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Crystallization progress and soft magnetic properties of Finemet alloy with Ge addition
Vacuum 104 (2014) 88e91
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Crystallization progress and soft magnetic properties of Finemet alloy with Ge addition Ruwu Wang a, c, *, Jing Liu a, b, Chun Zeng c, Fengquan Zhang c a
College of Materials Science and Metallurgical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China State Key Laboratory for Advanced Metals and Materials, Beijing University of Science and Technology, Beijing 100083, China c National Engineering Research Center for Silicon Steel, Wuhan 430080, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2013 Received in revised form 10 January 2014 Accepted 11 January 2014
The influence of the partial substitution of Si by Ge (6 at.%) on the structure, crystallization kinetics and magnetic properties of Finemet-type alloys were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimeter (DSC) and room-temperature hysteresis loops measurements. Amorphous ribbons were heat treated at different temperatures for 1 h in a vacuum furnace to induce nanocrystallization. It was found that a homogeneous structure of aFe3Si structure nanocrystals with a grain size less than 15 nm embedded in an amorphous matrix was obtained within the temperature range of 510e590 C. The crystallization activation energies calculated using Kissinger model were 331 and 356 kJ/mol for the first and the second crystallizations, respectively. The saturation magnetization of the as-quenched amorphous ribbon is 138 Am2/kg and decreases gradually with the increase of annealing temperatures from 125 Am2/kg for 510 C to 115 Am2/kg for 590 C. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Finemet Crystallization Magnetic properties
Fe-based nanocrystalline alloy, with the typical composition of Fe73.5Si13.5B9Cu1Nb3, was first found by Yoshizawa in 1988, by adding some Cu and M (M ¼ Nb, Mo, W, Ta, etc) into the amorphous alloy of FeeSieB [1]. The Fe73.5Si13.5B9Cu1Nb3 alloy, which is called Finemet alloy, displays excellent soft magnetic properties with high initial relative permeability (w105 at 1 kHz), relatively high saturation magnetization (1.2 T), very low coercive field (HC < 1 A/m) and near zero saturation magnetostriction (w106) [2,3]. The excellent soft magnetic properties are attributed to the ultrafine structure composed of a-Fe (Si) nanocrystallites embedded in remaining amorphous matrix. In such a structure, the effective magnetostriction constant of the whole materials is reduced about one order of magnitude because of the compensation effect between the negative magnetostriction of a-Fe (Si) phase (lS ¼ 6 106) and the positive value of the amorphous one (lS ¼ 22 106) [4,5]. Furthermore, since the size of nanocrystallites (w10e15 nm) is smaller than the ferromagnetic exchange interaction length (about 35 nm for FeSiBCuNb amorphous/ nanocomposite system), ferromagnetic exchange coupling between the nanocrystalline a-Fe (Si) grains through amorphous
* Corresponding author. National Engineering Research Center for Silicon Steel, Wuhan 430080, China. Tel.: þ86 27 86487871. E-mail address: [email protected] (R. Wang). 0042-207X/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2014.01.018
matrix results in an averaging out of the magnetocrystalline anisotropy (as explained by the random anisotropy model) [6]. The influence of the addition of Ge to Finemet-type alloy was thoroughly studied in the past years [7e15]. Some interesting results can be drawn from these works. The MS of the alloy remained independent of the Ge content. The Curie temperature of the amorphous increased with the addition of Ge for all the studied alloys reaching a maximum value of 664 K for the Fe73.5Ge15.5B7Nb3Cu1 alloy. The replacement of Si or B for Ge reduced the amount of amorphous matrix when heat treated at the same temperature. The partial substitution of Ge for B reduces the onset temperature of crystallization phase and increase the temperature of boride precipitation in a continuous heating process. However, there is little report about the effect of Ge substitution for Si on the crystallization kinetics of the Finemet-type amorphous alloy. In this paper, the crystallization kinetics of amorphous Fe73.5Si7.5Ge6B9Cu1Nb3 alloy is analyzed by using the Kissinger method. Crystallization of amorphous alloy is studied by XRD and TEM techniques. Meanwhile, the influence of heat treatment on the magnetic properties is investigated. The master alloy ingot of amorphous Fe73.5Si7.5Ge6B9Cu1Nb3 was prepared by arc melting purity metals under argon atmosphere. Amorphous ribbons, about 3 mm wide and 25 mm thick, were obtained from the ingot by melt spinning technique in air. The amorphous ribbons were isothermally annealed at different
