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Effect of Ga substitution on the crystallization behaviour and glass forming ability of Zr–Al–Cu–Ni alloys
Materials Science and Engineering A 527 (2010) 469–473
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of Ga substitution on the crystallization behaviour and glass forming ability of Zr–Al–Cu–Ni alloys Devinder Singh a , T.P. Yadav a , R.K. Mandal b , R.S. Tiwari a , O.N. Srivastava a,∗ a b
Department of Physics, Nano-Science and Technology Unit, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India Department of Metallurgical Engineering, Nano-Science and Technology Unit, Institute of Technology, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India
a r t i c l e
i n f o
Article history: Received 6 August 2009 Accepted 10 October 2009
Keywords: Metallic glasses Nano-quasicrystals Glass forming ability Crystallization Zr–Al–Ga–Cu–Ni alloys
a b s t r a c t The crystallization behaviour of melt spun Zr69.5 Al7.5−x Gax Cu12 Ni11 (x = 0–7.5; in at.%) metallic glasses has been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The DSC traces showed changes in crystallization behaviour with substitution of Ga. Formation of single nano-quasicrystalline phase by controlled crystallization of glasses has been found only for 0 ≤ x ≤ 1.5. Further increase of Ga content gives rise to formation of the quasicrystals together with Zr2 Cu type crystalline phase. In addition to this, the substitution of Ga influences the size and shape of nano-quasicrystals. The glass forming abilities (GFAs) of these metallic glasses were assessed by the recognition of glass forming ability indicators, i.e. reduced glass transition temperature (Trg ) and supercooled liquid region (Tx ). The glass transition temperature (Tg ) has been observed for all the melt spun ribbons. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Both quasicrystal forming alloys and bulk metallic glasses (BMGs) giving rise to quasicrystalline phase on annealing has attracted attention owing to their promise to qualify for many potential applications. They possess high hardness, low coefficient of sliding friction, low adhesion, excellent wear resistance, high corrosion resistance, etc. [1–6]. Inoue proposed three empirical rules [7] for the synthesis of amorphous alloys with high thermal stability and good glass forming ability (GFA): these rules relate to (a) multicomponent systems consisting of more than three elements, (b) significant difference in atomic size ratios above about 12% and (c) negative heats of mixing among the three main constituent elements. However, they were found to be insufficient [8] for determining the exact alloy composition having best GFA and highest thermal stability. Therefore, another criterion of average valence electron per atom (e/a ratio ∼1.4) and constant atomic size Ra (∼0.1496 nm) of an alloy was proposed [9–12]. Azad et al. [13] have classified BMGs with the help of Venn diagram. Multicomponent Zr-based bulk glass forming alloys exhibit icosahedral quasicrystalline precipitates in the amorphous matrix upon annealing for a variety of alloy compositions [14–20]. The first report on formation of quasicrystalline phase by partial crystallization of a Zr–Al–Cu–Ni metallic glass was given by Koster et al. [14]. The Zr–Al–Cu–Ni composition is one of the good cur-
∗ Corresponding author. Tel.: +91 5422368468; fax: +91 5422369889. E-mail address: [email protected] (O.N. Srivastava). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.10.020
rently known glass forming alloys [21]. Annealing of amorphous Zr69.5 Al7.5 Cu12 Ni11 alloy above the glass transition temperature leads to the formation of quasicrystalline phase [14,22]. The quasicrystalline phase has a fine grain size in the range of ∼50–100 nm. The dispersion of these nano-icosahedral particles in the bulk glassy alloy significantly enhances their mechanical properties [23,24]. Khare et al. [25] reported the influence of alloying on the formation of Zr-based quasicrystals. The precipitation of an icosahedral quasicrystalline phase (I-phase) upon devitrification has been reported for Zr–Al–Cu–Ni–Ti [15,26], Zr–Ti–Cu–Ni [27] and Zr–Ti–Cu–Ni–Be alloys [16]. In particular, further addition of elements, such as Ag, Pd, Au or Pt to Zr–Al–Cu–Ni glasses is believed to generate inhomogeneous atomic configuration regions including short range order of icosahedral configurations in the supercooled liquid [17–20,28], which then promotes the precipitation of an icosahedral phase. Recently the effect of Ti addition on the crystallization behaviour and glass forming ability of Zr–Al–Cu alloys has been investigated [29]. Although considerable work has been done on these alloys and some multicomponent alloys have been discovered with promising properties, still more substitution studies are required for the Zr–Al–Cu–Ni amorphous alloy from both the scientific and application points of view. Keeping the above said considerations in view, we have substituted Al by Ga in Zr–Al–Cu–Ni alloy system. It may be noted that the effect of Ga substitution on the glass/quasicrystal forming ability of Fe-based bulk glassy as well as Al–Pd–Mn alloys have already been reported [30,31]. The reason for substitution of Al by Ga is that both are lying in the same group of the periodic table and having same valancy (+3). Thus the substitution of Al by Ga does not change the
