COM

MUNIC

ATIO

N

DOI: 10.1002/adem.201000207

Improved Synthesis of Bulk Metallic Glasses byCurrent-Assisted Copper Mold Casting**

By Mihai Stoica*, Andras Bardos, Stefan Roth, Lajos K. Varga, Ludwig Schultz, Antal Lovas andJurgen Eckert

The amorphous alloys, especially the bulk metallic glasses (BMGs), possess special properties, makingthem very attractive for applications as structural and functional materials. The most importantcriterion for amorphization is that during cooling from the liquid state crystal nucleation and growth isavoided. The casting technologies currently available do not allow preparation of amorphous parts withcomplicated geometries. A DC electrical current applied between the molten metal and the mold duringthe ejection may inhibit heterogeneous nucleation by modifying the surface tension of the liquid alloy.More, by reducing the surface tension, the molten alloy flows better, filling even complicated molds.Using this new technique, the casting processes may become reproducible and can be relatively easilyscaled up to the industrial requirements for mass production.

[*] Dr. M. Stoica, Prof. J. EckertInstitute for Complex Materials, IFW Dresden, Helmholtzstr.20, D-01069 Dresden, GermanyE-mail: m.stoica@ifw-dresden.de

Dr. A. Bardos, Prof. A. LovasDepartment of Vehicle Manufacturing and Repairing, Berta-lan Lajos u. 2, BUTE, H-1111 Budapest, Hungary

Dr. S. Roth, Prof. L. SchultzInstitute for Metallic Materials, IFW Dresden, Helmholtzstr.20, D-01069 Dresden, Germany

Prof. L. K. VargaResearch Institute for Solid State Physics and Optics,Hungarian Academy of Sciences, P.O. Box 49, H–1525Budapest, Hungary

Prof. L. Schultz, Prof. J. EckertInstitute of Materials Science, University of TechnologyDresden, D-01062 Dresden, Germany

Dr. A. BardosPresent address: Kienleþ Spiess GmbH, Research and Devel-opment Department, Bahnhofstr. 23, D-74343 Sachsenheim,Germany

[**] This work was supported by the German Science Foundation(DFG) grant no. Ec 111/8–3 and by the National ScientificResearch Fund (OTKA) of Hungary through grants no.T-034666 and no. T-035278. The support of the EU throughRTN-networks on bulk metallic glasses (HPRN-CT-2000-00033) and ductile BMG composites (MRTN-CT-2003-504692) is also acknowledged. The authors thank H. Schulzeand B. Bartusch for technical assistance.

38 wileyonlinelibrary.com � 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2011, 13, No. 1--2

Bulk metallic glasses (BMGs) lack three-dimensional

atomic periodicity beyond a few atomic distances. As

consequence, they possess special properties, making them

very attractive for applications as structural and functional

materials;[1] however, the mass production of BMGs is

restricted by technological difficulties. The easiest preparation

way is rapidly cooling a master alloy from its molten state;[2]

however, the casting technologies currently available do not

allow preparation of amorphous parts with complicated

geometries. In order to retain the amorphous structure, the

crystal nucleation and growth must be avoided[3] when the

alloy is cooled from the liquid state. The nucleation may be

homogeneous, due to the undercooling, and/or heterogeneous,

when impurities present in the melt act as ‘‘seeds’’ for

nucleation. In reality, even the micro-imperfections of the

mold walls may be centers for heterogeneous nucleation.[3–4]

In this work, we will show that a DC electrical current applied

between the molten metal and the mold during the ejection

may inhibit heterogeneous nucleation by modifying the

surface tension of the liquid alloy. The modification of the

surface tension of themolten alloy helps to avoid solidification

before complete filling of themold cavity. All together, we have

a new phenomenon which is different when compared with

the electrowetting or electrocapillarity described by Lipp-

mann[5] almost 135 years ago. Using this new technique, the

casting processes may become reproducible even for compli-

cated geometries and can be relatively easily scaled up to the

industrial requirements for mass production.

