass (BMG) bonding with a thin elemental crystalline layer has been examined in terms of electro discharge technique. Thestimated temperature rise during electro discharge was up to about 3500 ◦C, and thus the interface involved a liquid phase. When the optimized

atabase was applied to fabricate the two-layered BMG (BMG/Zr/BMG), the interface between crystalline layer and BMG showed a well-bondednterface and the fracture toughness of the two-layered BMG exhibited higher value compared with monolithic BMG. The detailed calculation fornput energy and the main mechanism for the higher toughness are discussed. 2006 Elsevier B.V. All rights reserved.

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eywords: Composite; BMG; Electro discharge; Fracture toughness

. Introduction

The fabrication of bulk metallic glasses (BMGs) has receivedn attention due to the unique properties, such as high strength,arge elastic limit and high corrosion behavior [1,2]. The largelass forming ability (GFA) of amorphous alloys has enabled toabricate large size BMGs more than 20 mm in diameter with alow cooling rate as low as 1 K/s [1]. Since the large sized BMGan provide an increased feasibility for practical application ofMGs, attempts have been made in order to increase the size

imitation of BMGs by several bonding techniques [3,4], suchs friction welding, electron beam welding, explosive welding,elf-heat propagation and brazing. Recently, the bonding of crys-alline material with BMGs has been reported in order to increasehe potential applicability. In order to achieve promising bondingroperties of the BMGs/crystalline materials, a detailed study ofonding conditions is one critical factor as well as preservationf amorphous phase during the bonding time [5].

While several approaches have been made for successfulonding, the electro discharge technique has not been examinedn detail. The electro discharge technique has been intensively

∗ Corresponding author. Tel.: +82 2 3408 3779; fax: +82 2 3408 3664.E-mail address: whlee@sejong.ac.kr (W.H. Lee).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.10.028

nvestigated by Lee and co-workers [6–10]. It has been shownhat the technique is very efficient for the compaction/sinteringf Ti and Ti-6Al-4V powders in less then 400 �s without chang-ng their unique microstructures. In principle, the bonding pro-ess using the electro discharge technique has a benefit thathe objective sample bonding can be fabricated in a very shorteriod of time frame and thus the crystallization can be preventedhen a proper amorphous materials is selected [11]. Further-ore, with an establishment of optimizing input energy and

elected materials, the constructed database can be extendedowards other bonding technique, such as a brazing process.

hen the bonding condition between crystalline and amor-hous layers is optimized, the feasibility of the fabrication ofaminated composite composed of amorphous and crystallineayers can be maximized. For the BMG laminated compos-tes, it has been reported that the fracture resistance of theMG laminated composite increased compared with the mono-

ithic BMG, and thus an approach of a synthesis of laminatedomposites via bonding process can provide a large poten-ial for enhancement of the mechanical properties of BMGs11,12].

In this study, we first attempted bonding experiments withpecimen arrays of BMG/thin crystalline layer with using aharge electro technique in order to identify optimum condi-ions for the achievement of successful bonding. As a model

3 nd Engineering A 447 (2007) 319–323

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20 J.S. Park et al. / Materials Science a

ystem, a commercial Vitreloy 1 (Vit. 1) and several crystallinehin layers (Nb, Zr and Al) have been applied in order to examinehe effect of crystalline layers, such as melting temperature andpplied energy. The cross-sections of the bonded layer exhibitedomplete bonding without defects when an optimum conditionas selected. Here, we present the optimization criteria of input

nergy associate with materials properties of the fabricated lam-nated composites.

. Experimental procedure

30 g ingots with a nominal composition of monolithic Vit.1Zr41.2Ti13.8Cu12.5Ni10Be22.5) have been made by arc meltingf elemental components and Be–Cu–Ni alloys in a Ti-getteredr atmosphere. In order to fabricate a bulk form BMG a self-esigned plate-shaping equipment was used: after melting ofhe ingot by arc in an Ar atmosphere, the melt was sandwichedmmediately between two water-chilled cupper rods, resulting tohe fabrication of a BMG coupon. For the preparation of bondingrocess, the BMG coupon was cut into a rectangle shape withdimension of about 5 mm × 5 mm × 4 mm (width, length andeight). Thin crystalline layers of Zr (25 �m), Nb (50 �m) andl (25 �m) were purchased from Alfa Aesa®. The metallic layeras selected upon difference of melting temperatures. The thin

ayer was inserted between two pieces of the BMG.Each multilayered sample was put on the tungsten electrode

