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Stress and structural relaxation in amorphous ribbons along dynamic current annealing
Journal of Magnetism and Magnetic Materials 160 (1996) 291-292
journal of magnetism ~ H and magnetic materials
ELSEVIER
Stress and structural relaxation in amorphous ribbons along dynamic current annealing A. Houzali *, F. Alves, J.C. Perron LGEP-ESE, Plateau du Moulon, 91192 Gi['/ Ycette Cedex, France
Abstract In order to improve our knowledge of the effect of an applied mechanical stress on stress relaxation in Fe78SigBi3 amorphous alloy, measurements of the change of length were performed at 753 K corresponding to the optimum magnetic properties. The magnetic properties of samples annealed by this process achieved their optimum with an applied stress of 18.8 MPa and when ribbon contraction occurred. In addition, using Taub's model, the evolution of stress relaxation in terms of SRF is discussed. Kevwords: Joule heating: Mechanical behavior: Stress relaxation
1. Introduction
3. Results and discussion
Structural relaxation mechanisms and the induced anisotropy taking place during fast annealing with or without stress, particularly in Joule heating, are not well known. We have recently developed a new current annealing technique under tensile stress [1] in which the ribbon is moving. Previous results [2] have shown that Fe7sSi9B]3 amorphous alloy achieves optimum magnetic properties when the maximum peak temperature (Topt), applied stress ((top~) and feed rate are 753 K, 18.8 MPa and 1 c m / s , respectively. Using these conditions, we present here measurements of the change of ribbon length in order to understand the origin of the magnetic anisotropy induced during our process. The anisotropy seems to be associated with the stress contribution. Structural relaxation was also studied.
Fig. 1 shows the evolution of e,v for as-quenched (a.q.) ribbons under different stresses during the same heating (up to 753 K) and cooling cycle. We observe the disappearance of the length contraction from 28.2 MPa and an increase of the amplitude of viscoplastic deformation after cooling to room temperature. Measurements of e~v under a tensile stress of 18.8 MPa and at different temperatures were also performed. The results show that the contraction phenomenon, which reflects the structural relaxation effect [3], occurs when the temperature exceeds 473 K. This is in good agreement with DSC measurements, which indicate a significant increase of structural relaxation above 473 K [2] but below the crystallization temperature. We also examined the mechanical behavior of the
2. Experiments
.
.
.
.
i
.
.
.
.
i
.
.
.
.
i
.
.
.
.
i
,
0.3
The FeTsSi9 B ]3 amorphous ribbon was purchased from Allied Signal. Length measurements on motionless ribbons were performed in air using our annealing setup [1] by means of a linear voltage differential transformer (LVDT). As the temperature along the ribbon length is not constant, we only consider the average strain ( e , ~ - A I / I ) . The temperature is controlled by means of an infrared pyrometer [1 ]. The heating rate was limited by the heat capacity to 400 K / m i n .
0.2
0.1 -
. . . . 0
~
p
i . . . . 100
a
3
i . . . . 200
7
"
6M
P a
t . . . . 300
Time ( s ) Corresponding author. Fax: + 33-1-6941-8318.
Fig. 1. e,,,. vs. time for as-quenched ribbons.
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0304-885 3(96)00202-8
i , 400
,
292
A. Houzali et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 291-292
amorphous ribbon during Joule heating at Topt by observing the strain response to various stress cycles. Fig. 2 shows an example of a creep curve at stress cycles below 30 MPa. An elastic response (Eel) appears instantaneously and completely recovers on stress reduction. We also observed a slight anelastic deformation (can) and a contraction of the length of the ribbon after cooling to room temperature. As regards the stress cycle above 30 MPa, the results show a plastic deformation during the loading-unloading tests and the disappearance of the contraction phenomenon. This is probably due to the fact that, at high temperature (753 K), an applied stress of about 30 MPa is sufficient to exceed the elastic limit of the amorphous ribbon and therefore to cause a plastic deformation. It should be noted that the same contraction phenomenon as mentioned above is present in a.q. ribbons annealed classically (673 K, 2 h). However, this contraction of the ribbon is more pronounced (0.126%) than that obtained on the Joule heating setup under optimal conditions (0.022%). The problem of stress relief of amorphous alloys has been widely examined [4-7]. According to Taub's model [4], in which the stress relaxation is expressed in terms of a stress relief fraction (SRF) given by Eq. (1), we show in Fig. 3 the SRF computed from Eq. (1) and the experimental curve determined from Eq. (2): ~/t ] - ~ / 3 ~
sRF(t) = (1 +
'
where E is the elastic modulus of the alloy, 7/(0) is the viscosity at t = 0 and ~ is the rate of viscosity increase; SRF(t) = 1-
R r(t~'
(2)
where R and r ( t ) are, respectively, the radii of the annealing zone and the ribbon, after annealing at various times t. The experimental curve presents the same evolution as predicted by Eq. (1) and indicates effectively that the higher the annealing temperature, the larger the extent of . . . .
r
. . . .
I
. . . .
r
. . . .
I
. . . .