R. Wang et al. / Vacuum 104 (2014) 88e91
temperatures for 1 h in vacuum for nanocrystallization. The microstructure and mean grain size of the as-spun and the annealed samples were analyzed by the technique of XRD (PHILIPS Xpertpro diffractometer with Cu-Ka radiation) and TEM (JEM2100F). DSC was performed using Thermal Analysis System NETZSCH STA 449C under argon flow at different heating rates to analyze the thermal stability and the crystallization kinetic of the alloy. The hysteresis loops of different samples were measured by a vibrating sample magnetometer (VSM: Lakeshore Model 7404) at room temperature and at a maximum applied field of 2.17 T. Fig. 1 shows the room-temperature powder XRD patterns for the amorphous and the annealed samples at different temperatures. For the rapidly quenched ribbon (Fig. 1a), the diffraction pattern exhibits only one broad band centered at around 2q ¼ 45 and no any appreciable diffraction peaks corresponding to crystalline phases are detected, indicating that the quenched ribbon is fully amorphous. The ribbon annealed at 470 C remained amorphous but began to nanocrystallize at 510 C (Fig. 1b and c). The patterns of the ribbons annealed at 510 C, 550 C and 590 C remained almost unaffected with increasing temperatures except for the higher and narrower peaks, which indicates an increase of the phase volume fraction and a grain size growth, respectively. The lattice parameter of the nanocrystalline alloy annealed at 550 C (a ¼ 5.73 A) is larger compared with the pure Finemet alloy (a ¼ 5.68 A [12],) due to the difference in atomic size between Ge and Si. From a previous work [16] it is known that in the Fe71.5Si9.5Ge6B9Nb3Cu1 alloy a nanocrystalline a-Fe (Si, Ge) phase is formed. According to the above results, the patterns of the ribbons annealed at 510 C, 550 C and 590 C may be attributable to the a-Fe (Si, Ge) phase. A detailed study is in progress to determine this. From the XRD diffraction patterns the average grain size of nanocrystalline phase (D) can be determined with the Scherrer relationship [17]:
D ¼
0:9l s cos q
(1)
where l is the X-ray wavelength (l ¼ 1.54056 A), 2q is the angle of the dominant Bragg maximum and the s (rad) is the full-width at half-maximum of the diffraction peak. The average grain sizes are 7.7 nm, 10.9 nm and 15.0 nm for samples annealed at 510 C, 550 C and 590 C, respectively. The X-ray measurements were confirmed by TEM. Fig. 2 shows the bright field images with the corresponding diffraction patterns. No grain structures were observed in asquenched ribbon and the ribbon annealed at 470 C for 1 h due to the amorphous structure. When the annealing temperature is up
to 510 C, a-Fe3Si structure nanocrystallines would precipitate and distribute randomly in the amorphous matrix. When the annealing temperature reaches 590 C, numerous nanocrystallines precipitate from the amorphous matrix with a grain size less than 15 nm. Comparing the three TEM images, it can be seen that increasing annealing temperature results in an increase of grain size and an increase of the number of nanocrystallines. The typical DSC spectra obtained from as as-quenched ribbons during continuous heating at different heating rates from 5 to 20 C/min are shown in Fig. 3. The DSC curves exhibit clearly two exothermal peaks, corresponding to the crystallization stages during heating progress. The first peak is due to the crystallization of crystalline a-Fe3Si soft magnetic phase from amorphous matrix and the second peak is related to formation of the boride-type phases and recrystallization phenomena [18]. Thermal parameters such as two onset temperatures Tx1, Tx2 and two peak temperatures Tp1, Tp2 are determined from the DSC curves, whose values are listed in Table 1. As shown in Fig. 3, the thermal parameters Tx1, Tx2, Tp1 and Tp2 are affected by the heating rates. It is obvious that thermal parameters Tx1, Tx2, Tp1 and Tp2 shift to the higher temperatures with increasing heating rate, which means the nanocrystallization process has kinetic effects. It is found that the thermal parameters Tx1, Tx2, Tp1 and Tp2 are lower than that of pure Finemet alloy [19]. The precipitation temperature of the a-Fe3Si soft magnetic phase decreases by Ge doping, which means that the