Cu−Zr−Ti ternary bulk metallic glasses correlated with(L → Cu8Zr3 + Cu10Zr7) univariant eutectic reaction

Chun-Li Dai, Jing-Wei Deng, Ze-Xiu Zhang, and Jian Xua)

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academyof Sciences, Shenyang 110016, People’s Republic of China

(Received 12 July 2007; accepted 12 December 2007)

Starting from Cu60Zr30Ti10, the compositional dependence of bulk metallic glass(BMG) formation was revisited in the Cu−Zr−Ti ternary system. It was revealed thatthe optimal BMG-forming composition is located at Cu60Zr33Ti7, for which amonolithic BMG rod 4 mm in diameter can be fabricated using copper mold casting.This composition is along, although slightly off, the univariant eutectic groove for thereaction (L → Cu8Zr3 + Cu10Zr7). With respect to the corresponding Cu−Zr binaryalloys, Ti has a significant effect on further stabilizing the liquid, thus increasing theglass-forming ability. For the Cu60Zr40−yTiy (3 � y � 10) series BMGs, the glasstransition temperature Tg decreased with increasing Ti content, at a rate of about 2.8K/at.%. Among these BMGs, significant compositional dependence of compressiveplasticity is not observed, irrespective of the Tg change. Cu60Zr33Ti7 glass exhibitsmaximum fracture strength around 2160 MPa.

I. INTRODUCTION

Among the family of bulk metallic glasses (BMGs),Cu-based alloys are particularly interesting for practicalapplication as new structural materials due to their sig-nificant advantages, including low-cost, high fracturestrength around 2 GPa often coexisting with visible duc-tility, and feasibility for forming BMG-based compos-ites.1–11 So far, Cu-based BMGs with a critical size (Dc)on the centimeter scale have been discovered in severalalloy systems, such as the quaternary Cu46Zr42Al7Y5,7

Cu43Zr43Al7Be7,8 Cu44.25Ag14.75Zr36Ti5,9 andCu46.25Zr44.25Al7.5Er2,10 and even ternary Cu49Hf42Al9.11

In fact, the majority of the currently developed Cu-based BMGs are derived from binary Cu−Zr12–14 orCu−Hf14,15 as the base system, such as Cu−Zr−Ti,1,2

Cu−Zr−Al,4,7,16 Cu−Zr−Ag,17 Cu−Hf−Ti,2 andCu−Hf−Al.11,18 In the Cu−Zr binary system, it has beenfound that BMG with a Dc of 2 mm can be formed atCu64Zr36, which is located near a deep eutectic, L →Cu8Zr3 + Cu10Zr7.12,13 Also, it was noticed that theglass-forming ability (GFA) exhibits a strong compo-sition dependence, sensitive to changes as small as0.5 at.%.12 Furthermore, in the Cu−Zr−Ti ternary, Inoueand his collaborators discovered a BMG-forming com-

position at Cu60Zr30Ti10,2 even though residual crystal-lites remained in the 4-mm-diameter as-cast rod.19

However, their work did not explain whether the BMGformation is related to a particular type of eutectic reac-tion. The phase diagram of Cu−Zr−Ti ternary systemcurrently available was contributed by Woychik andMassalski.20 Unfortunately, the eutectic reaction aroundthe Cu60Zr30Ti10 composition remains ambiguous due tocomplexity. Thus, in terms of metallurgical insight, theorigin of BMG formation remains unclear.

On the other hand, it has become highly interestingrecently that the ductility and elastic constants of BMGshave been found to be composition-dependent in a givenalloy system.21–24 Duan et al.24 showed that the glasstransition temperature Tg, shear modulus G, and Pois-son’s ratio � for the Cu−Zr−Be BMGs are very sensitiveto change in compositions. For this system, as the (Zr +Ti) content increase, Tg and G decrease, while � exhibitsthe opposite trend. The BMG with lower G and higher �is expected to be more ductile than other compositionssince the shear flow barrier for an unstressed shear co-operative zone is relatively small.25

The purpose of this paper is threefold. First, the com-positional dependence of BMG formation was revisitedin the vicinity of the previous Cu60Zr30Ti10 composition(denoted as Inoue’s alloy) to locate the optimal BMG-forming composition in the Cu−Zr−Ti ternary system.Second, the BMG-forming composition zone is corre-lated with the eutectic reaction the alloy melt undergoesduring solidification by determining the phase selection

a)Address all correspondence to this author.e-mail: jianxu@imr.ac.cn.

