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Glass-forming ability and crystallization behavior of Nd60Al10Ni10Cu20−xFex (x = 0, 2, 4) bulk metallic glass with distinct glass transition
Materials Science and Engineering A 385 (2004) 38–43
Glass-forming ability and crystallization behavior of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) bulk metallic glass with distinct glass transition Z. Zhang a,b,∗ , W.H. Wang b , Y. Hirotsu a a
The institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan b Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Received 24 December 2003; received in revised form 29 March 2004
Abstract The effect of the iron addition on the glass-forming ability (GFA) of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) alloys has been investigated with respect to the glass transition and crystallization kinetics. The magnetic property of the as-cast samples varies from paramagnetic to ferromagnetic by partial substitution of Fe for Cu, indicating that the substitution can induce the formation of metastable phase (or cluster) and thus decreases the GFA of the alloys. The dependence of the glass transition on the heating rate was analyzed in terms of the Vogel–Fulcher–Tamman (VFT) equation. The glass-forming ability of these alloys is discussed based on the composition-dependent melting behaviour, the reduced glass transition temperature, and the fragility parameter. It is shown that substitution of Fe for Cu does not improve the glass-forming ability of the alloys and lowers the thermal stability of the alloys. The studied alloys are found to be rather strong glass formers and possess relatively low glass transition temperature. © 2004 Published by Elsevier B.V. Keywords: Glass formation ability; Glass transition; Crystallization kinetics; Thermal stability; Fragility parameter; Nd-based alloys
1. Introduction Nd-based bulk metallic glasses (BMGs) are known to have interesting hard magnetic properties combined with good glass-forming ability (GFA), promising the formation of bulk hard magnetic materials [1–8]. For some Fe-rich Nd-based alloys, experiments show that there are no distinct glass transitions in their differential scanning calorimetry (DSC) traces [2–4,6]. The absence of distinct glass transition of these alloys may indicate that either the glass transition is masked by the growth of quenched-in nanocrystallites or primary crystallization [6], because the microstructure of these rapid quenched alloys actually is the mixture of nanocrystalline clusters and glassy matrix [9,10]. However, a little addition of Fe content up to 5 at.% in some Nd-based alloys such as Nd60 Al15 Co10 Fe5 causes a large extension of the supercooled liquid region before crystallization [1]. The
∗
Corresponding author. E-mail address: [email protected] (Z. Zhang).
0921-5093/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.msea.2004.04.050
result suggests the glass transition of the Nd-base alloys is dependent on Fe content. The wide supercooled liquid region 45 K and relatively low glass transition temperature 430 K of the BMG make it especially suitable for the study of the kinetics of the glass transition. The glass transition is one of the most important and intensive investigated topics in the metallic glasses, the quantitative analysis of crystallization kinetics data can provide information about understanding the origin of the excellent GFA and essential parameters for controlled partial or full nanocrystallization. However, a comprehensive understanding of the glass transition for Nd-based alloys is still lacking, up to now, the effect of Fe on the glass transition as well as magnetic property is not clear. In this paper, the effect of Fe addition on the glass-forming ability and the magnetic properties of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) alloys are reported. In contrast to the Nd60 Al15 Cu10 Co10 Fe5 alloy [1], the substitution of Ni for Co was used to enhance the form ability of a bulk “amorphous” structure and Ni has similar contribution to magnetic property with Co. The glass transition, the
Z. Zhang et al. / Materials Science and Engineering A 385 (2004) 38–43
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thermal stability and the crystallization kinetics are analyzed in terms of the Vogel–Fulcher–Tamman (VFT) equation, and the value of the fragility parameter m is discussed in the framework of the general classification scheme of glass-forming liquids.
2. Experimental procedure Multicomponent Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) master alloys were prepared by arc-melting pure (99.9%) Nd, Al, Ni, Cu and Fe in a titanium-gettered argon atmosphere. To ensure homogeneity of the samples, the ingots were remelted several times. From the prealloyed ingots, bulk cylindrical specimens 3 mm or 5 mm in diameter and 80 mm in length were prepared by suction of the melt into a copper mold under argon atmosphere. X-ray diffraction (XRD) data of the alloys containing different Fe addition were analyzed by using a MAC M03 XHF diffractometer with Cu K␣ radiation. The samples were proven to be fully amorphous in the as-cast state by XRD measurements. The values of the onset glass transition temperature Tg , the onset crystallization temperature Txi , were determined by differential scanning calorimetry. The experiments were performed with a DSC-6200 (Seiko) under a continuous argon flow at different heating rates ranging from 5 to 80 K/min. The melting temperature, Tm and liquidus temperature, Tl were taken at the inflection point on the endotherm of the heating curve at the high temperature side. The M–H hysteresis loop was obtained at 300 K by a PPMS 6000 of Quantum Design Company.
3. Results 3.1. Magnetic properties The room temperature M–H hysteresis loops measured at a maximum applied field of 2000 kA/m for the Nd60 Al10 Ni10 Cu20−x Fex alloys are shown in Fig. 1. The magnetic properties of the alloys were a composition dependent. The alloys with 0 and 2 at.% Fe addition were paramagnetic, whereas at 4 at.% Fe, the alloy exhibits hard magnetic property with a coercivity of 70 KA/m. The value is smaller than the volue of Nd60 Al10 Fe20 Co10 hard magnetic alloy [7]. The present result is also useful for understanding the origin of the observed hard magnetic behavior in Re-based alloys. It is known that the hard magnetic properties of the Nd-based alloys can be correlated to the pre-existence of nanocrystalline phase [9,10]. The clusters with approximate composition of Fe3 Re (A1 phase) are reported to be responsible for the coercivity observed in the Nd-based alloys [11,12]. With increasing Fe content, the alloys have a tendency to form nano-clusters with Fe3 Re phase, which make them to exhibit ferromagnetic property. However, the present alloys are considered to be analogous
Fig. 1. Hysteresis J–H loops of the as-cast Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) alloys.
