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Amorphization of soft magnetic alloys by the mechanical alloying technique
1368
Materials Science and Engineering, A134 ( 1991 ) 1368-1371
Amorphization of soft magnetic alloys by the mechanical alloying technique S. Surifiach, M. D. Bar6, J. Segura and M. T. Clavaguera-Mora Fisica de Materials, Departament de Fisica, Universitat Aut6noma de Barcelona, 08193-Bellaterra (Spain
N. Clavaguera Departament Estructura i Constituents de la Mat~ria, Facultat de Fisica, Diagonal 647, Universitat de Barcelona, 08028-Barcelona (Spain)
Abstract The progress of amorphization by mechanical alloying on F e - B and F e - B - S i powders is studied by X-ray diffraction and differential scanning calorimetry. A p a r t from the well known broad exothermic effect, a well defined endothermic effect is present after milling times typically of 300 h. The enthalpy and activation energy of this endothermic peak are obtained and related to the crystallization ones. The results exclude diffusion as involved in the endothermic process.
1. Introduction At present, there is considerable interest in the formation of amorphous phases in soft magnetic alloy systems, because of their potential properties and processing advantages. Amorphous soft magnetic materials have traditionally been prepared by the rapid solidification of molten alloys (see refs. 1-4). Recently, new synthesis methods have been developed which are based on solid-state reaction between pure elements or compounds [5, 6]. In this paper the progress on the amorphization by mechanical alloying (MA) on Fe-B and Fe-B-Si powders will be presented as observed by X-ray diffraction, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). It will be compared with the behaviour of
the alloys prepared by rapid solidification (meltspinning) of the melts. 2. Experimental details In Table 1 are shown the nominal composition and precursors used for the two alloys studied. The Fe-B-Si composition was also obtained in ribbon form by melt-spinning. The powders were ball-milled using the sets of the vial and balls made of stainless CrNi steel in a planetary type mill (Fritsch P-5). The ball to powder ratio was 5:1. The MA powders were characterized and their thermal stability was tested by studying their heating behaviour in a computerized differential scanning calorimeter, Perkin-Elmer DSC II. The microstructural evolution during MA was followed by SEM.
TABLE 1 Nominal composition and precursors used in the mechanical alloying process and activation energies, E, of the endothermic peaks (after 350 h milling) and the crystallization peak Sample
Nominal composition
Precursors
E (eV) Endotherm
MA3X
FesoB20
Fe + FeB < 200/zm
1.5
MA11X
Fe75B~sSi10
Fe + B + Si < 200/zm
0.9
0921-5093/91/$3.50
Crystallization 4.5 ~ 3.3 (first peak) / 3.9 (second peak)
© Elsevier Sequoia/Printed in The Netherlands
1369
3. Results and discussion
3.1. X-ray diffraction The X-ray diffraction pattern of samples MA3X for selected milling time are presented in Fig. 1. The unmilled powder exhibits sharp diffraction lines. The intensity of the lines decreases and their width increases with milling time. Assuming a microcrystalline structure, the typical crystallite size after 350 h milling would be of 20 A. At long milling time, amorphization is clearly evident from X-ray results. Partial amorphization after a long milling time was also observed for the MAI IX samples. This result is
:
I i I[
in contradiction with those published by Narita and Sugimoto [6, 7]. These authors suggest that after 900 h milling the powder is not yet amorphous because the magnetization does not increase significantly on annealing.
3.2. Microscopy The morphology of the powders with milling time was followed by SEM. The typical lamellar structure found in other systems does not form [6]. A quick growth in size of the powders occurs on milling. Figure 2 shows the powder morphology of samples MA3X after 350 h milling. The mean particle size amounts from 3 to 15 ~m. This value is about 103 times the typical crystallite size determined from the X-ray diffraction pattern. Therefore, amorphization is quite effective after 350 h milling. Further evidence of the amorphization is obtained from DSC.
3.3. Differential scanning calorimetry b~
~'i
~'~
~'~,'~.~ , ~
ci
L~
Fig. 1. X-ray diffraction patterns of Fe~oB2o after different millinIa times• (a) l) h; (b) 10l) h; (c) 200 h; (d) 350 h.
We present in Fig. 3 the results obtained on sample MA3X after 100 h of milling for the asprepared sample and after preheating at 500 and 600 K. The crystallization exotherm is fairly insensitive to the preheating treatment. As the preheating temperature increases there is, however, a progressive reduction of the low-temperature part of the broad exothermic peak prior to @ X ....
