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Magnetic and mechanical properties of rapidly solidified FeSi 6.5 wt% alloys and their interpretation
Journal of Magnetism and Magnetic Materials 160 (1996) 315-317
~
journalof magnetism
and
magnetic
ELSEVIER
,i~
materials
Magnetic and mechanical properties of rapidly solidified Fe-Si 6.5 wt% alloys and their interpretation B. Viala a, j. Degauque ~, M. Baricco h, E. Ferrara ~, M. Pasquale ~, F. Fiorillo c,, ~ Laboraloire de Physique des Solide.~ (CNRS) INSA, 31077 Toulouse, France b Dipartimenlo eli Chimica IbM dell" Unit'ersit21, 10125 Torino, ltah' lstituto Elettrotecnico Naci(ma/e Galileo Ferrariv aml INFM GNSM, C.so M. el'Aceg/io 42, 10125 Torino, Italy
Abstract The structural, mechanical and magnetic properties of Fe Si 6.5 wt% rapidly solidified alloys have been investigated following recrystallization annealing and different rates of cooling through the B 2 + DO 3 ordering region (1 _< 7~< 1500OCmin t). A transition from ductile to brittle behavior is observed for "F< - 1000°Cmin i chiefly due to B 2 ordering and the associated formation of superlattice dislocations, having reduced glide and cross-slip capability. The magnetic behavior appears, however, to be weakly dependent on 7~, with the energy losses minimized for average grain size around 100-150 ~,m. Keywords: Fe-Si alloys; Rapidly solidified alloys: Energy losses: Mechanical properties
Fe Si 6.5 wt% rapidly solidified alloys are obtained as ductile ribbons in the as-quenched state and show excellent soft magnetic properties only after high temperature annealing, which induces stress relaxation and grain growth [1-3]. The final magnetically optimized alloys can, however, suffer a dramatic loss of ductility, related to the formation of the ordered B~ and DO.~ phases upon cooling, which can hinder their effective use in applications. Investigation of the microscopic mechanisms leading to embrittlement and assessing the corresponding role of the thermal treatment is a basic step in achieving the best compromise between magnetic and mechanical properties. In this paper we present a study of the mechanical properties of Fe-Si 6.5 wt% ribbons and their relationship with phase ordering after recrystallization annealing and cooling at different rates. To this end, transmission electron nilcroscopy (TEM) observations of dislocations aild ordered phases, correlated with tensile and bending tests, are exploited. Magnetic characterization demonstrates the relatively minor role of the cooling rate on losses and shows that ductility can be retained to some extent in magnetically excellent ribbons. Fe-Si 6.5 wt% ribbons, 5 mm wide and 30 txm thick, were prepared by planar flow casting in a He atmosphere
Corresponding author. Email: [email protected]: lhx: +39-11-6507-61 I.
using a high purity master alloy. Strip samples 200 mm long were subjected either to conventional annealing in vacuum (CA), using a resistive oven, o] infrared annealing (1R) in vacuum o1 H 2 atmosphere, at temperatures ranging between 800 and 1250°C. In the former case, the cooling rate throu*,h the olderin~ region ( ~ 800-_ 00 C) was ?~ I°Cmin ~. By IR annealing, 7~ could be varied from 100 to 1500°Cmin ~. Tensile tests were performed, using a suitably adapted Instron machine, at constant strain rate 10 5 s ~. while the parallel plate method was adopted lk}r the bending tests [4]. The associated bending ductility parameter A = d / ( D d) was determined, where d is the ribbon thickness and D the distance between the plates at fracture ( A - 1 for a ductile material). Investigations on phase ordering and dislocation structures were carried out by TEM analysis. Finally, magnetic losses were determined, from dc to 10 kHz, by means of a controlled induction waveform digital wattmeter [5]. According to previous observations [3,6], abnormal grain growth is achieved by 1 h annealing at temperatures T higher than about 950°C (CA) or 800°C (IR), which is associated with the development of a magnetically favourable (100) (0~'w) texture. At the same time, however. the ductile behavior of the material is impaired, but the present experiments show that rapid cooling after IR annealing is an effective way of restraining material embrittlement. Fig. la compares the behavior of the ductility parameter a vs. the average grain size s, that is vs. T,,, at
0304-8853/96/$15.00 Copyright ,~) 1996 Elsevier Science B.V. All rights reserved. PII S0304-8853(96)00209 0
316
B. Viala et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 315-317
I"
\
.ll
~.