substitution of Ge for Si facilitate the formation of a-Fe3Si structure nanocrystalline phase. In addition, The formation of the boride phase, which promotes the grain growth are reported to have adverse effect on the initial magnetic permeability and give rise to the decline of soft magnetic properties due to the increase of the magnetocrystalline anisotropy [20]. Hence, the shift of the second crystallization peak towards lower temperatures would be disadvantageous. The range of the peak temperature, DT ¼ Tp2 Tp1, changes slightly, which is 139 C for the pure Finemet alloy, 140 C for the present sample under the conditions of the same heating rate [21]. It indicates that the substitution of Ge for Si has nearly no effect on the precipitation temperature range of a-Fe3Si structure nanocrystalline phase. It is important to know the activation energy of the crystallization process. The activation energy generally defined as the threshold value of energy above which the energy fluctuation is sufficient for the elementary reaction to occur, and it should have a characteristic constant value for each particular reaction. The activation energy can be obtained from the DSC results using Kissinger equation [22], which is widely used for the calculation of activation energy in Finemet-type alloys. The Kissinger equation is as follows:
ln
Fig. 1. The room-temperature powder XRD patterns of Fe73.5Si7.5Ge6B9Cu1Nb3: (a) asquenched amorphous ribbon; (b,c,d,e) ribbons annealed at 470 C, 510 C, 550 C and 590 C for 1 h in a vacuum, respectively.
89
b T2
Ea ¼ þ const RT
(2)
where b is the heating rate, R is the universal gas universal gas constant, Ea is the activation energy for the crystallization of the amorphous phase and T is a specific absolute temperature such as peak temperature Tpi (i ¼ 1, 2). Fig. 4 shows the dependence of ln(b/ T2) on the inversed exothermal temperature, 1000/Tp. The linear dependence of the ln(b/T2) on the 1000/Tp confirms that the Kissinger model is reasonable for describing the crystallization in this study. As a result, the activation energies of Ea1 and Ea2 in this case can be easily determined by linear fitting the dependence in Fig. 4, and shown to be 331 and 356 kJ/mol, respectively. Here, the activation energies Ea1 and Ea2 indicate the participation of different phases in the first and second stages of continuous heating for the sample. G. Herzer showed that the activation energy (Ea1) for the crystallization of the pure Finemet alloy Fe73.5Si13.5B9Cu1Nb3 was 413 kJ/mol [3,4]. Thus the activation energy for the crystallization of
90
R. Wang et al. / Vacuum 104 (2014) 88e91
Fig. 2. TEM micrographs of the as-quenched sample (a) and the samples annealed at different temperatures for 1 h in a vacuum: (b) 470 C, (c) 510 C, (d) 550 C, (e) 590 C. The black spots in this figure is related to the formation of a-Fe3Si structure nanocrystalline phase.
decreases from 128 Am2/kg for 470 C, 125 Am2/kg for 510 C, 121 Am2/kg for 550 C to 115 Am2/kg for 590 C. Turtelli et al. [23] propose, for Finemet-type alloy, that Ms (T) can be approximated as
MS ðTÞ ¼ xMScr ðTÞ þ ð1 xÞMSam ðTÞ
(3)
Where x is the volume fraction of a-Fe (Si, Ge) nanograins, MScr ðMSam Þ is the saturation magnetization of the crystalline (amorphous) phase. MSam ðTÞ depends on temperature as follows:
T b MScr ðTÞfMScr 1 cr TC Fig. 3. DSC curves of as-quenched samples Fe73.5Si7.5Ge6B9Cu1Nb3 performed at various heating rates from 5 to 20 C/min.
the alloy of this study Fe73.5Si7.5Ge6B9Cu1Nb3 is less than that of the pure Finemet alloy, demonstrating that the Fe73.5Si7.5Ge6B9Cu1Nb3 alloy has less glass forming ability and thermal stability. Fig. 5 shows the room-temperature hysteresis loops of asquenched and annealed Fe73.5Si7.5Ge6B9Cu1Nb3 alloy at different temperatures for 1 h in a vacuum. All the samples reach a saturated magnetic state in the measured field scope. Based on the law of approach to saturation, saturation magnetization Ms of each sample is calculated. It is important to note that MS of the as-quenched sample is higher than that of the annealed samples, meanwhile, with the increase of annealing temperature, MS of the alloy
where TCcr is the magnetic ordering temperature of the crystalline phase and b is critical exponent. The saturation magnetization of the paramagnetic amorphous phase in the presence of an external field can be expressed as
Table 1 Crystallization onset temperatures and peak temperatures (Tx1, Tx2, Tp1 and Tp2) and DT ¼ Tp2 Tp1 of Fe73.5Si7.5Ge6B9Cu1Nb3 amorphous alloy obtained from DSC curves.
