Crystallization of amorphous Zr60Al15Ni25 alloy

Intermetallics 15 (2007) 1013e1019 www.elsevier.com/locate/intermet

Crystallization of amorphous Zr60Al15Ni25 alloy J.F. Li*, Z.H. Huang, Y.H. Zhou School of Materials Science and Engineering, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, PR China Received 1 August 2006; received in revised form 10 November 2006; accepted 8 December 2006 Available online 14 February 2007

Abstract The phase formation and crystallization kinetics during the thermal treatment of amorphous Zr60Al15Ni25 alloy were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). By lowering the isothermal annealing temperatures, it is revealed that the crystallization of the amorphous Zr60Al15Ni25 alloy consists of a primary transformation followed by a polymorphic transformation, corresponding to the precipitations of hexagonal Zr6Al2Ni and the Zr5AlNi4 with a U3Si2-typed superstructure. The primary phase being Zr6Al2Ni rather than Zr5AlNi4 in the crystallization is because the latter has a complex structure and its formation requires the diffusion of Al and Zr atoms on a large scale. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: B. Glasses, metallic; B. Phase transformations

1. Introduction Since the pioneering work of Duwez et al. [1] on the synthesis of metallic glasses, tremendous effort has been devoted to decrease the critical cooling rate with which an alloy melt can be frozen into glassy state. This brought about the discoveries of Pt- and Pd-based bulk metallic glasses (BMGs) in 1970’ [2], and subsequently the BMGs without noble metals, such as La-, Zr-, Mg-, Fe-, Fe-, Ni-, Ti-, Cu- and Co-based alloys after 1989 [3,4]. Among the BMGs produced so far, the Zr-based alloys exhibit excellent mechanical properties and good glass-forming ability, and have been used to fabricate sporting goods, medical devices, springs, parts of precision instruments and so on [4,5]. The first Zr-based BMG was made from ZreAleNi [6], based on which, a lot of important Zr-based bulk metallic glass-forming alloys have been designed. Careful experiments revealed that the composition with the best glass-forming

* Corresponding author. Tel.: þ86 21 62933785; fax: þ86 21 62932026. E-mail address: [email protected] (J.F. Li). 0966-9795/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2006.12.006

ability in this ternary system is Zr60Al15Ni25, whose supercooled liquid range defined as the temperature interval between the glass transition temperature and the crystallization temperature reaches as large as 77 K [6]. While attempts were made to determine the crystallization products of its glass, however, different conclusions were achieved. Ko¨ster and Meinhardt [7] thought that the metastable fcc Zr2Ni first formed as the primary phase in the crystallization, and subsequently transformed into the other phase. Choi et al. [8] argued that the products consisted of Al2Zr3, NiZr and Zr. But Li et al. [9] suggested that there were only two compounds Zr5Ni4Al and Zr6NiAl2 in the crystallized structure. Generally, the thermodynamically equilibrium structure of a glass-forming alloy contains intermetallic compounds. For a BMG composed of multi-components, the intermetallic compounds involved in its crystallization are often more than one type, and some of them possess complex crystal structure. Difficulties therefore arise in determining the crystallization products if at least two types of compounds simultaneously appear in the annealing treatment, and especially when unknown phases form. In the present work, we performed a series of isothermal annealing of amorphous

J.F. Li et al. / Intermetallics 15 (2007) 1013e1019

Zr60Al15Ni25, and observed two completely separated exothermic peaks at lower temperatures, by which the crystallization process of the amorphous Zr60Al15Ni25 alloy is revealed.

