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Non-isothermal crystallization kinetics and fragility of (Cu46Zr47Al7)97Ti3 bulk metallic glass investigated by differential scanning calorimetry
Thermochimica Acta 565 (2013) 132–136
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Non-isothermal crystallization kinetics and fragility of (Cu46 Zr47 Al7 )97 Ti3 bulk metallic glass investigated by differential scanning calorimetry Man Zhu a,∗ , Junjie Li b , Lijuan Yao a , Zengyun Jian a , Fang’e Chang a , Gencang Yang b a b
School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an, Shaanxi 710032, PR China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China
a r t i c l e
i n f o
Article history: Received 6 December 2012 Received in revised form 10 April 2013 Accepted 16 April 2013 Available online xxx Keywords: Bulk metallic glasses Differential scanning calorimetry Crystallization kinetics Kinetic fragility
a b s t r a c t In this paper, bulk metallic glasses with the composition of (Cu46 Zr47 Al7 )97 Ti3 were prepared by copper mold casting technique. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to investigate its structure and non-isothermal crystallization kinetics. DSC traces revealed that it undergoes two-stage crystallization. The activation energies corresponding to the characteristic temperatures have been calculated, and the results reveal that the as-cast alloys have a good thermal stability in thermodynamics. Based on Kissinger equation, the activation energies for glass transition, the first and second crystallization processes were obtained as 485 ± 16 kJ/mol, 331 ± 7 kJ/mol and 210 ± 3 kJ/mol, respectively, suggesting that the nucleation process is more difficult than the grain growth process. The fitting curves using Lasocka’s empirical relation show that the influence of the heating rate for crystallization is larger than glass transition. Furthermore, the kinetic fragility for (Cu46 Zr47 Al7 )97 Ti3 bulk metallic glasses is evaluated. Depending on the fragility index, (Cu46 Zr47 Al7 )97 Ti3 bulk metallic glasses should be considered as “intermediate glasses”. © 2013 Elsevier B.V. All rights reserved.
1. Introduction CuZr-based bulk metallic glasses (BMGs) have received tremendous attention in recent years [1–3], due to its unique properties such as high mechanical strength, good resistance against corrosion, and excellent glass forming ability (GFA). Thus, BMGs are quite attractive for industry application as a new class of structural or functional materials. Since the metallic glasses are metastable materials, it would transform into crystalline phase upon heating. From the technological point of view, the thermal stability of the metallic glasses is of great importance. Therefore, great attention was paid on the crystallization kinetics of metallic glasses, which helps us to understand the crystallization process of the metallic glasses and evaluation of glass forming ability. Recently, studies on the nonisothermal crystallization kinetics and isothermal crystallization kinetics have been carried out in a DSC equipment for a series of glassy alloys [4–10]. The crystallization process including the number of crystallization events, effective activation energy, crystallized volume fraction, width of supercooled liquid region, and transformation kinetics, can be analyzed. Qiao et al. [6] pointed out that the glass transition and crystallization both have remarkable
∗ Corresponding author. Tel.: +86 29 86173324; fax: +86 29 86173324. E-mail address: [email protected] (M. Zhu). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.04.017
kinetic effects for Cu46 Zr45 Al7 Y2 and Zr55 Cu30 Ni5 Al10 bulk metallic glasses. And the activation energies for crystallization event are 3.18 eV and 3.19 eV in Cu46 Zr45 Al7 Y2 and Zr55 Cu30 Ni5 Al10 glasses, respectively. Nowadays, CuZr-based amorphous alloys were intensively studied because of their potential applications. In present paper, non-isothermal crystallization kinetics of (Cu46 Zr47 Al7 )97 Ti3 BMGs was investigated by DSC at various heating rates. The different kinetic parameters are calculated in order to explain details of the nucleation and growth behaviors during the crystallization processes.
2. Experimental The master alloy ingots with nominal composition of (Cu46 Zr47 Al7 )97 Ti3 were prepared by arc melting high pure elements of Cu (99.999%), Zr (99.9%), Al (99.99%), and Ti (99.8%) under a Ti-gettered argon atmosphere. The alloy ingots were remelted for six times in order to ensure compositional homogeneity. Amorphous crylindrical rods with 2 mm in diameter were prepared by injecting the molten alloys contained in a quartz tube into a watercooled copper mold. X-ray diffractometry (Bruker; D8 Advance) with Cu K␣ as a radiation was carried out to verify the amorphicity of the samples. Thermal analysis was carried out in a DSC equipment (TA Company; SDT Q600) under a flow of high purified argon. In the case of continuous heating, a set of DSC scans was recorded at varied heating rates (10 K/min, 20 K/min, 40 K/min, and 60 K/min).
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Table 1 Thermal properties of the (Cu46 Zr47 Al7 )97 Ti3 BMGs at varied heating rates. Heating rate (K/min)
Tg (K)
Tx1 (K)
Tp1 (K)
Tx2 (K)
Tp2 (K)
Tm (K)
Tl (K)
Tx (K)
10 20 40 60
678 682 686 692
735 743 753 760
742 750 763 770
764 780 796 805
788 804 821 833
1125 1126 1128 1130
1146 1154 1164 1168
57 59 65 68
The samples were cut from the as-cast rods and Al2 O3 pans were utilized for continuous heating.