to the Re-based alloys, the addition of Fe decreases the GFA and thermal stability of the Nd-based alloys as shown below. 3.2. Glass-forming ability and thermal stability The DSC traces of the Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) as-cast alloys at a heating rate of 20 K/min are shown in Fig. 2. All curves exhibit the endothermic event characteristics of the glass transition, followed by a supercooled liquid region and several exothermic crystallization peaks at higher temperatures. The glass transition temperature, Tg , the onset temperatures of the first and second crystallization, Tx1 and Tx2 , the supercooled liquid region, T = Tx1 − Tg , for three alloys are listed in Table 1. For the alloy with x = 0, it shows a distinct glass transition with onset at Tg = 438 K and a large T = 40 K. The addition of Fe has a little effect on the glass transition temperature, but decrease the first crystallization temperature when the Fe content increases.
Fig. 2. DSC curves show the crystallization process of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2 and 4) at a heating rate of 20 K/min.
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Z. Zhang et al. / Materials Science and Engineering A 385 (2004) 38–43
Table 1 The crystallization temperatures (Tx1 , Tx2 ), the glass transition temperature (Tg ), supercooled liquid region (T), melting temperature (Tm , Tl ) and reduced glass transition temperature (Trg ) for Nd60 Al10 Ni10 Cu20−x Fex (x = 0,2 and 4) alloys x (at.%) Tg (K) Tx1 (K) Tx2 (K) T (K) Trg 0 2 4
438 442 438
478 482 472
572 573 578
40 40 34
Tm (K) Tl (K)
0.60 728 0.59 723 0.59 713
749 744
The width of the supercooled liquid region decreases from 40 K for the alloy for x = 0, 2 to 34 K for x = 4, indicating that the glass formation ability decreases. Two sharp crystallization events occur with onsets at Tx1 =478 and Tx2 = 572 K for x = 0. For the alloys with x = 2 and 4, their DSC traces also show similar glass transition and crystallization behaviour. But the first crystallization peak of Nd60 Al10 Ni10 Cu20 and Nd60 Al10 Ni10 Fe2 Cu18 show similar two different processes at low heating rates: a shoulder appears on the low temperature part of the crystallization peak. However, the mixed process of first crystallization peak is separated into two processes when Fe content x = 4, indicating that the Fe substitution of Cu changes the microstructure of the BMGs and crystallization process. The melting processes of the Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2 and 4) alloys are also shown in Fig. 2. The values of melting temperature, Tm , the liquidus temperature, Tl and the reduced glass transition temperature Trg (Trg = Tg /Tl ) of the alloys are listed in Table 1. As shown in Fig. 2 and Table 1, both the melting process and the value of Tm are dependent on Fe content. The curve corresponding to the Fe-free alloy exhibits a sharp single melting event indicative of an eutectic composition, while the curve corresponding to the alloys with x = 2, 4 display two clear melting peaks indicating that they are off eutectic. The thermal stability of the amorphous alloys shows a dependence on Fe content, since crystallization processes are affected by Fe substitution (see Table 1). Hruby proposed that a parameter, KH , obtained from differential thermal analysis (DTA) or DSC, indicates glass stability against crystallization on heating [13]. The glass stability parameter KH can be expressed as: KH =
Tx1 − Tg Tl − Tx1
3.3. The kinetics of the glass transition and the crystallization Fig. 3 shows the DSC traces obtained from Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) BMGs at different heating rates of 5, 10, 20, 40 and 80 K/min. Not only crystallizations but also glass transitions show the heating rate dependence during continuous heating. The kinetics of the glass transition and crystallization of amorphous alloys can be analyzed by using Kissinger’s equation [14]: Tp2 Ep Ep ln (2) = ln + β kB K 0 kB T p where β is the heating rate, Tp denotes the onset temperature the glass transition or crystallization, kB the Boltzmann constant and Ep the apparent activation energy. K0 is the frequency factor in Arrehenius law. The crystallization rate constant, Kx , is determined from Arrehenius law, Kx (Tp ) = K0 exp(2Ep /kB Tp ). Although there was some controversy on unreasonably high apparent attempt frequencies and high activation energies for glass transition compared with those for the crystallization stages derived from Kissinger’s equation [15], it has been widely used for understanding and prediction of thermal stability based on the knowledge of activation parameters: activation energy and apparent attempt frequency of crystallization for metallic glasses [16–18] (Fig. 4). The linear relationship between ln(Tp2 /β) and 1/Tp for the Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) BMGs are shown in Figure. The Ep and K0 for the glass transition and the crystallization, deduced from the slope and intercept, are listed
(1)
According to Hruby, the higher the value of KH of a certain glass, the higher its stability against crystallization on heating and, presumably, the higher its GFA [13]. The values of KH calculated in term of Eq. (1) for the BMGs with x = 0, 2 and 4 are 0.16, 0.15 and 0.13, respectively. KH decreases with increasing Fe content, indicating that glass stability and GFA of the Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, and 4) BMGs decrease when Fe content increases.
Fig. 3. DSC curves for Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) alloys at different heating rates (in K/min).