DOI: 10.1557/JMR.2008.0178

J. Mater. Res., Vol. 23, No. 5, May 2008 © 2008 Materials Research Society 1249

in the sample solidified at a cooling rate slightly lowerthan the critical cooling rate for BMG formation. Third,the effect of Ti on the Tg and compressive properties forthe Cu60Zr40–yTiy (3 � y �10) series BMGs was studiedto examine the compositional dependence in this system.

II. EXPERIMENTAL

Elemental pieces with purity better than 99.9% wereused as starting materials. The master alloy ingots of 20 g inweight with the nominal composition (in atomic percent-age) were prepared by arc melting under a Ti-gettered argonatmosphere in a water-cooled copper crucible. The alloyingots were melted several times to ensure compositionalhomogeneity. The weight change of the master alloysbefore and after arc-melting is less than 0.1 wt%. For thebulk samples with a diameter between 2 and 4 mm, themaster alloy was remelted in a quartz tube using induc-tion melting and injected in a purified inert atmosphereinto the copper mold that has internal rod-shaped cavitiesof about 50 mm in length. Bulk samples of 1 mm indiameter are fabricated using suction casting in a mini-arc melter for compression tests.

The arc-melted ingots and as-cast rods were sectionedtransversely and polished for scanning electron micros-copy (SEM) observation and x-ray diffraction (XRD)analysis. SEM observation of the samples was carried outin a LEO Supra 35 scanning electron microscope (Heiden-heim, Germany). The local compositions were semi-quantitatively determined using an electron probe micro-analyzer (EPMA; EPMA-1610, Shimadzu, Kyoto, Ja-pan). XRD analysis was performed using a RigakuD/max 2400 diffractometer (Tokyo, Japan) with mono-chromated Cu K� radiation. Samples for conventionaltransmission electron microscopy (TEM) observations ina JEM-2010 (JEOL, Tokyo, Japan) were prepared bytwin-jet electropolishing with a solution of 9 vol% nitricacid in a mixture of methanol and butoxyethanol (2:1).

The glass transition and crystallization behavior of theas-cast glassy rods were investigated in a differentialscanning calorimeter (DSC-diamond, Perkin-Elmer,Shelton, CT) with alumina container under flowing pu-rified argon at a heating rate of 0.67 Ks−1. A second rununder identical conditions was used to determine thebaseline after each run. To confirm the reproducibility ofthe experimental results, at least three samples weremeasured for each composition. All the measurements ofthe Tg and onset temperature of crystallization events(Tx1) were reproducible within the error of ±1 K. Theheat of crystallization �Hx for the glassy phase was de-termined by integrating the area under the DSC curve.The melting behavior of the alloys was measured in aNetzsch 404 DSC (NETZSCH-Gerätebau GmbH, Bay-ern, Germany) with alumina container, using the heatingand cooling rates of 0.33 Ks−1.

Compression test samples 2 mm in height were cutfrom the as-cast rods 1 mm in diameter. The loadingsurfaces were polished to be parallel to an accuracy ofless than 10 �m. Room-temperature compression tests(Instron 8871, Canton, MA) were carried out using a strainrate of 1 × 10−4 s−1. At least eight samples were meas-ured to ensure reproducible results. The strain was de-termined from the platen displacement after correctionfor machine compliance.

III. RESULTS

A. Composition dependence of BMG formationaround Cu60Zr30Ti10

Figure 1 displays the BMG-forming composition mapin the vicinity of the previous Inoue’s alloy. The selectedtriangular region involves only four compounds,Cu51Zr14, Cu8Zr3, Cu10Zr7, and Cu2ZrTi. The assumedtie-lines between the compounds are drawn as the solidor dashed lines. Dc of each composition was presentedwith different symbols. It was found that the best BMGformer is located at Cu60Zr33Ti7, for which fully glassyrod of 4 mm in diameter can be fabricated. Its GFA isbetter than the previous Inoue’s alloy,2 even though therewas only a 3 at.% change in Zr or Ti content.

Figure 2(a) displays the XRD patterns taken from thecross-sectional surfaces of as-cast rods (with diametersequal to Dc of the respective composition) for three rep-resentative BMGs, Cu60Zr37Ti3, Cu60Zr33Ti7, andCu60Zr30Ti10. The typical diffusive maxima between 2� �30° and 50° reflecting the amorphous nature, without de-tectable crystalline peaks within the XRD resolution, provethe complete glass formation at their respective sizes. The

FIG. 1. Composition map of BMG formation for as-cast rods aroundthe Cu60Zr30Ti10 composition. Dc of each composition was presentedwith different symbols. The assumed tie-lines between the compoundsare drawn as the dashed lines.