b/( C/min)
Tx1/ C
Tp1/ C
Tx2/ C
Tp2/ C
DT/ C
5 10 15 20
470 475 479 482
498 509 515 518
621 628 633 637
632 645 653 658
134 136 138 140
(4)
Fig. 4. Kissinger plots of as-quenched sample Fe73.5Si7.5Ge6B9Cu1Nb3.
R. Wang et al. / Vacuum 104 (2014) 88e91
91
in the atmosphere were heat treated at different temperatures for 1 h in a vacuum furnace to induce the nanocrystallization of the samples. It was found that a homogeneous structure of a-Fe3Si structure nanocrystals with a grain size less than 15 nm embedded in an amorphous matrix was obtained within the temperature range of 510e590 C. The crystallization activation energies (Ea1 and Ea2) for the growth of crystalline phase were 331 and 356 kJ/ mol for the first and the second crystallizations, respectively. The saturation magnetization of the as-quenched amorphous ribbon is 138 Am2/kg and decreases gradually with the increase of annealing temperatures from 125 Am2/kg for 510 C to 115 Am2/kg for 590 C. This work was funded by the State Key Lab of Advanced Metals and Materials (No. 2011ZD03) and High-tech Research and Development Program of China (863 Program, Grant No. 2012AA03A506). Fig. 5. The room-temperature hysteresis loops of as-quenched and annealed Fe73.5Si7.5Ge6B9Cu1Nb3 alloy at different temperatures for 1 h in a vacuum.
MSam ðTÞ ¼ M0 BS ðT; Ham Þ ¼ Nmam BS ðT; Ham Þ
(5)
where BS(T, Ham) is the Brillouin function, Ham is the field applied on the paramagnetic phase by the grains, N is the number of magnetic moment per unit volume and mam is the magnetic moment per atom in the amorphous phase. Hence the saturation magnetization of the sample becomes
MS ðTÞ ¼ xMScr ðTÞ þ ð1 xÞN mam BS ðyÞ
(6)
Where y ¼ Nm0gmBSHam/KBT with Ham f MScr ðTÞ. When the argument of the Brillouin function is small, the paramagnetic saturation magnetization can be approximated as
MSam ðTÞ ¼
MScr ðTÞC T
(7)
with C ¼ cNm0g2mBS(S þ 1)/3KB where mB is the Bohr magneton, g is the Lande factor and c is a constant less than one related to the geometric arrangement of the crystals. The influence of the partial substitution of Si by Ge (6 at.%) on the structure, crystallization kinetics and magnetic properties of Finemet-type alloys has been studied. Amorphous ribbons prepared by the standard single copper wheel melt spinning technique
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Yoshizawa Y, Oguma S, Yamauchi K. J Appl Phys 1988;64:6044. McHenry ME, Laughlin DE. Acta Mater 2000;48:223. Herzer G. Scr Metall Mater 1995;33(10/11):1741. Herzer G. Mater Sci Eng A 1991;133:1. Hernando A, Navarro I. In: Hadjipanayis GC, Siegel RW, editors. Nanophase materials. Netherlands: Kluwer Academic Publishers; 1994. p. 703. Herzer G. IEEE Trans Magn 1989;25:3327. Cremaschi V, Sanchez G, Sirkin H. Phys B 2004;354:213. Muraca D, Moya J, Cremaschi VJ, Sirkin HRM. Phys B 2007;398:325. Muraca D, Cremaschi V, Moya J, Sirkin H. J Magn Magn Mater 2008;320:1639. Muraca D, Silveyra J, Pagnola M, Cremaschi V. J Magn Magn Mater 2009;321: 3640. Muraca D, Cremaschi VJ, Sirkin H. J Magn Magn Mater 2007;311:618. Moya JA, Cremaschi VJ, Sirkin H. Phys B 2007;389:159. Moya JA. J Magn Magn Mater 2010;322:1784. Muraca D, Cremaschi V, Moya J, Sirkin H. J Magn Magn Mater 2008;320:810. Adam P, Michal W, Pawel U, Pavol S, Marek M. Vacuum 2009;83:S245. Cremaschi V, Saad A, Moya J, Arcondo B, Sirkin H. Phys B 2002;320:281. Cullity BD. Elements of X-ray diffraction. Reading, MA: Addison Wesley, Pub. Co. Inc.; 1987. p. 284. Phan MH, Peng HX, Wiscom MR, Yu SC, Chau N. Compos Part A 2006;37:191. Shahri F, Beitollahi A, Shabestari SG, Ghanaatshoar M, Tehranchi MM, Mohseni SM, et al. J Magn Magn Mater 2007;312:35. Phan MH, Peng HX, Wisnom MR. J Appl Phys 2005;98:014316. Shahri F, Beitollahi A, Shabestari SG, Ghanaatshoar M, Tehranchi MM, Mohseni SM, et al. J Magn Magn Mater 2007;312:35. Kissinger H. Anal Chem 1957;29:1702. Turtelli RS, Duong VH, Grossinger R, Schwetz M, Ferrara E, Pillmayr N. IEEE Trans Magn 2000;36(2):508.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
