Kinetics of crystallization process for Pd-based bulk metallic glasses

Intermetallics 17 (2009) 241–245

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Kinetics of crystallization process for Pd-based bulk metallic glasses Li Liu a, b, Xiangjin Zhao b, Chaoli Ma b, Tao Zhang b, * a

School of Environment and Materials Engineering, Yantai University, Yantai 264005, China Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 May 2008 Received in revised form 10 August 2008 Accepted 10 August 2008 Available online 17 December 2008

The crystallization behavior of Pd79Cu6Si10P5 and Pd79Cu4Au2Si10P5 bulk metallic alloys was studied using differential scanning calorimeter (DSC) under different heating conditions. Under isochronal heating, the onset temperatures of the glass transition and crystallization for these glasses exhibited strong heating-rate dependence. The activation energies for the glass transition and crystallization were determined, based on the Kissinger plots, to be 540 and 452 kJ/mol, respectively, for Pd79Cu6Si10P5 glassy alloy, and 640 and 318 kJ/ mol, respectively, for Pd79Cu4Au2Si10P5 glassy alloy. The isothermal kinetics was modeled by the Johnson– Mehl–Avrami equation. For the Pd79Cu6Si10P5 glassy alloy the Avrami exponent was mostly in the range from 2.23 to 2.52, which indicated a decreasing nucleation rate and a diffusion-controlled three-dimensional growth. For the Pd79Cu4Au2Si10P5 glassy alloy the Avrami exponent was in the range between 2.42 and 2.76, which indicated an increasing nucleation rate and a diffusion-controlled three-dimensional growth. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: B. Glasses, metallic B. Crystallography F. Calorimetry

1. Introduction Glass-forming ability (GFA) is one of the most important issues in the bulk metallic glass (BMG) science, because it involves many basic physical problems, and is a key factor for BMGs to be used practically [1–8]. To increase the GFA, it is essential to retard the solidification and/or the crystallization processes that occurred in a supercooled melt. Therefore, the knowledge about the crystallization behavior of a supercooled melt and/or crystallization processes that occurred in a heated metallic glass is of importance in understanding the mechanisms of phase transformation far from equilibrium and hence evaluating the GFA of a alloy [9–11]. Recently, it is found that several Pd-based Pd–Cu–Si–P alloys show wide supercooled liquid regions and high GFA. Bulk metallic glasses with a thickness of 5 mm are successfully produced by copper mold casting [12]. On the other hand, small substitution (2 at.%) of Au for the Cu element in Pd–Cu–Si–P alloys further increases the GFA and the attainable maximum thickness reaches up to 7 mm [13]. To reveal the reason for the high GFA and the retarded crystallization kinetics of these Pd–Cu–Si–P alloys, it is interesting to know the crystallization behavior of these alloys. Generally, the crystallization behavior of BMGs is very complicated and strongly dependent on the heating condition. In this paper, the isochronal and isothermal DSC annealing technique was employed to investigate the crystallization behavior of two

* Corresponding author. Tel./fax: þ86 10 82314869. E-mail address: [email protected] (T. Zhang). 0966-9795/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2008.08.010

Pd–Cu–Si–P glassy alloys of Pd79Cu6Si10P5 and Pd79Cu4Au2Si10P5, which possess very high GFA. The activation energies for glass transition and different crystallization events were calculated, and the crystallization mechanisms as well as the reason for the high GFA were discussed. 2. Experimental procedures The master alloys with compositions of Pd79Cu6Si10P5 and Pd79Cu4Au2Si10P5 were prepared by induction melting the mixture of pure elements of Pd, Cu, Au, Si and P with a purity of better than 99.9% under an Ar atmosphere. The weight loss of the ingots was measured to be less than 0.1%. It is therefore believed that the compositions of the ingots were very close to the designed compositions. From the alloy ingots, ribbon-shaped samples with a cross-section of 0.02  1 mm2 were prepared by a single-roller melt-spinning technique under an argon atmosphere. The amorphous features were verified by X-ray diffraction. The crystallization kinetics of the glassy ribbons was characterized by isochronal and isothermal annealing in a differential scanning calorimeter (DSC) under flowing high-purity Ar atmosphere. Continuous heating was performed at different heating rates from 0.083 K/s to 0.5 K/s. For the isothermal DSC analysis, the samples were first heated at a rate of 0.333 K/s up to a temperature that was 20 K below the isothermal test temperature, and then heated at a slow heating rate of 0.167 K/s to the test temperature, finally held the sample at this temperature for a certain period of time followed by rapidly cooling to room temperature. The DSC measurements were calibrated for temperature and enthalpy by

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using zinc and indium standards, giving an accuracy of 0.2 K and 0.02 mW, respectively. The crystallization products were verified by X-ray diffraction.