2. Experimental procedure The Zr60Al15Ni25 master alloy was prepared by arc melting a mixture of pure Zr (99.95 wt%), Al (99.99 wt%) and Ni (99.99 wt%) in titanium-gettered argon atmosphere. To reach a high homogeneity, the ingot was turned over and remelted six times. From the master alloy, a ribbon with a cross-section of about 2.5  0.025 mm2 was produced in a single roller melt-spinner under helium. Its composition was analyzed using chemical method. After the amorphous nature of the ribbon was identified by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), it was used to perform the crystallization experiment. Both isochronal and isothermal differential scanning calorimetry (DSC) measurements were carried out using a power compensated Perkin Elmer DSC Pyris-1. The applied heating rates for the isochronal experiment were 1, 5, 10, 20, 30, and 40 K/min. The temperature range for the isothermal treatment was from 703 to 773 K. The sample and reference pans were made of aluminum. The sample pan was filled with several pieces of ribbons weighing about 10 mg, and sealed with an aluminum cover. The reference pan without sample was provided with two aluminum covers so as to obtain a heat capacity of the reference comparable to that of the sample. A protective gas of pure argon was employed. The temperature and the heat flow were calibrated by measuring the melting temperature and the heat of fusion of pure In, Sn, and Zn. In the isochronal annealing experiment, two identical DSC runs were made successively for each sample. The second run, then with the sample in its crystalline state, served as a record of the baseline. Subtraction of this baseline from the first run realized the correction for the apparatus baseline shift, and as a result, the measurement curve was obtained. In the isothermal annealing experiment, the program was designed such that the sample was heated quickly up to 673 K, followed by a short plateau, and then heated again quickly to the predetermined isothermal temperature. The isothermal annealing time is long enough to ensure that crystallization was completed. Also, two identical DSC runs were made successively for every sample, and the second run served as the baseline. To study the structure at various stages of crystallization, additional isothermal anneals were conducted in the DSC calorimeter. Following the same heating program as the isothermal DSC measurement, pieces of the ribbon were heated to the predetermined temperature. Undergoing different times of annealing, the specimens were cooled rapidly to room temperature. Part of these specimens with different degrees of crystallization was then used for XRD measurement with a Philips X’pert Pro MRD diffractometer with Cu Ka radiation. The others were thinned by jet-polishing using 80 vol% methanol and 20 at.% perchloric acid at 248 K to conduct the transmission electron microscopy (TEM) observation.

The equilibrium ZreAleNi phase diagram at room temperature has not yet been established. According to the isothermal section of the phase diagram at 1073 K summarized by Nash and Pan [10], the possible compounds around the composition of Zr60Al15Ni25 are Zr5AlNi4, Zr6Al2Ni and Zr2Ni. To aid in determining the crystallization products during annealing the Zr60Al15Ni25 ribbon, fully amorphous Zr50Al10Ni40 and Zr66Al22Ni12 ribbons were also fabricated following the same procedure as used in the preparation of the Zr60Al15Ni25 ribbon, and their fully crystallized structure were analyzed by XRD. 3. Results 3.1. Ribbon samples The measured composition of the Zr60Al15Ni25 ribbon by chemical analysis agrees with the original integrant, indicating that the weight loss during the preparation process can be ignored. The oxygen content in the ribbon was determined to be 0.032  0.007 wt%. The XRD pattern of the as-cast Zr60Al15Ni25 ribbon is shown in Fig. 1a. It comprises two diffuse peaks, in which the main peak is located at about 2q ¼ 36.87 . The HRTEM observation reveals no lattice fringes in the image (Fig. 2). The selected area diffraction

Zr6Al2Ni Zr5AlNi4

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2θ Fig. 1. XRD patterns. (aec) As-cast ribbons of Zr60Al15Ni25, Zr66Al22Ni12 and Zr50Al10Ni40, respectively. (deg) Zr60Al15Ni25 ribbons annealed at 713 K for 53, 91, 145 and 200 min, which correspond to the peak and end of the first reaction and the peak and end of the second reaction in sequence. (h) Fully crystallized Zr66Al22Ni12 ribbon. (i) Fully crystallized Zr50Al10Ni40 ribbon.

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All the isochronal DSC scans of the Zr60Al15Ni25 ribbon at heating rates 1, 5, 10, 20, 30 and 40 K/min show a glass transition followed by the exothermic heat release event(s) due to crystallization. The supercooled liquid temperature range is a function of the heating rate. As the heating rate increases from 1 to 40 K/min, the glass transition temperature changes slightly from 689 to 700 K, while the crystallization temperature increases from 733 to 782 K. Namely, the supercooled liquid temperature range is enlarged from 50 to 82 K. The DSC curves related to crystallization are shown in Fig. 3. There is only one exothermic peak at a heating rate of 20 K/min or above. As the heating rate decreases to 10 K/min, a slight shoulder begins to appear on the left of the peak, and it becomes more obvious at 5 K/min. At 1 K/min two peaks partially overlapped are observed.

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peak (Fig. 4a). But as the annealing temperature decreases to 763 K, a small shoulder appears on the left of the peak. Further decreasing the temperature brings about the splitting of the crystallization event into two peaks. The time interval between the two peaks increases with the decrease of the annealing

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3.3. Isothermal DSC Based on the experimental results of the isochronal annealing, we performed the isothermal DSC measurement of the ascast Zr60Al15Ni25 ribbon in the temperature range from 705.5 to 773 K. The results are shown in Fig. 4. At the highest temperature of 773 K, crystallization leads to a single exothermic

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Fig. 3. Isochronal DSC curves of the amorphous Zr60Al15Ni25 ribbon.

