Electrical resistivity evolution in the annealed amorphous Fe78Si9B13 ribbons

Journal of Non-Crystalline Solids 354 (2008) 3612–3616

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Electrical resistivity evolution in the annealed amorphous Fe78Si9B13 ribbons W.M. Wang a,*, Y.C. Niu a, F. Wang b, J.C. Liang a, S.F. Jin a, W.G. Zhang a, X.F. Bian a a b

Key Laboratory of Liquid Structure and Heredity of Materials, Shandong University, Jinan 250061, China Shandong Labor Professional Technology College, Jinan 250061, China

a r t i c l e

i n f o

Article history: Received 25 March 2007 Received in revised form 21 January 2008 Available online 12 April 2008 PACS: 72.15.Cz 61.72.Dd Keywords: Alloys X-ray diffraction

a b s t r a c t The electrical resistivity was measured at room temperature for the amorphous Fe78Si9B13 ribbons annealed at various temperatures for different holding time. Although the annealed Fe78Si9B13 ribbons are in full amorphous state, their electrical resistivity obviously varies with the annealing time. At every annealed temperature, the electrical resistivity evolution can be divided into regions I, II, and III, respectively. Using X-ray diffraction (XRD), scanning electron microscope (SEM), and differential scanning calorimetry (DSC), we investigated the ribbons overlapping regions I and II (called the focused ribbons, FRs). The results show that the change of electrical resistivity, fracture morphology, thermal effect in DSC analysis of the focused ribbons (FRs) can be ascribed to the evolution of the short range order (SRO) in the amorphous alloy. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Amorphous Fe78Si9B13 alloy is one of the most important applied amorphous alloys due to its excellent magnetic properties, and consequently many researches have been carried out on its structure and properties [1,2]. For example, the fracture surface of the amorphous Fe78Si9B13 ribbon changes significantly with the annealing temperature, but its onset of crystallization temperature does not change obviously [3]. Meanwhile, the Curie temperature, Tc, of the mechanically pulverized Fe78Si9B13 glassy ribbon increases firstly and then decreases during the relaxation, which is ascribed to the enhancement of the short range order in the amorphous state [4] During the annealing treatments carried out step by step, the coercivity, Hc, of the amorphous Fe78Si9B13 ribbon exhibits a minimum value of 1.6 A/m after 3 min of annealing at 773 K, although it retains a full amorphous state from XRD patterns [5]. Its amorphous structure can be considered as the mixture of Fe–B regions and Fe–Si regions [6]. Hence, the evolution of the short range orders in Fe78Si9B13 glass is valuable to study. The electrical resistivity measurement seems more susceptible than other measurements for the microstructure of the amorphous alloy, because the electrical resistivity can give a more safe Tg and useful information on the scattering mechanism of intersititial atoms in the amorphous alloy [7,8]. In a low temperature range, amorphous alloys exhibit a variety of electrical resistivity, which is temperature dependent due to a lack of the long range order characteristic for the amorphous alloys [9]. The electrical resistiv-

* Corresponding author. Tel.: +86 531 88392749; fax: +86 531 88395011. E-mail address: [email protected] (W.M. Wang). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.03.016

ity vs. temperature curve of the amorphous Fe83B17 ribbon displays a discernible fall at some points below the crystallization temperature, ascribed to either some degree of ordering or to transformation of thermodynamically unstable/metastable phases into stable phases [10]. However, there is only a simple temperature coefficient of the electrical resistivity in the other Fe-based glasses [11,12]. Therefore, it is significant to measure the electrical resistivity of the Fe78Si9B13 ribbon after various annealing processes. Unfortunately, there is no detailed work on the relationship between the resistivity and short range orders in the amorphous Fe78Si9B13 ribbon after various annealing treatments. In present work, a 4-probes method was used to measure the electrical resistivity of the annealed Fe78Si9B13 ribbons after annealed at different temperatures for various times. Moreover, we chose several ribbons annealed at 473 K with a holding time of 3– 6.5 h as the focused ribbons (FRs), which were investigated by Xray diffraction (XRD), scanning electron microscope (SEM), and differential scanning calorimetry (DSC). Our aim is to obtain the deeper insight on the relationship between the electrical resistivity and microstructure of the glassy Fe78Si9B13 ribbons under various annealing conditions.