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Table 3 VFT parameters for the best fit of the DSC data according to Eq. (3), and fragility m of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) glasses
Fig. 4. Kissinger plots for Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) amorphous alloys.
in Table 2. The values of the apparent activation energy, Eg and the frequency factor, K0 of the glass transition decrease from 3.3 eV and 5 × 1038 s−1 to 1.6 eV and 6 × 1017 s−1 when Fe content increase from 0 to 4 at.%, the result may suggest that the larger Eg and the frequency factor K0 of a BMG, the higher the thermal stability. The crystallization reaction rate constant Kx at Txi calculated from the Arrhenius law from K0 , Ex , and Txi for every crystallization peak are also presented in Table 2. The study of glass transition kinetics can provide complementary information about the glass-forming ability of the amorphous alloys. The dependence of the glass transition temperature on the heating rate β was evaluated in terms of the Vogel–Fulcher–Tamman equation written in the form [19]: DT0g β(Tg ) = B exp (3) (Tg0 − Tg ) where Tg0 is the asymptotic value of Tg usually approximated as the onset of the glass transition within the limit of infinitely slow cooling and heating rate, B has the dimension of a heating rate and D is the strength parameter. The fitting of the experimental data was performed by the equation ln β(Tg ) = ln B − DT0g /(Tg − Tg0 ) with three adjustable VFT
x (at.%)
ln  (K/s)
Tg0 (K)
D
m
0 2 4
8 9 10
410 400 385
0.3 0.5 0.8
34 38 44
parameters: B, D, and Tg0 . The calculated values are given in Table 3 and the best VFT fits and the experimental points are displayed in Fig. 5. The values obtained for the strength parameter D are 0.3, 0.5 and 0.8 for the alloys with x = 0, 2 and 4, respectively. As GFA decreases, Tg0 decreases and D increases. The fragility concept provides a measure of the sensitivity of the structure of the liquid to temperature changes [20,21] and can be used in the classification of glass-forming materials into three general categories: strong, intermediate and fragile [20]. The fragility can be quantified by the strength parameter D in Eq. (3), which expresses the deviation from the Arrhenius behavior [20]. The Arrhenius when plotted as log10 τ versus Tg /T and the fragility parameter m can be evaluated as a slope at Tg [20] d log τ m= (4) d(Tg /T) T =Tg where τ is the average relaxation time. The equation presents a measure of the steepness of the relaxation curve as a function of temperature at Tg , which shows a larger value of m implying a greater deviation from Arrhenius behaviors with respect to the temperature dependence of relaxation time for a BMG. From the VFT fit, m at a particular Tg can be rewritten as [21]: m=
DT0g Tg
(5)
(Tg − Tg0 )2 ln 10
The m of the BMGs evaluated at a heating rate of 20 K/min are 34, 38 and 44, for the alloys with x = 0, 2 and 4, respectively, as shown in Table 3. 4. Discussion The magnetic property measurement reveals that the magnetic transition from paramagnetic to ferromagnetic by the
Table 2 Activation energy E, frequency factor K0 of the glass transition, the first and the second crystallization steps for Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) amorphous alloys (heating rate 20 K/min) x
0 2 4
Glass transition
First crystallization
Second crystallization
Eg (eV)
K0 (s−1 )
Ex1 (eV)
K0 (s−1 )
Kx1 (s−1 )
Ex2 (eV)
K0 (s−1 )
Kx2 (s−1 )
3.3 2.4 1.6
5 × 1038 4 × 1027 2 × 1019
1.4 1.4 1.7
2 × 1015 1 × 1015 6 × 1017
0.838 0.866 0.870
1.3 1.6 1.5
3 × 1011 1 × 1014 7 × 1013
0.701 0.876 6.905
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Z. Zhang et al. / Materials Science and Engineering A 385 (2004) 38–43
Fig. 5. Experimental points and VFT fits of the data in term of Eq. (4) (solid lines). Glass transition temperature Tg , as a function of the heating rate .
partial replacement of Cu by Fe (Fig. 1), indicating that metastable phase is formed with Fe addition, and thus the GFA of the alloys is decreased. However, it is deserved to point that we can not observe any magnetic property transformation with the addition of Fe from XRD and DSC measurements, the XRD pattern and DSC curve for x = 4 alloy is similar to that of x = 0 and 2, the results suggest that the size of metastable phases correlated to the magnetic property is so small that the they cannot be detected by XRD or DSC. Many criteria have been proposed to reflect relative GFA among BMGs on the basis of the characteristic temperatures measured by differential thermal calorimetry. Nevertheless, the most extensively used are the reduced glass transition temperature Trg and the supercooled liquid region T as well as Hruby parameter KH . It is suggested that the best glass-forming composition for an alloy is usually near deep eutectic, where the melt could be cooled to the underlying Tg with the smaller temperature interval [22,23]. It is known that the reduced glass transition temperature is a critical parameter in determining the glass-forming ability of an alloy according to Turnbull’s analysis [23], a liquid with Trg > 2/3 can only crystallize within a very narrow temperature range, and thus can be easily undercooled at a low cooling rate into the glassy state. The calculated value of Trg of about 0.60 for three alloys indicates that these systems can be easily undercooled at low cooling rate into a glass. For the Nd60 Al10 Ni10 Cu20 alloy, Trg is about 0.6, which classified the alloy as highly glass-forming alloy [23]. With increasing Fe content, on the other hand, one can see from the melting behavior that the composition of the alloy is gradually far from eutectic with Fe increasing (Fig. 2), and Trg decreased a little, thus the glass-forming ability is degraded. Usually, T is used as an indication of the devitrification tendency of a glass upon heating above Tg . A large T value may indicate that the supercooled liquid can exist in a wide
temperature range without crystallization and has a high resistance to the nucleation and growth of crystalline phases. Since crystallization is actually a competitive process with respect to glass formation, a large T would lead to a high GFA. In this sense, this temperature interval is somewhat related to GFA. The results show that T, Trg and KH decrease with Fe increasing, indicating that the GFA of the alloys decreases with Fe addition. In the process of crystallization, the activation energy Ex can be interpreted as the additional energy that an atom must acquire in order to be a part of the activated cluster [15,24]. The frequency factor Kx can be considered as a measure of the probability that an atom having energy Ex participates in a crystallization reaction [24]. It has been shown that the GFA of the BMGs can be represented by AT and Kx , there exist a relation between the Kx and GFA of the BMGs: the BMG with smaller Kx is of better GFA [16]. The Kx1 increases from 0.838 to 0.87 s−1 and Kx2 from 0.701 to 6.905 s−1 when Fe content change from 0 to 4 at.% (Table 2). Different from other alloys, for example, big differences in the value of Ex and Kc1 have been found in Zr41 Ti14 Cu12.5 Ni10−x Fex Be22.5 : a substitution of only 5 at.% of Ni by Fe causes a difference in Ex of 0.8 eV and in Kx0 of six orders of magnitude [16]. In comparison, addition of 4 at.% Fe for Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, and 4) BMGs increases 0.21 eV in Ex and two orders of magnitude of Kx0 , the result may indicate that the studied alloys have a similar GFA. As shown in Table 2, Kx1 increases with increasing Fe content as GFA decreases. The values of fragility parameter for three alloys indicate that the present alloys fall into the intermediate category according to Angell’s classification scheme, and are closer to the strong limit than to the fragile limit. Even though there is no direct connection between the value of m and the GFA of an alloy [25], however, the similar values of m and Trg stated for very good glass-forming liquids such as Zr46.75 Ti8.25 Cu7.4 Ni10 Be27.5 (m = 34) [26], Mg65 Cu25 Y10 (m = 41) [27] Pd40 Cu30 Ni10 P20 (m = 52) [28] confirms that the Nd-based BMGs can be classified into one of the best metallic glass formers. For three systems, the value of m was found to increase with Fe content (Table 3), this significant increase may reflected their GFA decreasing.