3. Results and discussion 3.1. Structural analysis of bulk metallic glass Fig. 1 shows the XRD pattern of the (Cu46 Zr47 Al7 )97 Ti3 amorphous rods with 2 mm in diameter. A broad amorphous halo is observed and no other diffraction peaks corresponding to crystalline phase were detected, indicating that fully amorphous structure can be obtained.
3.2. Differential scanning calorimeter Fig. 2 shows the non-isothermal DSC traces for the (Cu46 Zr47 Al7 )97 Ti3 BMGs at various heating rates ranging from 10 to 60 K/min. Fig. 2a and Fig. 2b present the low temperature region and high temperature region, respectively. It can be observed that all the DSC traces exhibit one endothermic event, a characteristic of the glass transition to undercooled liquid, followed by two exothermic reactions corresponding to the crystallization of the undercooled liquid. The crystallization process is directly related to the alloy composition. Chen et al. [5] confirmed that two-stage crystallization can be observed for Cu–Zr–Al–Y system containing 2–4 at %Y. The characteristic temperatures, i.e., glass transition temperature (Tg ), crystallization onset temperatures (Tx1 , Tx2 ), crystalline peak temperatures (Tp1 , Tp2 ), solidus temperature (Tm ), and liquidus temperature (Tl ), are marked by arrows in the DSC traces, and their values are listed in Table 1. As the heating rate increases from 10 to 60 K/min, all the characteristic temperatures (Tg , Tx1 , Tp1 , Tx2 , Tp2 , Tm , Tl ) are shifted to higher temperatures, indicating both glass transition and crystallization depend on the heating rate during the continuous heating. The value of Tg increases little and that of Tx1 increases rapidly with increasing the heating rate, thus resulting a increase of supercooled liquid region Tx (=Tx1 − Tg ), as shown in Table 1.
Fig. 2. DSC traces of the (Cu46 Zr47 Al7 )97 Ti3 BMGs at varied heating rates. (a) Low temperature region and (b) High temperature region.
On the basis of DSC data, the activation energy, E, corresponding to the characteristic temperatures can be determined by Kissinger equation [11]:
ln
T2 ˇ
=
E + constant RT
(1)
where ˇ is the continuous heating rate, R is the gas constant, and T stands for Tg , Tx , and Tp . Using Eq. (1) and the values of Tg , Tx1 , Tp1 , Tx2 , and Tp2 listed in Table 1, by plotting ln(T2 /ˇ) versus 1000/T, some approximately straight lines can be obtained with a slope of E/R using linear fitting method, as shown in Fig. 3. Thus, the effective activation energies for Eg , Ex1 , Ep1 , Ex2 , and Ep2 can be deduced (Table 2). Table 2 Activation energies calculated with different methods for the (Cu46 Zr47 Al7 )97 Ti3 BMGs. Activation energy (kJ/mol)
Fig. 1. XRD pattern of the (Cu46 Zr47 Al7 )97 Ti3 BMGs.
Equations
Eg
Ex1
Ep1
Ex2
Ep2
Kissinger Ozawa
485 ± 16 502 ± 11
331 ± 7 344 ± 6
283 ± 10 296 ± 10
210 ± 3 223 ± 3
205 ± 5 219 ± 4
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Fig. 3. Kissinger plots of ln(T2 /ˇ) verse 1000/T for (Cu46 Zr47 Al7 )97 Ti3 BMGs. Fig. 5. Relationship between temperature and ln ˇ for Tg , Tx1 , Tp1 , Tx2 , and Tp2 .
The activation energy can also be calculated from Ozawa equation [12]: ln ˇ = −
E + constant RT
(2)
Ozawa plots (Fig. 4) showing the relationship between ln ˇ versus 1000/T can also be obtained using Eq. (2). The effective activation energies for Eg , Ex1 , Ep1 , Ex2 , and Ep2 are also listed in Table 2. The results determined from two equations have the same tendency, and two equations yield almost the same effective activation energy. However, it can be found that the effective activation energies obtained from Ozawa equation were slightly larger than that obtained from Kissinger equation. The values of Eg calculated by Kissinger equation and Ozawa equation were 485 ± 16 kJ/mol and 502 ± 11 kJ/mol, respectively. Comparing with Eg of Cu52.5 Ti30 Zr11.5 Ni6 (356.86 kJ/mol) [13], Cu35 Ag15 Zr45 Al5 (483 kJ/mol) [14], and Zr61.5 Al10.7 Cu13.65 Ni14.15 (214.56 kJ/mol) [15], one can found that Eg for present alloy is much higher than others, implying that this alloy should overcome higher barrier for atomic rearrangement in the supercooled region. In addition, the values of Ex1 , Ex2 , Ep1 , and Ep2 calculated by Kissinger equation were 331 ± 7 kJ/mol, 210 ± 3 kJ/mol, 283 ± 10 kJ/mol, and 205 ± 5 kJ/mol, respectively (Table 2). It is known that the onset crystallization temperature is associated with the nucleation process, and the crystallization peak temperature is directly related to the grain growth process. Therefore, the value of Ex represents the effective activation energy for nucleation, while the other two values (Ep1 , Ep2 ) denote the effective activation energy for grain growth. From the results in Table 2, Ex is larger than Ep1 , suggesting that the nucleation process is more difficult than the grain growth process for the first crystallization process. However, Ep1 is larger than Ep2 , indicating that the grain