2. Experimental The amorphous Fe78Si9B13 ribbon, prepared by melt spinning, was supplied by the National Amorphous Nanocrystalline Alloy Engineering Research Center of China. The samples of the ribbon are about 10 mm wide and 27 ± 1 lm thick. The ribbons were annealed at a temperature range of 448–548 K for 0–24 h. For saving

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the operating room, ribbons were rolled into tubes and placed into the furnace chamber in the annealing treatment. The electrical resistivity measurements were carried out at room temperature using a DC four probe method with fixed distances between four probes. The potential drop was picked up on the 180 mm long specimen with an error of ±0.001 ohm. All the samples are measured for three times. The reduced electrical resistivity (qre) is defined as the ratio of the measured electrical resistivity against the value of the as quenched ribbon. The error of reduced resistivity qre is ±2% for the ribbons if their retained curvature is not large. Meanwhile, the structural feature of the annealed ribbons was examined by X-ray diffraction (XRD) analysis using a Rigaku D/ max-RB diffractometer with Cu Ka target. In order to get more structural information, we normalized the measured intensities, I, after considering the atomic and Compton scattering factors. Thus, we got the structure factor S(Q) of the alloys as following [13,14] SðQ Þ ¼

aI  hf ðQ Þi2  Icomp ; hf 2 ðQ Þi

ð1Þ

where the scattering vector Q equals to 4psin h/k, f(Q) and Icomp are the atomic scattering factor and Compton scattering factor of the elements in the alloy, respectively, and a is the normalized factor. The fracture morphology of the ribbons was investigated by scanning electron microscopy (SEM, Hitachi S-570). Thermal analysis was performed on a Shimadzu Type DSC-41 device, using aAl2O3 as the referenced pan under the flowing high purity nitrogen and a heating rate of 20 K/min. In order to investigate the structural change in the annealed ribbons, we conducted the DSC experiments in one day, and the measured and referenced crucibles were placed at one position respectively to decline the drift of base line from the position of measured and referenced crucibles. The weight of samples was remained similar to each other. The temperature for the structural change (Tr) at about 600 K is given by the intersection point of two tangent lines of the curves after and

before structural change. During the DSC experiments, we tried to fix the samples at one position. Hence, we assume approximately the area bounded by DSC curve, base line and line T = Tr as the extra thermal effect (DHex). It should be noted that DHex is contributed not only by structure relaxation of measured ribbons, but also by the difference of thermophysical properties between the measured ribbon and reference. Nevertheless, it is reasonable to consider that change of DHex reflects the structural evolution in the amorphous alloy under the conditions as mentioned above. 3. Results Fig. 1 shows the reduced resistivity qre of the Fe78Si9B13 ribbons after annealed at various temperatures (Ta) for different time (ta). For all curves, it is found that the evolution of qre can be divided roughly into regions I, II and III. There is a clear maximum of qre in regions I and II of each qre–ta curve, while several peaks appear in region III with a more noisy and irregular appearance. It should be pointed out that the ribbons in region III are brittle and bearing a large retained curvature, because the ribbons were rolled into tubes and placed into the furnace chamber before annealing. Therefore, the error of the qre of the samples in region III seems much larger than that in regions I and II. And the maxima in region III are neither so representative nor considered in this study. Hence, in order to understand the electrical resistivity evolution in the annealed ribbons, we chose the ribbons overlapping regions I and II as the focused ribbons (FRs), which were annealed at 473 K under a holding time of 3–6.5 h (Fig. 1). Fig. 2 displays the XRD patterns of the focused ribbons (FRs). Only two diffuse halos can be observed for all ribbons, suggesting that the ribbons remain in a fully glassy structure after annealing processes. The fracture morphology of the ribbons annealed for ta = 3 h and 4 h at 473 K are illustrated in Fig. 3. When annealed for 3 h, the surface exhibits a rough morphology displaying ridge-like features (Fig. 3(a)), which is the typical fracture feature for the metallic glasses [15]. In contrast, a flat surface with a few