t the bottom. An upper electrode was automatically machine-riven on the top of the sample followed by application of a 10 kgoad. The discharging chamber was evacuated to 2 × 10−3 Torrnd then filled with Ar gas at 1000 Torr. The capacitor bank of50 �F was charged with three different electrical input energies1.0, 1.5 and 2.0 kJ). The charged capacitor bank instantaneouslyischarged through the multilayered sample by an on/off highacuum switch, which closed the discharge circuit. The voltagend the current that the sample experienced when the circuit was

losed were simultaneously picked up by a high-voltage andhigh-current probe, respectively. Outputs from these probesere fed into a high-speed oscilloscope that stored them as a

unction of discharge time. A schematic diagram of the exper-

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Fig. 1. A schematic diagram of the experimental set-

ig. 2. (a) The schematic illustration of the sample shape and array and (b) theigital image of three-point bending tests.

mental set-up for the electro discharge bonding technique ishown in Fig. 1.

Following the discharge process, the specimen was cut per-endicular to the interface direction, and metallurgical esti-ations with a variation of input energy have been carried

ut by optical microscope (OM) and scanning electron micro-cope (SEM, Hitachi S-2700). X-ray diffraction (XRD, RigakuN2301) was used for the phase identification.

Following the identification of the bonding conditions, three-

oint bending tests of Vitreloy 1 and the composite with Zrayer have been performed with a strain rate of 2.5 × 10−4. Thechematic specimen dimension and digital image of the bendingests is shown in Fig. 2.

up for the electro discharge bonding technique.

J.S. Park et al. / Materials Science and Engineering A 447 (2007) 319–323 321

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Table 1The estimated enthalpy and temperature rise corresponding to the input energy

Input energy (kJ) �H (J) �T (◦C)

1.0 900 110012

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ig. 3. Traces of (a) X-ray diffraction and (b) DSC showing amorphous phasef Vitreloy 1.

. Results and discussion

Fig. 3 shows (a) XRD and (b) DSC of the as-cast Vit. 1 alloy,ndicating an amorphous halo at about 30◦. The glass transitionnd crystalline temperatures exhibited about 330 and 345 ◦C,imilar to the previously reported data [1]. Also, after the elec-ro discharge process, XRD did not show any crystallization of

he BMG. Fig. 4 shows optical micrographs of the cross-sectionsfter bonding process. The examined specimen showed a com-lete bonding layer when the applied energy was optimized.or the BMGs with an Al layer insert (Fig. 4(a)), the inter-

ig. 4. The cross-section area of BMG/Al/BMG with (a) 1.0 kJ and (b) 1.5 kJ;MG/Nb/BMG with (c) 1.0 KJ and (d) 2.0 kJ; BMG/Zr/BMG with (f) 1.5 kJ.

e) the enlargement of (d).

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aces between Al and BMG showed a fully bonded morphologyhen the input energy was set as 1.0 kJ. However, when the

nput energy was 1.5 kJ, several holes were observed (Fig. 4(b)),mplying that the Al layer underwent melting event due to aigh temperature rise during the discharge process. However,or the specimens with Nb or Zr insert layer, the holes wereot observed. This supports that the formation of the hole inhe insert layer is closely related to the temperature rise due tohe electric resistance associated with the input energy, inferringhat a controlled input energy does provide a critical solution foruccessful bonding with a crystalline layer.

For the BMGs with the Nb insert layer (Fig. 4(c)), when thenput energy was 1.5 kJ, poor interface bonding was observed.owever, when the input energy was 2.0 kJ, a well-bonded inter-

ace was observed. Also, there were not any noticeable phasesdentified at the interface as shown in Fig. 4(d and e). Finally, forhe specimen with a Zr layer, a complete bonding was observedhen the input energy (high energy) was 1.5 kJ (Fig. 4(f)). It

ppears that a relatively large amount of energy is needed forhe sample when the melting temperature of the insert layer isigh (>2000 ◦C). The bonding conditions corresponding to thenput energy was listed in Table 1. It appears that the success-ul bonding is closely related to the melting temperature of thensert layer and input energy.

Fig. 5 (a) shows the results of the three-point bending tests ofa) monolithic and (b) two-layered BMG. The fracture toughnessf the monolithic BMG shows similar values with previouslyublished value of Vit. 1 [13], while there is a few percent oflongation observed. At the same time, it clearly shows thathe fracture toughness of the two-layered composites show aigher value, i.e. 70 MPa m0.5, which is the higher value of theonolithic BMG. The elongation of the composite also showed

igher value. This insinuates that the bonding of the interfaceas optimized and the ductile crystalline layer (inserted layer)layed an important role for improving the toughness value.

ig. 5. Fracture toughness values of (a) as-cast and (b) two-layered BMG.