Table 1 The characteristic temperatures Tg, Tx, Tp, and DTx of Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) glassy alloys at different heating rates. Heating rate (K/min)

Tg (K)

Tx (K)

Tp (K)

DTx (K)

3. Results and discussion

(a) 5 10 15 20 25 30

607 608 610 611 614 616

665 671 672 673 677 680

671 680 683 684 687 688

58 63 62 62 63 64

(b) 5 10 15 20 25 30

605 607 609 611 612 613

669 677 682 684 688 690

670 679 684 687 689 693

64 70 73 73 76 77

Fig. 1 shows the DSC curves for Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) glassy alloys obtained at different heating rates. All the DSC curves have a single exothermic peak, and exhibit

a

Tx

5 K/min 10 K/min 15 K/min

Exothermic (a.u.)

20 K/min

a significant endothermic reaction characteristic of a glass transition (Tg) and extended supercooled liquid region before the onset of crystallization (Tx). The values of Tg, Tx, Tp (peak temperature) and the supercooled liquid region (DTx ¼ Tx  Tg) are listed in Table 1. It is noted that with heating rate increasing the Tg, Tx and Tp shifted to higher temperature, suggesting that both the glass transition and crystallization are heating-rate dependence. The apparent activation energies for the crystallization processes were determined by the Kissinger plot method [14]. It is

25 K/min 30 K/min Tg

Pd79Cu6Si10P5

a

Pd79Cu6Si10P5

-9.2 Tp 550

600

650

700

750

ln(Φ/T2)

500

-9.6

Temperature, T(K)

b

-10.0 -10.4

Pd79Cu4Au2Si10P5

Ex=452 kJ/mol

-10.8

Eg=540 kJ/mol

Tx 5 K/min

-11.2

10 K/min

-11.6 1.48

1.52

1.56

1.60

1.64

1000 / T(K-1)

b

20 K/min

-9.2

Pd79Cu4Au2Si10P5

25 K/min -9.6 30 K/min

-10.0

ln(Φ/T2)

Exothermic (a.u.)

15 K/min

Tg

-10.4 -10.8

-11.6

Tp

1.44 500

550

600

650

700

Eg=640 kJ/mol

Ex=318 kJ/mol

-11.2

750

1.48

1.52

1.56

1.60

1.64

1000 / T(K-1)

Temperature, T(K) Fig. 1. Isochronal heating DSC plots for Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) alloys at different heating rates.

Fig. 2. Plots of ln(F/T2) vs. 1000/T of the glass transition and crystallization of the Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) glassy alloys, from which the activation energies for glass transition and crystallization are obtained.

L. Liu et al. / Intermetallics 17 (2009) 241–245

658 K

a

Pd79Cu6Si10P5

0.10

Crystallization Fraction(x)

a

0.06

653 K exo.

Heat Flow (mw)

0.08

0.04

648 K 643 K

0.02

0.00 0

20

40

60

80

1.0 Pd79Cu6Si10P5

0.8

658 K 653 K 648 K 643 K

0.6

0.4

0.2

0.0

100

243

0

20

40

Time, t(min) 0.16 Pd79Cu4Au2Si10P5

658 K

b

1.0

Crystallization Fraction(x)

b

60

80

100

Time, t(min)

0.8

0.14

0.10

653 K

0.08 0.06

exo.

Heat Flow (mw)

0.12

648 K

0.04

643 K

0.02 0.00

0

20

40

60

80

known that for the crystallization transformation of metallic glasses continuously heated at heating rate B, the transformation rate, dx/dt, can be described by the equation [15]:

(1)

where x is the fraction transformed, T is the temperature, F(x) is a continuous function of x, kT is a rate constant which is related to the activation energy of crystallization, E, the gas constant, R, and the frequency factor, v, through the Arrhenius temperature dependence:

(2)

Based on Eqs. (1) and (2), Chen derived the following equation [15]:

ln



f

T2

 ¼ 

E þC RT

0.2

Pd79Cu4Au2Si10P5

0

20

40

60

80

100

Time, t(min)

Fig. 3. Isothermal DSC curves for Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) glassy alloys.