I
'
0.3 9.4 MPa
0.2 =>
f -~.~.~....................... i_............ ,............... r__
0.8~, 0.6
523
K
"".......... ". . . . . . . . . . . . . . . . . . . . . . .
s43K__
"..
0.4
..............
................................ s?3 K ....
02 \ 1[, i 0
m--
0
Experimental
. . . .
( 753
K-
18.8
MPa
)
-II
100 200 Annealing time ( s ) Fig. 3. SRF vs. time.
stress relaxation (decrease of SRF). However, contrary to the experimental data reported by Taub [4], amorphous ribbons, annealed by our process at 753 K, are not fully relaxed (the percentage of total relaxation is about 70 for a feed rate of 1 cm/s). We note that our results show clearly that, under optimal conditions (753 K, 18.8 MPa, 1 c m / s ) and taking into account the annealing time of about 20 s, no plastic deformation occurs. In addition, despite the fact that stress relief relaxation after Joule heating was not fully completed, better magnetic properties were attained. It seems that at high temperature (Topt) the applied stress has a beneficial effect on the magnetic properties when the ribbon is slightly contracted. Therefore, we believe that, in our case, the structural relaxation is the main process and the magnetic anisotropy is probably induced by the anelastic contribution only (o- < 20 MPa). Furthermore, a strong correlation exists between the annealing temperature, the applied stress and the destruction of ferromagnetic order. Indeed, at Topt above the Curie point (685 K), magnetic interactions decrease and the diffusivity of atoms bearing magnetic moments increases. Therefore, the applied stress can give rise to an atomic ordering favourable for a different reduction of free volume, improving the magnetic properties. In addition, we emphasize that DSC analysis shows differences in short range order 'SRO' (disappearance of the endothermic contribution) between dynamic current and furnace annealing [7]. This difference of SRO may be related to the lowering of brittleness in our current annealed ribbons.
References
0.1
-0.1
]
. . . .
i
. . . .
200
i
. . . .
400
i
. . . .
600 Time (s)
i
. . . .
800
Fig. 2. Mechanical behavior under stress changes.
i
,
1000
[1] [2] [3] [4] [5] [6] [7]
A. Houzali et al., Rev. Sci. Instr. 66 (1995) 4671. A. Houzali et al., Mater. Sci. Forum 179-181 (1995) 615. A.L. Mulder et al., Scripta Metall. 18 (1984) 515. A.I. Taub, IEEE Trans. Magn. 20 (1984) 564. A.I. Taub and F. Luborsky, Acta. Metall. 29 (1981) 1939. O.V. Nielsen et al., J. Magn. Magn. Mater. 24 (1981) 88. M.A. Escobar et al., IEEE Trans. Magn. 28 (1992) 1911.
journal of magnetism ~ H and magnetic materials
ELSEVIER
Stress and structural relaxation in amorphous ribbons along dynamic current annealing A. Houzali *, F. Alves, J.C. Perron LGEP-ESE, Plateau du Moulon, 91192 Gi['/ Ycette Cedex, France
Abstract In order to improve our knowledge of the effect of an applied mechanical stress on stress relaxation in Fe78SigBi3 amorphous alloy, measurements of the change of length were performed at 753 K corresponding to the optimum magnetic properties. The magnetic properties of samples annealed by this process achieved their optimum with an applied stress of 18.8 MPa and when ribbon contraction occurred. In addition, using Taub's model, the evolution of stress relaxation in terms of SRF is discussed. Kevwords: Joule heating: Mechanical behavior: Stress relaxation
1. Introduction
3. Results and discussion
Structural relaxation mechanisms and the induced anisotropy taking place during fast annealing with or without stress, particularly in Joule heating, are not well known. We have recently developed a new current annealing technique under tensile stress [1] in which the ribbon is moving. Previous results [2] have shown that Fe7sSi9B]3 amorphous alloy achieves optimum magnetic properties when the maximum peak temperature (Topt), applied stress ((top~) and feed rate are 753 K, 18.8 MPa and 1 c m / s , respectively. Using these conditions, we present here measurements of the change of ribbon length in order to understand the origin of the magnetic anisotropy induced during our process. The anisotropy seems to be associated with the stress contribution. Structural relaxation was also studied.
Fig. 1 shows the evolution of e,v for as-quenched (a.q.) ribbons under different stresses during the same heating (up to 753 K) and cooling cycle. We observe the disappearance of the length contraction from 28.2 MPa and an increase of the amplitude of viscoplastic deformation after cooling to room temperature. Measurements of e~v under a tensile stress of 18.8 MPa and at different temperatures were also performed. The results show that the contraction phenomenon, which reflects the structural relaxation effect [3], occurs when the temperature exceeds 473 K. This is in good agreement with DSC measurements, which indicate a significant increase of structural relaxation above 473 K [2] but below the crystallization temperature. We also examined the mechanical behavior of the
2. Experiments
.
.
.
.
i
.
.
.
.
i
.
.
.
.
i
.
.
.