Crystallization progress and soft magnetic properties of Finemet alloy with Ge addition Ruwu Wang a, c, *, Jing Liu a, b, Chun Zeng c, Fengquan Zhang c a
College of Materials Science and Metallurgical Engineering, Wuhan University of Science and Technology, Wuhan 430081, China State Key Laboratory for Advanced Metals and Materials, Beijing University of Science and Technology, Beijing 100083, China c National Engineering Research Center for Silicon Steel, Wuhan 430080, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2013 Received in revised form 10 January 2014 Accepted 11 January 2014
The influence of the partial substitution of Si by Ge (6 at.%) on the structure, crystallization kinetics and magnetic properties of Finemet-type alloys were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimeter (DSC) and room-temperature hysteresis loops measurements. Amorphous ribbons were heat treated at different temperatures for 1 h in a vacuum furnace to induce nanocrystallization. It was found that a homogeneous structure of aFe3Si structure nanocrystals with a grain size less than 15 nm embedded in an amorphous matrix was obtained within the temperature range of 510e590 C. The crystallization activation energies calculated using Kissinger model were 331 and 356 kJ/mol for the first and the second crystallizations, respectively. The saturation magnetization of the as-quenched amorphous ribbon is 138 Am2/kg and decreases gradually with the increase of annealing temperatures from 125 Am2/kg for 510 C to 115 Am2/kg for 590 C. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Finemet Crystallization Magnetic properties
Fe-based nanocrystalline alloy, with the typical composition of Fe73.5Si13.5B9Cu1Nb3, was first found by Yoshizawa in 1988, by adding some Cu and M (M ¼ Nb, Mo, W, Ta, etc) into the amorphous alloy of FeeSieB [1]. The Fe73.5Si13.5B9Cu1Nb3 alloy, which is called Finemet alloy, displays excellent soft magnetic properties with high initial relative permeability (w105 at 1 kHz), relatively high saturation magnetization (1.2 T), very low coercive field (HC < 1 A/m) and near zero saturation magnetostriction (w106) [2,3]. The excellent soft magnetic properties are attributed to the ultrafine structure composed of a-Fe (Si) nanocrystallites embedded in remaining amorphous matrix. In such a structure, the effective magnetostriction constant of the whole materials is reduced about one order of magnitude because of the compensation effect between the negative magnetostriction of a-Fe (Si) phase (lS ¼ 6 106) and the positive value of the amorphous one (lS ¼ 22 106) [4,5]. Furthermore, since the size of nanocrystallites (w10e15 nm) is smaller than the ferromagnetic exchange interaction length (about 35 nm for FeSiBCuNb amorphous/ nanocomposite system), ferromagnetic exchange coupling between the nanocrystalline a-Fe (Si) grains through amorphous
* Corresponding author. National Engineering Research Center for Silicon Steel, Wuhan 430080, China. Tel.: þ86 27 86487871. E-mail address: [email protected] (R. Wang). 0042-207X/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2014.01.018
matrix results in an averaging out of the magnetocrystalline anisotropy (as explained by the random anisotropy model) [6]. The influence of the addition of Ge to Finemet-type alloy was thoroughly studied in the past years [7e15]. Some interesting results can be drawn from these works. The MS of the alloy remained independent of the Ge content. The Curie temperature of the amorphous increased with the addition of Ge for all the studied alloys reaching a maximum value of 664 K for the Fe73.5Ge15.5B7Nb3Cu1 alloy. The replacement of Si or B for Ge reduced the amount of amorphous matrix when heat treated at the same temperature. The partial substitution of Ge for B reduces the onset temperature of crystallization phase and increase the temperature of boride precipitation in a