Journal of Non-Crystalline Solids 356 (2010) 1665–1669

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Journal of Non-Crystalline Solids

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Structure and crystallization of amorphous Cu–Zr–Al powders

Rubén Fernández ⁎, Wilson Carrasco, Alejandro ZúñigaDepartment of Mechanical Engineering, University of Chile, Beauchef 850, Santiago, Chile

⁎ Corresponding author.E-mail address: rufernan@ing.uchile.cl (R. Fernánde

0022-3093/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.jnoncrysol.2010.06.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 October 2009Received in revised form 28 May 2010Available online 16 July 2010

Keywords:Crystallization;Amorphous alloys

In this work, the structure and crystallization of amorphous (Cu50Zr50)100− xAlx, (x=0, 2.5, 5, 7.5, 10)powders produced by gas atomization were studied. X-ray diffraction patterns showed an amorphousstructure in the as-atomized powders; however, TEM observations revealed the presence of nanocrystals inall the samples. Differential Scanning Calorimetry (DSC) analyses revealed an improved glass forming ability(GFA) in the alloys containing Al, with the (Cu50Zr50)97.5Al2.5 alloy being the one with the best GFA. Kissingeranalyses showed that the activation energy Ea for crystallization increased with the Al content, and theAvrami parameter, n was higher compared to traditional crystalline metallic alloys. Structural analyses ofcrystallized samples revealed the formation of Cu10Zr7 nanocrystals.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

In the last decade, amorphous metallic alloys have attracted muchattention due to their uniquemechanical and physical properties suchas high strength and corrosion resistance [1,2]. Cu-based amorphousalloys are particularly interesting due to their high ductility, and lowercost compared to other amorphous metallic alloys.

Several processing techniques are available to produce metallicglasses, such as mechanical alloying/milling, casting and gas atom-ization. The casting technique allows obtaining an amorphousmaterial known as bulk metallic glasses (BMG), i.e., pieces ofamorphous metals that are large enough to be considered bulk, andnot particulate matter or thin films. The drawback of Cu-based BMG isthat the largest piece of material that can be obtained is in the order of10 mm. Amorphousmetallic alloys in powder form can be obtained bythe gas atomization method, and novel manufacturing techniques canbe applied to produce large components by depositing pre-alloyedpowders onto a substrate. On the other hand, Cu–Zr and Cu–Zr–Alalloys have shown extremely high compression resistance (50%)compared to other alloys [1,3]. This opens the possibility of applyingnew developing techniques to manufacture bulk Cu-based alloysstarting from amorphous powders.

Studies on Cu–Zr BMGhave shown the presence of tiny nanocrystalsinside the amorphous matrix, slightly reducing their strength [3].Similarly, other reports indicate that the GFA of the Cu–Zr alloys can beimproved by the addition of small amounts of Al to the alloy [4,5].

In the present article, structural and crystallization studies wereperformed on amorphous metallic (Cu50Zr50)100− xAlx, (x=0, 2.5, 5,

7.5, 10) alloy powders produced by gas atomization. Differentialscanning calorimetry (DSC) runs at four different heating rates wereused to obtain the activation energy of crystallization, Ea (using amodified Kissinger equation [6]), and the Avrami exponent, n, whichprovides the dimensionality of crystal growth [7]. The microstructuralstudies were carried out using X- ray diffraction and High-resolutionTransmission Electron Microscopy (HR-TEM).

2. Experimental

Cu50Zr50 and (Cu50Zr50)100−xAlx (x=2.5, 5, 7.5, 10) alloys powderswere produced by gas atomization using helium at 13 MPa. The alloyswere produced from master alloys and superheated 200 °C above themelting temperature prior to atomization. Finally, the atomizedpowder was sieved under 37 μm.

Thermal analyses were conducted using differential scanningcalorimetry using a DSC Q20 TA instrument, under an argonatmosphere. Each 80 mg-sample was mounted in an aluminumbasket, and the DSC scans were carried out from 293 K to 823 K,using four heating rates (10 K/min, 20 K/min, 30 K/min and 40 K/min). All the experiments were performed twice. The glass transitiontemperature (Tg) and crystallization temperature (Tx) were obtainedfrom the DSC curves. A Siemens D5000 X-Ray Diffractometer with CuKα (λ=0.1542 nm) radiation was used to study the structure of theas-atomized and crystallized powders. TEM samples were preparedmixing the powders with Gatan G1 Resin and cut into 3 mmdiscs. Thediscs were first mechanically grinded using a Fischione DimplingGrinder Model 200 machine, and then thinned to electron transpar-ency using a Fischione ion mill model 1010 cooled with liquidnitrogen to avoid crystallization [8]. For TEM analysis a HR-TEM FEITecnai G2 F-20 S-Twin microscope was used.