5. Conclusion The glass-forming ability of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) amorphous alloys with obvious glass transition was analyzed in terms of the composition-dependent melting behaviour, the reduced glass transition temperature and the Vogel–Fulcher–Tamman parameters. The magnetic property of the BMGs changes from paramagnetic to ferromagnetic when Fe increase up to about 4 at.%, indicating that the addition of Fe leads to the formation of metastable phase and thus the GFA of the alloys decreases. The indicative parameters of GFA, T, Trg and KH show a similar
Z. Zhang et al. / Materials Science and Engineering A 385 (2004) 38–43
trend within this series, they decrease with increasing Fe content. The values of the fragility parameters of the alloys fall into the intermediate category according to Angell’s classification scheme indicate that the present alloys are strong glass former. The addition of Fe does not improve the glass-forming ability of the alloys and lowers the thermal stability of the alloys.
References [1] Y. He, C.E. Price, S.J. Poon, Philos. Mag. Lett. 70 (1994) 371. [2] A. Inoue, T. Zhang, A. Takeuchi, W. Zhang, Mater. Trans. JIM 37 (1996) 636. [3] A. Inoue, A. Takeuchi, T. Zhang, Metall. Mater. Trans. 29A (1998) 1779. [4] Y. Li, S.C. Ng, Z.P. Lu, Y.P. Feng, K. Lu, Phil. Mag. Lett. 78 (1998) 213. [5] L.Q. Xing, J. Eckert, W. Löser, S. Roth, L. Schultz, J. Appl. Phys. 88 (2000) 3565. [6] G. Kumar, J. Eckert, S. Roth, W. Löser, L. Schultz, S. Ram, Acta Mater. 51 (2003) 29. [7] B.C. Wei, W.H. Wang, M.X. Pan, B.S. Han, Z.R. Zhang, W.R. Hu, Phys. Rev. B 64 (2001) 012406. [8] R. Politano, J.P. Nozieres, R. Perrier, M.P. Missell, IEEE Trans. Magn. 29 (1993) 2761.
43
[9] S. Schneider, P. Thiyagarajan, W.L. Johnson, Appl. Phys. Lett. 68 (1996) 493. [10] Z. Zhang, L. Xia, R.J. Wang, B.C. Wei, M.X. Pan, W.H. Wang, Appl. Phys. Lett. 81 (2002) 4371. [11] J. Delamare, D. Lemarchand, P.J. Vigier, Alloys Comp. 216 (1994) 273. [12] V.P. Menushenkov, A.S. Lileev, M.A. Oreshkin, S.A. Zhuravlev, J. Magn. Mag. Mater. 203 (1999) 149. [13] A. Hruby, Czech. J. Phys. Sect. B 22 (1972) 1187. [14] H.E. Kissinger, J. Res. Natl. Bur. Stand. 57 (1956) 217. [15] V.A. Khonik, K. Kitagawa, H. Morii, J. Appl. Phys. 87 (2000) 8440. [16] Y.X. Zhuang, W.H. Wang, Y. Zhang, M.X. Pan, D.Q. Zhao, Appl. Phys. Lett. 75 (1999) 2392. [17] N. Nishiyama, A. Inoue, Acta Mater. 47 (1999) 1487. [18] J.E. Shelby, J. Non-Cryst. Solids 34 (1979) 111. [19] R. Brüning, K. Samwer, Phys. Rev. B 46 (1992) 11318. [20] C.A. Angell, Science 267 (1995) 1924. [21] R. Böhmer, K.L. Ngai, C.A. Angell, D.J. Plazek, J. Chem. Phys. 99 (1993) 4201. [22] Z.P. Lu, C.T. Liu, Acta Mater. 50 (2002) 3501. [23] D. Turnbull, Contemp. Phys. 10 (1969) 473. [24] R.A. Ligero, J. Vazquez, P. Villares, R. Jimenez-garay, Mater. Lett. 8 (1989) 6. [25] J.M. Borrego, C.F. Conde, A. Conde, S. Roth, H. Grahl, A. Ostwald, J. Eckert, J. Appl. Phys. 92 (2002) 6607. [26] R. Busch, W.L. Johnson, Appl. Phys. Lett. 72 (1998) 2695. [27] R. Busch, W. Liu, W.L. Johnson, J. Appl. Phys. 83 (1998) 4134. [28] J.F. Loeffler, J. Schroers, W.L. Johnson, Appl. Phys. Lett. 77 (2000) 681.
