Scripta Materialia 55 (2006) 617–620

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Formation of glassy and icosahedral phases inas-cast (Zr9Ni4)75(Al1�xTix)25 alloys

J.B. Qiang,a,b,* W. Zhanga,b and A. Inouea

aInstitute for Materials Research (IMR), Tohoku University, Katahira 2-1-1, Aoba-Ku, Sendai 980-8577, JapanbOverseas IMR Research Center and School of Materials Science and Engineering,

Dalian University of Technology, Dalian 116024, China

Received 29 April 2006; accepted 13 June 2006Available online 3 July 2006

The icosahedral atomic cluster has been recognized as the building block of icosahedral quasicrystals (I-phase) and many bulkmetallic glasses (BMGs). With this in mind, a series of (Zr9Ni4)75(Al1�xTix)25 alloys were designed, and ribbons and rods of 3 mm indiameter were prepared. Structural analysis of these bulk samples revealed a pronounced alloying effect of Al and Ti on the forma-tion of BMG and I-phase. BMGs were formed at low Ti compositions (x 6 0.20). As the Ti concentration increased, an I-phasebegan to form in the glassy matrix, and a nearly monolithic I-phase alloy was obtained in the Al-free alloy. The alloying effect isdiscussed by considering the different roles of Al and Ti in the stabilization of local icosahedral atomic structures.� 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Bulk metallic glasses; Quasicrystals; Icosahedral atomic cluster; Alloying effect

An icosahedral short range order (ISRO) has beensuggested as the intrinsic local structure of metallicliquids and glasses [1–3]. It creates a thermodynamicbarrier to the formation of periodic crystals. Improvedglass-forming ability (GFA) is therefore a possibleconsequence of the enhanced stability of the ISRO[2–5]. Experimentally, the addition of other elementswhich have negative heats of mixing to the constituentelements of the basic alloy leads to enhanced GFAs,and bulk metallic glasses (BMGs) can be made [3].The icosahedral atomic cluster is a definitive solutionto the local structure of icosahedral quasicrystals (I-phases) and is arranged in an orderly manner on a longrange scale in the I-phases [6]. Recently, nanometerscaled I-phases have been found to be the primary crys-tallization products in many Zr-based BMG-formingalloys [4,7]. These experimental results imply a stronglink between an ISRO and the formation of BMGsand I-phases. To explore this topic, it is worthwhile firstto examine the experimental manifestation of the alloy-ing effect on their formations in a given system. In this

1359-6462/$ - see front matter � 2006 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2006.06.008

* Corresponding author. Present address: Institute for MaterialsResearch (IMR), Tohoku University, Katahira 2-1-1, Aoba-Ku,Sendai 980-8577, Japan. Tel.: +81 22 215 2470; fax: +81 22 2152381; e-mail: qiang@imr.tohoku.ac.jp

work, we set our sights on the known icosahedral atomiccluster composition, Zr9Ni4. It was taken as the basiccomposition and alloyed by Al and Ti. Zr–Ni-basedBMGs and I-phase formations are expected in thespecific alloy series under certain casting conditions. Theobserved alloying effect on the final phase selection is dis-cussed by emphasizing the different roles of Al and Ti.

The Zr9Ni4 icosahedron can be regarded as a link-age structural unit between Zr-based BMGs and icosa-hedral quasicrystals [8,9]. Noting the experimentalresults in Ref. [10], the ratio of the alloying elementsto the base composition close to 1:3 favors the forma-tion of quasicrystals. We used an analogous treatmentto fix the overall atomic per cent of Al and Ti at 25%.A series of (Zr9Ni4)75(Al1�xTix)25 alloys were designedon the basis of the Zr9Ni4 icosahedral clustercomposition.

Ingots with nominal compositions (Zr9Ni4)75(Al1�x-Tix)25 (at.%; x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) were pre-pared by arc-melting high purity constituent metals(>99 wt.%). Then ribbon samples with a cross sectionof about 0.05 · 1.0 mm2 were prepared by a single rollermelt-spinning method at a speed of 40 m/s, and alloyrods of 3 mm in diameter were fabricated by the coppermold casting method. Structural identification wascarried out by using a Rigaku RINT-ultima IIIsp dif-fractometer with CuKa irradiation (k = 0.15406 nm).

sevier Ltd. All rights reserved.

0.0 0.2 0.4 0.6 0.8 1.0

600

700

800

900

30

40

50

60

70

80

Tem

pera

ture

(K

)

ΔTX (K

)

ΔTx

heating rate 40 K/min

X value

Tx

gT

Figure 3. Composition dependence of glass transition temperature Tg,onset temperature of crystallization Tx and undercooled liquid regionDTx = Tx � Tg of these melt-spun (Zr9Ni4)75(Al1�xTix)25 glasses.

