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Soft magnetic properties and brittleness of boron-doped Sendust alloy
Journal of Magnetism and Magnetic Materials 117 (1992) 83-86 North-Holland
Soft magnetic properties and brittleness of boron-doped Sendust alloy Shao Y u a n - Z h i Department of Physics, Zhongshan University, Guangzhou 510275, China
Gu Sou-Ren and Chen Nan-Ping Department of Material Science and Engineering, Tsinghua University, Beijing 100084, China Received 12 December 1991; in revised form 4 May 1992
In this paper, static and dynamic magnetic properties as well as brittleness improvement of the boron-microalloyed Sendust ribbons prepared by rapid quenching were investigated. A discussion associated with soft magnetic properties and brittleness improvement of the boron-doped Sendust alloys is given in detail.
1. Introduction Sendust Fe3(Si, AI) consisting of nearly 85 wt% Fe, 9.6 wt% Si and 5.4 wt% AI composition is well known to have high magnetic flux density as well as excellent soft magnetic properties, and it is one of the best soft magnetic materials with lower cost. However, the brittleness of this bcc ordered intermetallic compounds limited its application. For making use of this Sendust alloy at high frequency, in order to reduce the eddy current effect, it is desired to get a thin sheet of this alloy. But it is impossible to roll Sendust alloy into a thin sheet by usual techniques due to its obvious brittleness. We have succeeded in making boron-doped Sendust thin ribbon by means of the rapid quenching method. To maintain the most excellent soft magnetic properties and high mechanical wear resistance of this alloy at the special composition (85Fe, 9.6Si, 5.4A1), better brittleness improvement of the Sendust alloy has been achieved through boron-microalloying than that through other microalloying elements [1]. Correspondence to: Dr. Y.-Z. Shao, Department of Physics, Zhongshan University, Guangzhou 510275, China. Elsevier Science Publishers B.V.
In this paper, we report about both brittleness improvement and soft magnetic properties of boron-doped Sendust ribbons.
2. Experimental procedure The alloys were prepared by arc-melting under argon repeatedly. The boron concentration is within range of 0-2000 ppm by weight. Ribbonform alloys were obtained by rapid quenching. The size of ribbons was 15 mm in width, 40 ~m in thickness and 3-5 m in length. The ribbons had a silver white luster, and were so mechanically flexible that it was possible to wind them easily around a rod of 5 mm in diameter. Annealed at 1000°C for 2 h, then cooled with a rate of 2.5 and 130°C/h respectively for ordering treatment. The deformation capacity of the ribbons both as-prepared and annealed was measured by bending them into semi-circular loops between two parallel plates of a micrometer. The tensile strain of the outer surface of the ribbon is given by ~ = t~ ( d - t), where d is the separation between two plates, and t is the ribbon thickness. The fracture
Y.-Z. Shao et al. / Brittleness of B-doped Sendust alloys
84
strain, ef, is defined as the strain at which catastrophic fracture occurs. Magnetic measurement was taken using toroidal ribbons of 12 mm in diameter. The frequency ranged from 10 to 1000 kHz and the magnetic flux density was selected at 0.002 T.
£fO/o -HF
S 0.6
2
•
/
~
i
3. Experimental results The boron-doped Sendust ribbons after quenching had much mechanical strain as well as many lattice defects which were formed and stored during ribbon formation, and as result, soft magnetic properties were unfavorable. Annealing is necessary to improve the soft magnetic properties. For different boron-doped ribbons, measurement of fracture strain was taken to evaluate their deformation capacity. Fig. 1 illustrates the relation of fracture strain of as-quenched and annealed ribbons with various boron content. It turns out that the fracture strain increase with raising boron concentration, the optimal result being obtained with nearly 150-200 ppm boron addition. The curve of the degree of long-range order, S, in fig. 1 also indicates that with increasing boron content the degree of long-range order S in this superlattice structure decreases up to 150-200 ppm boron content. Under the condition of optimal concentration of boron for deformation capacity, the minimum of the parameter, S, is obtained. In table 1 the static magnetic properties, microhardness and resistivity of partial ribbon specimens are listed. The frequency dependences of both the effective permeability and the
....... -o~\~\
0.2 0.1
LII
0
0.4
z J i,]*,d
~
,
10
i
llltltl
,
10 B (PPM)
* tl,l,,]
t
10
Fig. 1. T h e relationship of fracture strain and degree of long-range order with various boron content. Ao: ~f as-prepared; zx o: ~f cool rate 130 and 2.5°C/h; [] • : S cool rate 130 and 2.5°C/h.
