Materials Letters 60 (2006) 662–665www.elsevier.com/locate/matlet

Effect of Fe substitution on the crystallization and mechanical propertiesof Zr55Cu30Al10Ni5−xFex alloys

M. Iqbal a,b, J.I. Akhter a,⁎, W.S. Sun b, J.Z. Zhao b, M. Ahmad a, W. Wei b, Z.Q. Hu b, H.F. Zhang b

a Physics Research Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistanb Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, 110016, PR China

Received 4 August 2005; accepted 28 September 2005Available online 12 October 2005

Abstract

Melt-spun ribbons of composition Zr55Cu30Al10Ni5−xFex (x=1, 2 and 3) have been synthesized to study the effect of Fe substitution for Ni onthe crystallization and mechanical properties. Differential scanning calorimetry (DSC) was carried out to measure the glass transition temperature(Tg), onset crystallization temperature Tx and supercooled liquid region ΔTx. The activation energy of crystallization has been calculated usingKissinger equation. The alloys were heat-treated at 450, 490, 550 and 600 °C for 20 min to introduce the crystalline phases for the enhancementsof the mechanical properties. The substitution of Fe for Ni helps to nucleate the FeZr2, Al3Fe and Zr (Al, Fe) as well as change the crystallizationprocess. The nanohardness and elastic modulus of the alloys improved with the introduction of the nanocrystalline phases.© 2005 Elsevier B.V. All rights reserved.

Keywords: Metallic glasses; Activation energy; Crystallization; Nanohardness

1. Introduction

There has been great interest in the synthesis of amorphousalloys during the last few decades due to their excellentproperties such as high strength, high wear, corrosion resistance,high toughness, high viscous flow, high hardness and lowYoung's modulus [1]. These materials are having applications inthe golf clubs heads, electronics and as engineeringmaterials [2].Klement et al. [3] synthesized the first amorphous ribbon of Au–Si alloy in 1960 by rapid solidification technique with coolingrate above 105 K/s. The field of amorphous alloys gainedmomentum after the discovery of multicomponent systems thatcan be produced at cooling rate of the order of 0.1–10 K/s [4–6].Extensive research has been carried out on the Zr-basedmuticomponent amorphous alloys with the aim to achieve widesupercooled liquid region and to improve the glass formabilityby varying the composition of the constituents [7–12].

The major consideration in the selection of the constituentsof an amorphous alloy has been the crystallization kinetics andto produce a stabilized supercooled liquid region and high glass

⁎ Corresponding author. Tel.: +92 51 2207224; fax: +92 51 9290275.E-mail addresses: jiakhter@yahoo.com, akhterji@pinstech.org.pk

(J.I. Akhter).

0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2005.09.053

forming ability. The crystallization behaviour of amorphousalloys has been explored to address the subject of stability inthese alloys [1,9,13]. Among the Zr-based amorphous alloys,the Zr–Al–Ni–Cu system has been studied extensively becauseof its ability to retain amorphous structure even when producedwith low cooling rate. The crystallization behaviour of bulkamorphous alloy Zr55Cu30Al10Ni5 has been studied and phaseslike NiZr2, CuZr, CuZr2, and Cu10Zr7 have been reported [10].A number of additional elements such as Ti, Ag and Be havebeen substituted to investigate their effect on the glassformingability and thermal stability. It was found that thesubstitution of Ag for Cu reduces the supercooled liquid regionsubstantially [14]. On the other hand addition of Ti and Be wasshown to have a positive effect on glass formability [15].

It is a well known fact that homogeneous distribution ofnanoscale particles and secondary phases in the amorphousmatrix enhances the tensile strength as compared to thecorresponding single-phase bulk amorphous alloys [16,17]. Ithas also been reported [18] that substitution of 2 at.% Fe inZr41Ti14Cu12.5Ni10− xBe22.5Fex alloy enhanced the structuralrelaxation time for glass transition, thermal stability againstcrystallization with higher onset crystallization temperature andwidened the supercooled liquid region. There is no reportavailable in the literature on the effect of Fe addition in place of

Fig. 2. DSC plots of the alloys at heating rate of 0.67 K/s.

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any constituent in the Zr55Cu30Al10Ni5 alloy. The present studyhas been carried out to synthesize alloys with Fe substitution inplace of Ni in the Zr55Cu30Al10Ni5 alloy to investigate its effecton thermal stability, supercooled liquid region and crystal-lization behaviour. The effect of crystallization on the hardnessand elastic modulus is also examined.