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Fig. 2. HRTEM image of the as-cast Zr60Al15Ni25 ribbon. The inset is a selected area diffraction pattern.

using an electron beam of 1 mm in diameter shows a pattern consisting of a broad diffraction halo and faint larger one (the inset in Fig. 2). All these verify that the as-cast ribbon is fully amorphous. Crystalline diffraction peak is not found in the XRD patterns of the as-cast Zr66Al22Ni12 and Zr50Al10Ni40 ribbons (Fig. 1b and c). Compared with the Zr60Al15Ni25 ribbon, the position of the main diffuse peak of the Zr66Al22Ni12 ribbon decreases to 2q ¼ 36.28 , while that of the Zr50Al10Ni40 ribbon increases to 2q ¼ 38.88 . Crystals were not found during the HRTEM observation of these two ribbons (the results are not shown here). That is to say, the latter two ribbons also possess an amorphous nature.

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Time (s) Fig. 4. Isothermal DSC curves of the amorphous Zr60Al15Ni25 ribbon at higher (a) and lower (b) temperatures.

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temperature, and eventually they are separated completely at 713 K (Fig. 4b). At lower temperatures, the inoculation period for crystallization is increased, and the reaction is slowed down. 3.4. X-ray diffraction The crystalline phases formed in the isothermal annealing of the amorphous Zr60Al15Ni25 alloy at 713 K were examined by XRD. The results are illustrated in Fig. 1. During the first reaction, besides one group of diffraction peaks whose positions are fixed from the beginning to the end, there are no new diffraction peaks to occur at other positions (Fig. 1d and e). Elongation of the annealing time makes the diffraction strength enhanced. Additional diffraction peaks appear only when the second reaction has started. As the second reaction proceeds, no other diffraction peaks appear (Fig. 1f and g). These XRD results verify the conclusion obtained in the DSC measurement that the isothermal crystallization of the Zr60Al15Ni25 ribbon at temperature below 713 K consists of two crystallization reactions separated completely. The XRD patterns of the fully crystallized Zr50Al10Ni40 and Zr66Al22Ni12 ribbons (isochronally heated up to 798 K) are shown in Fig. 1h and i, respectively. Evidently, the crystalline phase formed in the first exothermic reaction of the Zr60Al15Ni25 ribbon is identical with the crystallization product of the Zr66Al22Ni12 ribbons, while that formed in the second reaction is identical with the crystallization product of the Zr50Al10Ni40 ribbon.

(P42/m) if the alloy is further annealed for 2 h at 1023 K. Detailed descriptions of the superstructure are presented in Ref. [11]. In the present work, the crystalline Zr50Al10Ni40 sample was obtained by continuously heating the amorphous Zr50Al10Ni40 ribbon at 20 K/min to 798 K. Its diffraction pattern can be well fitted by the U3Si2-typed superstructure. Therefore, we can conclude from the results shown in Fig. 1 that the phase formed in the first crystallization process of the amorphous Zr60Al15Ni25 alloy is Zr6Al2Ni, while the second crystallization process results in the superstructured Zr5AlNi4. 4.2. Crystallization kinetics For the analysis of transformation kinetics, enthalpy of the material subject to investigation is often traced as a function of time and temperature. The transformed fraction can be expressed by the ratio of the partial enthalpy to the total enthalpy of transformation so long as the transformation is iso-kinetic, namely proceeds under the same mechanism. The DSC curves shown in Fig. 4 clearly indicate that the crystallization of amorphous Zr60Al15Ni25 alloy is composed of two reactions that are separated completely when isothermal annealing is performed at low temperatures. Thus each reaction can be thought to be iso-kinetic, and the transformed fraction can be calculated directly by integrating the separate heat signals. The results are shown in Fig. 5. For each reaction, the transformed fraction x at a certain temperature should be described by the JohnsoneMehleAvrami equation [12,13].

4. Discussion

xðtÞ ¼ 1  expððKtÞn Þ

4.1. Identification of the crystallized phases

where t is the time, n is an exponent which reflects the crystallization mechanism as well as the dimensionality of this process, and K is a rate constant. K may be expected to exhibit an Arrhenius temperature dependence:

A lot of intermetallic compounds can form in the ZreAle Ni system. Although it has been established that Zr5AlNi4, Zr6Al2Ni and Zr2Ni are the equilibrium phases at 1073 K around the composition of Zr60Al15Ni25 [10], what will form during the crystallization is dependent on the kinetic conditions and can only be determined by experiments. By comparing the Bragg diffraction peaks of the crystallized Zr60Al15Ni25 ribbon with those of the crystalline Zr50Al10Ni40 and Zr66Al22Ni12 alloys, Li et al. [9] concluded that the crystallization of amorphous Zr60Al15Ni25 results in the simultaneous precipitation of Zr5AlNi4 and Zr6Al2Ni. Considering that the structure of Zr6Al2Ni had been known, their argument for the Zr6Al2Ni formation is credible. However, if it is noted that the data of the crystalline Zr5AlNi4 structure had not been established before their work, the formation of the Zr5AlNi4 phase hangs in doubt because in this case it is insufficient to claim that the Bragg diffraction peaks obtained from the crystalline Zr50Al10Ni40 alloy can be fitted by a crystalline phase. Such a doubt is eliminated owing to the recent work of Leineweber et al. [11]. According to their results, the phase formed in the isochronal crystallization of amorphous Zr50Al10Ni40 is a superstructure of the U3Si2 type (P42/mnm) which then transfers into a thermodynamically stable form

K ¼ K0 expð  E=RTÞ

ð1Þ

ð2Þ

where E is the effective activation energy, T is the absolute temperature and R is the gas constant. For isothermal crystallization, a plot of ln(ln (1  x )) vs ln t gives a straight line. Its slope is the Avrami exponent n (Fig. 6). The average value of n determined as such is 2.74 for the first reaction and 3.02 for the second one. As described above, the primary phase during the crystallization of amorphous Zr60Al15Ni25 alloy is the Zr6Al2Ni compound whose composition is different evidently from that of the glassy matrix. So its formation is a primary crystallization and is controlled by the volume diffusion of solute. An Avrami exponent of 2.74 indicates that the crystallization of Zr6Al2Ni is realized by an increasing nucleation rate [12]. The second crystallization process of amorphous Zr60Al15Ni25 alloy corresponds to the transformation of the residual amorphous phase into Zr5AlNi4. Since the Zr6Al2Ni phase has stopped growing in the second exothermic reaction, the formation of Zr5AlNi4 in the residual amorphous phase is a polymorphic reaction. An

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lnt (s) Fig. 5. Variation of crystallized volume fraction with time in the first (a) and second (b) reactions of amorphous Zr60Al15Ni25. The lines represent the fitting to the experimental results by the JMA equation.

Avrami exponent of 3.02 means that the crystallization of Zr5AlNi4 proceeds by the direct growth of the existing nuclei. Effective activation energy for a crystallization process can be obtained by analyzing the time necessary to obtain an identical amount of crystallized phases at various temperatures [13]. From the time intervals in which the volume fraction of the investigated phase increases from 20% up to 80% (denoted by t20 and t80, respectively), plots of ln(t80  t20) vs 1/T are drawn for the two crystallization processes of the amorphous Zr60Al15Ni25 alloy, and the results are shown in Fig. 7. Their slopes give an activation energy of 346 kJ/mol for Zr6Al2Ni and 330 kJ/mol for Zr5AlNi4. Fitting the experimental results of the crystallization fraction variation at various temperatures with the obtained n and E, a satisfied agreement is achieved (see Fig. 5), indicating a reasonable deduction of the values of n and E. During investigating the crystallization kinetics of an amorphous Zr50Al10Ni40 alloy [14], it has been revealed that the Avrami exponent for the Zr5AlNi4 formation is about 4, and the effective activation energy is 450 kJ/mol. An Avrami exponent of 4 in the polymorphic transformation means a constant nucleation rate [12]. Obviously, the crystallization mechanism

Fig. 6. Plots of ln(ln(1  x)) vs ln t for the first (a) and second (b) crystallization reactions of amorphous Zr60Al15Ni25.

of Zr5AlNi4 in the residual amorphous phase of the Zr60Al15Ni25 alloy is different from that in the amorphous Zr50Al10Ni40 alloy. The effective activation energy 330 kJ/ mol in the crystallization of amorphous Zr60Al15Ni25 alloy is only related with crystal growth. In the present state, although we cannot assure that the intermetallic compound formed in the second crystallization reaction of amorphous Zr60Al15Ni25 alloy has a stoichiometric composition of Zr50Al10Ni40, so large a difference between the values of activation energy 330 and 450 kJ/mol still indicates that the activation of atoms during the nucleation of the Zr5AlNi4 phase with a superstructure is very difficult. 4.3. Primary phase formation Metastable phases often occur in the crystallization of amorphous Zr-based alloys. During the investigation of the crystallization of amorphous NiZr2, Sutton et al. [15] found that a transient structural precursor, i.e. a poorly ordered version of the final crystalline structure, was present. This precursor identified to be fcc Zr2Ni phase exists at the interface between the