C-L. Dai et al.: Cu−Zr−Ti ternary bulk metallic glasses correlated with (L → Cu8Zr3 + Cu10Zr7) univariant eutectic reaction

J. Mater. Res., Vol. 23, No. 5, May 20081250

corresponding DSC traces for these glassy rods are givenin Fig. 2(b). In all cases, a pronounced endothermic sig-nal resulting from glass transition and at least two-stepcrystallization events are observed. The thermal proper-ties measured from DSC curves for the three alloys aresummarized in Table I, including the Tg, Tx1, width of the

supercooled liquid region �Tx (�Tx� Tx1− Tg), and�Hx. The �Tx of the best glass former Cu60Zr33Ti7 isabout 47 K. It is not the largest one among the investi-gated glasses. Figure 3 shows Tg and Tx1 as a function ofTi content for the Cu60Zr40−yTiy (3 � y � 10) seriesBMGs. Both temperatures decreased with increasing Tisubstitution for Zr. The rate of decrease in Tg is about2.8 K/at.% as the Ti content is increased.

Figures 4(a) and 4(b) display the DSC curves for theCu93−xZrxTi7 (31 � x � 36) and the Cu60Zr40−yTiy (3 �y � 10) series alloys, respectively, near and above theirmelting temperature during heating. The Tm and TL aremarked with arrows on each curve in the figures. The twoseries of alloys investigated, for which the Ti and Cucontent is fixed at 7 and 60 at.%, respectively, aremarked using dashed lines (lines I and II) in Fig. 1. Usingthese data, the Tm and TL values as a function of Zr andTi content are plotted in Figs. 5(a) and 5(b), respectively.Although the TL values obtained from such continuousheating are usually about 10−30 K higher than real TL inthe equilibrium state, current data of the TL can be treatedas an approximation to reflect a same trend of the liqui-dus variation. As shown in Fig. 5(a), the Tm of thesealloys remains nearly constant at 1140 K within experi-mental error, when the Ti content is fixed at 7 at.%. Withrespect to the binary invariant eutectic temperature of(L → Cu8Zr3 + Cu10Zr7) (denoted as e2 hereafter,Teut�1158 K, see phase diagram26), the Tm valuedropped by about 18 K. Nevertheless, it is of interest tonote that the TL variation shows a U-shaped dependenceon the Cu/Zr fraction. The lowest TL temperature (1176 K)is located at Cu59Zr34Ti7 [see Fig. 5(a)], which is believedto be located at the univariant eutectic groove, as markedusing an arrow in Fig. 5(a). The optimal BMG-formingcomposition is slightly off the eutectic groove and skewsFIG. 2. (a) XRD patterns taken from the cross-sectional surface and

(b) DSC scans (at a heating rate of 0.67 Ks−1) of the as-cast rods atrespective Dc for the Cu60Zr37Ti3, Cu60Zr33Ti7, and Cu60Zr30Ti10.

TABLE I. Thermal properties of Cu60Zr40−yTiy (y � 3, 5, 7, 8, 10)BMGs fabricated using copper mold casting, determined from DSCmeasurements.

Alloys (at.%)Tg

(K)Tx1

(K)�Tx

(K) Trg

�Hx

(kJ/mol)Dc

(mm)

Cu60Zr37Ti3 728 777 49 0.59 5.1 2Cu60Zr35Ti5 725 773 47 0.60 4.7 2Cu60Zr33Ti7 722 768 46 0.60 5.0 4Cu60Zr32Ti8 719 760 41 0.61 4.6 3Cu60Zr30Ti10 708 747 39 0.61 4.3 3

FIG. 3. Glass transition temperature Tg and onset temperature of crys-tallization Tx1 as a function of Ti content for the Cu60Zr40−yTiy (3 �y � 10) series alloys.