  E kT ¼ v exp  RT

658 K 653 K 648 K 643 K

0.4

0.0

100

Time, t(min)

dx dx ¼ f ¼ kT Fx dt dT

0.6

(3)

here T represents the specific temperature, such as the glass transition temperature (Tg), the onset temperature (Tx) or the peak temperature (Tp) of crystallization, and C is a constant. By plotting ln(F/T2) vs. 1/T, an approximately straight line with a slope of Eg and Ex can be obtained, as shown in Fig. 2, in which Eg is the apparent activation energy for the glass transition, and Ex the apparent activation energy for the crystallization. From the slope of the

Fig. 4. The crystallized volume fraction as a function of annealing time of Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) glassy alloys at different annealing temperatures.

straight lines we derived the value of activation energy and obtained Eg ¼ 540, Ex ¼ 452 kJ/mol for Pd79Cu6Si10P5 alloy, and Eg ¼ 640, Ex ¼ 318 kJ/mol for Pd79Cu4Au2Si10P5 alloy. Note that the activation energy of the glass transition is considerably larger than that of the crystallization process. This suggests that the energy barrier for the glass transformation is higher than that of crystallization. Fig. 3 shows the isothermal DSC curves for Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) glassy alloys at annealing temperatures of 643 K, 648 K, 653 K and 658 K. It is noted that all the DSC curves exhibit almost symmetric exothermic peak after a certain

Table 2 The kinetic parameters of Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) glassy alloys in the case of isothermal annealing. Annealing temperature (K)

Incubation time, s (min)

Avrami exponent, n

Reaction constant, K

(a) 643 648 653 658

23.3 11.3 4.8 2.5

2.48 2.52 2.48 2.23

0.03 0.06 0.10 0.22

(b) 643 648 653 658

49.7 25.3 12.5 6.4

2.52 2.76 2.77 2.42

0.05 0.08 0.13 0.25

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L. Liu et al. / Intermetallics 17 (2009) 241–245

incubation time and the incubation time becomes shorter when the annealing temperature is higher. These facts indicate that the samples are of fully amorphous structure and the amorphous phase crystallizes via a nucleation-and-growth process [16]. It has been suggested that the fractional area of the exothermic peak on a DSC curve is proportional to the crystallized volume fraction. Integrating the exothermal peaks shown in Fig. 3, we obtained the crystallized volume fraction, which is a function of annealing time. Fig. 4 shows the sigmoid relationship between crystallized volume fraction and annealing time. The isothermal crystallization kinetic is normally described by the Johnson–Mehl–Avrami (JMA) equation [17–21]:

 x ¼ 1  exp  ½Kðt  sÞn ;

(4)

in which, x is the transformed volume fraction, n is the Avrami exponent, reflecting the characteristics of nucleation-and-growth during crystallization, s is the incubation time taken as the time interval between the specimens reaching the annealing temperature and the start of the transformation, and K is the reaction rate constant which is related to the activation energy for the process, Ea, by

  Ea K ¼ K0 exp  RT

(5)

where K0 is a constant. Values for K and n can be determined using the relationship:

Pd79Cu6Si10P5

a

Cu-Kα

(6)

Plotting ln{ln[1/(1  x)]} against ln(t  s) for 0.15  x  0.85 and for different temperatures, the JMA plot is obtained. The Avrami exponents n are obtained from the slope of the straight lines. The Avrami exponent n and the reaction rate constant K calculated from the JMA plot are listed in Table 2. The Avrami exponent n varies from about 2.23 to 2.52, implying a decreasing nucleation rate and a diffusion-controlled three-dimensional growth [22] for Pd79Cu6Si10P5 alloy, and about 2.42 to 2.76 for Pd79Cu4Au2Si10P5 alloy, which implies an increasing nucleation rate and a diffusioncontrolled three-dimensional growth [22]. According to Eq. (5), the apparent activation energy for crystallization is calculated to be Ea ¼ 445 kJ/mol and Ea ¼ 326 kJ/mol respectively for the Pd79Cu6Si10P5 and Pd79Cu4Au2Si10P5 alloys. To study the crystalline phases formed during isochronal and isothermal annealing processes, we annealed samples to different stages and examined them separately. We used two annealing strategies to investigate phases related to a specific crystallization exotherm: (1) annealing isochronally (20 K/min) to the temperature (923 K) where that crystallization event ends; (2) annealing isothermally at 643 K for 0–120 min. XRD patterns of the isochronally and isothermally annealed samples are shown in Figs. 5 and 6. From these patterns, it is known that Pd, Pd4Si and Au3Si are the major crystalline phases after the isothermal annealing treatment

Pd79Cu6Si10P5

Pd4Si

Intensity (a. u.)

