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The role of Nb in the nanocrystallization of amorphous Fe–Si–B–Nb alloys
PII:
Acta mater. Vol. 46, No. 2, pp. 397±404, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 S1359-6454(97)00271-1
THE ROLE OF Nb IN THE NANOCRYSTALLIZATION OF AMORPHOUS Fe±Si±B±Nb ALLOYS T. NAOHARA Department of Materials Science and Engineering, Ehime University, Matsuyama 790, Japan (Received 12 June 1996; accepted 29 July 1997) AbstractÐThe aging behavior of amorphous Fe84 ÿ xSi6B10Nbx alloys was investigated using dierential scanning calorimetry (DSC), transmission electron microscopy, and high-frequency magnetic measurements. A marked change in the peak morphology was observed on the DSC curve for the alloys containing more than 3 at.% Nb. The right side of the ®rst exothermic peak extended to the higher temperature side, while the second exothermic peak was drastically reduced in its intensity. After aging for 3.6 ks in the ®rst exothermic temperature ranges, all the alloys with a Nb content up to 6 at.% were composed of a mixed structure of the a-Fe phase and an amorphous matrix; however, the addition of more than 3 at.% Nb caused a signi®cant grain re®nement of the crystallization-induced a-Fe particles. Furthermore, the lowtemperature aging produced a notable increase in the eective permeability only for the alloys containing more than 3 at.% Nb, even though the amorphous phase remained unchanged without crystallization. Based on these results, the mechanism of nanocrystallization was reasonably explained as being related to a change in the amorphous structure which occurs in the vicinity of 2±3 at.% Nb. # 1998 Acta Metallurgica Inc.
1. INTRODUCTION
As is generally known, the eective permeability (me) of Fe-based amorphous alloys is somewhat lower compared with that of Co-based amorphous alloys [1, 2], probably because of their large magnetostriction [3]. Less attention, therefore, has been paid to the practical use of high-frequency magnetic materials of the amorphous Fe±Si±B alloys, despite the fact that they possess a large saturation magnetization (Bs) greater than 1.50 T [4]. The development of soft magnetic materials with a large Bs as well as a high me is an important subject to meet the growing demands for higher eciency and resultant miniaturization of electronic components [5]; therefore, great eorts have been devoted to improvement in the soft magnetic properties of the amorphous Fe±Si±B alloys. Yoshizawa et al. [6] have recently determined that the crystallization of amorphous Fe±Si±B alloys containing Nb and Cu simultaneously causes the formation of a nanoscale a-Fe phase and a concurrent me enhancement. A number of research studies have already been performed to examine the nanocrystallization behavior of the amorphous Fe±Si±B±Nb±Cu alloys and its eects on the soft magnetic properties. According to previous reports [7, 8], the simultaneous addition of Nb and Cu is necessary for the appearance of the nanoscale a-Fe phase in the amorphous Fe±Si±B alloys. It has been explained that the existence of Cu assists in uniform nucleation of the a-Fe phase, while the addition of Nb suppresses grain growth [9, 10]. 397
Moreover, the reason for such an improvement in the soft magnetic properties has been attributed to the large reduction in the magnetocrystalline anisotropy, resulting from an ultra®ne equiaxed microstructure [11]. However, recent research by the author [12, 13] has revealed that the crystallization-induced a-Fe phase with a grain size of 030 nm appears on aging of an amorphous Cu-free Fe79Si6B10Nb5 alloy, suggesting that the nanocrystallization occurs due solely to addition of Nb. It has also been determined that a marked me enhancement is achieved in this amorphous alloy upon prolonged aging below 623 K, which precedes the nanocrystallization. Taking these data into consideration, a more detailed investigation from dierent viewpoints is indispensable for completely understanding the mechanism of nanocrystallization. In the present work, we have attempted to examine the aging behavior of the amorphous Fe±Si±B±Nb alloys from the viewpoint of the compositional eect of Nb. 2. EXPERIMENTAL PROCEDURE
The specimens used in the present work were Febased alloys containing 6 at.% Si, 10 at.% B, and Nb ranging from 0 to 6 at.%. Mixtures of pure metals and metalloids were melted under an argon atmosphere in an induction furnace. The melts were sucked into a quartz tube with an inner diameter of 5 mm and then allowed to solidify in air. Continuous ribbon-shaped specimens 020 mm thick
398
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
Fig. 1. DSC curves of amorphous Fe84 ÿ xSi6B10Nbx alloys with lower Nb content: (a) X = 0; (b) X = 1; (c) X = 2.
and 2 mm wide were prepared from these master alloys using a melt-spinning apparatus having a steel roller with a diameter of 200 mm. The crystallization behavior upon heating from room temperature to 1073 K was examined using dierential scanning calorimetry (DSC) at a heating rate of 0.17 K/s. Subsequently, the crystallization temperatures (Tp1 and Tp2) were determined from the top temperatures of the ®rst and second exothermic peaks, while the crystallization enthalpies DHTotal (=DH1+DH2) were evaluated from the areas of these exothermic peaks. The rapidly quenched specimens were subjected to two kinds of aging treatments. They were aged for 0.6 Ms at low temperatures below 623 K, and the high-temperature aging was carried out for 3.6 ks at Tp1 and Tp2. The microstructure of the specimens after aging was investigated using transmission electron microscopy (TEM). The me was measured at 1 kHz with a vector impedance analyzer in a driving ®eld of 0.8 Am/m.