322 J.S. Park et al. / Materials Science and Engineering A 447 (2007) 319–323

Table 2The bonding conditions of BMGs upon inset layer and input energy

Melting point (◦C) 1.0 KJ 1.5 KJ 2.0 KJ

Al 660 Good Bad BadZr 1852 Bad Good GoodN

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Compared Al (melting point: 660 ◦C) with Nb (melting point:468 ◦C), a higher energy is needed for the Nb insert layer thanhe Al insert layer for successful bonding. Since the completeonding by charge electro technique depends on the temperatureise during the process, the heat generated during discharge isiven by [14]:

H = ΣI2(t)R(t)�t (1)

here R is the power column resistance and t is the dischargeime. Since the current, I, and resistance, R, of the BMG as aunction of time, both I and R were obtained using high voltagend current probes and oscilloscope. The estimated enthalpy wasisted in Table 2, when the input energy was 1.0, 1.5 and 2.0 kJ.lso, the temperature rise at the interface, which is induced by

n input of energy during discharge, was estimated as [15]:

= mcp(t)�T (2)

here W is input energy, m mass, cp(t) heat capacity and �Ts difference between temperature rise during discharge. Fromq. (2), the estimated temperature rise during discharge was

isted in Table 1. It is clear that the increase of input energys in proportional to the temperature rise. Also, the extents ofhe corresponding interface bonding judged from observationsf cross-sections were shown in Table 2. It appeared that theelting temperature of the insert layer is one critical factor for

uccessful bonding. For example, when the melting temperaturef the insert layer is low, such as Al, high input energy does notrovide a beneficial effect for interface bonding. For example,ince the melting point of Al is 660 ◦C, pores were observed atoth the interface and inside of the layer, possibly due to theoo much temperature rise, inferring that a precise control of thenput energy is a key feature for the achievement of successfulonding.

Since the fracture toughness of the two-layered BMG showedigher value than that of the monolithic BMG, the interfaceetween BMG and crystalline layer has been investigated. Espe-ially, when the stacked number of the BMG layers reachesour layers, the specimen has not been fractured, even thoughhe load for fracture passed the maximum value (Fig. 6(a)). Inrder to identify the crack propagation along the interface, theross-section of the specimen has been examined and shownn Fig. 6(b). It clearly indicates that the crack propagation pathas changed into other direction when the crack penetrates crys-

alline layer as marked by circle as depicted in Fig. 6(c). It has

een reported that the crack redirection and fiber pulling is mainechanism for achieving high toughness for brittle materials,

uch as ceramics [16]. For the present case, the cross-section ofhe specimen clearly shows that the fracture mode is changed

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ig. 6. (a) The digital image of four layered BMG specimen, (b) optical imagef the interface and (c) the crack propagation of the specimen after three-pointending tests.

rom catastrophic to ductile behavior by presence of the ductile

rystalline layer. For the structural materials point of view, theracture behavior of the layered BMG is attractive, and gives aotential for structural application considering the catastrophicracture nature of the BMGs.

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J.S. Park et al. / Materials Science

. Summary and conclusion

The BMG bonding with a thin elemental crystalline layeras been examined in terms of electro discharge technique.t appears that the input energy provides a critical factor foruccessful bonding. Since the temperature rise during electronischarge was estimated up to about 3500 ◦C, and the inter-ace bonding involved liquid phase appeared as a main reasonor complete bonding. When Nb or Zr layer was used as annsert layer, a complete bonding was achieved. But, when Alayer was used as an insert layer with the input energy of 1.5 kJ,ores were observed, possibly due to temperature rise during dis-harge process. So, when a low melting point crystalline layers used as an insert layer, a precise control of input energy androper materials selections can lead to a successful bonding.hen the optimized database was applied to fabricate the two-

ayered BMG, the interface between crystalline layer and BMGhowed a well-bonded interface and the fracture toughness of thewo-layered BMG exhibited higher value compared with mono-ithic BMG. It appears that crack redirection along the interfaceayer provided main mechanism for the achievement of the highoughness.

cknowledgments

The authors are grateful for the financial support of the Cre-tive Research Initiatives of the Korean Ministry of Science and

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Engineering A 447 (2007) 319–323 323

echnology, and the ERC program of KOSEF (grant # R11-005-048-00000-0).

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