Glass-forming ability and crystallization behavior of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) bulk metallic glass with distinct glass transition Z. Zhang a,b,∗ , W.H. Wang b , Y. Hirotsu a a
The institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan b Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Received 24 December 2003; received in revised form 29 March 2004
Abstract The effect of the iron addition on the glass-forming ability (GFA) of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) alloys has been investigated with respect to the glass transition and crystallization kinetics. The magnetic property of the as-cast samples varies from paramagnetic to ferromagnetic by partial substitution of Fe for Cu, indicating that the substitution can induce the formation of metastable phase (or cluster) and thus decreases the GFA of the alloys. The dependence of the glass transition on the heating rate was analyzed in terms of the Vogel–Fulcher–Tamman (VFT) equation. The glass-forming ability of these alloys is discussed based on the composition-dependent melting behaviour, the reduced glass transition temperature, and the fragility parameter. It is shown that substitution of Fe for Cu does not improve the glass-forming ability of the alloys and lowers the thermal stability of the alloys. The studied alloys are found to be rather strong glass formers and possess relatively low glass transition temperature. © 2004 Published by Elsevier B.V. Keywords: Glass formation ability; Glass transition; Crystallization kinetics; Thermal stability; Fragility parameter; Nd-based alloys
1. Introduction Nd-based bulk metallic glasses (BMGs) are known to have interesting hard magnetic properties combined with good glass-forming ability (GFA), promising the formation of bulk hard magnetic materials [1–8]. For some Fe-rich Nd-based alloys, experiments show that there are no distinct glass transitions in their differential scanning calorimetry (DSC) traces [2–4,6]. The absence of distinct glass transition of these alloys may indicate that either the glass transition is masked by the growth of quenched-in nanocrystallites or primary crystallization [6], because the microstructure of these rapid quenched alloys actually is the mixture of nanocrystalline clusters and glassy matrix [9,10]. However, a little addition of Fe content up to 5 at.% in some Nd-based alloys such as Nd60 Al15 Co10 Fe5 causes a large extension of the supercooled liquid region before crystallization [1]. The
∗
Corresponding author. E-mail address: [email protected] (Z. Zhang).
0921-5093/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.msea.2004.04.050
result suggests the glass transition of the Nd-base alloys is dependent on Fe content. The wide supercooled liquid region 45 K and relatively low glass transition temperature 430 K of the BMG make it especially suitable for the study of the kinetics of the glass transition. The glass transition is one of the most important and intensive investigated topics in the metallic glasses, the quantitative analysis of crystallization kinetics data can provide information about understanding the origin of the excellent GFA and essential parameters for controlled partial or full nanocrystallization. However, a comprehensive understanding of the glass transition for Nd-based alloys is still lacking, up to now, the effect of Fe on the glass transition as well as magnetic property is not clear. In this paper, the effect of Fe addition on the glass-forming ability and the magnetic properties of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) alloys are reported. In contrast to the Nd60 Al15 Cu10 Co10 Fe5 alloy [1], the substitution of Ni for Co was used to enhance the form ability of a bulk “amorphous” structure and Ni has similar contribution to magnetic property with Co. The glass transition, the
Z. Zhang et al. / Materials Science and Engineering A 385 (2004) 38–43
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thermal stability and the crystallization kinetics are analyzed in terms of the Vogel–Fulcher–Tamman (VFT) equation, and the value of the fragility parameter m is discussed in the framework of the general classification scheme of glass-forming liquids.
2. Experimental procedure Multicomponent Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) master alloys were prepared by arc-melting pure (99.9%) Nd, Al, Ni, Cu and Fe in a titanium-gettered argon atmosphere. To ensure homogeneity of the samples, the ingots were remelted several times. From the prealloyed ingots, bulk cylindrical specimens 3 mm or 5 mm in diameter and 80 mm in length were prepared by suction of the melt into a copper mold under argon atmosphere. X-ray diffraction (XRD) data of the alloys containing different Fe addition were analyzed by using a MAC M03 XHF diffractometer with Cu K␣ radiation. The samples were proven to be fully amorphous in the as-cast state by XRD measurements. The values of the onset glass transition temperature Tg , the onset crystallization temperature Txi , were determined by differential scanning calorimetry. The experiments were performed with a DSC-6200 (Seiko) under a continuous argon flow at different heating rates ranging from 5 to 80 K/min. The melting temperature, Tm and liquidus temperature, Tl were taken at the inflection point on the endotherm of the heating curve at the high temperature side. The M–H hysteresis loop was obtained at 300 K by a PPMS 6000 of Quantum Design Company.
3. Results 3.1. Magnetic properties The room temperature M–H hysteresis loops measured at a maximum applied field of 2000 kA/m for the Nd60 Al10 Ni10 Cu20−x Fex alloys are shown in Fig. 1. The magnetic properties of the alloys were a composition dependent. The alloys with 0 and 2 at.% Fe addition were paramagnetic, whereas at 4 at.% Fe, the alloy exhibits hard magnetic property with a coercivity of 70 KA/m. The value is smaller than the volue of Nd60 Al10 Fe20 Co10 hard magnetic alloy [7]. The present result is also useful for understanding the origin of the observed hard magnetic behavior in Re-based alloys. It is known that the hard magnetic properties of the Nd-based alloys can be correlated to the pre-existence of nanocrystalline phase [9,10]. The clusters with approximate composition of Fe3 Re (A1 phase) are reported to be responsible for the coercivity observed in the Nd-based alloys [11,12]. With increasing Fe content, the alloys have a tendency to form nano-clusters with Fe3 Re phase, which make them to exhibit ferromagnetic property. However, the present alloys are considered to be analogous
Fig. 1. Hysteresis J–H loops of the as-cast Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) alloys.
to the Re-based alloys, the addition of Fe decreases the GFA and thermal stability of the Nd-based alloys as shown below. 3.2. Glass-forming ability and thermal stability The DSC traces of the Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) as-cast alloys at a heating rate of 20 K/min are shown in Fig. 2. All curves exhibit the endothermic event characteristics of the glass transition, followed by a supercooled liquid region and several exothermic crystallization peaks at higher temperatures. The glass transition temperature, Tg , the onset temperatures of the first and second crystallization, Tx1 and Tx2 , the supercooled liquid region, T = Tx1 − Tg , for three alloys are listed in Table 1. For the alloy with x = 0, it shows a distinct glass transition with onset at Tg = 438 K and a large T = 40 K. The addition of Fe has a little effect on the glass transition temperature, but decrease the first crystallization temperature when the Fe content increases.