C-L. Dai et al.: Cu−Zr−Ti ternary bulk metallic glasses correlated with (L → Cu8Zr3 + Cu10Zr7) univariant eutectic reaction

J. Mater. Res., Vol. 23, No. 5, May 2008 1251

to the side with the higher TL (terminal Cu8Zr3 phase)[see the dashed line in Fig. 5(a)]. In contrast, a pro-nounced depression of both Tm and TL occurs as the Ticontent is increased, when the Cu content is fixed at60 at.%, as seen in Fig. 5(b). Exceeding 9 at.% for the Ticontent, the Tm no longer changes much, remaining atabout 1110 K. This means that the composition probablyhas reached the boundary of a ternary invariant eutectic(L → Cu8Zr3 + Cu10Zr7 + Cu2ZrTi), which will beproven later. Moreover, it was observed that the ternaryinvariant eutectic temperature (∼1110 K) is about 48 Klower than the binary invariant eutectic temperature for(L → Cu8Zr3 + Cu10Zr7); see Cu−Zr binary phase dia-gram.26 This is an indication that introducing Ti signifi-cantly stabilizes the liquid relative to the Ti-free binaryCu−Zr alloys. Additionally, it was noticed that the opti-mal BMG-forming composition mainly undergoes the

univarant eutectic reaction, rather than involving the ter-nary invariant eutectic; see the dash line in Fig. 5(b).

Additionally, making use of the data of TL, the reducedglass transition temperature Trg (Trg� Tg/TL) of the in-vestigated Cu−Zr−Ti ternary BMGs are calculated andlisted in Table I as well. The Trg value of these BMGs isaround 0.60. This difference in the Trg value between theBMGs is not significant, even though their GFA is dif-ferent.

B. Microstructure of arc-meltedBMG-forming alloys

The cooling rate for the arc-melted BMG-forming al-loy ingot was estimated to be on the order of 1−10 Ks−1.Using the Eq. (2) and parameters for the Cu60Zr30Ti10

alloy provided in Ref. 27, the cooling rate of arc-meltedCu60Zr33Ti7 ingot was approximately estimated to be ina range of 2−15 Ks−1. Upon this cooling rate, the solidi-fication microstructure in the center of the alloy ingot can

FIG. 4. DSC scans near and above melting temperatures during heat-ing (at a heating rate of 0.33 Ks−1) for the (a) Cu93−xZrxTi7 (31 � x �36) and (b) Cu60Zr40−yTiy (3 � y � 10) series.

FIG. 5. Liquidus TL and solidus Tm from DSC measurements for the25 relevant alloys as a function of Zr/Ti content for the two series ofalloys: (a) Cu93−xZrxTi7 (31 � x � 36) and (b) Cu60Zr40−yTiy (3 �y � 10), respectively.

C-L. Dai et al.: Cu−Zr−Ti ternary bulk metallic glasses correlated with (L → Cu8Zr3 + Cu10Zr7) univariant eutectic reaction

J. Mater. Res., Vol. 23, No. 5, May 20081252

be directly correlated with the phase selection when thealloy melt crystallizes before freezing into a glass, i.e., itsnon-equilibrium solidification pathway prior to glass for-mation. Therefore, crystalline phases involved in the eu-tectic reaction that the liquid undergoes during solidifi-cation can be revealed.

Figure 6 illustrates the XRD patterns of the arc-meltedCu60Zr33Ti7 and Cu60Zr30Ti10 alloys. For theCu60Zr33Ti7 ternary, crystalline phases Cu8Zr3 (ortho-rhombic) and Cu10Zr7 (orthorhombic) can be identified.The Cu60Zr30Ti10 alloy exhibits a similar pattern, but atrace amount of ternary compound Cu2ZrTi (MgZn2-type) is detectable.

Figures 7(a)–7(c) show the backscattered SEM imagesof the arc-melted ternary Cu60Zr33Ti7 and Cu60Zr30Ti10

alloys, together with the binary eutectic Cu61.8Zr38.2 (e2)without Ti for comparison. The lateral surface of thesample was polished for SEM observation, under whichit was shown that the microstructure is uniform through-out the sample from the top to the bottom. Two phaseswith different contrasts can be seen in Fig. 7(a) for theternary Cu60Zr33Ti7, marked A and B, respectively. Thechemical composition for each area, determined withEPMA analysis, was listed in Table II. According to theEPMA analysis, the A and B phases can be verified asthe Cu10Zr7 and Cu8Zr3 compounds, respectively, eachhaving a supersaturated solubility of Ti. In other words,the Ti dissolved in the Cu10Zr7 and Cu8Zr3 is about 6 and3 at.%, respectively. The volume fraction of Cu10Zr7

phase is significantly higher than that of Cu8Zr3 phase.For the binary eutectic Cu61.8Zr38.2, rod-shaped Cu8Zr3

(darker areas) dispersed in the Cu10Zr7 matrix, as shownin Fig. 7(b) (marked as B and A, respectively). Theseresults indicate that the phase selection and morphologyof Cu60Zr33Ti7 is quite similar to the binary eutectic alloy

Cu61.8Zr38.2 when the alloys solidified at a cooling rateslightly lower than that of glass formation. The ternarycompound Cu2ZrTi was not observed in this alloy. Incontrast, three phases were observed for the ternary

FIG. 6. XRD patterns taken from the cross-sectional surface of arc-melted Cu60Zr33Ti7 and Cu60Zr30Ti10 ingots.