Fig. 1. XRD spectra of the (Cu50Zr50)100− xAlx (x=0, 2.5, 5, 7.5, and 10) alloys.

Fig. 3. HR-TEM of the (Cu50Zr50)95Al5 sample, showing the presence of nanocrystals(5 nm).

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3. Results

3.1. As-atomized powder

The structure of the as-atomized powders was studied using X-raydiffraction. Fig. 1 shows the XRD patterns of the (Cu50Zr50)100−xAlx,(x=0, 2.5, 5, 7.5, 10) alloys, which exhibit a broad diffuse peak betweenthe diffraction angles of 30° and 45°, indicating the amorphous state ofthe samples.

HR-TEM studies were also performed on the amorphous powdersto reveal the details of the structure. SAED micrographs of allcompositions (Cu50Zr50)1− xAlx (x=0, 2.5, 5, 7.5, 10) showed anamorphous pattern, as the one presented in Fig. 2 (for x=10). Eventhough the specimens show generally an amorphous structure in thediffraction experiments, HR-TEM observations showed the presenceof Cu10Zr7 nanocrystals (revealed by analyses of the electrondiffraction patterns) of approximately 5 nm in size in all the samples(Figs. 3 and 4).

3.2. Crystallization kinetics

The crystallization reaction was studied using differential scanningcalorimetry, the results are shown in Table 1. The glass transition

Fig. 2. SAED of the (Cu50Zr50)90Al10 alloy.

temperatures (Tg) and the onset temperatures of crystallization (Tx)were obtained at a heating rate of 10 K/min. Fig. 5 shows the DSCtraces for the (Cu50Zr50)100− xAlx (x=0, 2.5, 5, 7.5, and 10 at.%)obtained for the amorphous metallic powders. The alloys exhibitdistinct glass transitions, followed by the exothermic peakcorresponding to the crystallization reaction. When x=7.5, the DSCcurve shows two exothermic reactions (Fig. 6), which can eitherindicate a two step crystallization reaction, or a relaxation of theamorphous structure before crystallization. The extension of thesuper-cooled liquid region (SLR), defined as the section of thecalorimetric curve between Tg and Tx (ΔTx=Tx−Tg), is considered agood indicator of the Glass Forming Ability (GFA) in amorphous alloys[4,9]. The parameters Tg and Tx increased with increasing Al content.As shown in Table 1, the highest value ofΔTx is obtainedwhen x=2.5,indicating that the (Cu50Zr50)97,5Al2.5 has the best GFA. On the otherhand, the enthalpy released during the crystallization reaction can beestimated using the peak area. Table 1 summarizes the calorimetricdata, where the errors were calculated as the standard deviation of

Fig. 4. Dark field image of the (Cu50Zr50)97,5Al2,5 sample, showing the presence ofnanocrystals.

Table 1Calorimetric data of the (Cu50Zr50)100−xAlx powder alloys at a heating rate of 10 K/min.

Composition (at.%) Tg (K) Tx1 (K) Tx2 (K) ΔTx1 (K) ΔTx2 (K) ΔH (J/mol)

Cu50Zr50 678±1.33 707±0.74 – 29±2.07 – −4243.59±11.16(Cu50Zr50)97.5Al2.5 685±0.54 733±0.92 – 48±1.46 – −4934.53±15.11(Cu50Zr50)95Al5 701±1.07 744±0.64 – 44±1.71 – −4381.01±14.96(Cu50Zr50)92.5Al7.5 704±1.14 737±0.38 750±0.73 33±1.52 46±1.87 −3427.47±13.9(Cu50Zr50)90Al10 712±0.58 754±0.48 – 42±1.06 – −2084.64±14.10

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each experiment, and the values are in good agreement with otherreports [5,10]. Again, the (Cu50Zr50)97,5Al2.5 alloy released the highestvalue of enthalpy during the crystallization reaction, which can beattributed to a greater volume fraction of amorphous phase in thealloy, in agreement with this composition having the best GFA.