Region Region I

Region III

II

Ta= 448 K

1.2 1.0

FRs

1.2

Ta= 473 K

Reduced resistivity, ρ re

1.0 1.2

Ta= 485.5 K

1.0 1.2

Ta= 498 K

1.0 1.2

Ta= 510.5 K

1.0 1.2

Ta= 523 K

1.0 1.2

Ta= 548 K

1.0 0

4

8

12

16

20

24

Annealing time ta, h Fig. 1. The reduced resistivity qre of the Fe78Si9B13 ribbons after annealed at various temperatures (Ta) under different time (ta).

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Ta=473K

Region I

D

2.0

ta=3.5h

Region II

1.5

1.0

ta=4.0h

1.15

Intensity, a.u.

Ta = 473 K

(a)

ta=3.0h

ta=4.5h

(b)

e

1.10

1.05

ta=5.5h 1.00 3

4

5

6

7

ta, h

ta=6.5h

Fig. 4. (a) Fracture morphology’s fractal dimension D and (b) reduced resistivity qre of the focused ribbons (FRs).

20

40

60

80

2θ , deg Fig. 2. XRD patterns of the focused ribbons (FRs).

striations near both edges is the main feature for the ribbon annealed for 4 h (Fig. 3(b)). Similar to the work of Shek et al. [16], the profiles of the fracture morphologies of the FRs were analyzed by a computer program which counts the number of boxes (N) containing the profile pixels by using different yardstick lengths (g). Based on the SEM results, the yardstick length used in our measurement is from 0.1 lm to 60 lm. In our analysis, the pattern of SEM is assumed to obey N b ðrÞ 1 r D ;

ð2Þ

where D is the fractal dimension. Therefore, the fractal dimension is obtained by the following equation [17], D ¼ lim g!0

log NðgÞ :  log d

ð3Þ

Fig. 4 shows the fractal dimension D of the fracture morphologies for the FRs as well as the reduced electrical resistivity qre. It indi-

cates that the D–ta curve can be divided into regions I and II, within the ranges of 3–3.5 h and of 4–6.5 h, respectively, which is similar to qre–ta curve. The DSC curves of the focused ribbons (FRs) are displayed in Fig. 5. There are two exothermic peaks at 821 K and 837 K, respectively. The slope of the DSC curves changed obviously at about 600 K, as marked by the arrows and denoted by Tr. Moreover, checking the base line of the DSC apparatus, there is no slope change in the vicinity of 600 K. Hence, the slope change of DSC curves at about 600 K, i.e. Tr, should be ascribed to the effect of the sample. In addition, the DSC curves deviate from the base line by different degrees in the temperature range of 298 K–Tr, reflecting a variation of the thermal effect (DHex) in this temperature range. Fig. 6 gives the kink temperatures Tr and specific thermal effect DHex/m for the FRs. Here m is the mass of the measured samples. It is clear that the parameters can be divided into regions I and II within the ranges of 3–3.5 h and 4–6.5 h, respectively. This is consistent with the D–ta and qre–ta curves (Fig. 4). Fig. 7 displays the structure factor S(Q) for the FRs as well as the width DQ of the main peak and intensity ratio of the main

Fig. 3. SEM of the fracture morphology of the Fe78Si9B13 ribbons with (a) ta = 3 and (b) 4 h at 473 K.