FIG. 7. Backscattered SEM images taken from the cross-sectionalsurface of arc-melted ingots of the alloys: (a) Cu60Zr33Ti7, (b) binaryeutectic Cu61.8Zr38.2 (e2), and (c) Cu60Zr30Ti10.

C-L. Dai et al.: Cu−Zr−Ti ternary bulk metallic glasses correlated with (L → Cu8Zr3 + Cu10Zr7) univariant eutectic reaction

J. Mater. Res., Vol. 23, No. 5, May 2008 1253

Cu60Zr30Ti10, as seen in Fig. 7(c). The chemical compo-sition given in Table II allows the A, B, and C phases tobe identified as binary Cu10Zr7 and Cu8Zr3, and ternaryCu2ZrTi, respectively. As a result, it is probable that theternary invariant eutectic reaction of (L → Cu8Zr3 +Cu10Zr7 + Cu2ZrTi) was involved during solidificationfor this alloy. This is evidently different from the case ofCu60Zr33Ti7.

Furthermore, TEM observation for the arc-meltedCu60Zr33Ti7 reveals only the coexistence of Cu8Zr3 andCu10Zr7 compounds in the alloy. No other phases arepresent. The TEM bright-field image of this alloy in Fig.8(a) shows the two phases with different contrasts. Theselected-area electron diffraction (SAED) pattern takenalong the [011] zone axis for the lighter region, as shownin Fig. 8(b), proves that it is the Cu8Zr3 phase with anorthorhombic structure, of which the lattice parametersare a � 0.787 nm, b � 0.815 nm, and c � 0.998 nm.Figure 8(c) shows the SAED pattern for the darker re-gion. The diffraction spots are identified as the [011]diffraction patterns of orthorhombic Cu10Zr7 with thelattice parameters of a � 1.263 nm, b � 0.932 nm, andc � 0.933 nm. To determine whether there is any ori-entation relationship between two crystalline phases, fur-ther systematic examination remains necessary.

C. Compressive properties of CuZrTi BMGs withdifferent Ti content

Figure 9 shows the uniaxial compressive stress–straincurves for three monolithic glasses with different Ti con-tent, Cu60Zr40−yTiy (y � 3, 7, 10). As seen in Fig. 9, themonolithic glasses mainly exhibit elastic deformation be-fore facture, without much macroscopic yielding andplastic strain (�p). The fracture strengths �f of theCu60Zr37Ti3, Cu60Zr33Ti7 and Cu60Zr30Ti10 are about2090, 2160, and 2050 MPa, respectively.

Figure 10(a) shows the appearance of the fracturedsamples after compression tests for the Cu60Zr33Ti7 glassunder SEM observation. The angle between the fracturesurface and compressive loading axis is measured to be43°, indicating that the deformation and fracture of theglass are controlled by localized shear deformation. Fig-ure 10(b) displays the SEM images of fracture surfaces of

TABLE II. Chemical composition determined using EPMA for dif-ferent phases in the arc-melted Cu60Zr33Ti7 and Cu60Zr30Ti10 alloys.

Alloy AreaCu

(at.%)Zr

(at.%)Ti

(at.%)Normalized

phase

Cu60Zr33Ti7 A 57 38 5 Cu10Zr7

B 72 26 2 Cu8Zr3

Cu60Zr30Ti10 A 58 33 9 Cu10Zr7

B 71 26 3 Cu8Zr3

C 51 24 25 Cu2ZrTi

FIG. 8. (a) Bright-field TEM image and SAED patterns of theCu60Zr33Ti7 arc-melted ingot. (b) and (c) correspond to the Cu8Zr3

phase (bright region) and Cu10Zr7 phase (dark region) in (a), respec-tively.