. '\
{
0 ,..a r~
I}
<
I
350
'
I
a50
550
650
T(K)
750~
i !
~a5a_
Fig. 3. DSC curves of FesoB2o after 100 h of milking: (a) asprepared powders; (b) heat-treated at 500 K; (c) heat-treated at 600 K. 0 X .r.q o .-1
• lit
[.. < [M
£:1 v
350
i
450
'
5£o
6£0
75c
s5o
T(K)
Fig. 2. Typical powders of FesoB20 produced by ball milling for 350 h.
Fig. 4. DSC curves of Fe~oB2o after various milling times:
(a) 100 h; (b) 200 h.
1370
crystallization. This broad exothermic peak can be eliminated by heating the sample up to 675 K. T h e influence of long milling time on sample MA3X, as it results in the DSC curves, is shown in Fig. 4. After a milling time of 200 h, sample MA3X shows a clear endothermic peak superimposed on the broad exothermic peak already mentioned. We found no reference to such an endothermic effect after long milling times in M A but exothermal events have been reported [8]. The appearance of such a peak after long milling time is not exclusive to the Fe-B alloys. Effectively, the sample M A l l X (Fe-B-Si) also exhibits this behaviour after long milling times. Figure 5 shows the DSC curves of sample M A 1 1 X after milling times of 200, 350, 450 h. The endothermic peak is already present after 350 h milling and increases with milling time. The sequence of heat treatment necessary to eliminate both the exo- and endothermic effects is seen in Fig. 6. Curve a in Fig. 6 shows the behaviour of sample M A 1 1 X after 350 h of milling. Curve b corresponds to the same sample preheated previously at 525 K. The low-temperature part ( < 525 K) of
a
'
.<
v
350
450
550
650
T(K)
750
850
Fig. 5. DSC curves of Fe75BisSi~0 after various milling times: (a) 200 h; (b) 350 h; (c) 450 h.
\
/
the broad exothermic peak disappears with this heat treatment, but the endothermic peak remains unchanged. Curve c shows the DSC trace of the same sample after annealing for 14 h at 500 K. Curve d is obtained after preheating at 625 K. Both heat treatments completely eliminate the endothermic peak and partially eliminate the broad exothermic peak. Preheating at 775 K (curve e) is sufficient to erase the precrystallization effect. This behaviour indicates that the endothermic peak is independent of the exothermic one. To clarify the nature of the endothermic effect, the activation energies involved in both the endothermic effect and the crystallization process were evaluated by the peak method. The results are presented in Table 1. The activation energies for the endothermic processes are significantly lower than those of crystallization, the latter being comparable with the known values of the apparent activation energy of diffusion [9]. Therefore, diffusion is not involved in the endothermic process. Also, the enthalpy changes related to the endothermic peaks after 350 h milling on samples MA3X and M A l l X are 5 calg -1 and 9 cal g- ~, respectively. For comparison, the total enthalpy of crystallization enthalpies after 350 h milling are 26.0 cal g- ~ and 25.2 cal g- ~ on samples MA3X and MA11X, respectively. Further studies are in progress to elucidate the nature of the endothermic peak observed. The crystallization of the M A Fe-B-Si powders occurs at lower temperature than that of the melt-spun ribbons, but the activation energy is the same [10]. Curve a in Fig. 7 corresponds to M A 1 1 X after 350 h of milling and preheating at 775 K; curve b corresponds to a ribbon of the same nominal composition which was identified as amorphous by X-ray diffraction. The enthalpy of crystallization measured for MA11X is 25.2 cal g- 1, while that for the melt-spun ribbon is 34
/I
0
0
a o
M m v
0
/
!
v
500
T(K) Fig. 6. DSC curves of FeT~B15Si10 after 350 h milling: (a) asprepared; (b) heat-treated at 525K; (c) annealed 14h at 500 K; (d) heat-treated at 625 K; (e) heat-treated at 775 K.
6&o
7&o T(K)
800
900
Fig. 7. DSC curves of Fe75BtsSilo: (a) powders after 350h milling and preheating at 775 K; (b) amorphous ribbons obtained by melt-spinning.
1371
cal g ~. Therefore, we can estimate from these results that after 350 h of milling the amorphization has almost reached 74% of the sample. Of course, it is not excluded that during heating the milled sample in the DSC further amorphization occurred due to solid-state interdiffusion.