growth process for the first crystallization process is more difficult than that for the second crystallization process. 3.3. Influence of heating rate on nucleation and grain growth Fig. 5 depicts the variation of Tg , Tx1 , Tp1 , Tx2 , and Tp2 with ln ˇ for (Cu46 Zr47 Al7 )97 Ti3 BMGs, which can be described using Lasocka’s empirical relation [16] in the following form: T = A + B ln ˇ
(3)
where A and B are constants. The values of A and B obtained from least square method are listed in Table 3. The value of B is reported as an indication of the response of the configurational changes within the glass transition region to the heating rate. The more the B, the more is the sensitivity of the characteristic temperature to heating rate. The cooling rate of present experiment is higher than the heating rate used in the DSC equipment, and the values of Tg obtained on the DSC trace are much lower than real glass transition temperatures [16]. Thus, the values of Tg obtained on the DSC trace may be regarded as low-temperature range of the glass transition region. The value of B for Tg is the smallest, and the value of B for Tp2 is the biggest. Therefore, glass transition process is insensitive to heating rate, and the second crystallization process is the most sensitive to heating rate. 3.4. Local Avrami exponent n(x) The local Avrami exponent of crystallization is a key parameter to understand the mechanisms of nucleation and grain growth during phase transformation. For isothermal conditions the transformed volume fraction can be described by Johnson–Mehl–Avrami (JMA) equation [17–19]:
x = 1 − exp −(kt)
n
(4)
where x is the crystallized volume fraction at time t, n is the dimensionless Avrami exponent related to the nucleation and growth process, k is the reaction rate constant associated with absolute temperature described by Arrhenius equation:
k = k0 exp −
E RT
(5)
Table 3 Values of A and B for (Cu46 Zr47 Al7 )97 Ti3 BMGs.
Fig. 4. Ozawa plots of ln ˇ verse 1000/T for (Cu46 Zr47 Al7 )97 Ti3 BMGs.
A B
Tg
Tx1
Tp1
Tx2
Tp2
660.34 7.39
703.49 13.16
704.27 15.89
711.28 22.93
730.02 24.91
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strength when the fragility index m is in the range of 30–70 [24]. It is found that the value of m for the (Cu46 Zr47 Al7 )97 Ti3 BMGs is quite similar to those reported on other BMGs, such as Zr46.75 Ti8.25 Cu7.5 Ni10 Be27.5 (m = 44.2) [25], Cu46 Zr42 Al7 Y5 (m = 49) [26], Cu50.3 Zr47.7 Nb2 (m = 30) [27], Cu30 Zr55 Al10 Ni5 (m = 69) [6], implying that the (Cu46 Zr47 Al7 )97 Ti3 BMGs should be considered as “intermediate glasses”. 4. Conclusions The thermal stability and non-isothermal crystallization kinetics of (Cu46 Zr47 Al7 )97 Ti3 BMGs were investigated by XRD and DSC. The main conclusions are drawn as follows:
Fig. 6. Local Avrami exponent as a function of the crystallized volume fraction.
where k0 is a constant, E the effective activation energy of crystallization, R the gas constant, and T the absolute temperature. Using JMA equation, the Avrami exponent n can be obtained by plotting ln(−ln(1−x)) verse 1/T. The value of Avrami exponent n can be calculated if the value of E is known. However, it should be mentioned that this equation is only applicable for isothermal crystallization. The nucleation and grain growth rate do not remain constant during the whole crystallization process. For non-isothermal crystallization, the local Avrami exponent, n(x), was introduced to describe crystallization mechanism [4]: n (x) =
−R∂ ln [− ln (1 − x)]
E (x) ∂ 1/T
(6)
The local Avrami exponent n(x) can be calculated according to the variable local activation energy E(x). Since the error is too serious when the crystallized volume fraction is very low or high, we present the local Avrami exponent n(x) as a function of crystallization volume fraction (5% < x < 95%) at constant heating rate of 10 K/min (Fig. 6). It can be found that the value of n(x) gradually decreases as the crystallization fraction increases, implying nucleation rate decreases, and reaches a minimum value at about x = 90%, the rate of nucleation tends to a constant. 3.5. Kinetic fragility According to Angell’s suggestion [20–23], the fragility index m is expressed as:
m=
d log10 (T )
d(Tg /T )
(7) T =Tg
where (T) is the characteristic temperature-dependence relaxation time. Because the viscosity is proportional to a structural relaxation time, m can be estimated by replacing (T) with shear viscosity (T) in Eq. (7). Thus, m is a measure of the steepness of the slope of the viscosity curve at Tg when the temperature is scaled by Tg . If (T) can be described by the Arrhenius equation, then the fragility index m can be expressed as:
m=
d log10 (T )
d(Tg /T )
= T =Tg
Eg RTg ln 10
(8)