~'. IR --..~, \ ~
Io
,,.
\1~
/ ~00
2°°°c/rain /
OC/min
/
-/
\
,, 10"3
a
q 50
~ I oo
// 150
i 200
250
s (~m)
+- (011) <111>
e- (011) < I l I >
•
(011) <111>
Fig. 1. Fe-Si 6.5 wt% ribbons treated for 1 h at different temperatures, up to 77a = 1250°C, by infrared (IR) or conventional (CA) annealing, and cooled through the phase ordering region at different rates 7". (a) Dependence of the ductility parameter a on the average grain size s (i.e. 77~)for different values of ;/'. (b) Dissociated screw dislocations within a B 2 island after fast cooling (T ~ 500°Cmin-I). (c) Same as (b) alter slow cooling (T ~ I°C min- I ).
three different 7~ values. It can be seen that for 7~ 400OCmin i the ductility is preserved up to s ~ 120 ixm, which happens to be close to the condition of minimum loss and maximum permeability. On the other hand, a increases at large grain sizes (s > 200 ixm) in the CA samples, an effect due to partial loss of Si (down to ~ 5 wt%) upon high temperature annealing (T~,> 1200°C). TEM analysis shows that the ordered B 2 domains, already present in the as-quenched state, swell rapidly for T < 1000°Cmin - I , to become ~ 2 - 3 ixm wide upon slow cooling ( ; r ~ l ° C m i n ]). DO 3 domains are more effectively suppressed at high J" values and appear as pseudoprecipitates within the larger B 2 domains only when 7; < 500°Cmin i. Partial phase ordering is the source of the additional material hardening. This is apparent when the dependence of the tensile yield stress cro on the cooling rate is described by means of the Hall-Petch law o-o = o-L + O-ord + k s - ] / 2 which permits one, through experimental determination of the Hall-Petch constant /, and knowledge of the Peierls-Nabarro force cr L [7], to calculate the friction term %rd provided by ordering [6], which restricts the dislocation glide within the grains. It turns out that
such a term is higher when the cooling rate is lower, that is when the B. + DO 3 phases are more developed. This finding directly correlates with the dislocation structures actually observed, examples of which are reported in the TEM micrographs shown in Fig. lb and lc, corresponding to 7~~ 500°C min i and T ~ I°C min i, respectively. It is concluded from these observations that the internal stress buildup chiefly occurs because the dissociated superlattice dislocations in the B 2 domains are unable to relieve such a stress by cross-slip, eventually causing brittle fracture by cleavage mode. The type of annealing (CA vs. IR) and the cooling rate do not greatly affect the magnetic properties, provided the optimal grain size and texture are achieved and few morphological defects are introduced during the treatment [3]. An example is provided in Fig. 2, where one can see that the 50 Hz power losses attain a minimum value for s ~ 100-150 ixm. The rationale for this behavior in CA samples is given in Ref. [3]. IR annealing permits one to attain a good compromise between mechanical and magnetic behavior, but it remains a more critical methodology than CA. Fig. 2 shows, for instance, that after IR annealing in vacuum at 1100°C, such as to obtain s > I00 ixm, and fast cooling (400°Cmin i) the losses are remarkably increased, an effect likely due to the morphological deterioration of the sample during annealing and the increase of stresses during cooling. It is interesting to remark that, while the 50 Hz energy losses basically coincide with the hysteresis (quasi-static) component, the high frequency losses (say around 10 kHz) are chiefly associated with the excess loss component [8]. It can be concluded that, as suggested by the increase of excess losses with s [3,8] and the behavior of A shown in Fig. l a, an excellent compromise between mechanical behavior and high frequency magnetic properties can be found by limiting grain growth to s ~ 6 0 - 8 0 gm, for which IR annealing and natural cooling provide high ductility.