4. Conclusions The progress on the amorphization for several MA powders of Fe-B and Fe-B-Si was analyzed by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. The amorphous fraction after 350 h milling is typically - 7 5 % . Long milling times give rise to an endothermic effect, on subsequent heating, whose activation energy is ~ 1/3 of the activation energy of diffusion and whose change in enthalpy is 1/4 of the crystallization enthalpy. These results show that diffusion is not involved in the endothermic process but further studies are needed to obtain a definitive conclusion.
Acknowledgments This work was supported by the Comisi6n Interministerial de Ciencia y Tecnologfa
(CICYT) project MA88-439. The authors wish to thank Mr. X. Alcob6 of the "Serveis CientificoTecnics" of the University of Barcelona for the X-ray diffraction measurements.
References 1 Proc. 4th Int. ¢'onf on Rapidl_v Quenched Metals, Sendal, 1081, eds. T. Masumoto and K. Suzuki (Japan Inst. of Metals, 1982). 2 Proc. 5th Int. ('onj: on Rapidly Quenched Metals, Wiirzburg, 196'4, eds. S. Steeb and H. Warlimont (NorthHolland, Amsterdam, 1985 I. 3 Proe. 6th Int. Conf. on Rapidly Quenched Metals, Montreal, 198Z eds. R. W. Cochrane and J. O. Str6mOlsen. Mater. Sei. Eng.. 97-99 (I t)88). 4 Proc. 6th Int. ('onfi on Liquid and AmorlJhous Metal,s, Garmiseh-Partenkirchen 198'& eds. W. Gl~ser, F. Helsel and E. Liischer, Z. Phys. ('hem. N. t£, 156 157 (1988). 5 P. S. Gilman and J. S. Benjamin, Ann. Rev. Maler. A'ci., 13 t 1983') 279. 6 L. Schultz, in E. A~-ztand L. Schultz (eds.). New Malerial,s by Mechanical Alloying Techniques, DGM Informationsgesellschaft-Verlag, Oberursel, 1989, p. 53. 7 K. Narita and T. Sugimoto, A n a l Fis., in press. 8 G. Cocco, S. Enzo, L, Schiffini and L. Battezzati, in ref. 6, p. 343. 9 U. K~';ster and U. Hcrold, in H.-J. Giintherodl and H. Beck (eds.), Glas,sy Metals, Vol. 1, Springer. Berlin. 1981, p.225. 10 S. Surifiach. M. D. Bar6 and N. Clavaguera, in rcf. 4, Vol. 157, p. 395.
Materials Science and Engineering, A134 ( 1991 ) 1368-1371
Amorphization of soft magnetic alloys by the mechanical alloying technique S. Surifiach, M. D. Bar6, J. Segura and M. T. Clavaguera-Mora Fisica de Materials, Departament de Fisica, Universitat Aut6noma de Barcelona, 08193-Bellaterra (Spain
N. Clavaguera Departament Estructura i Constituents de la Mat~ria, Facultat de Fisica, Diagonal 647, Universitat de Barcelona, 08028-Barcelona (Spain)
Abstract The progress of amorphization by mechanical alloying on F e - B and F e - B - S i powders is studied by X-ray diffraction and differential scanning calorimetry. A p a r t from the well known broad exothermic effect, a well defined endothermic effect is present after milling times typically of 300 h. The enthalpy and activation energy of this endothermic peak are obtained and related to the crystallization ones. The results exclude diffusion as involved in the endothermic process.
1. Introduction At present, there is considerable interest in the formation of amorphous phases in soft magnetic alloy systems, because of their potential properties and processing advantages. Amorphous soft magnetic materials have traditionally been prepared by the rapid solidification of molten alloys (see refs. 1-4). Recently, new synthesis methods have been developed which are based on solid-state reaction between pure elements or compounds [5, 6]. In this paper the progress on the amorphization by mechanical alloying (MA) on Fe-B and Fe-B-Si powders will be presented as observed by X-ray diffraction, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). It will be compared with the behaviour of
the alloys prepared by rapid solidification (meltspinning) of the melts. 2. Experimental details In Table 1 are shown the nominal composition and precursors used for the two alloys studied. The Fe-B-Si composition was also obtained in ribbon form by melt-spinning. The powders were ball-milled using the sets of the vial and balls made of stainless CrNi steel in a planetary type mill (Fritsch P-5). The ball to powder ratio was 5:1. The MA powders were characterized and their thermal stability was tested by studying their heating behaviour in a computerized differential scanning calorimeter, Perkin-Elmer DSC II. The microstructural evolution during MA was followed by SEM.