.
i
,
0.3
The FeTsSi9 B ]3 amorphous ribbon was purchased from Allied Signal. Length measurements on motionless ribbons were performed in air using our annealing setup [1] by means of a linear voltage differential transformer (LVDT). As the temperature along the ribbon length is not constant, we only consider the average strain ( e , ~ - A I / I ) . The temperature is controlled by means of an infrared pyrometer [1 ]. The heating rate was limited by the heat capacity to 400 K / m i n .
0.2
0.1 -
. . . . 0
~
p
i . . . . 100
a
3
i . . . . 200
7
"
6M
P a
t . . . . 300
Time ( s ) Corresponding author. Fax: + 33-1-6941-8318.
Fig. 1. e,,,. vs. time for as-quenched ribbons.
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0304-885 3(96)00202-8
i , 400
,
292
A. Houzali et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 291-292
amorphous ribbon during Joule heating at Topt by observing the strain response to various stress cycles. Fig. 2 shows an example of a creep curve at stress cycles below 30 MPa. An elastic response (Eel) appears instantaneously and completely recovers on stress reduction. We also observed a slight anelastic deformation (can) and a contraction of the length of the ribbon after cooling to room temperature. As regards the stress cycle above 30 MPa, the results show a plastic deformation during the loading-unloading tests and the disappearance of the contraction phenomenon. This is probably due to the fact that, at high temperature (753 K), an applied stress of about 30 MPa is sufficient to exceed the elastic limit of the amorphous ribbon and therefore to cause a plastic deformation. It should be noted that the same contraction phenomenon as mentioned above is present in a.q. ribbons annealed classically (673 K, 2 h). However, this contraction of the ribbon is more pronounced (0.126%) than that obtained on the Joule heating setup under optimal conditions (0.022%). The problem of stress relief of amorphous alloys has been widely examined [4-7]. According to Taub's model [4], in which the stress relaxation is expressed in terms of a stress relief fraction (SRF) given by Eq. (1), we show in Fig. 3 the SRF computed from Eq. (1) and the experimental curve determined from Eq. (2): ~/t ] - ~ / 3 ~
sRF(t) = (1 +
'
where E is the elastic modulus of the alloy, 7/(0) is the viscosity at t = 0 and ~ is the rate of viscosity increase; SRF(t) = 1-
R r(t~'
(2)
where R and r ( t ) are, respectively, the radii of the annealing zone and the ribbon, after annealing at various times t. The experimental curve presents the same evolution as predicted by Eq. (1) and indicates effectively that the higher the annealing temperature, the larger the extent of . . . .
r
. . . .
I
. . . .
r
. . . .
I
. . . .
I
'
0.3 9.4 MPa
0.2 =>
f -~.~.~....................... i_............ ,............... r__
0.8~, 0.6
523
K
"".......... ". . . . . . . . . . . . . . . . . . . . . . .
s43K__
"..
0.4
..............
................................ s?3 K ....
02 \ 1[, i 0
m--
0
Experimental
. . . .
( 753
K-
18.8
MPa
)
-II
100 200 Annealing time ( s ) Fig. 3. SRF vs. time.
stress relaxation (decrease of SRF). However, contrary to the experimental data reported by Taub [4], amorphous ribbons, annealed by our process at 753 K, are not fully relaxed (the percentage of total relaxation is about 70 for a feed rate of 1 cm/s). We note that our results show clearly that, under optimal conditions (753 K, 18.8 MPa, 1 c m / s ) and taking into account the annealing time of about 20 s, no plastic deformation occurs. In addition, despite the fact that stress relief relaxation after Joule heating was not fully completed, better magnetic properties were attained. It seems that at high temperature (Topt) the applied stress has a beneficial effect on the magnetic properties when the ribbon is slightly contracted. Therefore, we believe that, in our case, the structural relaxation is the main process and the magnetic anisotropy is probably induced by the anelastic contribution only (o- < 20 MPa). Furthermore, a strong correlation exists between the annealing temperature, the applied stress and the destruction of ferromagnetic order. Indeed, at Topt above the Curie point (685 K), magnetic interactions decrease and the diffusivity of atoms bearing magnetic moments increases. Therefore, the applied stress can give rise to an atomic ordering favourable for a different reduction of free volume, improving the magnetic properties. In addition, we emphasize that DSC analysis shows differences in short range order 'SRO' (disappearance of the endothermic contribution) between dynamic current and furnace annealing [7]. This difference of SRO may be related to the lowering of brittleness in our current annealed ribbons.
References
0.1
-0.1
]
. . . .
i
. . . .
200
i
. . . .
400
i
. . . .
600 Time (s)
i
. . . .
800
Fig. 2. Mechanical behavior under stress changes.
i
,
1000
[1] [2] [3] [4] [5] [6] [7]
A. Houzali et al., Rev. Sci. Instr. 66 (1995) 4671. A. Houzali et al., Mater. Sci. Forum 179-181 (1995) 615. A.L. Mulder et al., Scripta Metall. 18 (1984) 515. A.I. Taub, IEEE Trans. Magn. 20 (1984) 564. A.I. Taub and F. Luborsky, Acta. Metall. 29 (1981) 1939. O.V. Nielsen et al., J. Magn. Magn. Mater. 24 (1981) 88. M.A. Escobar et al., IEEE Trans. Magn. 28 (1992) 1911.