2. Experimental

The ingots of the alloys of composition Zr55Cu30Al10Ni5− x

Fex (x=0, 1, 2 and 3) were prepared under Ar atmosphere bymelting the mixtures of 2 N pure elements in an arc furnace. Themetals were cleaned ultrasonically before melting and arcmelting was carried out under Ti gettered atmosphere of highpurity Ar. The ingots were melted several times to get thehomogeneous compositions. Attempts were made to synthesizebulk samples of all the alloys with various thicknesses.However, crystallization occurred in all the Fe containing alloys.Therefore amorphous ribbons of 30 μm thickness and 3 mmwidth were produced by spin melting technique. The ribbons ofthe alloys are designated as base alloy, alloy-1, alloy-2 and alloy-3 according to content of 0–3 at.% Fe. DSCwas conducted usingPerkin Elmer 7/Pyris-1 at heating rates of 0.167, 0.33, 0.50 and0.67 K/s under Ar atmosphere. In order to introduce phases,samples were annealed at 450, 490, 550 and 600 °C for 20min insealed quartz capsules under Ar inert atmosphere. Before sealingthe samples in quartz capsules, the capsules were evacuated andflushed with Ar several times to avoid oxidation duringannealing. The crystalline phases were characterized usingRegako X-ray diffractometer and Cu Kα1 radiation (λ=1.54051Å). MTS Nanoindenter XP was used to measure nanohardnessof as-cast and heat-treated samples with maximum load of 5mN.

3. Results and discussion

XRD patterns of three as-cast alloy ribbons are shown in Fig. 1. Abroad band is observed for all the three alloys, which indicates theamorphous nature of the alloys. In order to check the crystallizationprocess of the alloys, DSC was conducted and results are shown in Fig.

Fig. 1. XRD patterns of the as-cast alloys.

2 for heating rate of 0.67 K/s. DSC curves for the alloys show anendothermic reaction characteristic of glass transition temperature,followed by a single exothermic event corresponding to the crystal-lization process. Fig. 2 also shows that the substitution of Fe for Nireduces the intensity and broadens the crystallization peak, whichsuggests reduced energy for the nucleation of phases. Differentparameters such as glass transition temperature Tg, onset crystallizationtemperature Tx, peak temperature Tp and supercooled liquid regionΔTxare determined from DSC scans taken at 0.67 K/s and given in Table 1.It is clear that Tg does not vary appreciably with Fe substitution, whichis in agreement with results reported for Zr41Ti14Cu12.5Ni10− xBe22.5Fexalloy [18]. However there is an increase inΔTxwith the Fe substitution.The atomic radii of Zr, Cu, Al, Ni, and Fe are 0.16, 0.128, 0.143, 0.125and 0.124 nm, respectively [6]. The atomic radius of the Fe is slightly(0.8%) smaller than that of Ni and it may increase the atomic packingdensity of the alloys to some extent.

In order to calculate the activation energy for crystallization DSCwas conducted at different heating rates. Activation energy forcrystallization of the alloys was calculated from Kissinger equationusing Tp

lnðr=T2p Þ ¼ ð−E=RTpÞ þ C

Where R is the gas constant and its value is 8.3145 J/Kmol, Tp is peaktemperature, E is the activation energy, r is the heating rate and C is aconstant. Kissinger plot of ln (r /Tp

2) versus 1 /Tp is shown in Fig. 3 foralloy-2 as well as the base alloy, which is a straight line with slope of(−E /R). The calculated activation energies for the base alloy and thealloys with Fe addition are also given in Table 1. The activation energyof Zr55Cu30Al10Ni5 alloy reduces with the addition of Fe in place of

Table 1Glass transition temperature Tg, crystallization temperature Tx, peak temperatureTp for crystallization, supercooled liquid region ΔTx at 0.67 K/s and activationenergy E of the alloys

Alloys Tg (°C) Tx (°C) Tp (°C) ΔTx=Tx−Tg (°C)

E (kJ/mol)

Base alloy (bulkamorphous)

430 488 502 58 272.3

Alloy-1 426 503 508 77 244.3Alloy-2 424 504 510 80 231.7Alloy-3 420 506 515 86 192.3

Fig. 3. Kissinger plot using Tp for alloy-2 and the base alloy.