TABLE 1 Nominal composition and precursors used in the mechanical alloying process and activation energies, E, of the endothermic peaks (after 350 h milling) and the crystallization peak Sample
Nominal composition
Precursors
E (eV) Endotherm
MA3X
FesoB20
Fe + FeB < 200/zm
1.5
MA11X
Fe75B~sSi10
Fe + B + Si < 200/zm
0.9
0921-5093/91/$3.50
Crystallization 4.5 ~ 3.3 (first peak) / 3.9 (second peak)
© Elsevier Sequoia/Printed in The Netherlands
1369
3. Results and discussion
3.1. X-ray diffraction The X-ray diffraction pattern of samples MA3X for selected milling time are presented in Fig. 1. The unmilled powder exhibits sharp diffraction lines. The intensity of the lines decreases and their width increases with milling time. Assuming a microcrystalline structure, the typical crystallite size after 350 h milling would be of 20 A. At long milling time, amorphization is clearly evident from X-ray results. Partial amorphization after a long milling time was also observed for the MAI IX samples. This result is
:
I i I[
in contradiction with those published by Narita and Sugimoto [6, 7]. These authors suggest that after 900 h milling the powder is not yet amorphous because the magnetization does not increase significantly on annealing.
3.2. Microscopy The morphology of the powders with milling time was followed by SEM. The typical lamellar structure found in other systems does not form [6]. A quick growth in size of the powders occurs on milling. Figure 2 shows the powder morphology of samples MA3X after 350 h milling. The mean particle size amounts from 3 to 15 ~m. This value is about 103 times the typical crystallite size determined from the X-ray diffraction pattern. Therefore, amorphization is quite effective after 350 h milling. Further evidence of the amorphization is obtained from DSC.
3.3. Differential scanning calorimetry b~
~'i
~'~
~'~,'~.~ , ~
ci
L~
Fig. 1. X-ray diffraction patterns of Fe~oB2o after different millinIa times• (a) l) h; (b) 10l) h; (c) 200 h; (d) 350 h.
We present in Fig. 3 the results obtained on sample MA3X after 100 h of milling for the asprepared sample and after preheating at 500 and 600 K. The crystallization exotherm is fairly insensitive to the preheating treatment. As the preheating temperature increases there is, however, a progressive reduction of the low-temperature part of the broad exothermic peak prior to @ X ....
. '\
{
0 ,..a r~
I}
<
I
350
'
I
a50
550
650
T(K)
750~
i !
~a5a_
Fig. 3. DSC curves of FesoB2o after 100 h of milking: (a) asprepared powders; (b) heat-treated at 500 K; (c) heat-treated at 600 K. 0 X .r.q o .-1
• lit
[.. < [M
£:1 v
350
i
450
'
5£o
6£0
75c
s5o
T(K)
Fig. 2. Typical powders of FesoB20 produced by ball milling for 350 h.
Fig. 4. DSC curves of Fe~oB2o after various milling times:
(a) 100 h; (b) 200 h.
1370
crystallization. This broad exothermic peak can be eliminated by heating the sample up to 675 K. T h e influence of long milling time on sample MA3X, as it results in the DSC curves, is shown in Fig. 4. After a milling time of 200 h, sample MA3X shows a clear endothermic peak superimposed on the broad exothermic peak already mentioned. We found no reference to such an endothermic effect after long milling times in M A but exothermal events have been reported [8]. The appearance of such a peak after long milling time is not exclusive to the Fe-B alloys. Effectively, the sample M A l l X (Fe-B-Si) also exhibits this behaviour after long milling times. Figure 5 shows the DSC curves of sample M A 1 1 X after milling times of 200, 350, 450 h. The endothermic peak is already present after 350 h milling and increases with milling time. The sequence of heat treatment necessary to eliminate both the exo- and endothermic effects is seen in Fig. 6. Curve a in Fig. 6 shows the behaviour of sample M A 1 1 X after 350 h of milling. Curve b corresponds to the same sample preheated previously at 525 K. The low-temperature part ( < 525 K) of
a
'
.<
v
350
450
550
650
T(K)
750
850
Fig. 5. DSC curves of Fe75BisSi~0 after various milling times: (a) 200 h; (b) 350 h; (c) 450 h.