The activation energy Ea for the crystallization process wasestimated using the modified Kissinger's equation [6]:

lnT2p

ϕ

!= − Ea

RTp+ ln

EaRk0

� �ð1Þ

Where the shift in the peak crystallization Tp along with theheating rate ϕ is used to determine Ea if a lineal model fits to theparameters mentioned. Table 2 summarizes the activation energiesfor the major exothermic events. The values of the activation energyfor crystallization increased with the Al content, possibly because thisincrement makes the diffusion of atoms in the super-cooled liquidmore difficult.

The Avrami exponent, n, which provides information on the crystalgrowth rate, was estimated using the Mehl–Johnson–Avrami kineticmodel (MJA), which establishes that the transformed volume fractionof a reaction can be estimated using the expression:

f ðtÞ = 1− expð−ðktÞnÞ ð2Þ

where k is the reaction rate of the crystallization process, and isassumed to follow and Arrhenius activation law:

kðTÞ = k0 · exp − EaRT

� �ð3Þ

Where R is the universal gas constant and Ea and k0 are parametersestimated using the Kissinger model. In order to use the MJA model

Fig. 5. DSC curves (heating rate 10 K/min) of the (Cu50Zr50)100− xAlx (x=0, 2.5, 5, 7.5,and 10) amorphous powders.

under non-isothermal conditions, without dependence of time, it isnecessary to use the concept of reduced time θ:

θ =RT2

ϕEaexp − Ea

RT

� �ð4Þ

Where ϕ is the heating rate for the non-isothermal case.Developing Eq. (2) leads to the expression

ln ln1

1−x

� �� �= n lnðθÞ + n lnðk0Þ ð5Þ

where x denotes the transformed volume fraction taken as propor-tional to the fractional area of the exothermic peak:

x =Ai

AT

� �ð6Þ

Ai denotes the area of the peak up to a determined temperature,and AT corresponds to the total area of the exothermic peak. Table 2summarizes the Avrami parameter obtained for each alloy. TheAvrami parameter n found for the (Cu50Zr50)100− xAlx alloys with xbetween 0 and 7.5 was approximately 7, while the (Cu50Zr50)90Al10alloy showed an n value of 3.6.

3.3. Crystallized powder

XRD analyses were performed on crystallized (post-DSC run)samples of (Cu50Zr50)100− xAlx alloys to investigate the phases formedduring the crystallization process (Fig. 7). The Cu10Zr7 phase wasidentified in all samples, while the formation of some oxides such as(Al2O3 and ZrO2) was also detected. When x=5, the XRD patternshowed the formation of AlZr3 crystals, which were not detected inthe alloys with a lower Al content. When x=7.5 and x=10, the AlZr3

Fig. 6. DSC curves for (Cu50Zr50)92.5Al7.5 at 10, 20, 30 and 40 K/min. The crystallizationevent when x=7.5 occurs in two steps.

Table 2Kissinger and MJA analyses.

Composition (at.%) Ea (Kissinger) n

Cu50Zr50 221.895±2.85 6.9±0.18(Cu50Zr50)97.5Al2.5 231.007±3.25 7.2±0.23(Cu50Zr50)95Al5 233.751±0.46 7.8±0.11(Cu50Zr50)92.5Al7.5 264.997±7.53 6.9±0.09(Cu50Zr50)90Al10 331.096±4.34 3.6±0.05

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peaks became more intense, and other peaks corresponding to AlCu,Al2Zr crystals were also identified.

The TEM diffraction pattern obtained from the Cu50Zr50 and(Cu50Zr50)92.5Al7.5 specimens are shown in Fig. 8. Cu10Zr7 crystalswere identified in both samples, in agreement with the XRD analyses.

Fig. 8. Dark field image of the (Cu50Zr50)92,5Al7,5 sample, showing the presence ofCu10Zr7 crystals (20 nm).

4. Discussion

The HR-TEM image of the as-atomized powder (Fig. 3) showed theformation of Cu10Zr7 nanocrystals, and the presence of thesenanocrystals can be attributed to the cooling rate in the atomizationprocess, which could have not been fast enough to obtain a fullyamorphous structure.