618 J. B. Qiang et al. / Scripta Materialia 55 (2006) 617–620

TA-DSC Q100 type differential scanning calorimetry(DSC) and TA-STD Q600 type differential thermal anal-ysis (DTA) are employed to evaluate the thermal stabil-ity. The heating rates for DSC and DTA measurementswere 0.67 K s�1 and 0.167 K s�1, respectively.

The XRD patterns of melt-spun (Zr9Ni4)75(Al1�x-Tix)25 samples (Fig. 1) indicate that a fully amorphousstate was obtained by melt-quenching. The glassy naturewas confirmed by TEM. Figure 2(a) shows the DSCtraces obtained from these melt-spun samples. The Ti-free sample exhibits a distinct glass transition, followedby a single crystallization event. The addition of Tialtered the crystallization mode into multi-stage crystal-lization events. In particular, no glass transition wasdetectable at higher Ti concentrations (x = 0.6 and0.8). The DTA traces (Fig. 2(b)) of the low Ti concentra-tion (x = 0 and 0.2) samples show two separate meltingpeaks. The second melting peak becomes smeared athigher Ti concentrations, and finally a single peakmelting process appears at the Al-free composition.The onset melting temperature (Tm) and the liquidus

30 40 50 60 70 80

x = 1.00

x = 0.80

x = 0.60

x = 0.40

x = 0.20

x = 0CuKa (λ = 0.15406 nm)

Inte

nsity

(a.

u.)

2θ (degree)

Figure 1. XRD patterns of the melt-spun (Zr9Ni4)75(Al1�xTix)25

alloys.

400 500 600 700 800 900

(a)

x = 1.00

x = 0.80

x = 0.60

x = 0.40

x = 0.20

x = 0 Tg

Tx

heating rate 40 K/min

Exo

them

ic

Temperature (K)

1000 1100 1200 1300 1400

heating rate 10 K/min

Tl

Tm

x = 1.00

x = 0.80

x = 0.60

x = 0.40

x = 0.20

x = 0

(b)

Temperature (K)

End

othe

mic

Figure 2. DSC (a) and DTA (b) traces of the melt-spun(Zr9Ni4)75(Al1�xTix)25 alloys.

temperature (Tl) continuously decrease with the increas-ing Ti concentration.

The effect of Al and Ti additions on the thermal sta-bility of metallic glass is shown in Figure 3. One findsthat the glass transition temperature (Tg), the onset tem-perature of crystallization (Tx) and the undercooledliquid region DTx (= Tx � Tg) decrease with an increasein Ti concentration. The difference in GFA at these com-positions can be revealed by the well recognized indica-tors such as DTx [3], Tg/Tm [11], Tg/Tl [12], c = Tx/(Tg + Tl) [13]. The calculated values, together with thosecharacteristic temperatures, are summarized in Table 1.For the glasses associated with a distinct glass transi-tion, Tg shows a larger declining slope versus Ti concen-tration in comparison with those of Tm and Tl. Thus allthese indicators show a consistent tendency of the GFAto decrease (Fig. 4).

The difference in GFA was further revealed by thestructural analysis on the as-cast alloy rods. The XRDpatterns in Figure 5 show the formation of BMGs atlow Ti compositions (x 6 0.20). As the Ti concentrationfurther increases, I-phase begins to form in the glassymatrix. At the maximum Ti concentration, a nearlymonolithic I-phase alloy was made. The diffractionpeaks can be completely indexed by the I-phase follow-ing the scheme of Cahn [14,15]. The phase formationdetails of the as-cast rods were included in Table 1.Alloying effects on the formations of BMGs and I-phasewere found to be pronounced in the present (Zr9Ni4)75-(Al1�xTix)25 alloy series. It is evident that the Ti addi-tion frustrates GFA but favors the formation of I-phase,while the Al addition has the opposite effect.

The Al-alloying effect on the local structures of Zr-based Zr–Ni metallic glasses has been extensivelyexplored by many authors [16–18]. Al has strong nega-tive heats of mixing with Zr (�44 kJ mol�1) and Ni(�22 kJ mol�1) [19]. It has been well established thatin these metallic glasses strong Al–Zr and Al–Ni corre-lations exist, as signalled by the reduced Al–Zr andAl–Ni interatomic distances. Inoue et al. proposed thatthe negative heats of mixing between the constituentelements favor BMG formation [3]. In the present work,the BMG formation in Al-rich alloys may be thermody-namically attributed to the enhanced negative heateffect. In this situation a deeply depressed free energyof the undercooled liquid will be obtained, and the freeenergy difference between the liquid and the crystallinephases, i.e. the driving force available for crystallization,is reduced. An enhanced GFA is then achieved. The

Table 1. Thermal analysis data of melt-spun (Zr9Ni4)75(Al1�xTix)25 alloys and the phase components of the as-cast rods (3 mm in diameter)

