A study of non-isothermal primary crystallization kinetics and soft magnetic property of Co65Fe4Ni2Si15B14 amorphous alloy

Journal of Alloys and Compounds 539 (2012) 210–214

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A study of non-isothermal primary crystallization kinetics and soft magnetic property of Co65Fe4Ni2Si15B14 amorphous alloy Xiao Zhou a, Haitao Zhou a,⇑, Zhongkai Zhao a, Ruirui Liu a, Yong Zhou b a b

School of Materials Science and Engineering, Central South University, Changsha 410083, PR China Institute of Micro/Nano Science & Technology, Shanghai Jiao Tong University, Shanghai 200030, PR China

a r t i c l e

i n f o

Article history: Received 28 March 2012 Received in revised form 3 June 2012 Accepted 7 June 2012 Available online 17 June 2012 Keywords: Amorphous alloy Co based Crystallization kinetics Activation energy Avrami exponent

a b s t r a c t The non-isothermal primary crystallization kinetics and soft magnetic properties of Co65Fe4Ni2Si15B14 amorphous alloy have been investigated using differential scanning calorimeter (DSC), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). It is found that the apparent crystallization activation energy Ec is not the same as the calculation methods of Kissinger and Doyle–Ozawa, and is 471.68 and 461.50 kJ/mol, respectively. At the steady crystallization stage, the local crystallization energy decreases gradually, and the thermal stability of the residual amorphous matrix becomes weak finally. This is ascribed to the diffusion of boron atoms at elevated temperatures. As a result, this primary crystallization mechanism can be explained by JMA nucleation-and-growth model and normal grain growth kinetic mode. During crystallization, some of nanocrystals such as Co–B intermetallics, Co2Si, Fe23B6 and NiB precipitate from amorphous matrix after annealing at 773 K. This leads to soft magnetic properties being improved finally. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction

growth mechanism is described with Johnson–Mehl–Avrami (JMA) model [10–12]. That is,

Amorphous alloys can crystallize when annealed above the crystallization temperature for a sufficient time companying with a series changes of many prosperities such as heat capacity, volume, mechanical properties, electrical properties and magnetic properties [1]. In the last decade, a substantial increase in soft magnetic properties has been reported in the formation of nano-sized grains in Fe-based amorphous alloy such as Fe–Si–B–Nb–Cu, Fe– Zr–B–Cu and Fe–Mo–B [2–4]. However, it is found that a good soft magnetic property also exists in Co-based amorphous alloy, and they have wide practical applications due to their excellent soft properties. For example, amorphous alloy 2714A (Co65Fe4Ni2Si15B14) is a special soft magnetic amorphous alloy with a near zero magnetostriction, an ultra-high permeability, an extremely low core loss and a widely application in electric transformers and sensors [5]. Recently, it is found that annealing below or near the conventional crystallization temperature could give rise to the formation of nano-sized crystals along with a large increment of initial permeability and reduction of coercivity during the primary crystallization [6–9]. However, how to control the crystallization process is essential for obtaining good soft magnetic properties. Usually, a primary crystallization reflecting the nucleation and

f ðaÞ ¼ nð1  aÞ½ ln ð1  aÞðn1Þ=n

⇑ Corresponding author. Tel.: +86 731 86450817; fax: +86 731 88876692. E-mail address: [email protected] (H. Zhou). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.035

ð1Þ

where n is Avrami exponent reflecting the behaviors of nucleation and growth, and a is crystallization fraction. This equation is deduced under isothermal condition with some assumptions. However, Henderson [13,14] pointed out that the valid of JMA model could be also used under non-isothermal conditions to interpret crystallization mechanism as the entire nucleation behavior occurs at the early stage of the transformation. Besides, the Kissinger–Ozawa method could be used for calculation of apparent activation energy of crystallization under non-isothermal conditions [15,16]. However, because of the difference of nucleation and growth behavior in crystallization process, the activation energy for different crystallization fraction is not constant. Therefore, it is necessary to introduce the local activation energy related to crystallization fraction a in order to represent the activation energy at all crystallization stages on the basis of the Flynn–Wall–Ozawa (FWO) method [16,17]. In the present paper, Johnson–Mehl–Avrami (JMA) model is employed to investigate the nanocrystallization of Co65Fe4Ni2Si15B14 amorphous alloy according to the non-isothermal DSC signals, and also the Kissinger–Ozawa method and Flynn–Wall–Ozawa (FWO) method are used to calculate the apparent activation energy and the local activation energy of crystallization, respectively. We try to obtain the local Avrami exponent in detail to help us with estimating the primary crystallization mechanism of Co65Fe4Ni2Si15B14 amorphous alloy.