J.F. Li et al. / Intermetallics 15 (2007) 1013e1019

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short-range orders are assumed to be dominant in amorphous alloys [18]. Therefore, Zr6Al2Ni rather than Zr5AlNi4 forms as the primary phase in the crystallization of amorphous Zr60Al15Ni25. In the case where the crystallization is a polymorphic transformation, the activation energy for the Zr5AlNi4 formation has been as large as 450 kJ/mol. If Zr5AlNi4 were assumed to form as the primary phase in the crystallization of amorphous Zr60Al15Ni25 alloy, a larger activation energy would be predicted. The primary phase formation is also affected by the diffusion of elemental atoms. Although the compositions of both Zr6Al2Ni and Zr5AlNi4 are all different from Zr60Al15Ni25, their formation as the primary phase needs volume diffusion of different solute atoms. From their stoichiometric equations it can be found that 32.5% of atoms, comprised of 46% Zr and 54% Ni, is required to diffuse out of the new phase if Zr6Al2Ni crystallizes from the glassy matrix. When the primary phase is Zr5AlNi4, 37.5% of atoms, comprised of 77% Zr and 23% Al, must diffuse out of the new phase. It has been shown that the diffusion coefficients of Al and especially Zr in the supercooled Zr-based alloy melts are greatly lower than that of Ni when the temperature just exceeds the glass transition temperature [19]. So the solute diffusion accompanied by the growth of Zr6Al2Ni is also easier than that of Zr5AlNi4, and Zr6Al2Ni rather than Zr5AlNi4 which forms in the first crystallization stage of the amorphous Zr60Al15Ni25 alloy. 5. Conclusions

1.405

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1/T (10-3 K-1) Fig. 7. Plots for calculation of the effective crystallization energies in the first (a) and second (b) crystallization reactions of amorphous Zr60Al15Ni25.

growing crystal phase and the amorphous phase, and is formed because it is structurally closer to the amorphous than the equilibrium phase which exhibits bct crystal structure. The fcc Zr2Ni was also reported by Ko¨ster and Meinhardt [7] to form in the crystallization of amorphous Zr60Al15Ni25 alloy in the temperature range from 673 to 781 K. Our crystallization experiment was also conducted in this temperature range, but no trace of the metastable fcc Zr2Ni phase was found. The XRD patterns at every stage of crystallization can only be fitted by Zr6Al2Ni or/and Zr5AlNi4. A lot of experiments on the binary Zr-based alloys has demonstrated that the formation of metastable fcc Zr2Ni phase can be enhanced by the introduction of oxygen [15,16]. A similar phenomenon was also observed in the multi-component glass-forming alloys, where metastable icosahedral phase rather than fcc phase often formed as the primary product [17]. In our experiment the level of oxygen was controlled below 320 ppm. Such low content of oxygen is not enough to lead to the formation of the metastable fcc or icosahedral phases. Zr6Al2Ni is of hexagonal structure, but Zr5AlNi4 possesses a complex superstructure when the annealing is performed at lower temperatures. It has been known that, of all the crystalline structures, face-centered cubic and hexagonal structures are the nearest to the amorphous structure if icosahedral

(1) The crystallization of amorphous Zr60Al15Ni25 alloy comprises a primary reaction and subsequently a polymorphic reaction, corresponding to the precipitation of Zr6Al2Ni and Zr5AlNi4 that possesses the U3Si2-typed superstructure, respectively. The two reactions are completely separated during the isothermal annealing below 713 K, but partially overlapped at higher temperatures. (2) The Avrami exponents for the crystallization of Zr6Al2Ni and Zr5AlNi4 in the amorphous Zr60Al15Ni25 equal 2.74 and 3.02, indicating a decreasing nucleation rate and a zero nucleation during crystallization, respectively. The corresponding effective activation energies are determined to be 346 and 330 kJ/mol. (3) Zr6Al2Ni rather than Zr5AlNi4 crystallizes as a primary phase from the amorphous Zr60Al15Ni25 alloy owing to two reasons. First, the topological transition from an amorphous structure to the superstructured Zr5AlNi4 is more difficult than that to the hexagonal Zr6Al2Ni. Second, the formation of Zr5AlNi4 requires more Al and Zr atoms to diffuse, while their mobilities in the alloy are very low.

Acknowledgements Financial supports from the National Natural Science Foundation of China under Grant Nos. 50071032 and 50441014, and the Scientific Research Foundation for ROCS, SEM are acknowledged. J.F. Li thanks Prof. F. Sommer and

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Prof. E.J. Mittemeijer for their fruitful discussion and the permission to carry out part of the experiments at the Max-Planck Institute for Metals Research in Stuttgart, Germany. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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