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Scripta Materialia 54 (2006) 1123–1126

Formation and mechanical properties of (Ce–La–Pr–Nd)–Co–Albulk glassy alloys with superior glass-forming ability

Ran Li, Shujie Pang, Hua Men, Chaoli Ma, Tao Zhang *

Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, XueYuan Road No. 37,

HaiDian District, Beijing 100083, China

Received 11 October 2005; received in revised form 22 November 2005; accepted 24 November 2005Available online 28 December 2005

Abstract

Bulk (Ce–La–Pr–Nd)65Co25Al10 metallic glass with critical diameter of 15 mm and high mechanical strength was developed. The addi-tion of multi-lanthanide metals with diverse electron configurations and similar atomic sizes induced a sharply decreased liquidus tem-perature and remarkably improved glass-forming ability of (Ce–La–Pr–Nd)65Co25Al10 compared with single-lanthanide metal-basedalloys, e.g. La65Co25Al10 and Ce65Co25Al10.� 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Rare earth; Metallic glass; Ductility; Rapid solidification

1. Introduction

In 1989, the first lanthanide-based glassy rod with adiameter of 2.5 mm was fabricated by copper mold castingthrough adding Al to a simple binary La–Ni system [1].Following research indicated that the glass-forming ability(GFA) can be improved remarkably through multi-compo-nent addition to a recognized binary amorphous alloy sys-tem, e.g. multi-component additions to Ln–Al, Zr–Cu, (Zr,Ti)–Be, Fe–B or Ca–Mg system increased the critical thick-ness for glass formation from �50 lm to above 1 cm [2–7].In recent years, investigation of bulk metallic glasses(BMGs) based on lanthanide metals has become a fascinat-ing subject in the research field of metastable materials,because these glasses have low melting temperature [8],high glass-forming ability [2,9,10], good mechanical prop-erties [11], and hard magnetic properties [12,13], as wellas significant superplasticity and polymer-like thermoplas-ticity in the supercooled liquid region DTx (DTx = Tx � Tg,

1359-6462/$ - see front matter � 2005 Acta Materialia Inc. Published by Else

doi:10.1016/j.scriptamat.2005.11.074

* Corresponding author. Tel./fax: +86 10 82314869.E-mail address: zhangtao@buaa.edu.cn (T. Zhang).

where Tg is glass transition temperature and Tx is crystalli-zation temperature) [14,15]. Very recently, we developedCe-rich misch metal-based bulk metallic glasses with highglass-forming ability, in which the Ce-rich misch metal(Mm) consisted of 45.1 at.% Ce, 33.6 at.% La, 5.4 at.% Prand 15.9 at.% Nd by neglecting impurities [16]. Theseglassy alloys have potential for industrial applicationsbecause of their low melting temperature (�700 K), rela-tively low Tg (�430 K) and high glass-forming ability,besides an obvious advantage in price compared with Ce-based BMGs.

In this work, we adopted the mixture of pure lanthanidemetals (La, Ce, Pr and Nd) to substitute the Ce-rich mischmetal in the previous Mm65Co25Al10 alloy. The resulting(Ce–La–Pr–Nd)65Co25Al10 alloy exhibited high glass-form-ing ability evidenced by the formation of glassy rods withdiameters up to 15 mm. To investigate the effect of theaddition of multi-lanthanide metals on the glass-formingability, critical diameters and thermal parameters of the(Ce–La–Pr–Nd)65Co25Al10 BMG and the glassy alloysbased on single-lanthanide metal, Ln65Co25Al10 (Ln = La,Ce, Pr or Nd, respectively) were compared. Mechanicalproperties of these BMGs were also examined.

vier Ltd. All rights reserved.

Fig. 2. DSC curves of glassy (Ce–La–Pr–Nd)65Co25Al10 alloy ribbon androd with a diameter of 15 mm.