211

X. Zhou et al. / Journal of Alloys and Compounds 539 (2012) 210–214 2. Experimental procedure The amorphous ribbon Co65Fe4Ni2Si15B14 (Metglas 2714A) was 20 lm thick and 25 mm wide provided by Honeywell Electronics. Non-isothermal measurements were conducted by using Netzsch STA 449C, and the heating rates ranged from 1 to 50 K/min under the protection of high purity Ar. Rigaku D/Max 2550 with Cu Ka radiation was used for XRD analysis. Annealing processing was in vacuum (0.1 Pa) using a CVD (G)-06/50/2 electronic high-temperature tube furnace (argon gas protection) without magnetic field. Annealing temperatures were selected from 763 to 783 K according to DSC results. When annealing was completed, the samples were taken out and cooled quickly in the air. The magnetic properties were measured by LakeShore 7407 VSM.

10K/min 15K/min 20K/min EXO

Heat Flow (a.u.)

5K/min

35K/min

3. Results and discussion The X-ray diffraction patterns of the as-quenched Co65Fe4Ni2Si15B14 alloy annealed at different temperatures for 90 min are shown in Fig. 1. It can be seen that amorphous hump located at 2h  45°. This confirms that the as-cast sample is fully amorphous. From Fig. 1, the weak peak associated with cobalt with h.c.p. structure is detected at 2h  42°and 2h  47° on the amorphous hump implying that a composite structure of amorphous matrix and single crystalline phase is formed. As the annealing temperature is elevated to 773 and 783 K, the occurrence and growth of the peak corresponding to hcp–Co element, Co–Si and Co–B compounds can be observed clearly and the diffraction peaks are very sharp because of crystalline. This is consistent with the research in Ref. [18], i.e. crystallites of cubic (fcc) and hexagonal hcp–Co, of Co2B and Co3B. Therefore, the primary crystallization of Co65Fe4Ni2Si15B14 amorphous alloy is as follows: Am ? Am0 + hcp–Co ? Am00 + hcp– Co + Co–Si + Co–B ? hcp–Co + Co–Si + Co–B. Besides, it is suggested that the peak corresponding to hcp–Co element, Co–Si and Co–B compounds at 783 K is more intense than that at 773 K. This is because that higher temperature brings about an increase of crystalline driving force that makes the crystallization more complete. Fig. 2 shows the DSC curves of Co65Fe4Ni2Si15B14 amorphous alloy at different heating rates. The characteristic temperature and some criteria for evaluating the glass-forming ability (GFA) are shown in Table 1. As is shown in Fig. 2, all the curves have two similar characteristics: (1) a wide supercooled region before crystallization. (2) The primary crystallization corresponding to the nanocrystallization products presents asymmetrical profiles attributed to the sequent precipitation of different crystallization phases. Furthermore, with increasing of the heating rate, the exothermic peaks become more acute and move toward high temperature.

40K/min

300

400

500

600

800

900

1000

1100

Fig. 2. DSC curves of amorphous alloy Co65Fe4Ni2Si15B14 at different heating rates.

Table 1 Tg, Tx, Tp and DTx of amorphous alloy Co65Fe4Ni2Si15B14 at different heating rates. Heating rate (K/min)

Tg (K)

Tx (K)

Tp (K)

DTx (K)

5 10 15 20 35 40

796 801.5 806.3 809.0 811.1 811.8

811.1 819.0 823.8 828.5 833.4 834.8

813.9 821.2 826.8 830.7 836.2 837.3

15.1 17.5 17.5 19.5 22.3 23.0

The thermodynamic parameters of amorphous alloy Co65Fe4Ni2Si15B14 at different heating rates including glass transition temperature Tg, crystallization onset temperature Tx, peak temperature Tp and supercooled liquid region DTx are displayed in Table 1. With increasing of heating rate, the crystallization is carried out in higher temperature region accompanied with Tg, Tx, Tp, DTx raised. First, Kissinger’s method [15] is used as Eq. (2),