continuous heating process. However, there is little report about the effect of Ge substitution for Si on the crystallization kinetics of the Finemet-type amorphous alloy. In this paper, the crystallization kinetics of amorphous Fe73.5Si7.5Ge6B9Cu1Nb3 alloy is analyzed by using the Kissinger method. Crystallization of amorphous alloy is studied by XRD and TEM techniques. Meanwhile, the influence of heat treatment on the magnetic properties is investigated. The master alloy ingot of amorphous Fe73.5Si7.5Ge6B9Cu1Nb3 was prepared by arc melting purity metals under argon atmosphere. Amorphous ribbons, about 3 mm wide and 25 mm thick, were obtained from the ingot by melt spinning technique in air. The amorphous ribbons were isothermally annealed at different
R. Wang et al. / Vacuum 104 (2014) 88e91
temperatures for 1 h in vacuum for nanocrystallization. The microstructure and mean grain size of the as-spun and the annealed samples were analyzed by the technique of XRD (PHILIPS Xpertpro diffractometer with Cu-Ka radiation) and TEM (JEM2100F). DSC was performed using Thermal Analysis System NETZSCH STA 449C under argon flow at different heating rates to analyze the thermal stability and the crystallization kinetic of the alloy. The hysteresis loops of different samples were measured by a vibrating sample magnetometer (VSM: Lakeshore Model 7404) at room temperature and at a maximum applied field of 2.17 T. Fig. 1 shows the room-temperature powder XRD patterns for the amorphous and the annealed samples at different temperatures. For the rapidly quenched ribbon (Fig. 1a), the diffraction pattern exhibits only one broad band centered at around 2q ¼ 45 and no any appreciable diffraction peaks corresponding to crystalline phases are detected, indicating that the quenched ribbon is fully amorphous. The ribbon annealed at 470 C remained amorphous but began to nanocrystallize at 510 C (Fig. 1b and c). The patterns of the ribbons annealed at 510 C, 550 C and 590 C remained almost unaffected with increasing temperatures except for the higher and narrower peaks, which indicates an increase of the phase volume fraction and a grain size growth, respectively. The lattice parameter of the nanocrystalline alloy annealed at 550 C (a ¼ 5.73 A) is larger compared with the pure Finemet alloy (a ¼ 5.68 A [12],) due to the difference in atomic size between Ge and Si. From a previous work [16] it is known that in the Fe71.5Si9.5Ge6B9Nb3Cu1 alloy a nanocrystalline a-Fe (Si, Ge) phase is formed. According to the above results, the patterns of the ribbons annealed at 510 C, 550 C and 590 C may be attributable to the a-Fe (Si, Ge) phase. A detailed study is in progress to determine this. From the XRD diffraction patterns the average grain size of nanocrystalline phase (D) can be determined with the Scherrer relationship [17]:
D ¼
0:9l s cos q
(1)
where l is the X-ray wavelength (l ¼ 1.54056 A), 2q is the angle of the dominant Bragg maximum and the s (rad) is the full-width at half-maximum of the diffraction peak. The average grain sizes are 7.7 nm, 10.9 nm and 15.0 nm for samples annealed at 510 C, 550 C and 590 C, respectively. The X-ray measurements were confirmed by TEM. Fig. 2 shows the bright field images with the corresponding diffraction patterns. No grain structures were observed in asquenched ribbon and the ribbon annealed at 470 C for 1 h due to the amorphous structure. When the annealing temperature is up