where, Eg is the activation energy for Tg . In the present study, the kinetic fragility index m was calculated from Eq. (8) using the data of Eg (=485 kJ/mol) and Tg (=682 K). Therefore, the kinetic fragility index m for (Cu46 Zr47 Al7 )97 Ti3 BMGs is calculated to be 37. The fragility index m was classified into three general categories: strong, intermediate, and fragile. For metallic alloys, it exhibits an intermediate fragility
(1) (Cu46 Zr47 Al7 )97 Ti3 bulk metallic glass manifests two crystallization procedures. The glass transition and crystallization both have obvious kinetics effects. The characteristic temperatures are shifted to high temperatures as the heating rate increases. (2) Activation energies corresponding to the characteristic temperatures have been calculated. The effective activation energy Ex for the onset crystallization is higher than that for Ep1 , indicating that the nucleation process is more difficult than the growth process for the first crystallization transition. However, the second crystallization process is more sensitive to heating rate than the first crystallization process. The effective activation energy Ep1 for the first crystallization peak is larger than that Ep2 for the second crystallization peak, suggesting that the growth rate for the first crystallization process is more difficult than that for the second crystallization process. (3) The fitting curves between the characteristic temperatures and the heating rate proved that the second crystallization process is the most sensitive to heating rate. (4) Depending on the calculated value of the fragility index, (Cu46 Zr47 Al7 )97 Ti3 can be classified into “intermediate glasses”. Acknowledgements This work was supported by the Major State Basic Research Development Program of China (973 Program) (Grant no. 2011CB610403), the National Natural Science foundation of China (Grant nos. 51071115, 51171136), the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant no. SKLSP201221), and Natural Science Basic Research Plan in Shaanxi Province of China (Grant no. 2012JM6010). One of the authors (M. Zhu) expressed his great thanks to Mr. Jinyang Liu in Northwest Institute for Non-ferrous Metal Research. References [1] D.C. Hofmann, J.Y. Suh, A. Wiest, G. Duan, M.L. Lind, M.D. Demetriou, W.L. Johnson, Designing metallic glass matrix composites with high toughness and tensile ductility, Nature 45 (2008) 1085–1089. [2] A. Inoue, A. Takeuchi, Recent development and application products of bulk glassy alloys, Acta Mater. 59 (2011) 2243–2267. [3] A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys, Acta Mater. 48 (2000) 279–306. [4] W. Lu, B. Yan, W.H. Huang, Complex primary crystallization kinetics of amorphous Finemet alloy, J. Non-Cryst. Solids 351 (2005) 3320–3324. [5] S.F. Chen, S.L. Lin, J.K. Chen, Y.L. Lin, Thermal stability and corrosion behavior of Cu–Zr–Al–Y bulk metallic glass, Intermetallics 18 (2010) 1954–1957. [6] J.C. Qiao, J.M. Pelletier, Enthalpy relaxation in Cu46 Zr45 Al7 Y2 and Zr55 Cu30 Ni5 Al10 bulk metallic glasses by differential scanning calorimetry (DSC), Intermetallics 19 (2011) 9–18. [7] W. Yang, F. Liu, G.C. Yang, Z.F. Xu, J.H. Wang, Z.T. Wang, Analysis of crystallization kinetics of undercooled Fe–B hypereutectic alloy using DSC technique, Thermochim. Acta 527 (2012) 47–51. [8] Z.Z. Yuan, X.D. Chen, B.X. Wang, Z.J. Chen, Crystallization kinetics of melt-spun Co43 Fe20 Ta5.5 B31.5 amorphous alloy, J. Alloys Compd. 399 (2005) 166–172.
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[9] F. Xu, J.Z. Jiang, Q.P. Cao, Y.W. Du, Isothermal crystallization kinetics analysis of melt-spun Pd42.5 Cu30 Ni7.5 P20 amorphous ribbons, J. Alloys Compd. 392 (2005) 173–176. [10] B. Movahedi, M.H. Enayati, C.C. Wong, On the crystallization behavior of amorphous Fe–Cr–Mo–B–P–Si–C powder prepared by mechanical alloying, Mater. Lett. 64 (2010) 1055–1058. [11] H.E. Kissinger, Reaction kinetics in differential thermal analysis, Anal. Chem. 29 (1957) 1702–1704. [12] T. Ozawa, Kinetic analysis of derivative curves in thermal analysis, J. Therm. Anal. 2 (1970) 301–305. [13] Y.J. Yang, D.W. Xing, J. Shen, J.F. Sun, S.D. Wei, H.J. He, D.G. McCartney, Crystallization kinetics of a bulk amorphous Cu–Ti–Zr–Ni alloy investigated by differential scanning calorimetry, J. Alloys Compd. 415 (2006) 106–110. [14] X. Ou, G.Q. Zhang, X. Xu, L.N. Wang, J.F. Liu, J.Z. Jiang, Crystallization kinetics in Cu35 Ag15 Zr45 Al15 metallic glass, J.Alloys Compd. 441 (2007) 181–184. [15] A.H. Cai, W.K. An, Y. Luo, T.L. Li, X.S. Li, X. Xiong, Y. Liu, Glass forming ability, non-isothermal crystallization kinetics, and mechanical property of Zr61.5 Al10.7 Cu13.65 Ni14.15 metallic glass, J. Alloys Compd. 490 (2010) 642–646. [16] M. Lasocka, The effect of scanning rate on glass transition temperature of splatcooled Te85 Ge15 , Mater. Sci. Eng. 23 (1975) 173–177. [17] J.W. Chiristian, The theory of transformations in metals and alloys-part I and II, third ed., Pergamon, Oxford, 2002. [18] M. Avramin, Kinetics of phase change I: general theory, J. Chem. Phys. 7 (1939) 1103–1112.