Regarding the crystallization reaction and its kinetics, Table 2summarizes the MJA parameters obtained from the DSC experiments.The obtained values for the Avrami parameter are far from thetraditional transformations in metal alloys, with n approximatelyequal to 7, except for the (Cu50Zr50)90Al10 alloy, where n is approxi-mately 4. J.W. Christian [11] and Turnbull [12] suggested that if thenucleation rate increaseswith time, then nN4. The crystallization underthis condition occurs via an interface-controlled growth with anincreasing nucleation rate. The increasing nucleation rate has beenexplained by the phenomenon of self-heating of the specimen duringtransformation or a gradual short range-ordering of the glassy phase[13]. The n value for the (Cu50Zr50)90Al10 alloy (n∼4) suggests that thecrystallization process is diffusion controlled with a nucleation ratewhich decreases with time. Therefore, there is a critical Al content atwhich a change on the nature of the diffusion process might occur.Table 2 also shows that the (Cu50Zr50)97.5Al2.5 alloy has the highest ΔTxand enthalpy of reaction, indicating that this alloy has the best GFA. Thisbehavior can be attributed to the fact that Zr has stronger affinitywithAlthan Cu; therefore, as the Al content increases, the interaction betweenAl and Zr Atoms becomes dominant, hindering the formation of CuZr

Fig. 7. XRD patterns of the (Cu50Zr50)100− xAlx (x=0, 2.5, 5, 7.5, and 10) powders afterannealing at 823 K.

phases, thus stabilizing the amorphous state. As theAl content increasesabove 5%Al, the formationof Al-containing crystals is promoted, and theinfluence of Al on the GFA becomes only partially effective.

Regarding the two exothermic peaks found in the (Cu50Zr50)92.5-Al7.5 alloy, an interrupted DSC run performed just until the firstexothermic peak and a subsequent XRD analysis of this partiallytransformed sample (Fig. 9) showed that the structure remainsamorphous; therefore, this first exothermic peak can be attributed tothe relaxation of the structure before crystallization.

Finally, in the crystallized powders, XRD patterns and TEM imagesshowed that starting from x=7.5, the formation of Cu10Zr7, AlCu,Al2Zr and AlZr3 crystals was detected, and the presence of the lattercan be attributed to the higher Al content. It can be observed in Fig. 8(DF image) that the Cu10Zr7 crystal size is approximately 20 nm.

5. Conclusions

The structure and crystallization processes of a series of Cu-basedamorphous (Cu50Zr50)100−xAlx (x=0, 2.5, 5, 7.5, and 10 at.%) alloyswere studiedusingXRD, TEMandDSC. XRDmeasurements indicated anamorphous state of the as-received amorphous alloy; however, HR-TEM

Fig. 9. XRD pattern of the (Cu50Zr50)7.5Al2.5 powders after an interrupted DSC run(insert), showing an amorphous structure.

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observations confirmed the presence of Cu10Zr7 nanocrystals in eachsample. DSC analyses showed that the (Cu50Zr50)97.5Al2.5 alloy releasesthehigher amountof enthalpy during crystallization, indicating that thiscomposition has the best GFA of all the samples studied. The amount ofaluminumhas also a direct influence on the Tg and Tx temperatures; thehigher the aluminum content, the higher Tg and Tx, with the (Cu50-Zr50)97.5Al2.5 sample showing the higher value of ΔT. Kissinger analysisalso revealed that the higher the amount of aluminum, the higher theactivation energy for crystallization. The (Cu50Zr50)92.5Al7.5 alloyshowed two exothermic peaks, with the first peak being attributed tothe relaxation of the amorphous structure, and the second one to theformation of Cu10Zr7, AlCu, Al2Zr and AlZr3 crystals. Kinetic analysisusing theMehl–Johnson–Avramimethod established nN4; consequent-ly, the nucleation rate of the crystallized phase increased with time.

XRD analyses on post-DSC samples revealed the formation ofCu10Zr7 crystals for all compositions. It was also confirmed thatstarting from the (Cu50Zr50)92.5Al7.5 composition, new peaks appearedon the XRD patterns (attributed to Cu10Zr7, AlCu, Al2Zr and AlZr3crystals), while TEM observations showed the formation of Al3Zr4 inthe (Cu50Zr50)92.5Al7.5 composition.

Acknowledgements

The financial support of Fondecyt (Grant # 11075008) is acknowl-edged. The help of Prof. Enrique J. Lavernia for providing the powders,

and Eduardo Donoso for the DSCmeasurements are also acknowledged.Thanks are alsodue to the TransmissionElectronMicroscopy Laboratoryof the University of Chile for the HR-TEM measurements.

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