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D. Singh et al. / Materials Science and Engineering A 527 (2010) 469–473
e/a ratio of Zr–Al–Cu–Ni alloy system (e/a = 1.39). The atomic size of Ga (1.41 Å) and Al (1.43 Å) are also comparable. In our present investigation, Zr69.5 Al7.5−x Gax Cu12 Ni11 amorphous alloys with 0 ≤ x ≤ 7.5 have been studied. The aim of the present work is to examine the influence of Ga on the crystallization behaviour; glass forming ability and thermal stability of Zr–Al–Cu–Ni melt spun glassy ribbons. The changes observed with increasing Ga content on phase transformations are also reported. The effect of Ga substitution on the morphology of quasicrystalline phase during crystallization of melt spun ribbons will also be presented. 2. Experimental Alloy ingots of compositions Zr69.5 Al7.5−x Gax Cu12 Ni11 (x = 0, 0.5, 1.5, 2.5, 5.0, 7.5 at.%) were prepared in argon atmosphere by melting high purity Zr (99.9%), Al (99.96%), Ga (99.99%), Cu (99.99%) and Ni (99.99%) in a silica crucible using a radio-frequency induction furnace. The ingots were remelted several times to improve the homogeneity. To convert the ingots into ribbons, they were placed in a silica nozzle tube with a circular orifice of ∼1 mm diameter. The alloys were then melt spun onto a copper wheel (∼14 cm diameter) rotating at a speed of 40 m/s. During melt spinning the entire apparatus was enclosed in a steel enclosure through which argon gas was made to flow continuously. This was done to prevent oxidation of the ribbons after ejection from the nozzle. After melt spinning, long ribbons were formed. The length and thickness of the ribbons were ∼2 m and ∼40 m, respectively. The ribbons were very flexible and could be bent by 180◦ without breaking. These ribbons of Zr69.5 Al7.5−x Gax Cu12 Ni11 alloys were then packed in a Ta foil which was sealed in a silica ampoule under an argon atmosphere for annealing experiment. The thermal stability of the samples was investigated by differential scanning calorimetry (DSC) at heating rate of 20 K/min with a PerkinElmer DSC7 under a continuous flow of purified argon. The structural characterization was done by employing an Xray diffractometer (X’Pert Pro PANalytical diffractometer) with Cu K␣ radiation. The experimental conditions and parameters (scan speed, etc.) were kept same for all diffraction experiments performed on different samples. Isothermal annealing of the ribbons was carried out in a vacuum (10−6 Torr) using a Heraeus furnace with temperature control of ±1 ◦ C. The as-synthesized as well as annealed ribbons were thinned using an electrolyte, (90% methanol and 10% perchloric acid) at −30 ◦ C. In order to characterize the as-grown ribbons, transmission electron microscopy (TEM) of the ribbons was carried out. The samples were studied by TEM using Philips: CM-12 and FEI: Technai 20G2 electron microscope. 3. Results and discussion 3.1. Crystallization behaviour The Zr69.5 Al7.5−x Gax Cu12 Ni11 (x = 0–7.5 at.%) melt spun ribbons with a thickness of about 40 m were observed to be fully
Fig. 1. (a) XRD pattern of the as-synthesized Zr69.5 Al7.5−x Gax Cu12 Ni11 ribbons; (b) TEM image and the corresponding diffraction pattern of as-synthesized, Zr69.5 Al6 Ga1.5 Cu12 Ni11 alloy.
amorphous by means of X-ray diffraction as well as by TEM. Representative XRD patterns of melt spun alloys are shown in Fig. 1(a). This indicates that a fully amorphous state was obtained by melt quenching. Fig. 1(b) and the inset show the TEM micrograph and corresponding selected area (SAD) diffraction pattern displaying diffuse halos for Zr69.5 Al6 Ga1.5 Cu12 Ni11 (x = 1.5) alloy. We note the absence of residual contrast in the bright field image. This clearly indicates the formation of glassy phase in the system and similar features were observed for all the alloys (x = 0–7.5 at.%). Characterization of the thermal stability of the alloys, especially determination of glass transition temperature Tg and onset of crystallization temperature Tx were studied. Fig. 2(a) displays the DSC scans for the samples with x = 0–7.5 recorded at a heating rate of
Table 1 Thermal analysis data of the melt spun Zr69.5 Al7.5−x Gax Cu12 Ni11 (x = 0–7.5 at.%) ribbons. Alloys (at.%)
Tg (K)
Tx1 (K)
Tx2 (K)
Tx (K)
Tp1 (K)
Tp2 (K)
Tm (K)
Trg = Tg /Tm
x=0 x = 0.5 x = 1.5 x = 2.5 x = 5.0 x = 7.5
624 619 618 616 615 614
702 692 687 679 676 673
727 726 708 – – –
78 72 69 63 61 59
714 706 699 715 715 716
745 738 716 – – –
1064 1071 1077 1088 1114 1131
0.586 0.578 0.574 0.567 0.552 0.543
Tg : glass transition temperature; Tx1 : first crystallization temperature; Tx2 : second crystallization temperature; Tx : supercooled liquid region; Tp1 : first exothermic peak; Tp2 : second exothermic peak; Tm : melting temperature; Trg : reduced glass transition temperature.
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Fig. 3. XRD patterns of Zr69.5 Al7.5−x Gax Cu12 Ni11 ribbons annealed for 15 min.