50
....
, ....
, ....
, ....
r ....
f = 50 Hz 40 ~
Ip = 1 T
zx
l'\ ~3o~"
/ \ ", X
& \
/
%.,\\ °
,0
/
0
....
. ..~
\
El"
IR (400 °C'min)
/ °
. . . . .
IR (200 aC/min) 4 .... 50
\ i .... 100
i .... 150
CA (I °C/min) i .... 200
250
s (gm) Fig. 2. Same samples as in Fig. 1. Dependence of the energy losses at 50 Hz and l T on grain size.
B. Viala et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 315 317 Acknowledgement: This work was carried out in the framework of the CEC supported B R I T E / E U R A M project B E - 4 3 7 8 / 9 0 and the Franco-Itatian cooperation programme GALILEO.
References [1] M.J. Tenwick and H.A. Davies, in: Rapidly Quenched Metals, eds. S. Steeb and H. Warlimont (Elsevier. Amsterdam, 1985) p. 1639.
317
[2] K.I. Arai, H. Tsutsumitake and K. Ohmori, IEEE Trans. Magn. 20 (1984) 1463. [3] E. Ferrara et al., J. Magn. Magn. Mater. 133 (1994) 366. [4] F.E. Luborsky and J.L. Walter, J. Appl. Phys. 47 (1976) 3648. [5] G. Bertotti et al.. J. Appl. Phys. 73 (1993) 5375. [6] B, Viala, Ph.D. Thesis. Toulouse, 1994. [7] R. kabusch. Phys. Status Solidi 41 (1970) 659. [8] V. Basso et al., IEEE Trans. Magn. 30 (1994) 4893.
~
journalof magnetism
and
magnetic
ELSEVIER
,i~
materials
Magnetic and mechanical properties of rapidly solidified Fe-Si 6.5 wt% alloys and their interpretation B. Viala a, j. Degauque ~, M. Baricco h, E. Ferrara ~, M. Pasquale ~, F. Fiorillo c,, ~ Laboraloire de Physique des Solide.~ (CNRS) INSA, 31077 Toulouse, France b Dipartimenlo eli Chimica IbM dell" Unit'ersit21, 10125 Torino, ltah' lstituto Elettrotecnico Naci(ma/e Galileo Ferrariv aml INFM GNSM, C.so M. el'Aceg/io 42, 10125 Torino, Italy
Abstract The structural, mechanical and magnetic properties of Fe Si 6.5 wt% rapidly solidified alloys have been investigated following recrystallization annealing and different rates of cooling through the B 2 + DO 3 ordering region (1 _< 7~< 1500OCmin t). A transition from ductile to brittle behavior is observed for "F< - 1000°Cmin i chiefly due to B 2 ordering and the associated formation of superlattice dislocations, having reduced glide and cross-slip capability. The magnetic behavior appears, however, to be weakly dependent on 7~, with the energy losses minimized for average grain size around 100-150 ~,m. Keywords: Fe-Si alloys; Rapidly solidified alloys: Energy losses: Mechanical properties
Fe Si 6.5 wt% rapidly solidified alloys are obtained as ductile ribbons in the as-quenched state and show excellent soft magnetic properties only after high temperature annealing, which induces stress relaxation and grain growth [1-3]. The final magnetically optimized alloys can, however, suffer a dramatic loss of ductility, related to the formation of the ordered B~ and DO.~ phases upon cooling, which can hinder their effective use in applications. Investigation of the microscopic mechanisms leading to embrittlement and assessing the corresponding role of the thermal treatment is a basic step in achieving the best compromise between magnetic and mechanical properties. In this paper we present a study of the mechanical properties of Fe-Si 6.5 wt% ribbons and their relationship with phase ordering after recrystallization annealing and cooling at different rates. To this end, transmission electron nilcroscopy (TEM) observations of dislocations aild ordered phases, correlated with tensile and bending tests, are exploited. Magnetic characterization demonstrates the relatively minor role of the cooling rate on losses and shows that ductility can be retained to some extent in magnetically excellent ribbons. Fe-Si 6.5 wt% ribbons, 5 mm wide and 30 txm thick, were prepared by planar flow casting in a He atmosphere
Corresponding author. Email: [email protected]: lhx: +39-11-6507-61 I.