Fig. 2. DSC curves show the crystallization process of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2 and 4) at a heating rate of 20 K/min.
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Z. Zhang et al. / Materials Science and Engineering A 385 (2004) 38–43
Table 1 The crystallization temperatures (Tx1 , Tx2 ), the glass transition temperature (Tg ), supercooled liquid region (T), melting temperature (Tm , Tl ) and reduced glass transition temperature (Trg ) for Nd60 Al10 Ni10 Cu20−x Fex (x = 0,2 and 4) alloys x (at.%) Tg (K) Tx1 (K) Tx2 (K) T (K) Trg 0 2 4
438 442 438
478 482 472
572 573 578
40 40 34
Tm (K) Tl (K)
0.60 728 0.59 723 0.59 713
749 744
The width of the supercooled liquid region decreases from 40 K for the alloy for x = 0, 2 to 34 K for x = 4, indicating that the glass formation ability decreases. Two sharp crystallization events occur with onsets at Tx1 =478 and Tx2 = 572 K for x = 0. For the alloys with x = 2 and 4, their DSC traces also show similar glass transition and crystallization behaviour. But the first crystallization peak of Nd60 Al10 Ni10 Cu20 and Nd60 Al10 Ni10 Fe2 Cu18 show similar two different processes at low heating rates: a shoulder appears on the low temperature part of the crystallization peak. However, the mixed process of first crystallization peak is separated into two processes when Fe content x = 4, indicating that the Fe substitution of Cu changes the microstructure of the BMGs and crystallization process. The melting processes of the Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2 and 4) alloys are also shown in Fig. 2. The values of melting temperature, Tm , the liquidus temperature, Tl and the reduced glass transition temperature Trg (Trg = Tg /Tl ) of the alloys are listed in Table 1. As shown in Fig. 2 and Table 1, both the melting process and the value of Tm are dependent on Fe content. The curve corresponding to the Fe-free alloy exhibits a sharp single melting event indicative of an eutectic composition, while the curve corresponding to the alloys with x = 2, 4 display two clear melting peaks indicating that they are off eutectic. The thermal stability of the amorphous alloys shows a dependence on Fe content, since crystallization processes are affected by Fe substitution (see Table 1). Hruby proposed that a parameter, KH , obtained from differential thermal analysis (DTA) or DSC, indicates glass stability against crystallization on heating [13]. The glass stability parameter KH can be expressed as: KH =
Tx1 − Tg Tl − Tx1
3.3. The kinetics of the glass transition and the crystallization Fig. 3 shows the DSC traces obtained from Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) BMGs at different heating rates of 5, 10, 20, 40 and 80 K/min. Not only crystallizations but also glass transitions show the heating rate dependence during continuous heating. The kinetics of the glass transition and crystallization of amorphous alloys can be analyzed by using Kissinger’s equation [14]: Tp2 Ep Ep ln (2) = ln + β kB K 0 kB T p where β is the heating rate, Tp denotes the onset temperature the glass transition or crystallization, kB the Boltzmann constant and Ep the apparent activation energy. K0 is the frequency factor in Arrehenius law. The crystallization rate constant, Kx , is determined from Arrehenius law, Kx (Tp ) = K0 exp(2Ep /kB Tp ). Although there was some controversy on unreasonably high apparent attempt frequencies and high activation energies for glass transition compared with those for the crystallization stages derived from Kissinger’s equation [15], it has been widely used for understanding and prediction of thermal stability based on the knowledge of activation parameters: activation energy and apparent attempt frequency of crystallization for metallic glasses [16–18] (Fig. 4). The linear relationship between ln(Tp2 /β) and 1/Tp for the Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) BMGs are shown in Figure. The Ep and K0 for the glass transition and the crystallization, deduced from the slope and intercept, are listed
(1)
According to Hruby, the higher the value of KH of a certain glass, the higher its stability against crystallization on heating and, presumably, the higher its GFA [13]. The values of KH calculated in term of Eq. (1) for the BMGs with x = 0, 2 and 4 are 0.16, 0.15 and 0.13, respectively. KH decreases with increasing Fe content, indicating that glass stability and GFA of the Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, and 4) BMGs decrease when Fe content increases.
Fig. 3. DSC curves for Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) alloys at different heating rates (in K/min).