Fig. 2. (a) DSC traces of the melt spun Zr69.5 Al7.5−x Gax Cu12 Ni11 alloys at the heating rate of 20 K/min; (b) composition dependence of Tg , Tx and Tx = Tx − Tg of the melt spun Zr69.5 Al7.5−x Gax Cu12 Ni11 glassy alloys.
20 K/min. Table 1 summarises the thermal stability data for all the investigated samples. A smeared endothermic peak of the glass transition is observed at almost the same temperature for all the samples when DSC traces and thermal properties are considered (Fig. 2(b)). This confirms the results of Johnson and Peker [32] that Tg varies only slightly for an alloy system. In contrast, significant differences in crystallization behaviour, with increasing Ga content in the samples, were detected. It can be seen from Fig. 2 that two metastable stages I and II (MS-I and MS-II) having peak positions Tp1 and Tp2 exist. As Ga content increases, Tp2 shift towards Tp1 and finally second peak merged into first peak and appears as a single peak for x = 2.5–7.5. The detailed thermal data analysis is given in Table 1. Thus, the addition of Ga for x > 1.5 leads to single stage crystallization, which is different from the two stage crystallization for lower Ga content, i.e. x ≤ 1.5. The primary crystallization products of these melt spun glasses were examined after isothermal annealing. Fig. 3 compares XRD patterns for various alloys, heated either at a temperature slightly higher than the beginning of the first exothermic event Tx1 (for samples x = 0–1.5 at.%) or at the first exothermic peak Tp1 (for samples x = 2.5–7.5 at.%). The XRD pattern of the sample with Ga content of 2.5 at.% shows significant peak broadening in comparison to the samples of lower Ga content. These effects are pronounced by further increasing the Ga concentration. In particular broadening of the reflections is mainly caused by a noticeable change in the crystallite size. To evaluate the phase formation upon annealing for high Ga contents (x ≥ 2.5 at.%), the XRD patterns of samples
x ≥ 2.5 at.% heat treated near Tp1 were analysed. For such samples, reflection corresponding to crystalline phases is more pronounced. Formation of quasicrystalline phase was also observed in the crystallization processes of Ga bearing metallic glasses. For the samples with 0 ≤ x ≤ 1.5, only quasicrystalline phase was observed when heated to first crystallization temperature Tx1 . These samples display intense sharp peaks and several weaker peaks, which could be indexed on the basis of quasicrystalline phase [33]. The samples (x = 0–1.5 at.%) heated to the second crystallization temperature Tx2 leads to the formation of crystalline phase of Zr2 Cu type. A higher Ga content of 2.5 at.% leads to the precipitation of additional crystalline phase of Zr2 Cu type besides the quasicrystalline phase. At higher Ga contents (i.e. x ≥ 2.5) Zr2 Cu type crystalline phase showed up as the majority phase. The formation of quasicrystalline phase in these samples was further investigated by TEM. Fig. 4(a) shows nanometer sized grains of the annealed sample of Zr69.5 Al7.5 Cu12 Ni11 (x = 0) alloy. The corresponding selected area diffraction pattern along the 5-fold direction confirmed quasicrystalline phase. The size of these nano-quasicrystals is in the range 100–150 nm. Fig. 4(b) displays a nanometer scaled microstructure of the annealed Zr69.5 Al6 Ga1.5 Cu12 Ni11 (x = 1.5) alloy. It is interesting to note that the substitution of Ga changes the shape and size of nanoquasicrystals significantly. A non-faceted (nearly spherical) to faceted type of morphological transition has been observed (Fig. 4). Further, the average grain size of the quasicrystals decreases from ∼125 to ∼80 nm. The 5-fold, 3-fold and 2-fold zone axes diffraction patterns confirmed that the nanoparticles (Fig. 4b) correspond to quasicrystalline phase. The presence of diffuse ring in these diffraction patterns reveal that the nano-quasicrystals are embedded in the remaining glassy phase which does not completely transform during the exothermic MS-I event. Fig. 5(a) and (b) presents bright field TEM images of samples with x = 2.5 and 5.0 heated to Tp1 , illustrating the strong reduction of grain with increasing Ga. The bright field TEM image of the alloy with x = 2.5 reveals an inhomogeneous microstructure showing that quasicrystal and crystalline phases co-exist. These results indicate that Ga has a distinct effect on the precipitation of quasicrystals and decreases the size of the icosahedral particles in Zr–Al–Ga–Cu–Ni metallic glass. The changed crystallization mode and the products of the resultant metallic glass suggest a significant change in the local structure of their undercooled liquids. For x ≤ 1.5 the local structure may be possessing
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Fig. 5. Bright field TEM images of samples x = 2.5 (a) and x = 5.0 (b). Inset of (a) shows selected area diffraction pattern of the quasicrystalline (qc) phase along the 5-fold direction.