The term ‘‘metallic glass’’ usually refers to a metallic alloy

rapidly quenched in order to ‘‘freeze’’ its structure from the

liquid state. As a consequence, a metallic glass is a metastable

COM

MUNIC

ATIO

N

M. Stoica et al./Improved Synthesis of Bulk Metallic Glasses by Current-Assisted Copper . . .

Fig. 1. Schematic explanation of heterogeneous nucleation of a crystal embryo in anundercooled liquid. The catalytic surface (the heteronucleant) is considered to be flat (atthe beginning the catalyst is much bigger than the new embryo)

alloy which at room temperature shows an amorphous,

liquid-like structure. BMGs represent a particular class of

amorphous alloys. The BMGs are relatively young (first

mentioned in the 1980s)[1] and their most notable property is

the ultrahigh (near theoretical) strength and hardness.[6]

Depending on the composition BMGs may also exhibit

excellent corrosion resistance, high wear resistance, very

good soft magnetic properties,[1] and biocompatibility.[7] This

makes them very attractive for a variety of applications such

as (micro)gears and parts for motors, implants, magnetic

clutches, cases, springs, drilling heads, penetrators, or

sporting goods.[1] During cooling from the liquid state, an

alloy may become amorphous if crystal nucleation and/or

growth are avoided. Nucleation in an undercooled liquid may

be homogeneous or heterogeneous.[8] Homogeneous nuclea-

tion is provoked by the undercooling itself, while hetero-

geneous nucleation is triggered by ‘‘seeds’’ which are either

present accidentally or deliberately injected into the system.[3]

These seeds may be crystals of the material itself or other solid

materials, such as the container walls or particles suspended

in the liquid. At a given cooling rate the undercooling required

for heterogeneous nucleation varies widely with composition

and structure of the seed material.[3] Experimentally it is

difficult to circumvent the effects of seeds and thereby realize

homogeneous nucleation behavior. Unfortunately, the hetero-

geneous nucleation cannot be controlled and is even more

likely to appear when the used master alloy is made of cheap,

low-grade raw materials. In the following, we will show that

its effect can be minimized by current-assisted processing of

metallic melts. Regarding the cooling rate, besides the

physical characteristics as ejection temperature, thermal

conductivity of the mold (and the alloy) etc., an important

role is played by the viscosity and surface tension of the

molten alloy. In many cases BMG parts with complex

geometries cannot be obtained because the alloy ‘‘freezes’’

before filling all complicated small channels of the mold. Even

closed shapes as rings or toroids cannot be attained. In order to

achieve better mold filling, there are two options. One is to

decrease the viscosity, which means to increase the casting

temperature.[9] This is not desired, because casting from a

higher temperature is detrimental to the cooling rate. The

surface tension can be modified not only by adjusting the

temperature, but also by passing an electrical current through

the molten metal.[5,10] In this way, the current-assisted casting

can meet both purposes: avoiding heterogeneous nucleation

and improving the casting conditions. This casting method

was proved to drastically increase the castability of some

Fe-based alloys: (i) Fe65.5Cr4Mo4Ga4P12C5B5.5, (ii) eutectic cast

iron (CI) (FeCPBSiMn)-Phosphorus-Boron (CIPB), and (iii) a

mixture of both, with the final compositions close to

Fe70.7C6.7P10.4B5Si1.1Mn0.1Cr2Mo2Ga2. Fe-based alloys were

chosen for these experiments because their critical cooling

rates necessary for amorphization are quite high[1] and when

low-grade cheap materials are used, the alloys are prone to

crystallize heterogeneously (due to the impurities present in

the melt). The melting temperatures of these master alloys are

ADVANCED ENGINEERING MATERIALS 2011, 13, No. 1--2 � 2011 WILEY-VCH Ve

1350–1450K and they were produced by induction melting

industrial grade CI, FeP, FeGa, and FeB pre-alloys (purity

98%) under low vacuum (1mbar) atmosphere. The current-

assisted copper mold casting method is believed to work also

for other classes of alloys, especially for those which requires

for glass formation cooling rates of the order of 102–103K s�1.

According to the fundamentals of solidifications[8] the

thermodynamic barrier to nucleation is given by

DG� ¼ 16p

3

� �� s3

Dg2

� �� 2þ cosuð Þ 1�cosuð Þ2

4;

where s is the solid/liquid interfacial energy, Dg is the Gibbs

free energy difference between the liquid and the solid per

unit volume (always it is a negative term because it depends

on the undercooling DT), and u is the angle made by the

crystalline embryo with the substrate. Figure 1 explains the

situation in detail. There, a solid crystalline embryo is just

forming in a liquid, in contact with a catalytic surface (the

heteronucleant) already present in the liquid. The hetero-

nucleant is considered to have a flat surface, because at the

beginning the catalyst is much bigger than the new embryo.

scL and scS are the catalyst–liquid interfacial free energy and

the catalyst–solid interfacial free energy, respectively. Any

value of u between 08 and 1808 corresponds to a stable angle.