iron loss are shown in figs. l(a) and (b) respectively. The ratios of signal versus noise (S/N) of a magnetic head made of as-quenched microalloyed Sendust ribbon, cast Sendust alloy and ferrite are given in table 2. It can been seen that the magnetic head made of as-quenched microalloyed Sendust ribbon has the best ratio (S/N) by comparison with that of cast Sendust alloy and ferrite, no matter what tape velocity and frequency range were selected. The experimental results mentioned above demonstrate that the brittleness of the Sendust alloy with appropriate addition of boron can be improved obviously with negligible effects on its excellent soft magnetic properties and microhardness as well as special resistivity. As boron content rises further, the brittleness of boron-doped
Table 1 Measured values of static magnetic properties (saturated magnetic flux Bs, coercive force H c, both initial and maximal permeability /z0,/~m), microhardness H v and resistivity p of partial boron-doped Sendust alloys Sample
Boron content
Bs (T)
Hc (A m - t)
P0 (H m - 1)
~m (H m - i)
P (~,12 cm)
nv
1 2 3 4
0
0.9620 0.9240 0.9270 0.9450
3.820 5.093 5.809 8.435
0.018 0.035 0.016 0.026
0.163 0.086 0.078 0.052
119 123 133 138
537.0 515.0 533.9 550.5
150-250 950-1150 1600-1800
Y.-Z. Shao et aL / Brittleness of B-doped Sendust alloys Table 2 The ratios of signal versus noise ( S / N ) of different magnetic heads Velocity of tape Frequency range
1.52 m / s 3.04 m / s (400 Hz-1 MHz) (400 Hz-2 MHz)
As-quenched magnetic head Cast magnetic head Ferrite magnetic head
+ 30.7 dB + 30.5 dB + 30.0 dB
+ 31 dB + 26 dB + 30 dB
ribbon increases again, and soft magnetic properties deteriorate evidently.
4. Discussion
As shown in section 3, appropriately borondoped Sendust ribbons have an improved deformation capacity with negligible influence on its soft magnetic properties compared with standard Sendust alloy. The cause of brittleness improvement has much to do with the decrease of long range order [2]. It has been well-known that the standard Sendust alloy with its special composition 85wt% Fe, 9.6wt% Si, 5.4wt% A1 has the best soft magnetic
85
properties because both magnetic anisotropy K 1 and saturated magnetostriction As tend to minimize them toward zero simultaneously. The trace of boron added results in little deviation of alloy composition from standard Sendust composition. Therefore it is easy to explain why boron-microalloyed Sendust alloys almost have identically excellent soft magnetic properties with standard Sendust alloy. Sendust alloy, consisting of Fe3(Si , AI) single phase, belongs to bcc structure intermetallic compounds with type DO3 superlattice. Fe3(Si, A1) has seriously intrinsic brittleness at room temperature. Boron atom is of smaller atom radius and situated considerably in quasioctahedon interstice of bcc Fe3(Si, A1), which leads to variations of the nearest atom pairs and the next nearest atom pairs in the DO 3 superlattice. The variation of the microenvironment in the DO 3 superlattice causes reduction in DO3 long-range order. Analysis of the ultrafine field by M6ssbauer spectra has proved it [3]. Moreover, measurement of resistivity also indicates that addition of boron decreases the degree of long-range order in that reduction of long-range order results in increase of resistivity [4]. The decrease of long-range order weakens order-
P(w/kg)
/4
b
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10 o
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a
10-1
10- 2
10 - 2
10 - 3
10 - 4 , lO °
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I IIIIII
I
101
I
I
IIIIII
I
I
I
IIIlll
10 2
I
10 3
f(KHz)
10 - 3
I
10- 4
I
101
I
*
i iiiiii
I
I I*,1|1
10 2
Fig. 2. The frequency dependence of effective permeability (a) and iron loss (b).