Fig. 4. Bright field TEM images of samples x = 0 (a) and x = 1.5 (b) showing the influence of the Ga content on size and morphology of the nano-quasicrystalline phase formed upon heat treatment. The selected area diffraction patterns for x = 1.5 show the 5-fold, 3-fold and 2-fold diffractions of the quasicrystalline phase.
icosahedral symmetry giving rise to glass to quasicrystalline phase transformation at Tx1 . In contrast, for the samples having x > 1.5 quasicrystalline and crystalline phases form simultaneously. In view of the different types of phase formations and morphological transitions observed in the present studies after heat treatment of metallic glasses, it may be said that nucleation and growth characteristics of MS-I and MS-II phases are affected owing to Ga substitution. It may be pointed out here that interfacial energy per unit area of Al (∼1.2 J/m2 ) is higher than that of Ga (∼0.6 J/m2 ). Thus, Ga substitution may be reducing the interfacial energy between quasicrystal and remaining amorphous phase. 3.2. Glass forming ability The difference in GFA of these alloys was assessed by the recognized indicators such as Trg = Tg /Tm [34] and Tx = Tx − Tg [7], where Tm is the melting temperature of the alloy. The melting processes of these metallic glasses were studied using DSC (Fig. 2(a)). It was found that the onset melting temperature (Tm ) continuously
increases with Ga substitution. The calculated indicator values for GFA assessment are included in Table 1. The glass transition temperature (Tg ) has been observed for all the alloys. The supercooled liquid region Tx which reflects the thermal stability of the supercooled liquid towards crystallization, varies slightly with alloy composition (Fig. 2(b)) With increasing Ga content Tx1 shifts to lower temperatures and the lowest value has been determined for sample x = 7.5, Tx = 59 K. Trg slightly decreases as Ga content increases. Due to the lowered Tx of these glasses, Trg and Tx manifest a decreasing tendency with the increase of Ga content. The high thermal stability of the samples with low Ga content is connected with a low nucleation rate, due to a highly dense packed structure of the supercooled liquid and therefore, limited mobility of the atoms, i.e. limited diffusion. As shown in DSC traces the alloy with composition Zr69.5 Ga7.5 Cu12 Ni11 (x = 7.5) displays the glass transition temperature (Tg ) at 614 K which is lower than 624 K for Zr69.5 Al7.5 Cu12 Ni11 glassy alloy (x = 0). This means a new metallic glass composition with the complete substitution of Al by Ga has been discovered. If we compare Tx and Tg /Tm values of BMGs for Zr–Al–Cu–Ni systems, then we note that in our case the maximum value of Tx has been observed for x = 0. Further the ratio of Tg /Tm in our case is always less than those reported in Ref. [35]. It may be noted that Iqbal et al. [35] have utilised Cu-mould casting whereas we have employed rapid solidification technique for producing glasses. Thermal analysis data for these melt spun glassy alloys are given in Table 1. The well recognized GFA indicators Trg and Tx gave GFA assessment for the product alloys. To understand the very compositional sensitive crystallization behaviours of these metallic glasses, more experimental information on their crystallization kinetics is necessary. The present alloys, in which phase formation and transformation information is clear, are a useful system for further crystallization kinetics modelling. 3.3. Conclusions Varying the Ga content from 0 to 7.5 at.% in Zr69.5 Al7.5−x Gax Cu12 Ni11 alloys results in significantly differ-
D. Singh et al. / Materials Science and Engineering A 527 (2010) 469–473
ent devitrification behaviours of the samples. The substitution of Ga changes the morphology of nano-quasicrystals. A noticeable change in thermal stability and size of precipitates is observed with increasing Ga content. Formation of single quasicrystalline phase by annealing the glass has been found only for the Zr69.5 Al7.5−x Gax Cu12 Ni11 alloys with 0 ≤ x ≤ 1.5. Slightly higher Ga content (i.e. x > 1.5) results in almost simultaneous formation of the quasicrystalline phase together with crystalline phase of Zr2 Cu type. The well recognised GFA indicators Trg and Tx are only slightly affected by Ga substitution for the present alloys. Acknowledgements The authors of the paper are thankful to Prof. S. Ranganathan, Dr. N.K. Mukhoupadhyay and Dr. M.A. Shaz for many stimulating discussions. We are also thankful to Mr. Vijay Kumar for his technical assistance. Financial assistance from Department of Science and Technology (UNANST, DST), University Grant Commission (UGC) and Council of Scientific and Industrial Research (CSIR) are also gratefully acknowledged. References [1] J.M. Dubois, S.S. Kang, Y. Massiani, Journal of Non-Crystalline Solids 153 (1993) 443–445. [2] S.S. Kang, J.M. Dubois, J. Von Stebut, Journal of Materials Research 8 (1993) 2471–2481. [3] R. Wittmann, K. Urban, M. Schandl, E. Hombogen, Journal of Materials Research 6 (1991) 1165–1168. [4] A. Inoue, T. Zhang, J. Saida, M. Matsushita, Materials Transactions JIM 44 (2003) 1978. [5] K. Urban, M. Feverbacher, M. Wollgarten, MRS Bulletin 22 (1997) 65–68. [6] U. Kamachi Mudali, S. Scudino, U. Kuhu, J. Eckert, A. Gebert, Scripta Materialia 50 (2004) 1379–1384. [7] A. Inoue, Acta Materialia 48 (2000) 279–306. [8] C.T. Liu, Z.P. Lu, Intermetallics 13 (2005) 415–418. [9] W. Chen, Y. Wang, J. Qiang, C. Dong, Acta Materialia 51 (2003) 1899–1907. [10] Y.M. Wang, W.P. Xu, J.B. Qiang, C.H. Wang, C.H. Shek, C. Dong, Materials Science and Engineering A 375–377 (2004) 411–416.