C-L. Dai et al.: Cu−Zr−Ti ternary bulk metallic glasses correlated with (L → Cu8Zr3 + Cu10Zr7) univariant eutectic reaction

J. Mater. Res., Vol. 23, No. 5, May 20081254

this glass, showing vein-like patterns typical of the me-tallic glasses owing to locally reduced viscosity duringshear failure.

IV. DISCUSSION

Our findings indicate that the Cu60Zr33Ti7 alloy is lo-cated at the Cu8Zr3 side of the univariant eutectic grooveof (L → Cu8Zr3 + Cu10Zr7) in the phase diagram. (Ti hassolubility in the two terminal phases, Cu8Zr3 andCu10Zr7). The solidification pathway of its melt is ex-pected to undergo the following sequence: primary crys-tallization to precipitate the Cu8Zr3 phase, then followingthe univariant eutectic reaction. In other words, it meansthat Cu8Zr3 is the competing crystalline phase with glassformation. Starting from the eutectic point (e2) in Cu−Zrbinary subsystem, the univariant reaction tempera-ture was depressed with increasing Ti, along the univari-ant eutectic groove. This indicates that Ti involvementplays a role in stabilizing the alloy liquid, resulting in adeeper eutectic reaction in contrast to the simple Cu−Zrbinary, as seen in Fig. 5(b). This is expected to promotethe glass formation. The Dc is increased from 2 mm atCu64.5Zr35.5

12 to 4 mm at Cu60Zr33Ti7. Both composi-tions are off-eutectic (binary eutectic point or eutecticgroove), and skew to the Cu8Zr3 side with the higher TL,see Fig. 5(b). Such behavior can be understood based onthe skewed eutectic coupled zone idea due to the com-petitive growth of the participating phases.28,29 On fur-ther increasing the Ti content up to ∼9 at.%, the ternaryinvariant eutectic (L → Cu8Zr3 + Cu10Zr7 + Cu2ZrTi)will be involved; see Figs. 5(b) and 7(c). It is plausiblethat primary crystallization of Cu2ZrTi is favored, de-grading the GFA. Consequently, the GFA ofCu60Zr30Ti10 (Inoue’s previous alloy; Dc is only 3 mmunder our processing condition) is weaker than that of

Cu60Zr33Ti7. Figure 11 displays a schematic representa-tion of the BMG formation along, but slightly off, theunivariant eutectic groove.

As proposed by Boettinger,28 microstructures may inmany cases be the result of the competition of variousforms of crystal growth, and the formation of metallicglass can be promoted due to the limitations on eutecticgrowth kinetically controlled by diffusion. Such a con-cept can be used here to understand the glass formationnot only for the simple binary deep eutectics, but also for

FIG. 9. Compressive engineering stress–strain curves for the as-cast1-mm-diameter rods of the Cu60Zr37Ti3, Cu60Zr33Ti7, andCu60Zr30Ti10 bulk glass.

FIG. 10. (a) Side views of fractured samples under compression forthe Cu60Zr33Ti7 bulk glass and (b) its SEM images of the fracturedsurface.

C-L. Dai et al.: Cu−Zr−Ti ternary bulk metallic glasses correlated with (L → Cu8Zr3 + Cu10Zr7) univariant eutectic reaction

J. Mater. Res., Vol. 23, No. 5, May 2008 1255

the univariant eutectic in ternary systems. In addition, theobservation that the ternary alloys correlated with uni-variant eutectic reaction exhibit a higher GFA may alsoexplain why the GFA of binary alloys such as Cu−Zr canbe enhanced by microalloying.30

As presented above, significant compositional depend-ence of compressive plasticity is not observed for theCu60Zr40−yTiy (3 � y � 10) series BMGs, even thoughthe Tg decreased with increasing Ti content at a rate ofabout 2.8 K/at.%. It is different from the case ofCu−Zr−Be BMGs, where the Tg dropped about 3.8 K/at.%with increasing (Zr + Ti) content.24 The failure behavior ofthe BMGs with different Ti content is quite similar, alloperated and controlled by localized shear bands. Thefracture strength is comparable to those of other ter-nary Cu-based BMGs such as Cu−Zr−Al4,8,16 andCu−Zr−Ag17 but significantly lower than those ofCu−Hf−Ti2 and Cu−Hf−Al11 BMGs.