Fig. 5. XRD patterns of the alloy-2 at different temperatures.

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Ni. The crystallization of amorphous alloys requires rearrangement ofunlike atoms during which an atom has to overcome the bindingenergy with neighbors to take its lattice positions in the originalmaterial [19]. This shows that the effective activation energy forcrystallization of an amorphous alloy actually depends on theinteraction of atoms with each other. The heat of mixing of Zr–Cu,Zr–Ni, Zr–Fe, Al–Ni, Al–Cu, Al–Zr, Cu–Ni, Al–Fe, Cu–Fe and Fe–Ni atomic pairs is −23, −49, −25, −22, −0.8, −44, 4, −11,13 and −2kJ/mol, respectively [19,20]. The addition of Fe reduces the heat ofmixing due to Zr–Fe, Cu–Fe, Ni–Fe and Al–Fe bonds in place of Zr–Ni, Cu–Ni, Al–Ni, which makes the rearrangement of atoms easierduring the crystallization process. This indicates that the addition of asmall amount of Fe enhances the crystallization process and hencereduces the activation energy.

Alloys are heat-treated at 450, 490, 550 and 600 °C for 20 min toexplore the crystallization behaviour. XRD patterns of the alloy-1,alloy-2 and alloy-3 are shown in Figs. 4–6 at various temperatures. Thealloys remain amorphous at 450 °C, whereas Zhang et al. [10] observedNiZr2 crystalline phase at this temperature in the slowly cooled bulkZr55Cu30Al10Ni5 alloy. It clearly indicates that the increase in δTx in thesupercooled alloys helps to increase the incubation period of the phasesas compared to the bulk amorphous Zr55Cu30Al10Ni5 alloy.

Fig. 4. XRD patterns of the alloy-1 at different temperatures. Phases are denotedby NiZr2 (1), FeZr2 (2), Cu10Zr7 (3), CuZr2 (4), CuZr (5), Zr (Al, Fe) (6) andAl3Fe (7).

The phases identified by XRD at 490, 550 and 600 °C are NiZr2,CuZr, CuZr2, Cu10Zr7, FeZr2, Al3Fe and Zr (Al, Fe). The prominent ofthese phases is NiZr2 at all temperatures in alloy-1. However, thedensity of the CuZr2, Cu10Zr7 and FeZr2 increases as the temperature isincreased. The density of other phases is low. The prominent phase inalloy-2 and alloy-3 is also NiZr2 at 490 and 550 °C, whereas the majorphase at 600 °C is Cu10Zr7. The density of the phases Cu10Zr7, CuZr2and FeZr2 again increases with rise in temperature. Zhang et al. [10]reported formation of NiZr2, CuZr2 and Cu10Zr7 phases along with anunknown phase at 490 °C in Zr55Cu30Al10Ni5 bulk amorphous alloy.Their finding indicated that density of the CuZr2 was higher comparedto Cu10Zr7 at 700 °C and that of NiZr2 very low. They also reported thatthe external pressure enhances the density of phases in general due toincrease in Gibbs free energy and promotes the formation of Cu10Zr7and suppresses the NiZr2 phase at 600 °C. Our results suggest that withthe increase in Fe concentration, the density of Cu10Zr7, CuZr2 andFeZr2 increases at 600 °C and the density of NiZr2 decreases. Thevolume fraction of Cu10Zr7 is larger in alloy-3 at 550 °C as compared tothe alloy-1 and alloy-2. The reduction in the density of NiZr2 isobserved in alloy-2 and alloy-3 at 600 °C due to the decrease in Nicontent. The large volume fraction of Cu10Zr7 as compared to CuZr2 at

Fig. 6. XRD patterns of the alloy-3 at different temperatures.

Table 2Average nanohardness H (GPa) and elastic modulus E (GPa) of alloys

Heattreatment

Alloy-1 Alloy-2 Alloy-3

H E H E H E

As cast 5.5 84.2 5.3 83.8 5.5 83.9490 °C 7.3 94.4 7.4 93.1 7.0 90.5550 °C 7.7 90.9 7.4 96.8 7.0 92.0600 °C 7.7 98.2 6.8 84.3 7.2 86.3

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600 °C may be resulted due to the change in the Gibbs free energy byFe addition.