Composition(Zr9Ni4)75(Al1�xTix)25

Tg (K) Tx (K) Tm (K) Tl (K) DTx (K) Tg/Tm Tg/Tl c = Tx/(Tg + Tl) Phase

x = 0 779 835 1184 1281 56 0.658 0.608 0.405 BMGsx = 0.2 733 785 1133 1247 52 0.647 0.587 0.396 BMGsx = 0.4 718 751 1124 1238 33 0.638 0.580 0.384 G + Ix = 0.6 667 699 1094 1182 32 0.610 0.564 0.378 G + Ix = 0.8 706 1066 1089 Ix = 1.0 592 1067 1092 I

Glass transition temperature, Tg; onset temperature of crystallization, Tx; undercooled liquid region, DTx = Tx � Tg; onset temperature of melting,Tm; liquidus temperature, Tl; and reduced glass transition temperature, Tg/Tm, Tg/Tl and c = Tx/(Tg + Tl). Bulk metallic glass, BMGs; glass, G; andicosahedral quasicrystal, I.

30 40 50 60 70 80

φ3 mm as-cast rods

x = 1.00

x = 0.80

x = 0.60

x = 0.40

x = 0.20

x = 0

Inte

nsity

(a.

u.)

2θ (degree)

I-phase

Figure 5. XRD patterns of the as-cast (Zr9Ni4)75(Al1�xTix)25 rods of3 mm in diameter.

0.0 0.2 0.4 0.6 0.8 1.00.4

0.5

0.6

0.7

0.3

0.4

0.5

0.6

γ=

Tx/(T

g+T

l)

heating rate 10 K/min

X value

Tg/T

m

Tg/T

l

γTg/

Tm

,Tg/

Tl

Figure 4. Composition dependence of reduced glass transition tem-perature parameters Tg/Tm, Tg/Tl and c = Tx/(Tg + Tl) of these melt-spun (Zr9Ni4)75(Al1�xTix)25 glasses.

J. B. Qiang et al. / Scripta Materialia 55 (2006) 617–620 619

Goldschmidt radius of Al atom is 0.143 nm [20], whichhappens to be close to the mean value of Zr(0.160 nm) and Ni (0.125 nm). Taking the geometricalreasons into account, the Al-substitution can improvethe local atomic packing density by alternating the bondlength, and stabilizing an icosahedral atomic cluster.The comparison studies indicate that Al randomly occu-pies the substitutional atomic sites of the Zr–Ni matrix[18]. Such a manner of alloying introduces local chemi-cal disorder, and the Al-induced disorder will fabricate abarrier to the long range ordered arrangement of icosa-hedral clusters. As a consequence, the growth of the ico-sahedral short range regions in the undercooled liquidwill be suppressed, preventing an I-phase from forming.

Ti has an atomic size close to that of Al [19], andhence the same geometrical reason allows us to presumethat Ti-addition also stabilizes the local icosahedralorder in the matrix Zr–Ni glass. In comparison withthe strong Al–Zr correlation, the nearly zero heat ofmixing between Ti and Zr implies a weak chemical affin-

ity. Therefore, Ti would like to be preferentially bondedwith Ni. Considering the atomic size ratio for an idealicosahedral atomic cluster (R1/R2 = 1.1085) [6], thealternating local environment of Ni with Ti provides amore favorable geometrical condition for icosahedronformation as compared with that in the Zr9Ni4 alloy.On the other hand, such a bonding state may facilitatethe preferential propagation of the icosahedral atomiccluster, i.e., the long range icosahedral orientationalorder. Here, the enhancement of I-phase forming abilitywith Ti concentration indicates an easy long rangeextension of icosahedral symmetry. The formation ofthe nearly pure I-phase in the extreme composition(Al-free) provides evidence for this conjecture. A moredetailed examination of the local atomic structure ofthe Ti-bearing samples is needed to clarify the Ti alloy-ing mechanism.

In the light of increasing experimental evidence thaticosahedral symmetry prevails in some specific metallicliquids that can be cast into both metallic glasses andicosahedral quasicrystals, a series of (Zr9Ni4)75(Al1�x-Tix)25 alloys were synthesized by alloying the assumedicosahedral atomic cluster composition Zr9Ni4 with Aland Ti. By melt quenching, these alloys were made intoa fully glassy state. The copper mold casting of thesemetallic liquids into 3 mm diameter alloy rods resultedin a divergence in the final phase selection. A BMG for-mation was observed at the low Ti concentrations. TheI-phase forming ability was enhanced by increasing theTi concentration, and a nearly monolithic I-phase wasobtained at the Al-free composition. The forming abili-ties of BMGs and I-phase a showed strong composi-tional dependence in this apparent alloying manner.The alloying effect was ascribed to the different correla-tions of Al and Ti with the matrix elements Zr and Ni.The preferential bonding of Ti was suggested to favorthe long range arrangements of icosahedral atomic clus-ters for I-phase formation.

This work was financially supported by a Grant-in-Aid for Young Scientists (B) and the Research and Devel-opment Project on Advanced Metallic Glasses, InorganicMaterials and Joining Technology from the Ministry ofEducation, Science, Sports, and Culture of Japan.

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