10 3 f(KHz)
86
Y.-Z. Shao et al. / Brittleness of B-doped Sendust alloys
strengthening and results in improvement of deformation capacity [2]. Otherwise, as the boron content exceeds the optimal concentration for deformation capacity, i,e. 150-200 ppm, severe distortion of the DO 3 supperlattice induced by interstitial boron atoms is caused and segregation of boron on grain boundary takes place due to unstable high energy state. Boron-poor zones emerge inside grain interiors and beneficial effect of homogeneously distributed boron inside grain on lowering the energy of antiphase boundary is weakened. The degree of long-range order and brittleness increase once more [2].
5. Conclusions
1. The boron-doped Sendust alloys developed in the present research are eligible for materials
of wear-resistant audio-video magnetic recording heads or induction coils. 2. Brittleness improvement of boron-doped Sendust alloys is closely related to effect of added boron on microenvironment of the DO 3 superlattice and lowering its long range order.
References [1] Shao YuanZhi, Gu Shouren and Chen Nanping, J. Tsinghua Univ. 29 (1989) 25 (in Chinese). [2] Shao Yuanzhi, Gu Shouren and Chen Nanping, Proc. CMRS 90 Beijing, VoL 2 (1990). [3] Shao Yuanzhi, PhD dissertation, Tsinghua University (1990) Beijing, China. [4] R.W. Cahn and R. Feder, Phil. Mag. 5 (1960) 451.
Soft magnetic properties and brittleness of boron-doped Sendust alloy Shao Y u a n - Z h i Department of Physics, Zhongshan University, Guangzhou 510275, China
Gu Sou-Ren and Chen Nan-Ping Department of Material Science and Engineering, Tsinghua University, Beijing 100084, China Received 12 December 1991; in revised form 4 May 1992
In this paper, static and dynamic magnetic properties as well as brittleness improvement of the boron-microalloyed Sendust ribbons prepared by rapid quenching were investigated. A discussion associated with soft magnetic properties and brittleness improvement of the boron-doped Sendust alloys is given in detail.
1. Introduction Sendust Fe3(Si, AI) consisting of nearly 85 wt% Fe, 9.6 wt% Si and 5.4 wt% AI composition is well known to have high magnetic flux density as well as excellent soft magnetic properties, and it is one of the best soft magnetic materials with lower cost. However, the brittleness of this bcc ordered intermetallic compounds limited its application. For making use of this Sendust alloy at high frequency, in order to reduce the eddy current effect, it is desired to get a thin sheet of this alloy. But it is impossible to roll Sendust alloy into a thin sheet by usual techniques due to its obvious brittleness. We have succeeded in making boron-doped Sendust thin ribbon by means of the rapid quenching method. To maintain the most excellent soft magnetic properties and high mechanical wear resistance of this alloy at the special composition (85Fe, 9.6Si, 5.4A1), better brittleness improvement of the Sendust alloy has been achieved through boron-microalloying than that through other microalloying elements [1]. Correspondence to: Dr. Y.-Z. Shao, Department of Physics, Zhongshan University, Guangzhou 510275, China. Elsevier Science Publishers B.V.
In this paper, we report about both brittleness improvement and soft magnetic properties of boron-doped Sendust ribbons.