Fig. 2. DSC curves of amorphous Fe84 ÿ xSi6B10Nbx alloys with higher Nb content: (a) X = 3; (b) X = 4; (c) X = 5; (d) X = 6.
the other hand, the right side of the ®rst exothermic peak extends to the higher temperature side, accompanying a drastic decrease in the intensity of the second exothermic peak. It is apparent from these results that the peak morphology of the DSC curves changes in the vicinity of 2±3 at.% Nb. Figure 3 shows the compositional dependence of the DH values evaluated from the previously
3. RESULTS
3.1. Thermal analysis Figures 1 and 2 show the DSC curves of the rapidly quenched Fe84 ÿ xSi6B10Nbx alloys containing less than 2 at.% Nb and more than 3 at.% Nb, respectively. In comparing these ®gures, we notice that there is a distinct dierence in the peak morphology of the DSC curves. The former curves exhibit two exothermic peaks with broad and sharp shapes, and they shift to a higher temperature side with increasing Nb content. In the latter curves, on
Fig. 3. Changes in the DHTotal, DH1, and DH2 as a function of Nb content for rapidly quenched Fe84 ÿ xSi6B10Nbx alloys.
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
399
Fig. 4. Bright ®eld electron micrographs and selected area diraction patterns of Fe84 ÿ xSi6B10Nbx alloys aged for 3.6 ks at Tp1 after rapid quenching: (a),(b) X = 0; (c),(d) X = 1; (e),(f) X = 2.
mentioned DSC curves. While the change in DHTotal is presented as a function of Nb content in Fig. 3(a), those in each DH1 and DH2 value are displayed in Fig. 3(b). It is seen in the former that the DHTotal tends to gradually decrease with increasing Nb content up to 4 at.%, and then it slightly increases through the further addition of Nb. In the latter, the DH1 and DH2 values decrease in parallel at a Nb content of below 2 at.%. However, the addition of 3 at.% Nb produces a marked increase in DH1 up to ÿ5.6 kJ/ mol, corresponding to a decrease in DH2 to ÿ0.2 kJ/mol. Moreover, the DH1 and DH2 exhibit symmetrical behavior at Nb contents ranging from 3 to 6 at.%. Consequently, the compositional dependence of the DH1 and DH2 found in the alloys with the lower Nb content is dierent from those with the higher Nb content. 3.2. TEM observations Figures 4 and 5 show the bright ®eld electron micrographs and selected area diraction patterns of the Fe84 ÿ xSi6B10Nbx alloys aged for 3.6 ks at Tp1 after rapid quenching. The microstructure after aging consists of the a-Fe particles and an amorphous matrix for all the alloys; however, a
notable dierence in the a-Fe phase morphology is determined between these ®gures. Their particles possess dendritic morphology with a grain size of 0.2±0.3 mm in the alloys containing less than 2 at.% Nb. As seen in the latter, the aged alloys containing more than 3 at.% Nb have a-Fe particles with spherical morphology, which indicates that the increase in the Nb content causes a drastic grain re®nement. Correspondingly, a number of ring-like diraction spots appear in the selected area diraction patterns of Fig. 5(d), 5(f), and 5(h). The average grain size of the crystallization-induced a-Fe particles reaches an extremely small value of 20 nm at 6 at.% Nb as demonstrated in Fig. 5(g). 3.3. High-frequency magnetic properties Figures 6 and 7 show the changes in me at 1 kHz as a function of aging temperature for the rapidly quenched Fe84 ÿ xSi6B10Nbx alloys with Nb contents of less than 2 at.% and more than 3 at.%, respectively. It is apparent from these ®gures that the dierence in the Nb content considerably aects the aging behavior of me. It is noted in the former that the me behavior is almost the same for all the alloys over a wide temperature range from 423 K to Tp2. As shown by the open circle in
400
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
Fig. 5. Bright ®eld electron micrographs and selected area diraction patterns of Fe84 ÿ xSi6B10Nbx alloys aged for 3.6 ks at Tp1 after rapid quenching : (a),(b) X = 3; (c),(d) X = 4; (e),(f) X = 5; (g),(h) X = 6.