[19] G. Hampel, A. Pundt, J. Hesse, Crystallization of Fe73.5 Cu1 Nb3 Si13,5 B9 : structure and kinetics examined by X-ray diffraction and Mössbauer effect spectroscopy, J. Phys.: Condens. Matter 4 (1992) 3195–3214. [20] C.A. Angell, Glass-formers and viscous liquid slowdown since David Turnbull: enduring puzzles and new twists, MRS Bull. 33 (2008) 544–555. [21] L.M. Martinez, C.A. Angell, A thermodynamic connection to the fragility of glassforming liquids, Nature 410 (2001) 663–667. [22] K. Ito, C.T. Moynihan, C.A. Angell, Thermodynamic determination of fragility in liquids and a fragile-to-strong liquid transition in water, Nature 398 (1999) 492–495. [23] C.A. Angell, Structural instability and relaxation in liquid and glassy phases near the fragile liquid limit, J. Non-Cryst. Solids 102 (1988) 205–221. [24] D.N. Perera, Compilation of the fragility parameters for several glass-forming metallic alloys, J. Phys.: Condens. Matter 11 (1999) 3807–3812. [25] H. Kato, T. Wada, M. Hasegawa, J. Saida, A. Inoue, H.S. Chen, Fragility and thermal stability of Pt- and Pd-based bulk glass forming liquids and their correlation with deformability, Scr. Mater. 54 (2006) 2023–2027. [26] E.S. Park, J.H. Na, D.H. Kim, Correlation between fragility and glass-forming ability/plasticity in metallic glass-forming alloys, Appl. Phys. Lett. 91 (2007) 031907. [27] Q. Fang, Q. Wang, Z.L. Zhao, Y.D. Dong, Effect of Nb addition on the glass transition and crystallization kinetics of bulk Cu–Zr metallic glasses, Acta Phys. Sin. 56 (2007) 1292–1296.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Non-isothermal crystallization kinetics and fragility of (Cu46 Zr47 Al7 )97 Ti3 bulk metallic glass investigated by differential scanning calorimetry Man Zhu a,∗ , Junjie Li b , Lijuan Yao a , Zengyun Jian a , Fang’e Chang a , Gencang Yang b a b
School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an, Shaanxi 710032, PR China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China
a r t i c l e
i n f o
Article history: Received 6 December 2012 Received in revised form 10 April 2013 Accepted 16 April 2013 Available online xxx Keywords: Bulk metallic glasses Differential scanning calorimetry Crystallization kinetics Kinetic fragility
a b s t r a c t In this paper, bulk metallic glasses with the composition of (Cu46 Zr47 Al7 )97 Ti3 were prepared by copper mold casting technique. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to investigate its structure and non-isothermal crystallization kinetics. DSC traces revealed that it undergoes two-stage crystallization. The activation energies corresponding to the characteristic temperatures have been calculated, and the results reveal that the as-cast alloys have a good thermal stability in thermodynamics. Based on Kissinger equation, the activation energies for glass transition, the first and second crystallization processes were obtained as 485 ± 16 kJ/mol, 331 ± 7 kJ/mol and 210 ± 3 kJ/mol, respectively, suggesting that the nucleation process is more difficult than the grain growth process. The fitting curves using Lasocka’s empirical relation show that the influence of the heating rate for crystallization is larger than glass transition. Furthermore, the kinetic fragility for (Cu46 Zr47 Al7 )97 Ti3 bulk metallic glasses is evaluated. Depending on the fragility index, (Cu46 Zr47 Al7 )97 Ti3 bulk metallic glasses should be considered as “intermediate glasses”. © 2013 Elsevier B.V. All rights reserved.
1. Introduction CuZr-based bulk metallic glasses (BMGs) have received tremendous attention in recent years [1–3], due to its unique properties such as high mechanical strength, good resistance against corrosion, and excellent glass forming ability (GFA). Thus, BMGs are quite attractive for industry application as a new class of structural or functional materials. Since the metallic glasses are metastable materials, it would transform into crystalline phase upon heating. From the technological point of view, the thermal stability of the metallic glasses is of great importance. Therefore, great attention was paid on the crystallization kinetics of metallic glasses, which helps us to understand the crystallization process of the metallic glasses and evaluation of glass forming ability. Recently, studies on the nonisothermal crystallization kinetics and isothermal crystallization kinetics have been carried out in a DSC equipment for a series of glassy alloys [4–10]. The crystallization process including the number of crystallization events, effective activation energy, crystallized volume fraction, width of supercooled liquid region, and transformation kinetics, can be analyzed. Qiao et al. [6] pointed out that the glass transition and crystallization both have remarkable
∗ Corresponding author. Tel.: +86 29 86173324; fax: +86 29 86173324. E-mail address: [email protected] (M. Zhu). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.04.017
kinetic effects for Cu46 Zr45 Al7 Y2 and Zr55 Cu30 Ni5 Al10 bulk metallic glasses. And the activation energies for crystallization event are 3.18 eV and 3.19 eV in Cu46 Zr45 Al7 Y2 and Zr55 Cu30 Ni5 Al10 glasses, respectively. Nowadays, CuZr-based amorphous alloys were intensively studied because of their potential applications. In present paper, non-isothermal crystallization kinetics of (Cu46 Zr47 Al7 )97 Ti3 BMGs was investigated by DSC at various heating rates. The different kinetic parameters are calculated in order to explain details of the nucleation and growth behaviors during the crystallization processes.