\
/
the broad exothermic peak disappears with this heat treatment, but the endothermic peak remains unchanged. Curve c shows the DSC trace of the same sample after annealing for 14 h at 500 K. Curve d is obtained after preheating at 625 K. Both heat treatments completely eliminate the endothermic peak and partially eliminate the broad exothermic peak. Preheating at 775 K (curve e) is sufficient to erase the precrystallization effect. This behaviour indicates that the endothermic peak is independent of the exothermic one. To clarify the nature of the endothermic effect, the activation energies involved in both the endothermic effect and the crystallization process were evaluated by the peak method. The results are presented in Table 1. The activation energies for the endothermic processes are significantly lower than those of crystallization, the latter being comparable with the known values of the apparent activation energy of diffusion [9]. Therefore, diffusion is not involved in the endothermic process. Also, the enthalpy changes related to the endothermic peaks after 350 h milling on samples MA3X and M A l l X are 5 calg -1 and 9 cal g- ~, respectively. For comparison, the total enthalpy of crystallization enthalpies after 350 h milling are 26.0 cal g- ~ and 25.2 cal g- ~ on samples MA3X and MA11X, respectively. Further studies are in progress to elucidate the nature of the endothermic peak observed. The crystallization of the M A Fe-B-Si powders occurs at lower temperature than that of the melt-spun ribbons, but the activation energy is the same [10]. Curve a in Fig. 7 corresponds to M A 1 1 X after 350 h of milling and preheating at 775 K; curve b corresponds to a ribbon of the same nominal composition which was identified as amorphous by X-ray diffraction. The enthalpy of crystallization measured for MA11X is 25.2 cal g- 1, while that for the melt-spun ribbon is 34
/I
0
0
a o
M m v
0
/
!
v
500
T(K) Fig. 6. DSC curves of FeT~B15Si10 after 350 h milling: (a) asprepared; (b) heat-treated at 525K; (c) annealed 14h at 500 K; (d) heat-treated at 625 K; (e) heat-treated at 775 K.
6&o
7&o T(K)
800
900
Fig. 7. DSC curves of Fe75BtsSilo: (a) powders after 350h milling and preheating at 775 K; (b) amorphous ribbons obtained by melt-spinning.
1371
cal g ~. Therefore, we can estimate from these results that after 350 h of milling the amorphization has almost reached 74% of the sample. Of course, it is not excluded that during heating the milled sample in the DSC further amorphization occurred due to solid-state interdiffusion.
4. Conclusions The progress on the amorphization for several MA powders of Fe-B and Fe-B-Si was analyzed by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. The amorphous fraction after 350 h milling is typically - 7 5 % . Long milling times give rise to an endothermic effect, on subsequent heating, whose activation energy is ~ 1/3 of the activation energy of diffusion and whose change in enthalpy is 1/4 of the crystallization enthalpy. These results show that diffusion is not involved in the endothermic process but further studies are needed to obtain a definitive conclusion.
Acknowledgments This work was supported by the Comisi6n Interministerial de Ciencia y Tecnologfa
(CICYT) project MA88-439. The authors wish to thank Mr. X. Alcob6 of the "Serveis CientificoTecnics" of the University of Barcelona for the X-ray diffraction measurements.
References 1 Proc. 4th Int. ¢'onf on Rapidl_v Quenched Metals, Sendal, 1081, eds. T. Masumoto and K. Suzuki (Japan Inst. of Metals, 1982). 2 Proc. 5th Int. ('onj: on Rapidly Quenched Metals, Wiirzburg, 196'4, eds. S. Steeb and H. Warlimont (NorthHolland, Amsterdam, 1985 I. 3 Proe. 6th Int. Conf. on Rapidly Quenched Metals, Montreal, 198Z eds. R. W. Cochrane and J. O. Str6mOlsen. Mater. Sei. Eng.. 97-99 (I t)88). 4 Proc. 6th Int. ('onfi on Liquid and AmorlJhous Metal,s, Garmiseh-Partenkirchen 198'& eds. W. Gl~ser, F. Helsel and E. Liischer, Z. Phys. ('hem. N. t£, 156 157 (1988). 5 P. S. Gilman and J. S. Benjamin, Ann. Rev. Maler. A'ci., 13 t 1983') 279. 6 L. Schultz, in E. A~-ztand L. Schultz (eds.). New Malerial,s by Mechanical Alloying Techniques, DGM Informationsgesellschaft-Verlag, Oberursel, 1989, p. 53. 7 K. Narita and T. Sugimoto, A n a l Fis., in press. 8 G. Cocco, S. Enzo, L, Schiffini and L. Battezzati, in ref. 6, p. 343. 9 U. K~';ster and U. Hcrold, in H.-J. Giintherodl and H. Beck (eds.), Glas,sy Metals, Vol. 1, Springer. Berlin. 1981, p.225. 10 S. Surifiach. M. D. Bar6 and N. Clavaguera, in rcf. 4, Vol. 157, p. 395.