to 510 C, a-Fe3Si structure nanocrystallines would precipitate and distribute randomly in the amorphous matrix. When the annealing temperature reaches 590 C, numerous nanocrystallines precipitate from the amorphous matrix with a grain size less than 15 nm. Comparing the three TEM images, it can be seen that increasing annealing temperature results in an increase of grain size and an increase of the number of nanocrystallines. The typical DSC spectra obtained from as as-quenched ribbons during continuous heating at different heating rates from 5 to 20 C/min are shown in Fig. 3. The DSC curves exhibit clearly two exothermal peaks, corresponding to the crystallization stages during heating progress. The first peak is due to the crystallization of crystalline a-Fe3Si soft magnetic phase from amorphous matrix and the second peak is related to formation of the boride-type phases and recrystallization phenomena [18]. Thermal parameters such as two onset temperatures Tx1, Tx2 and two peak temperatures Tp1, Tp2 are determined from the DSC curves, whose values are listed in Table 1. As shown in Fig. 3, the thermal parameters Tx1, Tx2, Tp1 and Tp2 are affected by the heating rates. It is obvious that thermal parameters Tx1, Tx2, Tp1 and Tp2 shift to the higher temperatures with increasing heating rate, which means the nanocrystallization process has kinetic effects. It is found that the thermal parameters Tx1, Tx2, Tp1 and Tp2 are lower than that of pure Finemet alloy [19]. The precipitation temperature of the a-Fe3Si soft magnetic phase decreases by Ge doping, which means that the substitution of Ge for Si facilitate the formation of a-Fe3Si structure nanocrystalline phase. In addition, The formation of the boride phase, which promotes the grain growth are reported to have adverse effect on the initial magnetic permeability and give rise to the decline of soft magnetic properties due to the increase of the magnetocrystalline anisotropy [20]. Hence, the shift of the second crystallization peak towards lower temperatures would be disadvantageous. The range of the peak temperature, DT ¼ Tp2 Tp1, changes slightly, which is 139 C for the pure Finemet alloy, 140 C for the present sample under the conditions of the same heating rate [21]. It indicates that the substitution of Ge for Si has nearly no effect on the precipitation temperature range of a-Fe3Si structure nanocrystalline phase. It is important to know the activation energy of the crystallization process. The activation energy generally defined as the threshold value of energy above which the energy fluctuation is sufficient for the elementary reaction to occur, and it should have a characteristic constant value for each particular reaction. The activation energy can be obtained from the DSC results using Kissinger equation [22], which is widely used for the calculation of activation energy in Finemet-type alloys. The Kissinger equation is as follows:
ln
Fig. 1. The room-temperature powder XRD patterns of Fe73.5Si7.5Ge6B9Cu1Nb3: (a) asquenched amorphous ribbon; (b,c,d,e) ribbons annealed at 470 C, 510 C, 550 C and 590 C for 1 h in a vacuum, respectively.
89
b T2
Ea ¼ þ const RT
(2)
where b is the heating rate, R is the universal gas universal gas constant, Ea is the activation energy for the crystallization of the amorphous phase and T is a specific absolute temperature such as peak temperature Tpi (i ¼ 1, 2). Fig. 4 shows the dependence of ln(b/ T2) on the inversed exothermal temperature, 1000/Tp. The linear dependence of the ln(b/T2) on the 1000/Tp confirms that the Kissinger model is reasonable for describing the crystallization in this study. As a result, the activation energies of Ea1 and Ea2 in this case can be easily determined by linear fitting the dependence in Fig. 4, and shown to be 331 and 356 kJ/mol, respectively. Here, the activation energies Ea1 and Ea2 indicate the participation of different phases in the first and second stages of continuous heating for the sample. G. Herzer showed that the activation energy (Ea1) for the crystallization of the pure Finemet alloy Fe73.5Si13.5B9Cu1Nb3 was 413 kJ/mol [3,4]. Thus the activation energy for the crystallization of
90
R. Wang et al. / Vacuum 104 (2014) 88e91
Fig. 2. TEM micrographs of the as-quenched sample (a) and the samples annealed at different temperatures for 1 h in a vacuum: (b) 470 C, (c) 510 C, (d) 550 C, (e) 590 C. The black spots in this figure is related to the formation of a-Fe3Si structure nanocrystalline phase.