2. Experimental The master alloy ingots with nominal composition of (Cu46 Zr47 Al7 )97 Ti3 were prepared by arc melting high pure elements of Cu (99.999%), Zr (99.9%), Al (99.99%), and Ti (99.8%) under a Ti-gettered argon atmosphere. The alloy ingots were remelted for six times in order to ensure compositional homogeneity. Amorphous crylindrical rods with 2 mm in diameter were prepared by injecting the molten alloys contained in a quartz tube into a watercooled copper mold. X-ray diffractometry (Bruker; D8 Advance) with Cu K␣ as a radiation was carried out to verify the amorphicity of the samples. Thermal analysis was carried out in a DSC equipment (TA Company; SDT Q600) under a flow of high purified argon. In the case of continuous heating, a set of DSC scans was recorded at varied heating rates (10 K/min, 20 K/min, 40 K/min, and 60 K/min).
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Table 1 Thermal properties of the (Cu46 Zr47 Al7 )97 Ti3 BMGs at varied heating rates. Heating rate (K/min)
Tg (K)
Tx1 (K)
Tp1 (K)
Tx2 (K)
Tp2 (K)
Tm (K)
Tl (K)
Tx (K)
10 20 40 60
678 682 686 692
735 743 753 760
742 750 763 770
764 780 796 805
788 804 821 833
1125 1126 1128 1130
1146 1154 1164 1168
57 59 65 68
The samples were cut from the as-cast rods and Al2 O3 pans were utilized for continuous heating.
3. Results and discussion 3.1. Structural analysis of bulk metallic glass Fig. 1 shows the XRD pattern of the (Cu46 Zr47 Al7 )97 Ti3 amorphous rods with 2 mm in diameter. A broad amorphous halo is observed and no other diffraction peaks corresponding to crystalline phase were detected, indicating that fully amorphous structure can be obtained.
3.2. Differential scanning calorimeter Fig. 2 shows the non-isothermal DSC traces for the (Cu46 Zr47 Al7 )97 Ti3 BMGs at various heating rates ranging from 10 to 60 K/min. Fig. 2a and Fig. 2b present the low temperature region and high temperature region, respectively. It can be observed that all the DSC traces exhibit one endothermic event, a characteristic of the glass transition to undercooled liquid, followed by two exothermic reactions corresponding to the crystallization of the undercooled liquid. The crystallization process is directly related to the alloy composition. Chen et al. [5] confirmed that two-stage crystallization can be observed for Cu–Zr–Al–Y system containing 2–4 at %Y. The characteristic temperatures, i.e., glass transition temperature (Tg ), crystallization onset temperatures (Tx1 , Tx2 ), crystalline peak temperatures (Tp1 , Tp2 ), solidus temperature (Tm ), and liquidus temperature (Tl ), are marked by arrows in the DSC traces, and their values are listed in Table 1. As the heating rate increases from 10 to 60 K/min, all the characteristic temperatures (Tg , Tx1 , Tp1 , Tx2 , Tp2 , Tm , Tl ) are shifted to higher temperatures, indicating both glass transition and crystallization depend on the heating rate during the continuous heating. The value of Tg increases little and that of Tx1 increases rapidly with increasing the heating rate, thus resulting a increase of supercooled liquid region Tx (=Tx1 − Tg ), as shown in Table 1.
Fig. 2. DSC traces of the (Cu46 Zr47 Al7 )97 Ti3 BMGs at varied heating rates. (a) Low temperature region and (b) High temperature region.
On the basis of DSC data, the activation energy, E, corresponding to the characteristic temperatures can be determined by Kissinger equation [11]:
ln
T2 ˇ
=
E + constant RT
(1)
where ˇ is the continuous heating rate, R is the gas constant, and T stands for Tg , Tx , and Tp . Using Eq. (1) and the values of Tg , Tx1 , Tp1 , Tx2 , and Tp2 listed in Table 1, by plotting ln(T2 /ˇ) versus 1000/T, some approximately straight lines can be obtained with a slope of E/R using linear fitting method, as shown in Fig. 3. Thus, the effective activation energies for Eg , Ex1 , Ep1 , Ex2 , and Ep2 can be deduced (Table 2). Table 2 Activation energies calculated with different methods for the (Cu46 Zr47 Al7 )97 Ti3 BMGs. Activation energy (kJ/mol)
Fig. 1. XRD pattern of the (Cu46 Zr47 Al7 )97 Ti3 BMGs.
Equations
Eg
Ex1
Ep1
Ex2
Ep2
Kissinger Ozawa
485 ± 16 502 ± 11
331 ± 7 344 ± 6
283 ± 10 296 ± 10
210 ± 3 223 ± 3
205 ± 5 219 ± 4
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Fig. 3. Kissinger plots of ln(T2 /ˇ) verse 1000/T for (Cu46 Zr47 Al7 )97 Ti3 BMGs. Fig. 5. Relationship between temperature and ln ˇ for Tg , Tx1 , Tp1 , Tx2 , and Tp2 .