2. Experimental procedure The alloys were prepared by arc-melting under argon repeatedly. The boron concentration is within range of 0-2000 ppm by weight. Ribbonform alloys were obtained by rapid quenching. The size of ribbons was 15 mm in width, 40 ~m in thickness and 3-5 m in length. The ribbons had a silver white luster, and were so mechanically flexible that it was possible to wind them easily around a rod of 5 mm in diameter. Annealed at 1000°C for 2 h, then cooled with a rate of 2.5 and 130°C/h respectively for ordering treatment. The deformation capacity of the ribbons both as-prepared and annealed was measured by bending them into semi-circular loops between two parallel plates of a micrometer. The tensile strain of the outer surface of the ribbon is given by ~ = t~ ( d - t), where d is the separation between two plates, and t is the ribbon thickness. The fracture
Y.-Z. Shao et al. / Brittleness of B-doped Sendust alloys
84
strain, ef, is defined as the strain at which catastrophic fracture occurs. Magnetic measurement was taken using toroidal ribbons of 12 mm in diameter. The frequency ranged from 10 to 1000 kHz and the magnetic flux density was selected at 0.002 T.
£fO/o -HF
S 0.6
2
•
/
~
i
3. Experimental results The boron-doped Sendust ribbons after quenching had much mechanical strain as well as many lattice defects which were formed and stored during ribbon formation, and as result, soft magnetic properties were unfavorable. Annealing is necessary to improve the soft magnetic properties. For different boron-doped ribbons, measurement of fracture strain was taken to evaluate their deformation capacity. Fig. 1 illustrates the relation of fracture strain of as-quenched and annealed ribbons with various boron content. It turns out that the fracture strain increase with raising boron concentration, the optimal result being obtained with nearly 150-200 ppm boron addition. The curve of the degree of long-range order, S, in fig. 1 also indicates that with increasing boron content the degree of long-range order S in this superlattice structure decreases up to 150-200 ppm boron content. Under the condition of optimal concentration of boron for deformation capacity, the minimum of the parameter, S, is obtained. In table 1 the static magnetic properties, microhardness and resistivity of partial ribbon specimens are listed. The frequency dependences of both the effective permeability and the
....... -o~\~\
0.2 0.1
LII
0
0.4
z J i,]*,d
~
,
10
i
llltltl
,
10 B (PPM)
* tl,l,,]
t
10
Fig. 1. T h e relationship of fracture strain and degree of long-range order with various boron content. Ao: ~f as-prepared; zx o: ~f cool rate 130 and 2.5°C/h; [] • : S cool rate 130 and 2.5°C/h.
iron loss are shown in figs. l(a) and (b) respectively. The ratios of signal versus noise (S/N) of a magnetic head made of as-quenched microalloyed Sendust ribbon, cast Sendust alloy and ferrite are given in table 2. It can been seen that the magnetic head made of as-quenched microalloyed Sendust ribbon has the best ratio (S/N) by comparison with that of cast Sendust alloy and ferrite, no matter what tape velocity and frequency range were selected. The experimental results mentioned above demonstrate that the brittleness of the Sendust alloy with appropriate addition of boron can be improved obviously with negligible effects on its excellent soft magnetic properties and microhardness as well as special resistivity. As boron content rises further, the brittleness of boron-doped
Table 1 Measured values of static magnetic properties (saturated magnetic flux Bs, coercive force H c, both initial and maximal permeability /z0,/~m), microhardness H v and resistivity p of partial boron-doped Sendust alloys Sample
Boron content
Bs (T)
Hc (A m - t)
P0 (H m - 1)
~m (H m - i)
P (~,12 cm)
nv
1 2 3 4
0
0.9620 0.9240 0.9270 0.9450
3.820 5.093 5.809 8.435
0.018 0.035 0.016 0.026
0.163 0.086 0.078 0.052
119 123 133 138
537.0 515.0 533.9 550.5
150-250 950-1150 1600-1800
Y.-Z. Shao et aL / Brittleness of B-doped Sendust alloys Table 2 The ratios of signal versus noise ( S / N ) of different magnetic heads Velocity of tape Frequency range
1.52 m / s 3.04 m / s (400 Hz-1 MHz) (400 Hz-2 MHz)
As-quenched magnetic head Cast magnetic head Ferrite magnetic head
+ 30.7 dB + 30.5 dB + 30.0 dB
+ 31 dB + 26 dB + 30 dB
ribbon increases again, and soft magnetic properties deteriorate evidently.