decreases from 128 Am2/kg for 470 C, 125 Am2/kg for 510 C, 121 Am2/kg for 550 C to 115 Am2/kg for 590 C. Turtelli et al. [23] propose, for Finemet-type alloy, that Ms (T) can be approximated as
MS ðTÞ ¼ xMScr ðTÞ þ ð1 xÞMSam ðTÞ
(3)
Where x is the volume fraction of a-Fe (Si, Ge) nanograins, MScr ðMSam Þ is the saturation magnetization of the crystalline (amorphous) phase. MSam ðTÞ depends on temperature as follows:
T b MScr ðTÞfMScr 1 cr TC Fig. 3. DSC curves of as-quenched samples Fe73.5Si7.5Ge6B9Cu1Nb3 performed at various heating rates from 5 to 20 C/min.
the alloy of this study Fe73.5Si7.5Ge6B9Cu1Nb3 is less than that of the pure Finemet alloy, demonstrating that the Fe73.5Si7.5Ge6B9Cu1Nb3 alloy has less glass forming ability and thermal stability. Fig. 5 shows the room-temperature hysteresis loops of asquenched and annealed Fe73.5Si7.5Ge6B9Cu1Nb3 alloy at different temperatures for 1 h in a vacuum. All the samples reach a saturated magnetic state in the measured field scope. Based on the law of approach to saturation, saturation magnetization Ms of each sample is calculated. It is important to note that MS of the as-quenched sample is higher than that of the annealed samples, meanwhile, with the increase of annealing temperature, MS of the alloy
where TCcr is the magnetic ordering temperature of the crystalline phase and b is critical exponent. The saturation magnetization of the paramagnetic amorphous phase in the presence of an external field can be expressed as
Table 1 Crystallization onset temperatures and peak temperatures (Tx1, Tx2, Tp1 and Tp2) and DT ¼ Tp2 Tp1 of Fe73.5Si7.5Ge6B9Cu1Nb3 amorphous alloy obtained from DSC curves.
b/( C/min)
Tx1/ C
Tp1/ C
Tx2/ C
Tp2/ C
DT/ C
5 10 15 20
470 475 479 482
498 509 515 518
621 628 633 637
632 645 653 658
134 136 138 140
(4)
Fig. 4. Kissinger plots of as-quenched sample Fe73.5Si7.5Ge6B9Cu1Nb3.
R. Wang et al. / Vacuum 104 (2014) 88e91
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in the atmosphere were heat treated at different temperatures for 1 h in a vacuum furnace to induce the nanocrystallization of the samples. It was found that a homogeneous structure of a-Fe3Si structure nanocrystals with a grain size less than 15 nm embedded in an amorphous matrix was obtained within the temperature range of 510e590 C. The crystallization activation energies (Ea1 and Ea2) for the growth of crystalline phase were 331 and 356 kJ/ mol for the first and the second crystallizations, respectively. The saturation magnetization of the as-quenched amorphous ribbon is 138 Am2/kg and decreases gradually with the increase of annealing temperatures from 125 Am2/kg for 510 C to 115 Am2/kg for 590 C. This work was funded by the State Key Lab of Advanced Metals and Materials (No. 2011ZD03) and High-tech Research and Development Program of China (863 Program, Grant No. 2012AA03A506). Fig. 5. The room-temperature hysteresis loops of as-quenched and annealed Fe73.5Si7.5Ge6B9Cu1Nb3 alloy at different temperatures for 1 h in a vacuum.
MSam ðTÞ ¼ M0 BS ðT; Ham Þ ¼ Nmam BS ðT; Ham Þ
(5)
where BS(T, Ham) is the Brillouin function, Ham is the field applied on the paramagnetic phase by the grains, N is the number of magnetic moment per unit volume and mam is the magnetic moment per atom in the amorphous phase. Hence the saturation magnetization of the sample becomes
MS ðTÞ ¼ xMScr ðTÞ þ ð1 xÞN mam BS ðyÞ
(6)
Where y ¼ Nm0gmBSHam/KBT with Ham f MScr ðTÞ. When the argument of the Brillouin function is small, the paramagnetic saturation magnetization can be approximated as
MSam ðTÞ ¼
MScr ðTÞC T
(7)
with C ¼ cNm0g2mBS(S þ 1)/3KB where mB is the Bohr magneton, g is the Lande factor and c is a constant less than one related to the geometric arrangement of the crystals. The influence of the partial substitution of Si by Ge (6 at.%) on the structure, crystallization kinetics and magnetic properties of Finemet-type alloys has been studied. Amorphous ribbons prepared by the standard single copper wheel melt spinning technique
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