When u¼ 1808, the solid does not interact with the substrate,

f(u)¼ 1 and homogeneous nucleation is obtained. When u¼ 08,the solid ‘‘wets’’ the substrate, f(u)¼ 0 and DG�¼ 0. As a result,

solidification can begin immediately when the liquid cools to

the freezing point. From the point of view of classical

heterogeneous nucleation, a good nucleant corresponds to a

small contact angle between the nucleating particle and the

growing solid. However, in general, the values of both scS and

scL are not known and, therefore, it is rather difficult to predict

the potential catalytic effectiveness of a nucleant. In fact, there

is no clear insight into what determines u and how it varies

with (i) lattice disregistry between substrate and the stable

phase, (ii) topography of the catalytic substrate surface, (iii)

chemical nature of the catalytic surface, and (iv) absorbed

films on the catalytic substrate surface.[4]

A second aspect related to bulk glass formation is related to

the ability of the molten alloy to fill a cavity a mold.

Amorphization requires a rather high cooling rate, which

basically requires (i) a very good contact with themold and (ii)

rapid heat extraction from the alloy. If for the latter criterion, a

mold made from a material with high thermal conductive is

rlag GmbH & Co. KGaA, Weinheim http://www.aem-journal.com 39

COM

MUNIC

ATIO

N

M. Stoica et al./Improved Synthesis of Bulk Metallic Glasses by Current-Assisted Copper . . .

Fig. 2. Molten Ga on a Cu plate, at 400 K, (a) without and (b), (c) with applied electricalDC current. Here the conductive Cu plate was the cathode (�) and the electrode whichcan be seen on top of the droplet the anode (þ).

Fig. 3. The contact angle a function of the applied DC current. For every polarity atleast one local minimum can be observed, which is not necessarily the global minimum.

used, the good thermal contact implies a good wetting of the

mold cavity, which in fact hampers completemold filling. This

is the main reason why BMGs are very difficult to cast in

complicated geometries. The ability to fill the mold is strongly

determined by the viscosity h and the surface tension s

(interfacial free energy) of the molten alloy. Usually, the

viscosity of an alloy in the molten state decreases several

orders of magnitude at a temperature immediately above the

liquidus temperature,[9,11] decreasing further only slowly as

the overheating increases. Typical casting temperatures are

above the liquidus and the overheating may reach even 250K.

Such technological aspects are barely described in the

scientific literature, basically due to the fact that there is no

‘‘general recipe’’ and/or set of well established parameters for

casting. In most cases, each class of alloys behaves differently

and therefore the casting parameters have to be adjusted

accordingly.

Liquid metallic alloys which are able to form BMGs do not

wet the walls of a metallic mold, the angle u presented in

Figure 1 being larger than 908. It was shown previously that, in

theory, heterogeneous nucleation can be completely avoided

if u reaches 1808.[8] Hence, in order to minimize the effect of

heterogeneous nucleation it is enough to increase the angle u

as much as possible. Due to the high metal content, the alloys

able to form BMGs are electrically conductive. When a

conductive liquid is placed on a conductive substrate, the

surface tension can be modified by applying a current

between the liquid and the substrate.[5,10] As a consequence,

the angle u will change and will depend on the magnitude of

the applied electrical current. The details of these phenomena

were studied with the help of molten Ga on a heated

electrically conductive surface. The viscosity of Fe-based glass

formers is usually on the order of 5–20mPa � s,[9,12] similar to

the value reported for Ga in the liquid state.[13] Figure 2(a), (b),

and (c) show the molten Ga kept on a hot stage at 400K, with

and without applied electrical current. One can observe that

without current (a) the contact angle between metal and

substrate is 578. Once the current increases to 2 A [Fig. 2(b)] or

even more [7 A in Fig. 2(c)], the angle becomes smaller, 498 oreven less than 408. According to the previous formalism

(Fig. 1), here the marked angle is a¼ 1808– u, so it is desired to

have this a angle as small as possible. As can be seen in

Figure 2, the angle depends on the amplitude of the applied

current. Figure 3 summarizes our experimental data, when

only DC electrical current was used. The plot shows for both

polarities the variation of the contact angle measured for

molten Ga. From the plot it is clear that the contact angle has at

least one local minimum for every polarity. Due to technical

limitation, the experiment was stopped when currents of 20 A

were attained. Therefore, it cannot be concluded that the

shown local minima are also the global minima. Here should

be mentioned that the electrical current was applied only for

very short time (few seconds), in order to minimize Joule

heating during the experiment.