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Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of Ga substitution on the crystallization behaviour and glass forming ability of Zr–Al–Cu–Ni alloys Devinder Singh a , T.P. Yadav a , R.K. Mandal b , R.S. Tiwari a , O.N. Srivastava a,∗ a b
Department of Physics, Nano-Science and Technology Unit, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India Department of Metallurgical Engineering, Nano-Science and Technology Unit, Institute of Technology, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India
a r t i c l e
i n f o
Article history: Received 6 August 2009 Accepted 10 October 2009
Keywords: Metallic glasses Nano-quasicrystals Glass forming ability Crystallization Zr–Al–Ga–Cu–Ni alloys
a b s t r a c t The crystallization behaviour of melt spun Zr69.5 Al7.5−x Gax Cu12 Ni11 (x = 0–7.5; in at.%) metallic glasses has been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The DSC traces showed changes in crystallization behaviour with substitution of Ga. Formation of single nano-quasicrystalline phase by controlled crystallization of glasses has been found only for 0 ≤ x ≤ 1.5. Further increase of Ga content gives rise to formation of the quasicrystals together with Zr2 Cu type crystalline phase. In addition to this, the substitution of Ga influences the size and shape of nano-quasicrystals. The glass forming abilities (GFAs) of these metallic glasses were assessed by the recognition of glass forming ability indicators, i.e. reduced glass transition temperature (Trg ) and supercooled liquid region (Tx ). The glass transition temperature (Tg ) has been observed for all the melt spun ribbons. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Both quasicrystal forming alloys and bulk metallic glasses (BMGs) giving rise to quasicrystalline phase on annealing has attracted attention owing to their promise to qualify for many potential applications. They possess high hardness, low coefficient of sliding friction, low adhesion, excellent wear resistance, high corrosion resistance, etc. [1–6]. Inoue proposed three empirical rules [7] for the synthesis of amorphous alloys with high thermal stability and good glass forming ability (GFA): these rules relate to (a) multicomponent systems consisting of more than three elements, (b) significant difference in atomic size ratios above about 12% and (c) negative heats of mixing among the three main constituent elements. However, they were found to be insufficient [8] for determining the exact alloy composition having best GFA and highest thermal stability. Therefore, another criterion of average valence electron per atom (e/a ratio ∼1.4) and constant atomic size Ra (∼0.1496 nm) of an alloy was proposed [9–12]. Azad et al. [13] have classified BMGs with the help of Venn diagram. Multicomponent Zr-based bulk glass forming alloys exhibit icosahedral quasicrystalline precipitates in the amorphous matrix upon annealing for a variety of alloy compositions [14–20]. The first report on formation of quasicrystalline phase by partial crystallization of a Zr–Al–Cu–Ni metallic glass was given by Koster et al. [14]. The Zr–Al–Cu–Ni composition is one of the good cur-
∗ Corresponding author. Tel.: +91 5422368468; fax: +91 5422369889. E-mail address: [email protected] (O.N. Srivastava). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.10.020
rently known glass forming alloys [21]. Annealing of amorphous Zr69.5 Al7.5 Cu12 Ni11 alloy above the glass transition temperature leads to the formation of quasicrystalline phase [14,22]. The quasicrystalline phase has a fine grain size in the range of ∼50–100 nm. The dispersion of these nano-icosahedral particles in the bulk glassy alloy significantly enhances their mechanical properties [23,24]. Khare et al. [25] reported the influence of alloying on the formation of Zr-based quasicrystals. The precipitation of an icosahedral quasicrystalline phase (I-phase) upon devitrification has been reported for Zr–Al–Cu–Ni–Ti [15,26], Zr–Ti–Cu–Ni [27] and Zr–Ti–Cu–Ni–Be alloys [16]. In particular, further addition of elements, such as Ag, Pd, Au or Pt to Zr–Al–Cu–Ni glasses is believed to generate inhomogeneous atomic configuration regions including short range order of icosahedral configurations in the supercooled liquid [17–20,28], which then promotes the precipitation of an icosahedral phase. Recently the effect of Ti addition on the crystallization behaviour and glass forming ability of Zr–Al–Cu alloys has been investigated [29]. Although considerable work has been done on these alloys and some multicomponent alloys have been discovered with promising properties, still more substitution studies are required for the Zr–Al–Cu–Ni amorphous alloy from both the scientific and application points of view. Keeping the above said considerations in view, we have substituted Al by Ga in Zr–Al–Cu–Ni alloy system. It may be noted that the effect of Ga substitution on the glass/quasicrystal forming ability of Fe-based bulk glassy as well as Al–Pd–Mn alloys have already been reported [30,31]. The reason for substitution of Al by Ga is that both are lying in the same group of the periodic table and having same valancy (+3). Thus the substitution of Al by Ga does not change the