1124 R. Li et al. / Scripta Materialia 54 (2006) 1123–1126

2. Experimental procedure

Alloy ingots of (Ce–La–Pr–Nd)65Co25Al10, in which Ce–La–Pr–Nd is a mixture of 45.1 at.% Ce, 33.6 at.% La, 5.4at.% Pr and 15.9 at.% Nd, were prepared by arc-meltingthe mixture of pure Co (99.9 mass%), Al (99.99 mass%)and lanthanide elemental metals (above 99.5 mass%) in ahighly pure argon atmosphere. Ln65Co25Al10 (Ln = La,Ce, Pr or Nd, respectively) alloys based on single-lantha-nide metals were also produced under the same conditions.From the master alloys, ribbons were prepared by single-roller melt-spinning and cylindrical rod samples wereprepared by copper mold casting in an argon atmosphere.Structure of the samples was examined by X-ray diffraction(XRD) using a D/max2200PC Rigaku X-ray diffractome-ter with Cu-Ka radiation. Thermal stability of the sampleswas investigated by a Perkin–Elmer DSC-6 differentialscanning calorimeter (DSC) at a heating rate of 0.33 K/s.Melting behaviors of the alloys were characterized usinga NETZSCH DSC 404 C at a heating rate of 0.33 K/s.Microhardness was determined by a Vickers hardnessindenter with a load of 2 N. Compressive stress–straincurves were measured using as-cast glassy rods with thegauge dimension of 2 mm in diameter and 4 mm in lengthat a strain rate _e of 4.17 · 10�4 s�1 at room temperatureand the fracture surface was observed by scanning electronmicroscopy (SEM).

3. Results and discussion

Outer shape and surface appearance of the as-cast(Ce–La–Pr–Nd)65Co25Al10 rods of 12 mm and 15 mm indiameter are shown in Fig. 1, with the correspondingXRD patterns. The surfaces of the rods are smooth andneither concaveness nor cavity can be found. The firstand second diffuse diffraction bands in their XRD patternsare smooth, which indicates the absence of crystallinephase in the glassy rods at the sensitivity of XRD. Furtherexaminations to confirm the glassy structure of the as-cast

Fig. 1. Outer shape, surface appearance and XRD patterns of as-cast(Ce–La–Pr–Nd)65Co25Al10 rods with diameters of 12 mm and 15 mm.

rods were carried out by comparing Tg, Tx and the totalheat release of the main crystallization peaks (DHx)between the as-cast rods and ribbon. As shown in Fig. 2,no distinct difference in Tg, Tx and DHx between the rodof 15 mm in diameter and the ribbon was detected, furtherconfirming that the bulk (Ce–La–Pr–Nd)65Co25Al10 rodconsists of a fully glassy structure.

To investigate the effect of the addition of multi-lantha-nide metals on the glass-forming ability, glassy samples ofsingle lanthanide-based alloys, Ln65Co25Al10 (Ln = La, Ce,Pr or Nd, respectively), were fabricated. Fig. 3 shows theDSC curves of the (Ce–La–Pr–Nd)65Co25Al10 andLn65Co25Al10 ribbons. Ln65Co25Al10 rods were alsoprepared by copper mold casting to estimate their criticaldiameters for glass formation. The thermal parameters,such as Tg, DTx, Tg/Tm (Tm is melting temperature) andTg/Tl (Tl is liquidus temperature), and the critical diametersof the (Ce–La–Pr–Nd)65Co25Al10 and Ln65Co25Al10 glassyalloys are shown in Table 1. It is seen that the glass-form-ing ability of (Ce–La–Pr–Nd)65Co25Al10 (dc = 15 mm) ismuch higher than those of the Ln65Co25Al10 alloys(dc 6 4 mm). The Tg, Tx and Tm of the Ln65Co25Al10 alloysincrease in the following sequence: Ce–Co–Al, La–Co–Al,Pr–Co–Al and Nd–Co–Al. The Tg, Tx and Tm of (Ce–La–Pr–Nd)65Co25Al10, which are 431, 455 and 701 K,respectively, are similar to those of La65Co25Al10 andCe65Co25Al10. The melting behaviors indicate thatLa65Co25Al10 and Ce65Co25Al10 alloys are far from eutecticcomposition, whereas with the addition of multi-lanthanidemetals, the resulting (Ce–La–Pr–Nd)65Co25Al10 alloy is aeutectic or close to eutectic composition. This could beresponsible for the superior GFA of the (Ce–La–Pr–Nd)65Co25Al10 alloy, since high GFA can be achieved atdeep eutectics or near eutectic compositions in a givenalloy system [17–20]. However, this interpretation seemsinsufficient, because judging from the melting behaviorsthe Pr65Co25Al10 and Nd65Co25Al10 alloys are also of