Pd15P2 Pd6P

Intensity (a.u.)

a

lnfln½1=ð1  xÞg ¼ n lnðKÞ þ n lnðt  sÞ;

Cu-Kα

643 K, 100 min

643 K, 80 min 643 K, 13 min 643 K, 6 min 643 K, 2 min melt ribbon

20°

30°

40°

50°

60°

70°

20°

80°

30°

40°

b

Pd79Cu4Au2Si10P5

Pd Pd4Si

50°

60°

70°

80°

70°

80°

2θ

2θ

b

Cu-Kα

Pd79Cu4Au2Si10P5

Pd Au3Si Pd4Si

Cu-Kα

Intensity (a. u.)

Intensity (a. u.)

643 K, 120 min

643 K, 80 min 643 K, 40 min 643 K, 20 min 643 K, 10 min 643 K, 5 min 643 K, 0 min

20°

30°

40°

50°

60°

70°

80°

2θ Fig. 5. XRD patterns of the isochronally annealed samples for Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) glassy alloys.

20°

30°

40°

50°

60°

2θ Fig. 6. XRD patterns of the isothermally annealed samples for Pd79Cu6Si10P5 (a) and Pd79Cu4Au2Si10P5 (b) glassy alloys.

L. Liu et al. / Intermetallics 17 (2009) 241–245

for Pd79Cu4Au2Si10P5 alloy, suggesting a very complex crystallization process. Whereas only Pd4Si phase precipitated from Pd79Cu6Si10P5 glass phase during isothermal annealing process, suggesting a relatively simple crystallization process. The complex crystallization process for the Au added alloy (Pd79Cu4Au2Si10P5) implies that the atomic diffusion is more difficult comparing to the Au-free alloy. This is in agreement with our previous suggestion that the minor Au addition might facilitate the formation of new local atomic structure, which is favorable to stabilize the supercooled liquid, consequently, dramatically enhances the GFA [13].

4. Conclusions The crystallization kinetics of two bulk amorphous alloys, Pd79Cu6Si10P5 and Pd79Cu4Au2Si10P5, was studied by isochronal and isothermal annealing in a differential scanning calorimeter. The activation energies for glass transition and crystallization were obtained using Kissinger’s method. The isothermal kinetics was modeled by the Johnson–Mehl–Avrami equation and the Avrami exponent indicates a decreasing nucleation rate and a diffusioncontrolled three-dimensional growth for Pd79Cu6Si10P5 alloy and an increasing nucleation rate and a diffusion-controlled threedimensional growth for Pd79Cu4Au2Si10P5 alloy. Palladium, Pd4Si and Au3Si phases are the major phases produced during isothermal annealing process for Pd79Cu4Au2Si10P5 alloy, but only Pd4Si phase precipitated from Pd79Cu6Si10P5 alloy.

245

Acknowledgments This work was supported by the National Natural Science Foundation of China (50631010, 50771006), the National Basic Research Program of China (2007CB613900), and PCSIRT (IRT0512). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

Johnson WL. MRS Bull 1999;24:42. Xu D, Duan G, Johnson WL. Phys Rev Lett. 2004;92:245504. He Y, Poon SJ, Shiflet GJ. Science 1988;241:1640. Ponnambalam V, Poon SJ, Shiflet GJ. J Mater Res 2004;19:1320. Zhang B, Wang RJ, Zhao DQ, Pan MX, Wang WH. Phys Rev B 2004;70:224208. Das J, Tang MB, Kim KB, Theissmann R, Baier F, Wang WH, et al. Phys Rev Lett 2005;94:205501. Sheng HW, Luo WK, Alamgir FM, Bai JM, Ma E. Nature (London) 2006;439:419. Xu D, Lohwongwatana B, Duan G, Johnson WL, Garland C. Acta Mater 2004; 52:2621. Greer AL. Mater Sci Eng 1996;A41:179–80. Raval KG, Kirit NL, Pratap A, Awasthi AM, Bhardwaj S. Thermochimica Acta 2005;425:47. Weinberg MC. Thermochim Acta 1996;63:280–1. Liu L, Inoue A, Zhang T. Mater Trans 2005;46:376–8. Liu L, Pang SJ, Ma CL, Zhang T. Mater Trans 2005;46:2945–8. Kissinger HE. Anal Chem 1957;29:1702. Chen HS. J NonCryst Solids 1978;27:257. Chen LC, Spaepen F. Nature 1988;336:366. Johnson WA, Mehl RF. Abr. Trans Am Inst Min Metall Pet Eng 1939;135:416. Avrami M. J Chem Phys 1939;7:1103. Avrami M. J Chem Phys 1940;8:212. Avrami M. J Chem Phys 1941;9:177. Yinnon H, Uhlmann DR. J NonCryst Solids 1983;54:253. Christian JW. The theory of transformation in metals and alloys. 1st ed. Oxford: Pergamon; 1965. p. 489.