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e/a ratio of Zr–Al–Cu–Ni alloy system (e/a = 1.39). The atomic size of Ga (1.41 Å) and Al (1.43 Å) are also comparable. In our present investigation, Zr69.5 Al7.5−x Gax Cu12 Ni11 amorphous alloys with 0 ≤ x ≤ 7.5 have been studied. The aim of the present work is to examine the influence of Ga on the crystallization behaviour; glass forming ability and thermal stability of Zr–Al–Cu–Ni melt spun glassy ribbons. The changes observed with increasing Ga content on phase transformations are also reported. The effect of Ga substitution on the morphology of quasicrystalline phase during crystallization of melt spun ribbons will also be presented. 2. Experimental Alloy ingots of compositions Zr69.5 Al7.5−x Gax Cu12 Ni11 (x = 0, 0.5, 1.5, 2.5, 5.0, 7.5 at.%) were prepared in argon atmosphere by melting high purity Zr (99.9%), Al (99.96%), Ga (99.99%), Cu (99.99%) and Ni (99.99%) in a silica crucible using a radio-frequency induction furnace. The ingots were remelted several times to improve the homogeneity. To convert the ingots into ribbons, they were placed in a silica nozzle tube with a circular orifice of ∼1 mm diameter. The alloys were then melt spun onto a copper wheel (∼14 cm diameter) rotating at a speed of 40 m/s. During melt spinning the entire apparatus was enclosed in a steel enclosure through which argon gas was made to flow continuously. This was done to prevent oxidation of the ribbons after ejection from the nozzle. After melt spinning, long ribbons were formed. The length and thickness of the ribbons were ∼2 m and ∼40 m, respectively. The ribbons were very flexible and could be bent by 180◦ without breaking. These ribbons of Zr69.5 Al7.5−x Gax Cu12 Ni11 alloys were then packed in a Ta foil which was sealed in a silica ampoule under an argon atmosphere for annealing experiment. The thermal stability of the samples was investigated by differential scanning calorimetry (DSC) at heating rate of 20 K/min with a PerkinElmer DSC7 under a continuous flow of purified argon. The structural characterization was done by employing an Xray diffractometer (X’Pert Pro PANalytical diffractometer) with Cu K␣ radiation. The experimental conditions and parameters (scan speed, etc.) were kept same for all diffraction experiments performed on different samples. Isothermal annealing of the ribbons was carried out in a vacuum (10−6 Torr) using a Heraeus furnace with temperature control of ±1 ◦ C. The as-synthesized as well as annealed ribbons were thinned using an electrolyte, (90% methanol and 10% perchloric acid) at −30 ◦ C. In order to characterize the as-grown ribbons, transmission electron microscopy (TEM) of the ribbons was carried out. The samples were studied by TEM using Philips: CM-12 and FEI: Technai 20G2 electron microscope. 3. Results and discussion 3.1. Crystallization behaviour The Zr69.5 Al7.5−x Gax Cu12 Ni11 (x = 0–7.5 at.%) melt spun ribbons with a thickness of about 40 m were observed to be fully
Fig. 1. (a) XRD pattern of the as-synthesized Zr69.5 Al7.5−x Gax Cu12 Ni11 ribbons; (b) TEM image and the corresponding diffraction pattern of as-synthesized, Zr69.5 Al6 Ga1.5 Cu12 Ni11 alloy.
amorphous by means of X-ray diffraction as well as by TEM. Representative XRD patterns of melt spun alloys are shown in Fig. 1(a). This indicates that a fully amorphous state was obtained by melt quenching. Fig. 1(b) and the inset show the TEM micrograph and corresponding selected area (SAD) diffraction pattern displaying diffuse halos for Zr69.5 Al6 Ga1.5 Cu12 Ni11 (x = 1.5) alloy. We note the absence of residual contrast in the bright field image. This clearly indicates the formation of glassy phase in the system and similar features were observed for all the alloys (x = 0–7.5 at.%). Characterization of the thermal stability of the alloys, especially determination of glass transition temperature Tg and onset of crystallization temperature Tx were studied. Fig. 2(a) displays the DSC scans for the samples with x = 0–7.5 recorded at a heating rate of
Table 1 Thermal analysis data of the melt spun Zr69.5 Al7.5−x Gax Cu12 Ni11 (x = 0–7.5 at.%) ribbons. Alloys (at.%)
Tg (K)
Tx1 (K)
Tx2 (K)
Tx (K)
Tp1 (K)
Tp2 (K)
Tm (K)
Trg = Tg /Tm
x=0 x = 0.5 x = 1.5 x = 2.5 x = 5.0 x = 7.5
624 619 618 616 615 614
702 692 687 679 676 673
727 726 708 – – –
78 72 69 63 61 59
714 706 699 715 715 716
745 738 716 – – –
1064 1071 1077 1088 1114 1131
0.586 0.578 0.574 0.567 0.552 0.543
Tg : glass transition temperature; Tx1 : first crystallization temperature; Tx2 : second crystallization temperature; Tx : supercooled liquid region; Tp1 : first exothermic peak; Tp2 : second exothermic peak; Tm : melting temperature; Trg : reduced glass transition temperature.
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Fig. 3. XRD patterns of Zr69.5 Al7.5−x Gax Cu12 Ni11 ribbons annealed for 15 min.