The Fe-based alloys used for the current assisted casting

experiments were prepared in several steps. First, master

40 http://www.aem-journal.com � 2011 WILEY-VCH Verlag GmbH & Co.

alloys with the nominal composition Fe65.5Cr4Mo4-Ga4P12C5B5.5 were prepared by induction melting relatively

pure elements (99.8% or better). The eutectic cast iron-

phosphorous-boron (CIPB) was made by induction melting

together regular industrial CI (FeCPBSiMn) with crystalline

boron and red phosphorous, in such way that finally there are

17 at% metalloids (C, B, P). However, the CIPB glassy alloy is

KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2011, 13, No. 1--2

COM

MUNIC

ATIO

N

M. Stoica et al./Improved Synthesis of Bulk Metallic Glasses by Current-Assisted Copper . . .

very brittle and fragile,[14] while the Fe65.5Cr4Mo4Ga4P12C5B5.5

BMG displays small plastic deformation and a very high

toughness.[15,16] Therefore, these base alloys were mixed in

order to combine the desirable properties for net-shape

casting. Hence, the data presented further refer only to the

resulting multicomponent Fe70.7C6.7P10.4B5Si1.1Mn0.1Cr2-Mo2Ga2 alloy. The newly mixed alloy inherited the advanta-

geous properties of the base alloys. However, all these alloys

show relatively limited glass forming ability. They can be cast

as BMGs only as small rods, with diameters of 1–2mm. With

the current assisted casting method we were able to cast fully

amorphous toroids with external diameter of 22mm, internal

diameter 18mm, and a thickness of 1mm.

Each master alloy was crushed into small pieces and these

were cast in a copper mold using the injection casting method.

The device used for casting was a slightly in-house modified

commercially available INDURET-S facility produced by

Reitel Feinwerktechnik GmbH, Germany. The experimental

set-up is sketched in Figure 4. The device consists of two

chambers: the upper chamber in which the induction coil and

the working crucible are placed, and the lower chamber,

which contains the copper mold. Both chambers can be

evacuated to 1mbar using a membrane vacuum pump and

filled/flushed with 99.9% pure Ar. For melting, a ceramic

(Al2O3) cylindrical cruciblewas used, with a diameter of about

40mm and a conical nozzle, having in its lower part a hole of

approximately 6mm diameter. The hole is closed during

melting with a boron nitride stopper which can be lifted

automatically, opening the hole and giving the possibility to

eject the molten alloy. The ejection is supported by an argon

overpressure, set between 0 and 3 bars, which can be applied

at the samemoment as the withdrawal of the stopper or with a

Fig. 4. A sketch of the experimental apparatus used in this work. It shows schematicallythe situation during casting. The tungsten electrode is effectively inserted in the moltenmetal only during the casting moments.

ADVANCED ENGINEERING MATERIALS 2011, 13, No. 1--2 � 2011 WILEY-VCH Ve

delay time adjustable between zero and one second. The

induction generator can deliver an adjustable power between

0 and 3 kW at a frequency of 30 kHz. The temperature is

monitored by an external two-color optical pyrometer

visualizing the upper surface of the molten alloy through a

glass window. The maximum achievable temperature is

around 1650 8C. The lower chamber contains the copper mold.

The mold entrance and the ejection hole of the crucible are

carefully aligned. Besides the conical opening, the copper

mold has a small channel which assures the evacuation of the

gases which may be trapped during casting. In order to

modify the surface tension during casting, two electrodes

were attached. The one, in contact with the molten alloy, was

placed in the melting crucible. The electrode material is

tungsten. The second electrode is a Cu-plate, placed below

and in direct contact with the Cu-mold. The current flows

through the molten alloy only when the alloy is poured

(injected) into the mold. The electrodes are put in contact with

a power supply by proper wiring which passes to a special

opening safe for vacuum. The polarity of the applied current

was conventionally considered to be positive when the

tungsten electrode (and, implicitly, the molten alloy) was in

contact with the positive pole of the DC power supply.

Figure 5 shows examples of Fe70.7C6.7P10.4B5Si1.1Mn0.1Cr2-Mo2Ga2 BMG samples cast from the same temperature (a)

without and (b), (c) with applied current. The value of the

current used for sample (b) (þ2 A) was lower than the one

used for experiment (c) (þ7 A). As expected, there seems to be

aminimum value of the electrical current for which the results

are satisfactory. This value depends on the alloy composition

and the casting temperature, because the electrical current

affects a property (surface tension) which depends on the

nature of the alloy and the temperature. Other issues may be

related to the magnitude and the polarity of the applied

current. Conventionally, the polarity was considered positive

when themolten alloy played the anode role. The amorphicity

of all three casting products was checked by means of X-ray

diffraction (Co-Ka radiation, l¼ 0.178896 nm) and the results

are presented in Figure 6. All samples are mainly amorphous,

but at least in the case of the first uncompleted geometrical

ring crystalline inclusionsmay be observed, while the samples

cast under applied current (b and c) show no obvious Bragg

peaks.

