Materials Letters 64 (2010) 96–98

Contents lists available at ScienceDirect

Materials Letters

j ourna l homepage: www.e lsev ie r.com/ locate /mat le t

Ion beam mixing to study the metallic glass formation of the Cu–Zr system

T.L. Wang, W.T. Huang, W.C. Wang, B.X. Liu ⁎Advanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

⁎ Corresponding author. Tel.: +86 10 6277 2557; faxE-mail address: dmslbx@tsinghua.edu.cn (B.X. Liu).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.10.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 August 2009Accepted 8 October 2009Available online 15 October 2009

Keywords:Metals and alloysIon beam technologyIntermetallic alloys and compounds

An ion beam mixing experiment for Cu–Zr system was conducted and two supersaturated solid solutionswere observed with compositions of 16 atom% Zr in Cu and 17 atom% Cu in Zr, respectively, which are muchgreater than almost nil found from the equilibrium phase diagram of Cu–Zr system. The observationindicates that Cu–Zr metallic glasses could possibly be obtained in composition range bounded by the twoobserved solid solubilities, i.e. 16–83 atom% Zr. Besides, a unique Cu65Zr35 metallic glass was obtained by ionbeam mixing and its composition is very close to the so-called best composition referred in the literature.

: +86 10 6277 1160.

ll rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

During the past decades, metallic glasses have attracted consid-erable attentions because of their novel properties in many aspects,and some non-equilibriummaterials processing techniques have beendeveloped to produce metallic glasses [1]. Among these techniques,ion beam mixing of multiple metal layers has proved to be veryeffective in producing metallic alloys, and a large number of metallicglasses were obtained by this method [2].With the discovery ofmetallic glasses, a very basic issue has been raised, that is, in whichsystem and at what alloy composition metallic glass could beobtained. From a physical point of view, the very issue is to answerin which system and within what composition range, metallic glasscould be obtained, i.e., to predict the glass-forming range (GFR) of ametal system. Technically, the related issue is to predict at whatspecific alloy composition of a system, metallic glass could mostreadily be produced, i.e., to evaluate the glass-forming ability (GFA) ofan alloy. Up to now, several empirical criteria have been proposed topredict the GFR/GFA of ametal system [3–8]. Among these criteria, themost successful one is the structural difference rule, which was laterextended by defining a single parameter, namely, the maximumpossible amorphization range (MPAR) to predict the GFR of a system.The MPAR corresponds to the total width of the two-phase region(s)of a system, i.e., 100% minus the two maximum terminal solidsolubilities of the system [2,3]. Within MPAR, metallic glass formationis favored comparing to that of crystalline solid solution(s). Inevaluating the GFA of an alloy, Xia's proposal and Wang's modifica-tion, based on well documented Miedema's model and Alonso'smethod, work quite well for the transition metal systems [4,7–9].

In the present study, we focus our attention on the Cu–Zr system.The Cu–Zr binary metal system is one of the most extensively studied

systems, because of its extremely good GFA [10–12]. Besides, Cu–ZrBMGs have also been obtained by many research teams and havetriggered many others' interest [13–16]. However, the GFR/GFA of theCu–Zr system is still an open issue. In this respect, ion beammixing ofmultiple metal layers could serve as a powerful tool to reveal the GFR/GFA of the Cu–Zr system.

2. Experimental

Four sets of Cu–Zr multilayered films were prepared by depositingalternatively pure Cu (99.99%) and Zr (99.99%) at a rate of 0.2 Å/s ontoNaCl single crystals as substrates in an e-gun evaporation systemwitha vacuum level of 10−6 Pa. The prepared sampleswere then irradiatedby 200 keV xenon ions in an implanter with a vacuum level betterthan 5×10−4 Pa and the irradiation dose was in a range from 8×1014

to 9×1015 Xe+/cm2. During irradiation, the sample holder was cooledby liquid nitrogen (77 K) and the current density was confined to beabout 2 μA/cm2 to avoid an otherwise overheating effect. Forstructural characterization, all the Cu–Zr multilayered films wereexamined by selected area diffraction (SAD) of transmission electronmicroscopy and X-ray fluorescence examinations were employed toascertain the compositions of the films as well as the real composi-tions of the formed phases upon IBM.