ln

T 2p b

! ¼ ln

  Ec Ec  ln m þ R RT p

ð2Þ

where b is the heating rate; Ec is the activation energy; Tp is the peak temperature; m is the frequency factor in Arrehenius law and R is gas constant: 8.314 J/mol. Plotting ln ðT 2p =bÞ as y-axis and 1/Tp as x-axis, a straight fit line can be obtained with the slope of Ec/R and m can be obtained from intercept calculating by least square method. Therefore, m is 8.1389  1026 and the activation energy can be calculated as shown in Fig. 3(a). Second, the crystallization activation energies are also calculated by Doyle–Ozawa’s means [16] as Eq. (3),

ln b ¼ 1:0516 Ec =RT p þ A

Fig. 1. XRD patterns of Co65Fe4Ni2Si15B14 amorphous alloy at different annealing temperatures for 90 min.

700

Temperature (K)

ð3Þ

where A is a constant. A straight line can also be obtained by plotting ln b as y-axis and 1/T as x-axis with the slope of 1.0516Ec/R. The activation energy can then be calculated as seen in Fig. 3(b). The crystallization process of amorphous alloys experiences the nucleation and growth. This leads to the variety of the microstructures closely related with the properties. For nano-scale devitrification, primary crystallization is required. However, this condition is not sufficient to create nanocrystalline structures from the amorphous matrix. The morphology of the primary crystallized phase from amorphous precursors is variable, for instance, they may be spherical or dendritic. On the other hand, the crystal size can also change over a large range. This depends on the composition, the transfer rate of atoms across the interface and the diffusion rate

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12.0

40

10.5

10.0

103/Tp

20

300 200

10

100 0 0

9.5 1.190 1.195 1.200 1.205 1.210 1.215

30

400

LogA

11.0

p

ln(T 2/β )

Local activition energy, Ec

(a) 11.5

500

Ec=471.68KJ/mol Slope=56.73328 R=0.99757

0.0

0.2

0.4

0.6

0.8

1.0

Crystallization fraction α

1.220 1.225 1.230

)

(K-1

Fig. 4. The relationship between the local activation energy Ec and crystallization fraction a calculated by FWO method.

4.5

(b)

E =467.60kJ/mol c Slope=59.1451 R=0.99791

2.5

(Fe > Co > Ni = 0.162 nm, Si = 0.146 nm, B = 0.117 nm) exists, the boron atoms with a much less atom size can diffuse in long range and this diffusion is enhanced at elevated temperatures, which is another important mechanism to make the residual amorphous matrix less stable. According to Málek’s kinetics analysis method [22] for amorphous materials, in non-isothermal conditions, y(a) and z(a) functions are defined as follows:

2.0

yðaÞ  / exp

4.0 3.5

lnβ

3.0

1.5

Ec RT



zðaÞ  /T 2

1.190 1.195 1.200 1.205 1.210 1.215 1.220 1.225 1.230

1000/Tp (K-1) Fig. 3. Apparent crystallization activation energy Ec calculated by: (a) Kissinger method; (b) Doyle–Ozawa method.

of atoms towards or away from the interface [19]. Due to the difference of nucleation and growth behavior in crystallization process, the activation energy for different crystallization fraction is not constant. Therefore, local activation energy Ec(a) is introduced at all crystallization stages. Flynn–Wall–Ozawa (FWO) method [16,17] as shown in Eq. (4) is utilized to calculate local activation energy Ec(a) as a function of crystallization fraction a.

ln b ¼ 1:0516



ðEc Þi þA RT i

ð4Þ

Fig. 4 gives the curves of local activation energy and pre-exponent factor as a function of crystallization fraction. It can be seen that the energy transformation for primary crystallization of Co65Fe4Ni2Si15B14 amorphous alloy can be divided into three stages. At the beginning (0 < a < 0.1), the local activation energy rises rapidly to the value about 495 kJ/mol. And at the middle stage (0.2 < a < 0.5), the value of local activation energy is stabilized at about 465 kJ/mol and then drops gradually at the later stage (0.5 < a < 0.95). This implies that the stability of residual amorphous matrix decreases. The negative mixed enthalpy between Co–B, Fe–B and Ni–B are 24, 26 and 24 kJ/mol, respectively [20] and the boron-enriched phases such as Co2B, Co3B2, Fe3B and NiB [21] precipitate from amorphous matrix. The component of the large negative heats of mixing is reduced resulting in the instability of residual amorphous matrix. Moreover, crystallization is a heat activation process. And in this system, the complexity of atomic sizes