Z. Zhang et al. / Materials Science and Engineering A 385 (2004) 38–43
41
Table 3 VFT parameters for the best fit of the DSC data according to Eq. (3), and fragility m of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) glasses
Fig. 4. Kissinger plots for Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) amorphous alloys.
in Table 2. The values of the apparent activation energy, Eg and the frequency factor, K0 of the glass transition decrease from 3.3 eV and 5 × 1038 s−1 to 1.6 eV and 6 × 1017 s−1 when Fe content increase from 0 to 4 at.%, the result may suggest that the larger Eg and the frequency factor K0 of a BMG, the higher the thermal stability. The crystallization reaction rate constant Kx at Txi calculated from the Arrhenius law from K0 , Ex , and Txi for every crystallization peak are also presented in Table 2. The study of glass transition kinetics can provide complementary information about the glass-forming ability of the amorphous alloys. The dependence of the glass transition temperature on the heating rate β was evaluated in terms of the Vogel–Fulcher–Tamman equation written in the form [19]: DT0g β(Tg ) = B exp (3) (Tg0 − Tg ) where Tg0 is the asymptotic value of Tg usually approximated as the onset of the glass transition within the limit of infinitely slow cooling and heating rate, B has the dimension of a heating rate and D is the strength parameter. The fitting of the experimental data was performed by the equation ln β(Tg ) = ln B − DT0g /(Tg − Tg0 ) with three adjustable VFT
x (at.%)
ln  (K/s)
Tg0 (K)
D
m
0 2 4
8 9 10
410 400 385
0.3 0.5 0.8
34 38 44
parameters: B, D, and Tg0 . The calculated values are given in Table 3 and the best VFT fits and the experimental points are displayed in Fig. 5. The values obtained for the strength parameter D are 0.3, 0.5 and 0.8 for the alloys with x = 0, 2 and 4, respectively. As GFA decreases, Tg0 decreases and D increases. The fragility concept provides a measure of the sensitivity of the structure of the liquid to temperature changes [20,21] and can be used in the classification of glass-forming materials into three general categories: strong, intermediate and fragile [20]. The fragility can be quantified by the strength parameter D in Eq. (3), which expresses the deviation from the Arrhenius behavior [20]. The Arrhenius when plotted as log10 τ versus Tg /T and the fragility parameter m can be evaluated as a slope at Tg [20] d log τ m= (4) d(Tg /T) T =Tg where τ is the average relaxation time. The equation presents a measure of the steepness of the relaxation curve as a function of temperature at Tg , which shows a larger value of m implying a greater deviation from Arrhenius behaviors with respect to the temperature dependence of relaxation time for a BMG. From the VFT fit, m at a particular Tg can be rewritten as [21]: m=
DT0g Tg
(5)
(Tg − Tg0 )2 ln 10
The m of the BMGs evaluated at a heating rate of 20 K/min are 34, 38 and 44, for the alloys with x = 0, 2 and 4, respectively, as shown in Table 3. 4. Discussion The magnetic property measurement reveals that the magnetic transition from paramagnetic to ferromagnetic by the
Table 2 Activation energy E, frequency factor K0 of the glass transition, the first and the second crystallization steps for Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) amorphous alloys (heating rate 20 K/min) x
0 2 4
Glass transition
First crystallization
Second crystallization
Eg (eV)
K0 (s−1 )
Ex1 (eV)
K0 (s−1 )
Kx1 (s−1 )
Ex2 (eV)
K0 (s−1 )
Kx2 (s−1 )
3.3 2.4 1.6
5 × 1038 4 × 1027 2 × 1019
1.4 1.4 1.7
2 × 1015 1 × 1015 6 × 1017
0.838 0.866 0.870
1.3 1.6 1.5
3 × 1011 1 × 1014 7 × 1013
0.701 0.876 6.905
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Z. Zhang et al. / Materials Science and Engineering A 385 (2004) 38–43
Fig. 5. Experimental points and VFT fits of the data in term of Eq. (4) (solid lines). Glass transition temperature Tg , as a function of the heating rate .
partial replacement of Cu by Fe (Fig. 1), indicating that metastable phase is formed with Fe addition, and thus the GFA of the alloys is decreased. However, it is deserved to point that we can not observe any magnetic property transformation with the addition of Fe from XRD and DSC measurements, the XRD pattern and DSC curve for x = 4 alloy is similar to that of x = 0 and 2, the results suggest that the size of metastable phases correlated to the magnetic property is so small that the they cannot be detected by XRD or DSC. Many criteria have been proposed to reflect relative GFA among BMGs on the basis of the characteristic temperatures measured by differential thermal calorimetry. Nevertheless, the most extensively used are the reduced glass transition temperature Trg and the supercooled liquid region T as well as Hruby parameter KH . It is suggested that the best glass-forming composition for an alloy is usually near deep eutectic, where the melt could be cooled to the underlying Tg with the smaller temperature interval [22,23]. It is known that the reduced glass transition temperature is a critical parameter in determining the glass-forming ability of an alloy according to Turnbull’s analysis [23], a liquid with Trg > 2/3 can only crystallize within a very narrow temperature range, and thus can be easily undercooled at a low cooling rate into the glassy state. The calculated value of Trg of about 0.60 for three alloys indicates that these systems can be easily undercooled at low cooling rate into a glass. For the Nd60 Al10 Ni10 Cu20 alloy, Trg is about 0.6, which classified the alloy as highly glass-forming alloy [23]. With increasing Fe content, on the other hand, one can see from the melting behavior that the composition of the alloy is gradually far from eutectic with Fe increasing (Fig. 2), and Trg decreased a little, thus the glass-forming ability is degraded. Usually, T is used as an indication of the devitrification tendency of a glass upon heating above Tg . A large T value may indicate that the supercooled liquid can exist in a wide
temperature range without crystallization and has a high resistance to the nucleation and growth of crystalline phases. Since crystallization is actually a competitive process with respect to glass formation, a large T would lead to a high GFA. In this sense, this temperature interval is somewhat related to GFA. The results show that T, Trg and KH decrease with Fe increasing, indicating that the GFA of the alloys decreases with Fe addition. In the process of crystallization, the activation energy Ex can be interpreted as the additional energy that an atom must acquire in order to be a part of the activated cluster [15,24]. The frequency factor Kx can be considered as a measure of the probability that an atom having energy Ex participates in a crystallization reaction [24]. It has been shown that the GFA of the BMGs can be represented by AT and Kx , there exist a relation between the Kx and GFA of the BMGs: the BMG with smaller Kx is of better GFA [16]. The Kx1 increases from 0.838 to 0.87 s−1 and Kx2 from 0.701 to 6.905 s−1 when Fe content change from 0 to 4 at.% (Table 2). Different from other alloys, for example, big differences in the value of Ex and Kc1 have been found in Zr41 Ti14 Cu12.5 Ni10−x Fex Be22.5 : a substitution of only 5 at.% of Ni by Fe causes a difference in Ex of 0.8 eV and in Kx0 of six orders of magnitude [16]. In comparison, addition of 4 at.% Fe for Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, and 4) BMGs increases 0.21 eV in Ex and two orders of magnitude of Kx0 , the result may indicate that the studied alloys have a similar GFA. As shown in Table 2, Kx1 increases with increasing Fe content as GFA decreases. The values of fragility parameter for three alloys indicate that the present alloys fall into the intermediate category according to Angell’s classification scheme, and are closer to the strong limit than to the fragile limit. Even though there is no direct connection between the value of m and the GFA of an alloy [25], however, the similar values of m and Trg stated for very good glass-forming liquids such as Zr46.75 Ti8.25 Cu7.4 Ni10 Be27.5 (m = 34) [26], Mg65 Cu25 Y10 (m = 41) [27] Pd40 Cu30 Ni10 P20 (m = 52) [28] confirms that the Nd-based BMGs can be classified into one of the best metallic glass formers. For three systems, the value of m was found to increase with Fe content (Table 3), this significant increase may reflected their GFA decreasing.