Fig. 5. Examples of Fe70.7C6.7P10.4B5Si1.1Mn0.1Cr2Mo2Ga2 BMGs cast (a) without and(b), (c) with applied DC electrical current.

rlag GmbH & Co. KGaA, Weinheim http://www.aem-journal.com 41

COM

MUNIC

ATIO

N

M. Stoica et al./Improved Synthesis of Bulk Metallic Glasses by Current-Assisted Copper . . .

Fig. 6. Diffraction patterns of the samples cast under different values of applied DCcurrent. The patterns correspond to the three samples presented in Figure 5.

By modifying the contact angle, the global behavior of the

molten alloy was drastically changed. Due to a reduced

contact angle (i) the alloy flows better into the mold and (ii) for

the short casting time the cooling rate is smaller and the alloy

remains liquid until the cavity is completely filled. During

casting/ejection/mold filling the temperature decreases and

the alloy can even be undercooled. The temperature decrease

is accompanied with an increase in viscosity but homo-

geneous nucleation does not occur. Heterogeneous nucleation

also does not occur, because the applied electrical current,

which is still flowing, inhibits the nucleation. The experi-

mental set-up assures that when the entire quantity of alloy is

out of the crucible the current become zero. In this moment the

alloy wets again the container walls and solidifies at a high

rate, keeping the structure from the liquid state.

This casting method was proved to help the synthesis of

Fe-based BMGs, but it can be applied for casting any other

metallic melt. For example, it could be used for casting high

entropy alloys, assuring a continuous and reliable industrial

production of new materials with new properties and

42 http://www.aem-journal.com � 2011 WILEY-VCH Verlag GmbH & Co.

complicated geometries, suitable for applications in which

conventional metallic materials fail.

Received: July 8, 2010

Final Version: August 3, 2010

Published online: November 10, 2010

[1] A. Inoue, Acta Mater. 2000, 48, 279.

[2] T. R. Anantharaman, C. Suryanarayana, in Rapidly Soli-

dified Metals, a Technological Overview, Trans Tech Pub-

lications, Switzerland-Germany-USA 1987, p. 25.

[3] D. Turnbull, Contemp. Phys. 1969, 10, 473.

[4] H. Biloni, W. J. Boettinger, in Physical Metallurgy, Vol. I,

Ch. 8 Solidification (Eds: R. W. Cahn, P. Haasen), Elsevier

Science B.V, North Holland, Amsterdam 1996, p. 669.

[5] G. Lippmann, Ann. Chim. Phys. 1875 5, 494.

[6] M. F. Ashby, A. L. Greer, Scr. Mater. 2006, 54, 321.

[7] B. Zberg, P. J. Uggowitzer, J. F. Loffler, Nat. Mater. 2009,

8, 887.

[8] W. Kurz, D. J. Fisher, Fundamentals of Solidification, Trans

Tech Publications, Switzerland-Germany-UK-USA

1989.

[9] H. Chiriac, M. Tomut, J. Alloys Compd. 1994, 215, 289.

[10] F. Mugele, J. C. Baret, J. Phys.: Condens. Matter 2005, 17,

R705.

[11] C.Way, P.Wadhwa, R. Busch, Acta Mater. 2007, 55, 2977.

[12] T. Yamasaki, N. Yufune, H. Ushio, D. Okai, T. Fukami,

H.M. Kimura, A. Inoue, Mater. Sci. Eng. A 2004, 375–377,

705.

[13] S. V. Prokhorenko, Mater. Sci. 2005, 41, 271.

[14] A. Bardos, A. Lovas, S. Roth, M. Stoica, L. K. Varga,

Czech. J. of Phys. 2005, 55, 593.

[15] M. Stoica, J. Eckert, S. Roth, Z. F. Zhang, L. Schultz,W.H.

Wang, Intermetallics 2005, 13, 764.

[16] Z. F. Zhang, F. F. Wu, W. Gao, J. Tan, Z. G. Wang, M.

Stoica, J. Das, J. Eckert, B. L. Shen, A. Inoue, Appl. Phys.

Lett. 2006, 89, 251917.

KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2011, 13, No. 1--2