3. Results and discussions

The compositions of the as-deposited films were confirmed to beCu90Zr10, Cu78Zr22, Cu33Zr67, and Cu15Zr85, respectively. SAD patternsshowed that all the as-deposited films have both fcc and hcp diffractioncharacteristics, i.e., all of them aremixtures of Cu and Zr. After the Cu–Zrmultilayered films were irradiated to various doses, several metastablephases were obtained. For the Cu90Zr10 sample, Fig. 1(a) shows the SADpattern for an as-deposited state and Fig. 1(b) shows the SAD patterntaken after irradiation to a dose of 2×1015 Xe+/cm2. Indexing the SAD

Fig. 1. SAD patterns of (a) the as-deposited films with composition of Cu90Zr10 and(b) after it was irradiated to a dose of 2×1015 Xe+/cm2 with a real composition ofCu84Zr16.

Fig. 2. (a) the bright field image and (b) the SAD pattern of the obtained glass phaseafter a sample was irradiated to a dose of 5×1015 Xe+/cm2 and with a real compositionof Cu65Zr35.

97T.L. Wang et al. / Materials Letters 64 (2010) 96–98

pattern of Fig. 1(b) shows that a unique fcc phase with a latticeparameter around 3.76 Å was obtained. The real composition of theirradiated sample was confirmed to be Cu84Zr16. The discrepancies ofthe compositions between the as-deposited and the irradiated ones canbe attributed to the sputteringduring ionbeammixing. According to theVegard's law, a substitutional solid solution of Cu84Zr16 would have alattice parameter around 3.75 Å,while the obtained fcc phase has a latticeparameter of 3.76 Å. The agreement between the two is quite well,indicating that it is a Cu-based solid solution. For the Cu15Zr85 sample,when it was irradiated to a dose of 8×1014 and 9×1015 Xe+/cm2, uniqueZr-based hcp structure solid solutions were obtained in both cases. Thereal compositions of the irradiated sampleswere confirmed tobeCu15Zr85and Cu17Zr83, respectively. Examining the equilibrium Cu–Zr phasediagram, the solubilities of Zr in Cu and Cu in Zr are both nearly zero atroom temperature [17], and the formed Cu-based solid solution and theZr-based solid solution are therefore supersaturated. In general, metallicglass cannot be formed within the alloy compositions close to the solidsolidus, because ion beam mixing could extend the solid solubilities andformmetastable crystalline phases. As a result, the real GFR of a system isusually a little smaller than MPAR observed from the equilibrium phasediagram [2]. For the Cu–Zr system, the two supersaturated solid solutionsobtained in the present work indicate that the solid solubilities of Zr in Cuand Cu in Zr in non-equilibrium process should not less than 16 atom% Zrand 17 atom% Cu, respectively. Therefore, metallic glasses could only beobtained in a composition range bounded by the two determinedsupersaturated solubilities, i.e., 16–83 atom% Zr.

In fact, for the Cu78Zr22 sample, a unique metallic glass phase wasindeed obtained when it was irradiated to a dose of 5×1015 Xe+/cm2.Fig. 2(a) and (b) show the bright field image and the correspondingSAD pattern of the obtainedmetallic glass phase, respectively. The realcomposition of the metallic glass phase was confirmed to be Cu65Zr35.Interestingly, it is very much close to a well-known composition ofCu64.5Zr35.5, which was reported to have the best GFA in the Cu–Zrsystem [7,12,13]. Meanwhile, the composition is also in the vicinity ofthat has the best GFA predicted by Xia's proposal and Wang'smodification [7,8]. For the Cu33Zr67 sample, metallic glasses were alsoobserved coexist with Zr-base hcp phase.