5. Conclusion The glass-forming ability of Nd60 Al10 Ni10 Cu20−x Fex (x = 0, 2, 4) amorphous alloys with obvious glass transition was analyzed in terms of the composition-dependent melting behaviour, the reduced glass transition temperature and the Vogel–Fulcher–Tamman parameters. The magnetic property of the BMGs changes from paramagnetic to ferromagnetic when Fe increase up to about 4 at.%, indicating that the addition of Fe leads to the formation of metastable phase and thus the GFA of the alloys decreases. The indicative parameters of GFA, T, Trg and KH show a similar
Z. Zhang et al. / Materials Science and Engineering A 385 (2004) 38–43
trend within this series, they decrease with increasing Fe content. The values of the fragility parameters of the alloys fall into the intermediate category according to Angell’s classification scheme indicate that the present alloys are strong glass former. The addition of Fe does not improve the glass-forming ability of the alloys and lowers the thermal stability of the alloys.
References [1] Y. He, C.E. Price, S.J. Poon, Philos. Mag. Lett. 70 (1994) 371. [2] A. Inoue, T. Zhang, A. Takeuchi, W. Zhang, Mater. Trans. JIM 37 (1996) 636. [3] A. Inoue, A. Takeuchi, T. Zhang, Metall. Mater. Trans. 29A (1998) 1779. [4] Y. Li, S.C. Ng, Z.P. Lu, Y.P. Feng, K. Lu, Phil. Mag. Lett. 78 (1998) 213. [5] L.Q. Xing, J. Eckert, W. Löser, S. Roth, L. Schultz, J. Appl. Phys. 88 (2000) 3565. [6] G. Kumar, J. Eckert, S. Roth, W. Löser, L. Schultz, S. Ram, Acta Mater. 51 (2003) 29. [7] B.C. Wei, W.H. Wang, M.X. Pan, B.S. Han, Z.R. Zhang, W.R. Hu, Phys. Rev. B 64 (2001) 012406. [8] R. Politano, J.P. Nozieres, R. Perrier, M.P. Missell, IEEE Trans. Magn. 29 (1993) 2761.
43
[9] S. Schneider, P. Thiyagarajan, W.L. Johnson, Appl. Phys. Lett. 68 (1996) 493. [10] Z. Zhang, L. Xia, R.J. Wang, B.C. Wei, M.X. Pan, W.H. Wang, Appl. Phys. Lett. 81 (2002) 4371. [11] J. Delamare, D. Lemarchand, P.J. Vigier, Alloys Comp. 216 (1994) 273. [12] V.P. Menushenkov, A.S. Lileev, M.A. Oreshkin, S.A. Zhuravlev, J. Magn. Mag. Mater. 203 (1999) 149. [13] A. Hruby, Czech. J. Phys. Sect. B 22 (1972) 1187. [14] H.E. Kissinger, J. Res. Natl. Bur. Stand. 57 (1956) 217. [15] V.A. Khonik, K. Kitagawa, H. Morii, J. Appl. Phys. 87 (2000) 8440. [16] Y.X. Zhuang, W.H. Wang, Y. Zhang, M.X. Pan, D.Q. Zhao, Appl. Phys. Lett. 75 (1999) 2392. [17] N. Nishiyama, A. Inoue, Acta Mater. 47 (1999) 1487. [18] J.E. Shelby, J. Non-Cryst. Solids 34 (1979) 111. [19] R. Brüning, K. Samwer, Phys. Rev. B 46 (1992) 11318. [20] C.A. Angell, Science 267 (1995) 1924. [21] R. Böhmer, K.L. Ngai, C.A. Angell, D.J. Plazek, J. Chem. Phys. 99 (1993) 4201. [22] Z.P. Lu, C.T. Liu, Acta Mater. 50 (2002) 3501. [23] D. Turnbull, Contemp. Phys. 10 (1969) 473. [24] R.A. Ligero, J. Vazquez, P. Villares, R. Jimenez-garay, Mater. Lett. 8 (1989) 6. [25] J.M. Borrego, C.F. Conde, A. Conde, S. Roth, H. Grahl, A. Ostwald, J. Eckert, J. Appl. Phys. 92 (2002) 6607. [26] R. Busch, W.L. Johnson, Appl. Phys. Lett. 72 (1998) 2695. [27] R. Busch, W. Liu, W.L. Johnson, J. Appl. Phys. 83 (1998) 4134. [28] J.F. Loeffler, J. Schroers, W.L. Johnson, Appl. Phys. Lett. 77 (2000) 681.