ð5Þ ð6Þ

where / is the measured specific heat flow. These functions are normalized in the range between 0 and 1. The value ap of conversion corresponding to the maximum of z(a) function is 0.632 for JMA model. However, according to Fig. 5 and Table 2, the ap is far lower than the value range (0.61–0.65) from JMA model. It seems that one or more conditions for the applicability of the JMA model is not completed, i.e., this Co65Fe4Ni2Si15B14 amorphous alloy does not show only one simple crystallization kinetics as expressed by JMA model in the whole transformation range. Such a behavior is perhaps a consequence of the fast increase of the initial crystallization rate. This acceleration can be due to a secondary nucleation induced by the crystal growth.

ln

  da Ec þ ¼ ln ½Af ðaÞ dt RT

ð7Þ

As above, it is conventionally to use Suriñach analysis for studying the crystallization kinetics of Co65Fe4Ni2Si15B14 amorphous alloy [23]. The Eq. (7) gives the Suriñach curve fitting procedure. On the right side, the f(a) function elucidates the nucleation and growth mechanism, while on the left one, the (da/dt) is obtained from the DSC signals and the local activation energy Ec is calculated by one mean such as Flynn–Wall–Ozawa (FWO) method in this paper. And Table 3 gives crystallization kinetic model equations. In Fig. 6, using the least-squares minimization curve fitting, the kinetics function is deduced according to Table 3. Evidently, the crystallization begins with the JMA-like kinetics with the Avrami exponent n = 4.2 in the range a < 12.9%, and then the value of n decreases to 2.8 for the range 12.9 < a < 45%, and later crystallization kinetics follows the NGG-like model with the m = 0.939 for the range 44.6 < a < 98%. Furthermore, the crystallization of amorphous alloys contains the process of nucleation and grain growth. In the initial stage, i.e. the temperature is little higher than the crystallization

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X. Zhou et al. / Journal of Alloys and Compounds 539 (2012) 210–214

1.0

-3

(a)

-4

n=2.82309 lnA=3.66001

-5

0.6

ln[Af(α)]

Normalized y(α)

0.8

5K/min 10K/min 15K/min 20K/min 35K/min 40K/min

0.4

0.2

0.0 0.0

0.2

-6

m=0.93958 lnA=3.0516

-7

-8

0.4

0.6

0.8

1.0

0.0

Crystallization volume fraction, α

0.47

(b) JMA

1.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

-ln(1-α)

1.2

Normalized Z(α)

n=4.18085 lnA=3.74739

Fig. 6. Suriñach representations of non-isothermal DSC curves of amorphous alloy Co65Fe4Ni2Si15B14 at the heating rate between 5 and 40 K/min. Continuous lines are the plots for the theoretical JMA kinetics with different n value. Dashed line is the plot for the NGG mode.

0.8 0.6

5K/min 10K/min 15K/min 20K/min 35K/min 40K/min

0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

Crystallization volume fraction, α Fig. 5. Normalized (a) y(a)and (b) z(a)function obtained by transformation of nonisothermal data for the crystallization of amorphous alloy Co65Fe4Ni2Si15B14.

Table 2 The maximum of the function y(a) and z(a) at different heating rates. b (K/min)

5

10

15

20

35

40

am ap

0.471 0.474

0.339 0.336

0.311 0.306

0.180 0.183

0.199 0.196

0.139 0.149

Table 3 Crystallization kinetic model equations. Model

f(a)

Johnson–Mehl–Avrami (JMA) Normal-grain-growth (NGG) Sestak–Berggren (SB)

n(1  a)[ln (1  a)](n (1  a)m + 1 am(1  a)n

Label  1)/n

n m m, n

temperature so that the crystallization nucleation rate reaches maximum in no time and then decreases rapidly, while the grain growth rate increases rapidly and achieves maximum when the temperature is much higher than the initial crystallization temperature [24]. The nucleation and growth model is considered to be a rule in the early stage of crystallization. According to JMA model, the Avrami exponent is 4.1 close to 4 at the beginning and then decreases to 2.8 close to 3. It is suggested that the initial growth mechanism changes from the interface-controlled three-dimensional growth with increasing nucleation rate to the one with zero