According to the atomic collision theory [2], the process in themultilayered films is thought to occur as follows: initially, a sequenceof ballistic collision is triggered in the Cu–Zr multilayered films by200 keV xenon ions and is responsible for inducing the interfacialdiffusion of Cu atoms into its partner Zr lattices and vice versa,

resulting in forming a highly energetic Cu–Zr mixture. When theatomic collision cascade is eventually terminated, the highly energeticstate has to relax towards equilibrium. However, because therelaxation period is extremely short, i.e. lasting for 10−10 s [2], onlyvery minor atomic rearrangement could take place. Consequently,either simple structured crystalline phase, such as fcc, bcc and hcp,could be grown or the disordered structure prefers to preserve, thusforming an amorphous phase. When the overall composition is lessthan the non-equilibrium solid solubility of the system, a solid solutioncould be obtained. While the overall composition of a multilayeredfilm exceeds the non-equilibrium solid solubility, it is easy to formamorphous phases.

To further gain an insight of the formation of metallic glasses in theCu–Zr system, it is expected starting with an n-body potential of asystem, molecular dynamics simulation is a more or less precisemethod to determine the GFA/GFR of a system. The method is basedon checking the stabilities of solid solutions [2], and the observation oftwo supersaturated solid solutions in the Cu–Zr system is helpful forconducting molecular dynamics simulations for the system. Therelated study is currently undertaken by the present authors' group.

4. Conclusions

In summary, by themethodof IBM, two supersaturated solid solutionswith compositions of 16 atom% Zr and 17 atom% Cu, respectively, wereobserved for the Cu–Zr system, suggesting that Cu–Zr metallic glassescould only be obtained in a composition range bounded by the twodetermined supersaturated solubilities, i.e., 16–83 atom% Zr. Besides, aunique metallic glass was obtained at an alloy composition of Cu65Zr35,which is very close the best metallic glass composition predicted byprevious studies.

Acknowledgements

The authors are grateful to the financial support from the NationalNatural Science Foundation of China (50531040, 50871058), theMinistry of Science and Technology of China (2006CB605201), andthe Administration of Tsinghua University.

References

[1] Greer AL. Science 1995;267:1947–53.[2] Liu BX, Lai WS, Zhang Q. Mater Sci Eng R 2000;29:1–48.

98 T.L. Wang et al. / Materials Letters 64 (2010) 96–98

[3] Liu BX, Johnson WL, Nicolet M-A, Lau SS. Appl Phys Lett 1983;42:45–50.[4] Alonso JA, Lopez JM. Mater Lett 1986;4:316–9.[5] Park ES, Kim DH. Appl Phys Lett 2005;86:201912–4.[6] Senkov ON, Miracle DB, Mullens JM. J Appl Phys 2005;97:103502–8.[7] Xia L, Fang SS, Wang Q, Dong YD, Liu CT. Appl Phys Lett 2006;88:171905–7.[8] Wang TL, Li JH, Liu BX. Phys Chem Chem Phys 2009;11:2371–3.[9] De Boer FR, Boom R, Mattens WC, Miedema AR, Niessen AK. Cohesion in Metals:

Transition Metal Alloys. Amsterdam: North-Holland; 1988.[10] Wang N, Li CR, Du ZM, Wang FM, Zhang WJ. Calphad 2006;30:461–9.

[11] Buschow KHJ. J Appl Phys 1981;52:3319–23.[12] Sha ZD,WuRQ, Lu YH, Shen L, YangM, Cai YQ, et al. J Appl Phys 2009;105:043521–4.[13] Wang D, Li Y, Sun BB, Sui ML, Lu K, Ma E. Appl Phys Lett 2004;84:204029–31.[14] Tang MB, Zhao DQ, Pan MX, Wang WH. Chin Phys Lett 2004;21:901–3.[15] Xu DH, Lohwongwatana B, Duan G, Johnson WL, Garland C. Acta Mater

2004;52:2621–4.[16] Inoue A, Zhang W. Mater Trans JIM 2004;45:584–7.[17] Massalski TB, Okamoto H, Subramanian PR, Kacprzak L. Binary Alloy Phase

Diagrams, Second edition. Ohio: ASM International; 1990.