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0.42

Region I

Region II

-1

ta = 3.0 h

ta=5.5h

ΔQ, 10nm

10

ta = 3.5 h

8

ta=4.5h

ta = 4.0 h

6

0.40

0.38 3

ta=4h 4

4

ta = 4.5 h ta = 5.5 h

2

Tx

Δ Hex

0

Region II

2.0

1.8

ta = 6.5 h

1

1.6 3

Base line

-2 400

4

800

2

4

T, K

6

8

Q, 10nm

Fig. 5. DSC scans of the focused ribbons (FRs) with a heating rate of 20 K/min. Arrows denote the temperature of the slope change in DSC scans, i.e. Tr. Dash line is base line. Area of the diagonal zone corresponds to the extra thermal effect DHex.

5

6

ta, h

0

600

6

(b) Region I

ta=3.5h

ta=3h

5

ta, h

S (Q)

DSC, mW

Ta = 473 K

(a)

Tr

ta=6.5h

S(Q1)/S(Q2 )

exothermal

12

10

12

14

-1

Fig. 7. The structure factors S(Q) of the focused ribbons (FRs) as well as (inset a) width DQ of the main peak and (inset b) intensity ratio of the main peak relative to the second peak, S(Q1)/S(Q2).

4. Discussion 640

It has been reported that the electrical resistivity evolution against temperature in the amorphous Al78W22 film is ascribed to the structure relaxation [18]. In our case, for the ribbons in region I of all qre–ta curves, qre increases at the beginning and then decreases with ta, resulting in a maximum of qre (Fig. 1). This is similar to the change behavior during the formation of the GP zone in Al–Li–Cu alloys [19,20]. The increment of electrical resistivity Dq can be given by X Dq ¼ C s q0s þ N c gðnÞ; ð4Þ

(a) Region I I

Region I

r

620

600

580

where q0s and g(n) refer to the contributions to resistivity by residual solute atoms per mole percent and by one cluster (GP zone in the Al–Cu alloy) with average size n, respectively. Nc is the number of clusters and Cs is the concentration of element s. g(n) is expressed in Al–Cu alloys as [19,20]

Hex/m, J/g

(b) 20

gðnÞ ¼ ð2:1  1035 Þn0:8 :

10

3

4

5

6

7

ta, h Fig. 6. (a) Kink temperature Tr and (b) specific extra thermal effect DHex/m in DSC curves of the focused ribbons (FRs).

peak relative to the second peak, S(Q1)/S(Q2). The width DQ for the FRs, namely the full width half maximum (FWTH), can be divided into two regions within the ranges of 3–3.5 h and 4–6.5 h, respectively (Fig. 7, inset (a)). The DQ is almost unchangeable in the first region and increases with ta in the second region. Similarly, the intensity ratio S(Q1)/S(Q2) can also be divided into regions I and II (Fig. 7 inset (b)). The changing behavior of S(Q1)/ S(Q2)–ta curve is opposite to that of DQ–ta curve. Moreover, the shape of DQ–ta curve accords well with the previous qre–ta, D– ta, Tr–ta, and DHex–ta curves.

ð5Þ

In the beginning of the GP zone formation, Cu atoms leave the a (Al) solution and N increases gradually, hence, Dq increase gradually. Afterwards, low concentration of Cu atoms in a (Al) solution and growth of GP zones result in the decline of Cs and Nc, similar to the consideration on the abnormal resistivity behavior with scattering attenuation in GP zones [21]. This indicates that Dq decreases with increasing the annealing time and size of GP zone. Therefore, the qre–ta curve within region I (Fig. 1) can be explained by formation of a new type cluster or short range order (SRO) in the amorphous ribbon. According to the argument by Thompson et al. [16], the fracture toughness KIc is directly related to the increment of fractal dimension D of the fracture morphology by pffiffiffiffiffi pffiffiffiffiffiffiffi K Ic ¼ K 0 þ ðE a0 Þ DD; ð6Þ where K0 is defined as the toughness for a smooth planer fracture, DD the fractal dimension increment, E the elastic modulus and a0 a characteristic length involved in the fracture process. Therefore, the fracture toughness of the FRs changes with ta according to the