4. Discussion
As shown in section 3, appropriately borondoped Sendust ribbons have an improved deformation capacity with negligible influence on its soft magnetic properties compared with standard Sendust alloy. The cause of brittleness improvement has much to do with the decrease of long range order [2]. It has been well-known that the standard Sendust alloy with its special composition 85wt% Fe, 9.6wt% Si, 5.4wt% A1 has the best soft magnetic
85
properties because both magnetic anisotropy K 1 and saturated magnetostriction As tend to minimize them toward zero simultaneously. The trace of boron added results in little deviation of alloy composition from standard Sendust composition. Therefore it is easy to explain why boron-microalloyed Sendust alloys almost have identically excellent soft magnetic properties with standard Sendust alloy. Sendust alloy, consisting of Fe3(Si , AI) single phase, belongs to bcc structure intermetallic compounds with type DO3 superlattice. Fe3(Si, A1) has seriously intrinsic brittleness at room temperature. Boron atom is of smaller atom radius and situated considerably in quasioctahedon interstice of bcc Fe3(Si, A1), which leads to variations of the nearest atom pairs and the next nearest atom pairs in the DO 3 superlattice. The variation of the microenvironment in the DO 3 superlattice causes reduction in DO3 long-range order. Analysis of the ultrafine field by M6ssbauer spectra has proved it [3]. Moreover, measurement of resistivity also indicates that addition of boron decreases the degree of long-range order in that reduction of long-range order results in increase of resistivity [4]. The decrease of long-range order weakens order-
P(w/kg)
/4
b
.i/iJ,,
10 o
~eCH/rn) 10-~
a
10-1
10- 2
10 - 2
10 - 3
10 - 4 , lO °
"-,2..:. ,3 I
I IIIIII
I
101
I
I
IIIIII
I
I
I
IIIlll
10 2
I
10 3
f(KHz)
10 - 3
I
10- 4
I
101
I
*
i iiiiii
I
I I*,1|1
10 2
Fig. 2. The frequency dependence of effective permeability (a) and iron loss (b).
10 3 f(KHz)
86
Y.-Z. Shao et al. / Brittleness of B-doped Sendust alloys
strengthening and results in improvement of deformation capacity [2]. Otherwise, as the boron content exceeds the optimal concentration for deformation capacity, i,e. 150-200 ppm, severe distortion of the DO 3 supperlattice induced by interstitial boron atoms is caused and segregation of boron on grain boundary takes place due to unstable high energy state. Boron-poor zones emerge inside grain interiors and beneficial effect of homogeneously distributed boron inside grain on lowering the energy of antiphase boundary is weakened. The degree of long-range order and brittleness increase once more [2].
5. Conclusions
1. The boron-doped Sendust alloys developed in the present research are eligible for materials
of wear-resistant audio-video magnetic recording heads or induction coils. 2. Brittleness improvement of boron-doped Sendust alloys is closely related to effect of added boron on microenvironment of the DO 3 superlattice and lowering its long range order.
References [1] Shao YuanZhi, Gu Shouren and Chen Nanping, J. Tsinghua Univ. 29 (1989) 25 (in Chinese). [2] Shao Yuanzhi, Gu Shouren and Chen Nanping, Proc. CMRS 90 Beijing, VoL 2 (1990). [3] Shao Yuanzhi, PhD dissertation, Tsinghua University (1990) Beijing, China. [4] R.W. Cahn and R. Feder, Phil. Mag. 5 (1960) 451.