V. SUMMARY

Around the Cu60Zr30Ti10 composition discovered pre-viously by Inoue and collaborators, a search located thebest glass-forming composition at Cu60Zr33Ti7, forwhich a monolithic BMG rod 4 mm in diameter can befabricated using copper mold casting. This compositionis near the univariant eutectic groove of (L → Cu8Zr3 +Cu10Zr7) and skews to the side with the higher TL

(Cu8Zr3 phase terminal). Compared with the Cu−Zr bi-nary BMG, which is correlated with the eutectic reactionof (L → Cu8Zr3 + Cu10Zr7), the improvement of theglass-forming ability is caused by the fact that Ti signifi-cantly stabilizes the liquid. Our finding that the glass

formation was favored in the vicinity of the univarianteutectic was attributed to the constraints on the coupledgrowth of two-phases during solidification of the liquid.For the Cu60Zr40−yTiy (3 � y � 10) series bulk glasses,significant compositional dependence of compressiveplasticity is not observed, even though the glass transi-tion temperature Tg decreased much at about 2.8 K/at.%Ti. The compressive fracture strength of Cu60Zr37Ti3,Cu60Zr33Ti7, and Cu60Zr30Ti10 bulk metallic glass isaround 2090, 2160, and 2050 MPa, respectively.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the stimulatingdiscussion with Professor E. Ma and Professor Y. Li.This work was supported by the National Natural Sci-ence Foundation of China under Grant Nos. 50671105and 50621091.

REFERENCES

1. X.H. Lin and W.L. Johnson: Formation of Ti–Zr–Cu–Ni bulkmetallic glasses. J. Appl. Phys. 78, 6514 (1995).

2. A. Inoue, W. Zhang, T. Zhang, and K. Kurosaka: High-strengthCu-based bulk glassy alloys in Cu–Zr–Ti and Cu–Hf–Ti ternarysystems. Acta Mater. 49, 2645 (2001).

3. J.C. Lee, Y.C. Kim, J.P. Ahn, S. Lee, and B.J. Lee: Strain hard-ening of an amorphous matrix composite due to deformation in-duced nanocrystallization during quasistatic compression. Appl.Phys. Lett. 84, 2781 (2004).

4. J. Das, M.B. Tang, K.B. Kim, R. Theissmann, F. Baier, W.H. Wang,and J. Eckert: “Work-hardenable” ductile bulk metallic glass.Phys. Rev. Lett. 94, 205501 (2005).

5. P. Wesseling, T.G. Nieh, W.H. Wang, and J.J. Lewandowski:Preliminary assessment of flow, notch toughness, and high tem-perature behavior of Cu60Zr20Hf10Ti10 bulk metallic glass. ScriptaMater. 51, 151 (2004).

6. A. Inoue, W. Zhang, T. Tsurui, A.R. Yavari, and A.L. Greer:Unusual room-temperature compressive plasticity in nanocrystal-toughened bulk copper-zirconium glass. Philos. Mag. Lett. 85,221 (2005).

7. D.H. Xu, G. Duan, and W.L. Johnson: Unusual glass-formingability of bulk amorphous alloys based on ordinary metal copper.Phys. Rev. Lett. 92, 245504 (2004).

8. S.W. Lee, M.Y. Huh, E. Fleury, and J.C. Lee: Crystallization-induced plasticity of Cu–Zr containing bulk amorphous alloys.Acta Mater. 54, 349 (2006).

9. C.L. Dai, H. Guo, Y. Shen, Y. Li, E. Ma, and J. Xu: A newcentimeter-diameter Cu-based bulk metallic glass. Scripta Mater.54, 1403 (2006).

10. H. Men, J.Y. Fu, S.J. Pang, C.L. Ma, and T. Zhang: Formation andthermal stability of Cu46.25Zr44.25Al7.5Er2 bulk metallic glass witha diameter of 12 mm. Mater. Trans. 47, 2882 (2006).

11. P. Jia, H. Guo, Y. Li, J. Xu, and E. Ma: A new Cu–Hf–Al ternarybulk metallic glass with high glass forming ability and ductility.Scripta Mater. 54, 2165 (2006).

12. D. Wang, Y. Li, B.B. Sun, M.L. Sui, K. Lu, and E. Ma: Bulkmetallic glass formation in the binary Cu–Zr system. Appl. Phys.Lett. 84, 4029 (2004).

13. D.H. Xu, B. Lohwongwatana, G. Duan, W.L. Johnson, andC. Garland: Bulk metallic glass formation in binary Cu-rich alloy

FIG. 11. Schematic representation of the BMG-forming compositionzone (Dc > 1 mm) along the univariant eutectic groove, shown as alight green zone. The optimal composition Cu60Zr33Ti7 is presented asa star.