The activation energy can also be calculated from Ozawa equation [12]: ln ˇ = −
E + constant RT
(2)
Ozawa plots (Fig. 4) showing the relationship between ln ˇ versus 1000/T can also be obtained using Eq. (2). The effective activation energies for Eg , Ex1 , Ep1 , Ex2 , and Ep2 are also listed in Table 2. The results determined from two equations have the same tendency, and two equations yield almost the same effective activation energy. However, it can be found that the effective activation energies obtained from Ozawa equation were slightly larger than that obtained from Kissinger equation. The values of Eg calculated by Kissinger equation and Ozawa equation were 485 ± 16 kJ/mol and 502 ± 11 kJ/mol, respectively. Comparing with Eg of Cu52.5 Ti30 Zr11.5 Ni6 (356.86 kJ/mol) [13], Cu35 Ag15 Zr45 Al5 (483 kJ/mol) [14], and Zr61.5 Al10.7 Cu13.65 Ni14.15 (214.56 kJ/mol) [15], one can found that Eg for present alloy is much higher than others, implying that this alloy should overcome higher barrier for atomic rearrangement in the supercooled region. In addition, the values of Ex1 , Ex2 , Ep1 , and Ep2 calculated by Kissinger equation were 331 ± 7 kJ/mol, 210 ± 3 kJ/mol, 283 ± 10 kJ/mol, and 205 ± 5 kJ/mol, respectively (Table 2). It is known that the onset crystallization temperature is associated with the nucleation process, and the crystallization peak temperature is directly related to the grain growth process. Therefore, the value of Ex represents the effective activation energy for nucleation, while the other two values (Ep1 , Ep2 ) denote the effective activation energy for grain growth. From the results in Table 2, Ex is larger than Ep1 , suggesting that the nucleation process is more difficult than the grain growth process for the first crystallization process. However, Ep1 is larger than Ep2 , indicating that the grain
growth process for the first crystallization process is more difficult than that for the second crystallization process. 3.3. Influence of heating rate on nucleation and grain growth Fig. 5 depicts the variation of Tg , Tx1 , Tp1 , Tx2 , and Tp2 with ln ˇ for (Cu46 Zr47 Al7 )97 Ti3 BMGs, which can be described using Lasocka’s empirical relation [16] in the following form: T = A + B ln ˇ
(3)
where A and B are constants. The values of A and B obtained from least square method are listed in Table 3. The value of B is reported as an indication of the response of the configurational changes within the glass transition region to the heating rate. The more the B, the more is the sensitivity of the characteristic temperature to heating rate. The cooling rate of present experiment is higher than the heating rate used in the DSC equipment, and the values of Tg obtained on the DSC trace are much lower than real glass transition temperatures [16]. Thus, the values of Tg obtained on the DSC trace may be regarded as low-temperature range of the glass transition region. The value of B for Tg is the smallest, and the value of B for Tp2 is the biggest. Therefore, glass transition process is insensitive to heating rate, and the second crystallization process is the most sensitive to heating rate. 3.4. Local Avrami exponent n(x) The local Avrami exponent of crystallization is a key parameter to understand the mechanisms of nucleation and grain growth during phase transformation. For isothermal conditions the transformed volume fraction can be described by Johnson–Mehl–Avrami (JMA) equation [17–19]:
x = 1 − exp −(kt)
n
(4)
where x is the crystallized volume fraction at time t, n is the dimensionless Avrami exponent related to the nucleation and growth process, k is the reaction rate constant associated with absolute temperature described by Arrhenius equation:
k = k0 exp −
E RT
(5)
Table 3 Values of A and B for (Cu46 Zr47 Al7 )97 Ti3 BMGs.
Fig. 4. Ozawa plots of ln ˇ verse 1000/T for (Cu46 Zr47 Al7 )97 Ti3 BMGs.
A B
Tg
Tx1
Tp1
Tx2
Tp2
660.34 7.39
703.49 13.16
704.27 15.89
711.28 22.93
730.02 24.91
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strength when the fragility index m is in the range of 30–70 [24]. It is found that the value of m for the (Cu46 Zr47 Al7 )97 Ti3 BMGs is quite similar to those reported on other BMGs, such as Zr46.75 Ti8.25 Cu7.5 Ni10 Be27.5 (m = 44.2) [25], Cu46 Zr42 Al7 Y5 (m = 49) [26], Cu50.3 Zr47.7 Nb2 (m = 30) [27], Cu30 Zr55 Al10 Ni5 (m = 69) [6], implying that the (Cu46 Zr47 Al7 )97 Ti3 BMGs should be considered as “intermediate glasses”. 4. Conclusions The thermal stability and non-isothermal crystallization kinetics of (Cu46 Zr47 Al7 )97 Ti3 BMGs were investigated by XRD and DSC. The main conclusions are drawn as follows:
Fig. 6. Local Avrami exponent as a function of the crystallized volume fraction.