Fig. 2. (a) DSC traces of the melt spun Zr69.5 Al7.5−x Gax Cu12 Ni11 alloys at the heating rate of 20 K/min; (b) composition dependence of Tg , Tx and Tx = Tx − Tg of the melt spun Zr69.5 Al7.5−x Gax Cu12 Ni11 glassy alloys.
20 K/min. Table 1 summarises the thermal stability data for all the investigated samples. A smeared endothermic peak of the glass transition is observed at almost the same temperature for all the samples when DSC traces and thermal properties are considered (Fig. 2(b)). This confirms the results of Johnson and Peker [32] that Tg varies only slightly for an alloy system. In contrast, significant differences in crystallization behaviour, with increasing Ga content in the samples, were detected. It can be seen from Fig. 2 that two metastable stages I and II (MS-I and MS-II) having peak positions Tp1 and Tp2 exist. As Ga content increases, Tp2 shift towards Tp1 and finally second peak merged into first peak and appears as a single peak for x = 2.5–7.5. The detailed thermal data analysis is given in Table 1. Thus, the addition of Ga for x > 1.5 leads to single stage crystallization, which is different from the two stage crystallization for lower Ga content, i.e. x ≤ 1.5. The primary crystallization products of these melt spun glasses were examined after isothermal annealing. Fig. 3 compares XRD patterns for various alloys, heated either at a temperature slightly higher than the beginning of the first exothermic event Tx1 (for samples x = 0–1.5 at.%) or at the first exothermic peak Tp1 (for samples x = 2.5–7.5 at.%). The XRD pattern of the sample with Ga content of 2.5 at.% shows significant peak broadening in comparison to the samples of lower Ga content. These effects are pronounced by further increasing the Ga concentration. In particular broadening of the reflections is mainly caused by a noticeable change in the crystallite size. To evaluate the phase formation upon annealing for high Ga contents (x ≥ 2.5 at.%), the XRD patterns of samples
x ≥ 2.5 at.% heat treated near Tp1 were analysed. For such samples, reflection corresponding to crystalline phases is more pronounced. Formation of quasicrystalline phase was also observed in the crystallization processes of Ga bearing metallic glasses. For the samples with 0 ≤ x ≤ 1.5, only quasicrystalline phase was observed when heated to first crystallization temperature Tx1 . These samples display intense sharp peaks and several weaker peaks, which could be indexed on the basis of quasicrystalline phase [33]. The samples (x = 0–1.5 at.%) heated to the second crystallization temperature Tx2 leads to the formation of crystalline phase of Zr2 Cu type. A higher Ga content of 2.5 at.% leads to the precipitation of additional crystalline phase of Zr2 Cu type besides the quasicrystalline phase. At higher Ga contents (i.e. x ≥ 2.5) Zr2 Cu type crystalline phase showed up as the majority phase. The formation of quasicrystalline phase in these samples was further investigated by TEM. Fig. 4(a) shows nanometer sized grains of the annealed sample of Zr69.5 Al7.5 Cu12 Ni11 (x = 0) alloy. The corresponding selected area diffraction pattern along the 5-fold direction confirmed quasicrystalline phase. The size of these nano-quasicrystals is in the range 100–150 nm. Fig. 4(b) displays a nanometer scaled microstructure of the annealed Zr69.5 Al6 Ga1.5 Cu12 Ni11 (x = 1.5) alloy. It is interesting to note that the substitution of Ga changes the shape and size of nanoquasicrystals significantly. A non-faceted (nearly spherical) to faceted type of morphological transition has been observed (Fig. 4). Further, the average grain size of the quasicrystals decreases from ∼125 to ∼80 nm. The 5-fold, 3-fold and 2-fold zone axes diffraction patterns confirmed that the nanoparticles (Fig. 4b) correspond to quasicrystalline phase. The presence of diffuse ring in these diffraction patterns reveal that the nano-quasicrystals are embedded in the remaining glassy phase which does not completely transform during the exothermic MS-I event. Fig. 5(a) and (b) presents bright field TEM images of samples with x = 2.5 and 5.0 heated to Tp1 , illustrating the strong reduction of grain with increasing Ga. The bright field TEM image of the alloy with x = 2.5 reveals an inhomogeneous microstructure showing that quasicrystal and crystalline phases co-exist. These results indicate that Ga has a distinct effect on the precipitation of quasicrystals and decreases the size of the icosahedral particles in Zr–Al–Ga–Cu–Ni metallic glass. The changed crystallization mode and the products of the resultant metallic glass suggest a significant change in the local structure of their undercooled liquids. For x ≤ 1.5 the local structure may be possessing
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Fig. 5. Bright field TEM images of samples x = 2.5 (a) and x = 5.0 (b). Inset of (a) shows selected area diffraction pattern of the quasicrystalline (qc) phase along the 5-fold direction.