C-L. Dai et al.: Cu−Zr−Ti ternary bulk metallic glasses correlated with (L → Cu8Zr3 + Cu10Zr7) univariant eutectic reaction

J. Mater. Res., Vol. 23, No. 5, May 20081256

series-Cu100−xZrx (x � 34, 36 38.2, 40 at.%) and mechanicalproperties of bulk Cu64Zr36 glass. Acta Mater. 52, 2621 (2004).

14. A. Inoue and W. Zhang: Formation, thermal stability and me-chanical properties of Cu–Zr and Cu–Hf binary glassy alloy rods.Mater. Trans. 45, 584 (2004).

15. G. Duan, D.H. Xu, and W.L. Johnson: High copper content bulkglass formation in bimetallic Cu–Hf system. Metall. Mater. Trans.A 36, 455 (2005).

16. A. Inoue and W. Zhang: Formation, thermal stability and me-chanical properties of Cu–Zr–Al bulk glassy alloys. Mater. Trans.43, 2921 (2002).

17. W. Zhang and A. Inoue: High glass-forming ability and goodmechanical properties of new bulk glassy alloys in Cu–Zr–Agternary system. J. Mater. Res. 21, 234 (2006).

18. A. Inoue and W. Zhang: Formation and mechanical properties ofCu–Hf–Al bulk glassy alloys with a large supercooled liquid re-gion of over 90 K. J. Mater. Res. 18, 1435 (2003).

19. J.Z. Jiang, J. Saida, H. Kato, T. Ohsuna, and A. Inoue: IsCu60Ti10Zr30 a bulk glass-forming alloy? Appl. Phys. Lett. 82,4041 (2003).

20. C.G. Woychik and T.B. Massalski: Phase diagram relationships inthe system Cu–Zr–Ti. Z. Metallkd. 79, 149 (1988).

21. R.D. Conner and W.L. Johnson: Composition dependent ductilityin the amorphous Zr–Ti–Ni–Cu–Be alloy system. Scripta Mater.55, 645 (2006).

22. Y.H. Liu, G. Wang, R.J. Wang, D.Q. Zhao, M.X. Pan, andW.H. Wang: Super plastic bulk metallic glasses at room tempera-ture. Science 315, 1385 (2007).

23. G. Kumar, T. Ohkubo, T. Mukai, and K. Hono: Plasticity andmicrostructure of Zr–Cu–Al bulk metallic glasses. Scripta Mater.57, 173 (2007).

24. G. Duan, M.L. Lind, K. De Blauwe, A. Wiest, and W.L. Johnson:Thermal and elastic properties of Cu–Zr–Be bulk metallic glassforming alloys. Appl. Phys. Lett. 90, 211901 (2007).

25. W.L. Johnson and K. Samwer: A universal criterion for plasticyielding of metallic glasses with a (T/Tg)2/3 temperature depend-ence. Phys. Rev. Lett. 95, 195501 (2005).

26. D. Arias and J.P. Abriata: Cu–Zr, in Binary Alloy Phase Dia-grams, edited by T.B. Massalski, H. Okamoto, P.R. Subramanian,and L. Kacprzak (ASM International, Materials Park, OH, 1990),p. 1511.

27. D.V. Louzguine-Luzgin, A.D. Setyawan, H. Kato, and A. Inoue:Influence of thermal conductivity on the glass-forming ability ofNi-based and Cu-based alloys. Appl. Phys. Lett. 88, 251902(2006).

28. W.J. Boettinger: Growth kinetic limitations during rapid solidifi-cation, in Rapidly Solidified Amorphous and Crystalline Alloys,edited by B.H. Kear, B.C. Giessen, and M. Cohen (Elsevier Sci-ence Publishing, North Holland, New York, 1982), p. 15.

29. Y. Li: Bulk metallic glasses: Eutectic coupled zone and amor-phous formation. JOM 57, 60 (2005).

30. J.H. Xia, J.B. Qiang, Y.M. Wang, Q. Wang, and C. Dong: Ternarybulk metallic glasses formed by minor alloying of Cu8Zr5 icosa-hedron. Appl. Phys. Lett. 88, 101907 (2006).

C-L. Dai et al.: Cu−Zr−Ti ternary bulk metallic glasses correlated with (L → Cu8Zr3 + Cu10Zr7) univariant eutectic reaction

J. Mater. Res., Vol. 23, No. 5, May 2008 1257