where k0 is a constant, E the effective activation energy of crystallization, R the gas constant, and T the absolute temperature. Using JMA equation, the Avrami exponent n can be obtained by plotting ln(−ln(1−x)) verse 1/T. The value of Avrami exponent n can be calculated if the value of E is known. However, it should be mentioned that this equation is only applicable for isothermal crystallization. The nucleation and grain growth rate do not remain constant during the whole crystallization process. For non-isothermal crystallization, the local Avrami exponent, n(x), was introduced to describe crystallization mechanism [4]: n (x) =
−R∂ ln [− ln (1 − x)]
E (x) ∂ 1/T
(6)
The local Avrami exponent n(x) can be calculated according to the variable local activation energy E(x). Since the error is too serious when the crystallized volume fraction is very low or high, we present the local Avrami exponent n(x) as a function of crystallization volume fraction (5% < x < 95%) at constant heating rate of 10 K/min (Fig. 6). It can be found that the value of n(x) gradually decreases as the crystallization fraction increases, implying nucleation rate decreases, and reaches a minimum value at about x = 90%, the rate of nucleation tends to a constant. 3.5. Kinetic fragility According to Angell’s suggestion [20–23], the fragility index m is expressed as:
m=
d log10 (T )
d(Tg /T )
(7) T =Tg
where (T) is the characteristic temperature-dependence relaxation time. Because the viscosity is proportional to a structural relaxation time, m can be estimated by replacing (T) with shear viscosity (T) in Eq. (7). Thus, m is a measure of the steepness of the slope of the viscosity curve at Tg when the temperature is scaled by Tg . If (T) can be described by the Arrhenius equation, then the fragility index m can be expressed as:
m=
d log10 (T )
d(Tg /T )
= T =Tg
Eg RTg ln 10
(8)
where, Eg is the activation energy for Tg . In the present study, the kinetic fragility index m was calculated from Eq. (8) using the data of Eg (=485 kJ/mol) and Tg (=682 K). Therefore, the kinetic fragility index m for (Cu46 Zr47 Al7 )97 Ti3 BMGs is calculated to be 37. The fragility index m was classified into three general categories: strong, intermediate, and fragile. For metallic alloys, it exhibits an intermediate fragility
(1) (Cu46 Zr47 Al7 )97 Ti3 bulk metallic glass manifests two crystallization procedures. The glass transition and crystallization both have obvious kinetics effects. The characteristic temperatures are shifted to high temperatures as the heating rate increases. (2) Activation energies corresponding to the characteristic temperatures have been calculated. The effective activation energy Ex for the onset crystallization is higher than that for Ep1 , indicating that the nucleation process is more difficult than the growth process for the first crystallization transition. However, the second crystallization process is more sensitive to heating rate than the first crystallization process. The effective activation energy Ep1 for the first crystallization peak is larger than that Ep2 for the second crystallization peak, suggesting that the growth rate for the first crystallization process is more difficult than that for the second crystallization process. (3) The fitting curves between the characteristic temperatures and the heating rate proved that the second crystallization process is the most sensitive to heating rate. (4) Depending on the calculated value of the fragility index, (Cu46 Zr47 Al7 )97 Ti3 can be classified into “intermediate glasses”. Acknowledgements This work was supported by the Major State Basic Research Development Program of China (973 Program) (Grant no. 2011CB610403), the National Natural Science foundation of China (Grant nos. 51071115, 51171136), the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant no. SKLSP201221), and Natural Science Basic Research Plan in Shaanxi Province of China (Grant no. 2012JM6010). One of the authors (M. Zhu) expressed his great thanks to Mr. Jinyang Liu in Northwest Institute for Non-ferrous Metal Research. References [1] D.C. Hofmann, J.Y. Suh, A. Wiest, G. Duan, M.L. Lind, M.D. Demetriou, W.L. Johnson, Designing metallic glass matrix composites with high toughness and tensile ductility, Nature 45 (2008) 1085–1089. [2] A. Inoue, A. Takeuchi, Recent development and application products of bulk glassy alloys, Acta Mater. 59 (2011) 2243–2267. [3] A. Inoue, Stabilization of metallic supercooled liquid and bulk amorphous alloys, Acta Mater. 48 (2000) 279–306. [4] W. Lu, B. Yan, W.H. Huang, Complex primary crystallization kinetics of amorphous Finemet alloy, J. Non-Cryst. Solids 351 (2005) 3320–3324. [5] S.F. Chen, S.L. Lin, J.K. Chen, Y.L. Lin, Thermal stability and corrosion behavior of Cu–Zr–Al–Y bulk metallic glass, Intermetallics 18 (2010) 1954–1957. [6] J.C. Qiao, J.M. Pelletier, Enthalpy relaxation in Cu46 Zr45 Al7 Y2 and Zr55 Cu30 Ni5 Al10 bulk metallic glasses by differential scanning calorimetry (DSC), Intermetallics 19 (2011) 9–18. [7] W. Yang, F. Liu, G.C. Yang, Z.F. Xu, J.H. Wang, Z.T. Wang, Analysis of crystallization kinetics of undercooled Fe–B hypereutectic alloy using DSC technique, Thermochim. Acta 527 (2012) 47–51. [8] Z.Z. Yuan, X.D. Chen, B.X. Wang, Z.J. Chen, Crystallization kinetics of melt-spun Co43 Fe20 Ta5.5 B31.5 amorphous alloy, J. Alloys Compd. 399 (2005) 166–172.
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