Fig. 4. Compressive stress–strain curves of bulk glassy (Ce–La–Pr–Nd)65Co25Al10, Ce65Co25Al10 and La65Co25Al10 rods with a diameter of2 mm (Ce–La–Pr–Nd)65Co25Al10 is denoted by RE65Co25Al10 in thisfigure.

Fig. 3. DSC curves of (Ce–La–Pr–Nd)65Co25Al10 and Ln65Co25Al10(Ln = La, Ce, Pr or Nd, respectively) ribbons: (a) crystallization and(b) melting (Ce–La–Pr–Nd)65Co25Al10 is denoted by RE65Co25Al10 inthis figure.

R. Li et al. / Scripta Materialia 54 (2006) 1123–1126 1125

compositions close to eutectics, and their Tg/Tm andTg/Tl are only slightly lower than those of (Ce–La–Pr–Nd)65Co25Al10. On the other hand, it has been reportedthat optimized distribution of the atomic sizes in multi-component alloy systems can increase the degree of topo-logical disorder so as to improve the GFA [9]. However,it is not applicable to the (Ce–La–Pr–Nd)–Co–Al systembecause the lanthanide metals have a tiny difference inatomic sizes [21]. Theoretically, electronic effects also gov-

Table 1Thermal parameters and critical diameters (dc) of the (Ce–La–Pr–Nd)65Co25A

Alloy (at.%) dc/mm Tg/K Tx/K

(Ce–La–Pr–Nd)65Co25Al10 15 431 455La65Co25Al10 2 434 457Ce65Co25Al10 2 396 410Pr65Co25Al10 4 454 477Nd65Co25Al10 3 463 494

ern the formation and stability of metallic glasses [22–24].Considering the increase of valence electrons in thesequence La, Ce, Pr and Nd, we suggest that the similarphysical properties (e.g. atomic radii, melting point, ther-mal conductivity and Pauling’s electronegativity) anddiverse electron configurations of the lanthanide metalsmay increase the chemical disorder in the (Ce–La–Pr–Nd)65Co25Al10 alloy, and thus stabilize the glassy phaseduring the solidification from liquid to solid state andimprove the GFA obviously. The present experimentalresults imply that the GFA of some glassy alloy systemsmight be improved by the addition of other elements withdiverse electronic structures and similar atomic sizes tothose of the alloy constituent elements.

The compressive strain–stress curves shown in Fig. 4indicate that the bulk glassy (Ce–La–Pr–Nd)65Co25Al10alloy exhibits high fracture strength of 740 MPa, similarto those of Ce65Co25Al10 (750 MPa) and La65Co25Al10(670 MPa). Besides elastic strain of �2%, plastic deforma-tion of �0.5% can be observed for the (Ce–La–Pr–Nd)65Co25Al10 glass. Fig. 5 shows SEM images of thecompressive fracture of the glassy (Ce–La–Pr–Nd)65Co25-Al10 rod. The fracture took place along the maximum shearplane, which declines at about a 45� angle to the directionof the applied load. The fracture surface exhibits a veinpattern, typical of ductile bulk glassy alloys [25]. The Vick-ers hardness of (Ce–La–Pr–Nd)65Co25Al10 was measuredto be about 184. The good mechanical properties and

l10 and Ln65Co25Al10 (Ln = La, Ce, Pr or Nd, respectively) glassy alloys

Tm/K Tl/K DTx/K Tg/Tm Tg/Tl

701 729 24 0.61 0.59694 1041 23 0.63 0.42676 758 14 0.59 0.52758 786 23 0.60 0.58797 824 31 0.58 0.56

Fig. 5. SEM images of compressive fracture of bulk glassy (Ce–La–Pr–Nd)65Co25Al10 alloy.