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D–ta curve (Fig. 4). Although the drastic ductile–brittle transition in the Fe78Si9B13 ribbon occurs at 523 K or higher temperatures [3,22], it is possible for the ribbons to embrittle below 523 K after long time annealing, like the kinetic glass transition observed below the calorimetric glass transition temperature in the Zr-base glass [23]. In addition, an obvious reduction of the fracture toughness of the Zr-based and Fe46Ni32V2Si14B6 glasses was observed after a thermal exposure, which was explained by the formation of new type clusters or phases [15,16]. In the annealing treatment, the amorphous alloy undergoes two processes: short range local structure relaxation (SLSR) and medium range cooperative structure relaxation (MCSR) [24]. The relaxation process involves the diffusion of atoms like SLSR, which is an endothermal process and reversible; the relaxation process involves the ordering like MCSR, similar to the solidification of a melt and crystallization of a glass, which is an exothermal process and irreversible [18,24]. The DSC curve for ta = 3.5 h (Fig. 5) shows a maximum DHex, implying that the exothermal process in the corresponding ribbon is stronger than that of other ribbons. This indicates that a drastic change occurs in the case of ta = 3.5 h, suggesting that an MCSR occurs and that a new type SRO forms in this case. It is known that the main peak in the structure factor S(Q) is associated with the SRO in glasses and that the so called first sharp diffraction peak (FSDP, i.e. prepeak) is associated with the medium range order (MRO) [25]. For estimations of the correlation length d of the chemical SRO or MRO, a simple expression is often used by [13,26] d ¼ 2p=DQ ;

ð7Þ

here DQ is the half width of the FSDP. Although the FSDP is not visible in the present S(Q) for the FRs (Fig. 7), the ratio of the half width DQ1 of the main peak in the S(Q) relative to its position Q1, DQ1/Q1, varies from 0.127 to 0.134, close to the value of the FSDP for the glass LaSF9 [26]. Thus, the correlation length of SRO for the FRs can be given by Eq. (7) as following: 1.54, 1.54, 1.59, 1.55, 1.52, and 1.50 nm for ta = 3.0, 3.5, 4.0, 4.5, 5.5, and 6.5 h, respectively. These lengths are consistent with those for Zr–Ti–Al–Cu–Ni glasses [27]. In the glasses with FSDP, the higher S(Q1)/S(Q2) is, the larger the correlation length d is [13,14]. Hence, it is possible to explain the opposition relationship between the S(Q1)/S(Q2)–ta and DQ–ta curves (Fig. 7). Based on the above analysis as well as the formation of the second amorphous phase in the Fe73Al5Ga2P11C5B4 glass [7], the local structure change in regions I and II can be possibly ascribed to formation of SRO (I) and SRO (II), respectively. To say the least, the variation of resistivity qre and fracture morphology can be attributed to the change of the local SRO in the amorphous Fe78Si9B13 ribbon. 5. Conclusions The electric resistivity qre of the amorphous Fe78Si9B13 ribbons, annealed at various temperatures Ta for different holding time ta,

were measured at room temperature using 4-probes method. The evolution of the qre happens after the annealing process, which can be divided into regions I, II and III. The variation of qre of the focused ribbons (FRs, overlapping regions I and II) accords with the fracture morphology, thermal effect in DSC analysis, and parameters in structure factors S(Q) of the FRs. Based on the measured results, the evolution of qre can be attributed to the change of short range order (SRO) in the annealed amorphous Fe78Si9B13 ribbons. Acknowledgements The authors give grateful thanks for the support of the National Natural Science Foundation of China, under Grant No. 50301008 and 50471052, the Shandong Science and Research Foundation, under Grant No. 2004BS04014, and the project NCET-06-584. W.M.W. is grateful to Dr L.C. Zhang for stimulative discussions, as well as to Dr A. Gebert, Professor Schultz, and Alexander von Humboldt Foundation for supplying academic supports. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

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