Fig. 4. Bright field TEM images of samples x = 0 (a) and x = 1.5 (b) showing the influence of the Ga content on size and morphology of the nano-quasicrystalline phase formed upon heat treatment. The selected area diffraction patterns for x = 1.5 show the 5-fold, 3-fold and 2-fold diffractions of the quasicrystalline phase.
icosahedral symmetry giving rise to glass to quasicrystalline phase transformation at Tx1 . In contrast, for the samples having x > 1.5 quasicrystalline and crystalline phases form simultaneously. In view of the different types of phase formations and morphological transitions observed in the present studies after heat treatment of metallic glasses, it may be said that nucleation and growth characteristics of MS-I and MS-II phases are affected owing to Ga substitution. It may be pointed out here that interfacial energy per unit area of Al (∼1.2 J/m2 ) is higher than that of Ga (∼0.6 J/m2 ). Thus, Ga substitution may be reducing the interfacial energy between quasicrystal and remaining amorphous phase. 3.2. Glass forming ability The difference in GFA of these alloys was assessed by the recognized indicators such as Trg = Tg /Tm [34] and Tx = Tx − Tg [7], where Tm is the melting temperature of the alloy. The melting processes of these metallic glasses were studied using DSC (Fig. 2(a)). It was found that the onset melting temperature (Tm ) continuously
increases with Ga substitution. The calculated indicator values for GFA assessment are included in Table 1. The glass transition temperature (Tg ) has been observed for all the alloys. The supercooled liquid region Tx which reflects the thermal stability of the supercooled liquid towards crystallization, varies slightly with alloy composition (Fig. 2(b)) With increasing Ga content Tx1 shifts to lower temperatures and the lowest value has been determined for sample x = 7.5, Tx = 59 K. Trg slightly decreases as Ga content increases. Due to the lowered Tx of these glasses, Trg and Tx manifest a decreasing tendency with the increase of Ga content. The high thermal stability of the samples with low Ga content is connected with a low nucleation rate, due to a highly dense packed structure of the supercooled liquid and therefore, limited mobility of the atoms, i.e. limited diffusion. As shown in DSC traces the alloy with composition Zr69.5 Ga7.5 Cu12 Ni11 (x = 7.5) displays the glass transition temperature (Tg ) at 614 K which is lower than 624 K for Zr69.5 Al7.5 Cu12 Ni11 glassy alloy (x = 0). This means a new metallic glass composition with the complete substitution of Al by Ga has been discovered. If we compare Tx and Tg /Tm values of BMGs for Zr–Al–Cu–Ni systems, then we note that in our case the maximum value of Tx has been observed for x = 0. Further the ratio of Tg /Tm in our case is always less than those reported in Ref. [35]. It may be noted that Iqbal et al. [35] have utilised Cu-mould casting whereas we have employed rapid solidification technique for producing glasses. Thermal analysis data for these melt spun glassy alloys are given in Table 1. The well recognized GFA indicators Trg and Tx gave GFA assessment for the product alloys. To understand the very compositional sensitive crystallization behaviours of these metallic glasses, more experimental information on their crystallization kinetics is necessary. The present alloys, in which phase formation and transformation information is clear, are a useful system for further crystallization kinetics modelling. 3.3. Conclusions Varying the Ga content from 0 to 7.5 at.% in Zr69.5 Al7.5−x Gax Cu12 Ni11 alloys results in significantly differ-
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ent devitrification behaviours of the samples. The substitution of Ga changes the morphology of nano-quasicrystals. A noticeable change in thermal stability and size of precipitates is observed with increasing Ga content. Formation of single quasicrystalline phase by annealing the glass has been found only for the Zr69.5 Al7.5−x Gax Cu12 Ni11 alloys with 0 ≤ x ≤ 1.5. Slightly higher Ga content (i.e. x > 1.5) results in almost simultaneous formation of the quasicrystalline phase together with crystalline phase of Zr2 Cu type. The well recognised GFA indicators Trg and Tx are only slightly affected by Ga substitution for the present alloys. Acknowledgements The authors of the paper are thankful to Prof. S. Ranganathan, Dr. N.K. Mukhoupadhyay and Dr. M.A. Shaz for many stimulating discussions. We are also thankful to Mr. Vijay Kumar for his technical assistance. Financial assistance from Department of Science and Technology (UNANST, DST), University Grant Commission (UGC) and Council of Scientific and Industrial Research (CSIR) are also gratefully acknowledged. References [1] J.M. Dubois, S.S. Kang, Y. Massiani, Journal of Non-Crystalline Solids 153 (1993) 443–445. [2] S.S. Kang, J.M. Dubois, J. Von Stebut, Journal of Materials Research 8 (1993) 2471–2481. [3] R. Wittmann, K. Urban, M. Schandl, E. Hombogen, Journal of Materials Research 6 (1991) 1165–1168. [4] A. Inoue, T. Zhang, J. Saida, M. Matsushita, Materials Transactions JIM 44 (2003) 1978. [5] K. Urban, M. Feverbacher, M. Wollgarten, MRS Bulletin 22 (1997) 65–68. [6] U. Kamachi Mudali, S. Scudino, U. Kuhu, J. Eckert, A. Gebert, Scripta Materialia 50 (2004) 1379–1384. [7] A. Inoue, Acta Materialia 48 (2000) 279–306. [8] C.T. Liu, Z.P. Lu, Intermetallics 13 (2005) 415–418. [9] W. Chen, Y. Wang, J. Qiang, C. Dong, Acta Materialia 51 (2003) 1899–1907. [10] Y.M. Wang, W.P. Xu, J.B. Qiang, C.H. Wang, C.H. Shek, C. Dong, Materials Science and Engineering A 375–377 (2004) 411–416.
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