nucleation rates. And this decrease of nucleation rate results in the normal grain growth in the advance stage of crystallization of Co65Fe4Ni2Si15B14 amorphous alloy. Therefore, the primary crystallization mechanism of amorphous alloy Co65Fe4Ni2Si15B14 is made up of two stages or types of nucleation and grain growth mechanisms, i.e. the JMA-like model and NGG-like model. And these processes seem similar to the kinetics characteristics of Finemet and Co43Fe20Ta5.5B31.5 alloys [25,26]. Fig. 7 shows the hysteresis loops and magnetic properties of amorphous alloy Co65Fe4Ni2Si15B14 annealed at 773 K for different time. According to the Fig. 7, hysteresis loop of the sample with annealing at 773 K for 30 min is almost coincident and the residual magnetization is almost zero. Namely, there is no residual magnetization and hysteresis loss for the sample annealed at 773 K for 30 min. As the annealing time rising from 30 min to 90 min, it is evident that the hysteresis loops appear ring area and the coercivity increases. However, this phenomenon does not occur when annealing time is less than 30 min, which indicates that the magnetic properties of amorphous alloy Co65Fe4Ni2Si15B14 become deteriorating with increasing the annealing time. The main reason is that when crystallization occurs, the domain walls are suffering from strong pinning effect because of phase precipitation. Besides, from the insert of Fig. 7, the initial permeability increases as the amorphous alloy undergoes annealing at 773 K for 30 min. After 30 min, the initial permeability decreases. According to Herzer’s random anisotropy theory [4,27], the values of the anisotropy constants K and anisotropy fields of the nanostructured magnetic amorphous alloys decrease due to the reduction of internal stresses in the amorphous matrix and to the small size of crystallites which is less than the exchange correlation length. When annealing time is less than 30 min, the crystalline grain is very small and the internal stress is very low. As the annealing time rising from 30 min to 90 min, a great many crystalline grains nucleate and grow, which results in increasing of internal stress and makes the initial permeability reducing. The initial permeability can be described as the Eq. (8),

li ¼ pl

J 2s J 2 A3  pl s 4 6 l0 hji l0 K D

ð8Þ

where li is the initial permeability; D is the grain diameter; Js is the average saturation magnetization of the material and pl is dimensionless pre-factor of the unit. It is quite important to construct crystallization kinetics, judge the nucleation and growth

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X. Zhou et al. / Journal of Alloys and Compounds 539 (2012) 210–214

100

60 40

Acknowledgements As-cast 773K, 30min 773K, 60min 773K, 90min

The authors thank Mrs. Wu Tai-sha in Thermal Analysis Laboratory of MSE in CSU for the great help in measurements and data analysis. This work was supported by 863 Program (2009AA03Z301) from Ministry of Science and Technology of China and Nanotechnology Special Program (Grant No.065nm004) from Shanghai Science and Technology Committee.

20

i

0 -20

Permeability μ

Mangentization M (emu/g)

80

-40 -60 -80 -100 -300

References

0

-200

-100

0

30

60

90

Annealing time (min)

100

200

300

Applied field H (Oe) Fig. 7. Hysteresis loops of amorphous alloy Co65Fe4Ni2Si15B14 annealed at 773 K for different time.

mechanism of crystallization phases and control the annealing processing with excellent soft magnetic properties. 4. Conclusions (1) The apparent crystallization activation energy Ec is calculated to be 471.68 and 461.50 kJ/mol, respectively, by Kissinger and Doyle–Ozawa method for Co65Fe4Ni2Si15B14 amorphous alloy. (2) The primary crystallization of Co65Fe4Ni2Si15B14 amorphous alloy is a multi-stage continuous nucleation. The thermal stability of residual amorphous matrix becomes weak at the steady crystallization stage, which is ascribed to the longrange diffusion of boron atoms at elevated temperatures. (3) The crystallization mechanism of Co65Fe4Ni2Si15B14 amorphous alloy can be explained by JMA nucleation-and-growth model and normal grain growth kinetic mode. (4) Through controlling the process of annealing, the suitable magnetic property of Co65Fe4Ni2Si15B14 amorphous alloy could be gained and the best soft magnetic property is reached after annealed at 773 K for 30 min.

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