Nanohardness measurements are carried out for three designatedalloys to investigate the effect of crystalline phases. Average of sixreadings for each sample was taken and results are given Table 2 for theas-cast and heat-treated samples of the Fe containing ribbons. Hardnessand elastic modulus do not change with the increase in Fe concentrationin the as-cast alloy. However, the elastic modulus of our alloys is highercompared to the bulk amorphous base alloy [21]. The results indicatesthat introduction of crystalline phases by heat treatment enhances thenanohardness as well as the elastic modulus of the alloys synthesized.Although the hardness and the elastic modulus tend to increase with theincrease in temperature in the alloy-1, these quantities increase with theincrease in temperature up to 550 °C and a small decrease is observed at600 °C in alloy-2 and alloy-3. This decrease may be due to the increasein the density of Cu10Zr7 phase and decrease in NiZr2.

4. Conclusions

1. Substitution of Fe in place of Ni in the Zr–Cu–Al–Ni alloyrestricted the formation of bulk samples with amorphousstructure. However amorphous ribbons of thickness ∼30 μmwere synthesized successfully with increased ΔTx.

2. Activation energy of crystallization is reduced with Feaddition, which indicates enhancement of the crystallizationprocess.

3. Substitution of Fe in place of Ni helps to decrease NiZr2 phaseand introduces phases like FeZr2, Al3Fe and Zr (Al, Fe).

4. Nanohardness and elastic modulus increase with theintroduction of the crystalline phases during heat treatment.

Acknowledgments

M. Iqbal is thankful to Third World Academy of Sciences(TWAS) for providing travel grant as well as financial supportand to the Institute of Metal Research (IMR) Shenyang, ChineseAcademy of Sciences for providing local hospitality under thegrant from National Natural Science Foundation of China(50395104). Assistance provided by Ms. G.Y. Chen, X.F.Wang, J.T. Wang, Z.Y. Li, J. He, J. Liu, and Miss L.L. Gao atIMR is highly appreciated. Special thanks to Dr. M.A. Shaikh,Mr. Akhtar, Ghafar Ali and Mr. Naeem Akhter at (RDG) PRDPINSTECH for fruitful discussions.

References

[1] A. Inoue, Acta Mater. 48 (2000) 279.[2] M. Telford, Mater. Today 7 (2004) 36.[3] W. Klement, R.H. Willens, P. Duwez, Nature 187 (1960) 869.[4] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans., JIM 31 (1990) 177.[5] A. Inoue, T. Zhang, A. Takeuchi, Mat. Sci. Forum 269–272 (1998) 855.[6] Y. Li, J. Mater. Sci. Technol. 15 (1999) 97.[7] A. Inoue, T. Zhang, N. Nishiyama, K. Ohba, T. Masumoto, Mater. Trans.,

JIM 34 (1993) 1234.[8] A. Inoue, T. Zhang, Mater. Trans., JIM 37 (1996) 185.[9] L. Liu, Z.F. Wu, J. Zhang, J. Alloys Comp. 339 (2002) 90.[10] J. Zhang, Y.H. Wei, K.Q. Qiu, H.F. Zhang, M.X. Quan, Z.Q. Hu, Mater.

Sci. Eng., A 357 (2003) 386.[11] A. Inoue, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process.

267 (1999) 171.[12] W. Chen, Y. Wang, J. Qiang, C. Dong, Acta Mater. 51 (2003) 1899.[13] J. Eckert, N. Mattern, M. Zinkevitch, M. Seidel, Mater. Trans., JIM 39

(1998) 623.[14] A. Inoue, T. Zhang, Y.H. Kim, Mater. Trans., JIM 38 (1997) 749.[15] A. Peker, W.L. Johnson, App. Phys. Lett. 63 (1993) 2342.[16] A. Inoue, Mater. Sci. Eng., A 179/A180 (1994) 57.[17] H. Kato, T. Hirano, A. Matsuo, Y. Kawamura, A. Inoue, Scr. Mater. 43

(2000) 503.[18] Q. Wang, J.M. Pelletier, Y.D. Dong, Y.F. Ji, Mater. Sci. Eng., A 370 (2004)

316.[19] Z.J. Yan, S.R. He, J.R. Li, Y.H. Zhou, J. Alloys Comp. 368 (2004) 175.[20] A. Takeuchi, A. Inoue, Mater. Trans., JIM 41 (2000) 1372.[21] J.G. Wang, B.W. Choi, T.G. Nieh, C.T. Liu, J. Mater. Res. 15 (2000) 798.