using a high purity master alloy. Strip samples 200 mm long were subjected either to conventional annealing in vacuum (CA), using a resistive oven, o] infrared annealing (1R) in vacuum o1 H 2 atmosphere, at temperatures ranging between 800 and 1250°C. In the former case, the cooling rate throu*,h the olderin~ region ( ~ 800-_ 00 C) was ?~ I°Cmin ~. By IR annealing, 7~ could be varied from 100 to 1500°Cmin ~. Tensile tests were performed, using a suitably adapted Instron machine, at constant strain rate 10 5 s ~. while the parallel plate method was adopted lk}r the bending tests [4]. The associated bending ductility parameter A = d / ( D d) was determined, where d is the ribbon thickness and D the distance between the plates at fracture ( A - 1 for a ductile material). Investigations on phase ordering and dislocation structures were carried out by TEM analysis. Finally, magnetic losses were determined, from dc to 10 kHz, by means of a controlled induction waveform digital wattmeter [5]. According to previous observations [3,6], abnormal grain growth is achieved by 1 h annealing at temperatures T higher than about 950°C (CA) or 800°C (IR), which is associated with the development of a magnetically favourable (100) (0~'w) texture. At the same time, however. the ductile behavior of the material is impaired, but the present experiments show that rapid cooling after IR annealing is an effective way of restraining material embrittlement. Fig. la compares the behavior of the ductility parameter a vs. the average grain size s, that is vs. T,,, at
0304-8853/96/$15.00 Copyright ,~) 1996 Elsevier Science B.V. All rights reserved. PII S0304-8853(96)00209 0
316
B. Viala et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 315-317
I"
\
.ll
~.
~'. IR --..~, \ ~
Io
,,.
\1~
/ ~00
2°°°c/rain /
OC/min
/
-/
\
,, 10"3
a
q 50
~ I oo
// 150
i 200
250
s (~m)
+- (011) <111>
e- (011) < I l I >
•
(011) <111>
Fig. 1. Fe-Si 6.5 wt% ribbons treated for 1 h at different temperatures, up to 77a = 1250°C, by infrared (IR) or conventional (CA) annealing, and cooled through the phase ordering region at different rates 7". (a) Dependence of the ductility parameter a on the average grain size s (i.e. 77~)for different values of ;/'. (b) Dissociated screw dislocations within a B 2 island after fast cooling (T ~ 500°Cmin-I). (c) Same as (b) alter slow cooling (T ~ I°C min- I ).