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superior GFA of the glassy (Ce–La–Pr–Nd)–Co–Al alloygive it potential for future industrial applications.

4. Conclusions

Bulk (Ce–La–Pr–Nd)65Co25Al10 metallic glass withsuperior glass-forming ability and high mechanical strengthwas developed. Glassy (Ce–La–Pr–Nd)65Co25Al10 rodswith diameters up to 15 mm can be fabricated by coppermold casting. By comparing with single-lanthanide metal-based glassy alloys, Ln65Co25Al10 (Ln = La, Ce, Pr orNd, respectively), the glass-forming ability was found tobe improved considerably by the addition of multi-lantha-nide metals. Tg, DTx and Tg/Tl of the glassy (Ce–La–Pr–Nd)65Co25Al10 alloy were measured to be 431, 24 and0.59 K, respectively. The glassy (Ce–La–Pr–Nd)–Co–Alalloy with high fracture strength of 740 MPa, low meltingtemperature of 701 K and relatively low Tg may have use-ful industrial applications in the future.

Acknowledgement

This work was financially supported by National NatureScience Foundation of China (No. 50225103 and No.50471001).

References

[1] Inoue A, Zhang T, Masumoto T. Mater Trans JIM 1990;31:425.[2] Zhang Y, Tan H, Li Y. Mater Sci Eng A 2004;375–377:436.[3] Inoue A, Zhang T. Mater Trans JIM 1996;37:185.[4] Peker A, Johnson WL. Appl Phys Lett 1993;63:2342.[5] Guo FQ, Wang HJ, Poon SJ, Shiflet GJ. Appl Phys Lett 2005;86:

091907.[6] Ponnambalam V, Poon SJ, Shiflet GJ. J Mater Res 2004;19(5):1320.[7] Park ES, Kim DH. Appl Phys Lett 2005;86:201912.[8] Inoue A, Zhang T, Masumoto T. Mater Trans JIM 1989;30:965.[9] Guo F, Poon SJ. Appl Phys Lett 2003;83:2575.[10] Inoue A, Zhang T, Takeuchi A, Zhang W. Mater Trans JIM 1996;37:

636.[11] Inoue A, Nakamura T, Sugita T, Zhang T, Masumoto T. Trans JIM

1993;34:351.[12] Inoue A, Zhang T, Zhang W, Takeuchi A. Mater Trans JIM 1996;

37:99.[13] Inoue A, Zhang T, Takeuchi A. Mater Trans JIM 1996;37:1731.[14] Zhang T, Tsai AP, Inoue A, Masumoto T. Sci Rep RITU 1992;A36:

261.[15] Zhang B, Zhao DQ, Pan MX, Wang WH, Greer AL. Phys Rev Lett

2005;94:205502.[16] Li R, Pang SJ, Men H, Zhang T. Mater Trans 2005;46:2291.[17] Davies HA. In: Luborsky FE, editor. Amorphous metallic alloys.

London: Butterworth; 1983. p. 12.[18] Turnbull D. Contemp Phys 1969;10(5):473.[19] Turnbull D, Cohen MH. J Chem Phys 1961;34:120.[20] Chen HS. Acta Metall 1974;22:1505.[21] Dean JA. Lange’s handbook of chemistry. 15th ed. New York

(NY): McGraw-Hill; 1999. p. 4.30.[22] Nagel SR, Tauc J. Phys Rev Lett 1975;35:380.[23] Szofran FR, Gruzalski GR, Weymouth JW, Sellmyer DJ, Giessen

BC. Phys Rev B 1976;14:2160.[24] Buschow KHJ, Beekmans NM. Phys Rev B 1979;19:3843.[25] Inoue A, Zhang T, Masumoto T. Mater Trans JIM 1995;36:391.