three different 7~ values. It can be seen that for 7~ 400OCmin i the ductility is preserved up to s ~ 120 ixm, which happens to be close to the condition of minimum loss and maximum permeability. On the other hand, a increases at large grain sizes (s > 200 ixm) in the CA samples, an effect due to partial loss of Si (down to ~ 5 wt%) upon high temperature annealing (T~,> 1200°C). TEM analysis shows that the ordered B 2 domains, already present in the as-quenched state, swell rapidly for T < 1000°Cmin - I , to become ~ 2 - 3 ixm wide upon slow cooling ( ; r ~ l ° C m i n ]). DO 3 domains are more effectively suppressed at high J" values and appear as pseudoprecipitates within the larger B 2 domains only when 7; < 500°Cmin i. Partial phase ordering is the source of the additional material hardening. This is apparent when the dependence of the tensile yield stress cro on the cooling rate is described by means of the Hall-Petch law o-o = o-L + O-ord + k s - ] / 2 which permits one, through experimental determination of the Hall-Petch constant /, and knowledge of the Peierls-Nabarro force cr L [7], to calculate the friction term %rd provided by ordering [6], which restricts the dislocation glide within the grains. It turns out that
such a term is higher when the cooling rate is lower, that is when the B. + DO 3 phases are more developed. This finding directly correlates with the dislocation structures actually observed, examples of which are reported in the TEM micrographs shown in Fig. lb and lc, corresponding to 7~~ 500°C min i and T ~ I°C min i, respectively. It is concluded from these observations that the internal stress buildup chiefly occurs because the dissociated superlattice dislocations in the B 2 domains are unable to relieve such a stress by cross-slip, eventually causing brittle fracture by cleavage mode. The type of annealing (CA vs. IR) and the cooling rate do not greatly affect the magnetic properties, provided the optimal grain size and texture are achieved and few morphological defects are introduced during the treatment [3]. An example is provided in Fig. 2, where one can see that the 50 Hz power losses attain a minimum value for s ~ 100-150 ixm. The rationale for this behavior in CA samples is given in Ref. [3]. IR annealing permits one to attain a good compromise between mechanical and magnetic behavior, but it remains a more critical methodology than CA. Fig. 2 shows, for instance, that after IR annealing in vacuum at 1100°C, such as to obtain s > I00 ixm, and fast cooling (400°Cmin i) the losses are remarkably increased, an effect likely due to the morphological deterioration of the sample during annealing and the increase of stresses during cooling. It is interesting to remark that, while the 50 Hz energy losses basically coincide with the hysteresis (quasi-static) component, the high frequency losses (say around 10 kHz) are chiefly associated with the excess loss component [8]. It can be concluded that, as suggested by the increase of excess losses with s [3,8] and the behavior of A shown in Fig. l a, an excellent compromise between mechanical behavior and high frequency magnetic properties can be found by limiting grain growth to s ~ 6 0 - 8 0 gm, for which IR annealing and natural cooling provide high ductility.
50
....
, ....
, ....
, ....
r ....
f = 50 Hz 40 ~
Ip = 1 T
zx
l'\ ~3o~"
/ \ ", X
& \
/
%.,\\ °
,0
/
0
....
. ..~
\
El"
IR (400 °C'min)
/ °
. . . . .
IR (200 aC/min) 4 .... 50
\ i .... 100
i .... 150
CA (I °C/min) i .... 200
250
s (gm) Fig. 2. Same samples as in Fig. 1. Dependence of the energy losses at 50 Hz and l T on grain size.
B. Viala et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 315 317 Acknowledgement: This work was carried out in the framework of the CEC supported B R I T E / E U R A M project B E - 4 3 7 8 / 9 0 and the Franco-Itatian cooperation programme GALILEO.
References [1] M.J. Tenwick and H.A. Davies, in: Rapidly Quenched Metals, eds. S. Steeb and H. Warlimont (Elsevier. Amsterdam, 1985) p. 1639.
317
[2] K.I. Arai, H. Tsutsumitake and K. Ohmori, IEEE Trans. Magn. 20 (1984) 1463. [3] E. Ferrara et al., J. Magn. Magn. Mater. 133 (1994) 366. [4] F.E. Luborsky and J.L. Walter, J. Appl. Phys. 47 (1976) 3648. [5] G. Bertotti et al.. J. Appl. Phys. 73 (1993) 5375. [6] B, Viala, Ph.D. Thesis. Toulouse, 1994. [7] R. kabusch. Phys. Status Solidi 41 (1970) 659. [8] V. Basso et al., IEEE Trans. Magn. 30 (1994) 4893.