Fig. 6(a) the me value of the Nb-free Fe84Si6B10 alloy is 1800 in the as-quenched state. No appreciable change in me value is observed upon prolonged aging at 423 and 473 K, maintaining the me values of 2200 and 2150, respectively; however, aging at 523 K causes a decrease in me to 250. The me value is as small as 100 after aging over temperature ranges from 573 K to Tp2. In addition, an abrupt decrease in me occurs at higher temperatures between 523 and 573 K for the alloys containing 1 and 2 at.% Nb, as presented in Fig. 6(b) and (c). In the latter, the low-temperature aging produces a peculiar me behavior for the alloys containing more than 3 at.% Nb. The me value of these alloys
is approximately 3000 in the as-quenched state; however, a notable me increase is determined upon prolonged aging below 623 K. It is obvious from Fig. 7(a) that the me value of the Fe81Si6B10Nb3 alloy increases up to 5500 through aging at 573 K. As seen in Fig. 7(b) and 7(c), the aging at 623 K causes a me enhancement, which reaches a large value of 9200 for the Fe80Si6B10Nb4 alloy, while the Fe79Si6B10Nb5 alloy aged at 573 K possesses a maximum me value of 10,000. A steep decrease in me, however, is found to occur on the high-temperature aging of the alloys with higher Nb content. For instance, the me values of the Fe80Si6B10Nb4 alloy aged at Tp1 and Tp2 are at most 1000 and 90, respectively.
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
Fig. 6. Aging temperature dependence of me at 1 kHz in rapidly quenching Fe84 ÿ xSi6B10Nbx alloys: (a) X = 0; (b) X = 1; (c) X = 2. 4. DISCUSSION
4.1. Structural model of the amorphous Fe±Si±B alloys Figure 8(a) shows the compositional region of an amorphous phase formation in the Fe±Si±B ternary alloys [14]. The amorphous phase is obtained by rapid quenching over a wide compositional region, and it can be divided into two ranges possessing dierent amorphous structures as labelled in the ®gure [15]. According to the structural model originally proposed by Dubois et al. [16±18], the amorphous structures of [Range I] and [Range II] are illustrated in Fig. 8(b). Based on the principle that no metalloid atom is allowed to be the nearest neighbor of an other metalloid atom, the [Range I] is composed of both the AFe and AB regions. The former consists of Fe and Si atoms and the latter is occupied by Fe and B atoms. On the other hand, the [Range II] is composed of solely the AB region as a result of the structural change in the amorphous phase, leading to the further dissolution of Si atoms. It is appropriate to consider that new B-free environments are locally created in the AB region, in order to carry on the substitution of Si atoms. Thus, the AB region of [Range II] appears to include the Fe, Si, and B atoms, avoiding the nearest neighbor between the metalloid atoms. Due to recent research by the author [15], it is particularly emphasized that the eects of low-temperature aging on the microstructure and soft magnetic
401
Fig. 7. Aging temperature dependence of me at 1 kHz in rapidly quenching Fe84 ÿ xSi6B10Nbx alloys: (a) X = 3; (b) X = 4; (c) X = 5.
properties of the amorphous Fe88 ÿ xSi12Bx alloys are reasonably explained in terms of the previously mentioned structural model. As marked with an arrow in Fig. 8(a), the alloy composition, Fe84Si6B10,
Fig. 8. Compositional range for amorphous formation (a) and schematic representation of the amorphous structure (b) in Fe±Si±B ternary alloys.
402
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
Fig. 9. Schematic representation of the change in the amorphous structure for rapidly quenched Fe84 ÿ xSi6B10Nbx alloys.
employed in the present study is located near the edge of [Range I]; therefore, this alloy seems to originally possess an amorphous structure consisting of the AFe and AB regions. This strongly suggests that the ®rst and second exothermic reactions found in Fig. 1(a) correspond to the crystallization of the AFe and AB regions, respectively. 4.2. Eect of additional Nb on the amorphous structure The comparison of Fig. 1 with Fig. 2 clearly reveals that the Nb addition has a substantial eect on the peak morphology in the DSC curves. From the data exhibited in Fig. 1, the amorphous structure of the alloys containing 1 and 2 at.% Nb is thought to be essentially the same as that of the Nb-free Fe84Si6B10 alloy. However, a marked change in the peak morphology is observed in Fig. 2. While the ®rst exothermic peak extends to the high temperature side losing its symmetry, the second exothermic peak is drastically reduced in its intensity. As is obvious from Fig. 3(b), the compositional dependence of DH1 and DH2 exhibits such an anomalous behavior that a Nb addition of more than 3 at.% causes an abrupt increase in DH1, accompanying a decrease in DH2. These results allow us to consider that the structural change in the amorphous phase, which is illustrated in Fig. 9, occurs in the vicinity of 2±3 at.% Nb for the rapidly quenched Fe84 ÿ xSi6B10Nbx alloys. As described in the upper part of the ®gure, the metallic Nb atoms simultaneously penetrate into both the AFe and AB regions, because they should secure their sites by substituting those of Fe atoms. The
addition of more than 3 at.% Nb, however, seems to cause a remarkable extension in the AFe region, as illustrated on the right side of Fig. 9 [13]. It is presumed that the preservation of the amorphous structure illustrated on the left side of Fig. 9 becomes impossible with the addition of a large concentration of Nb atoms, probably due to their large size. Furthermore, it is emphasized that the AFe region in the alloys containing more than 3 at.% Nb includes a variety of elements such as Fe, Nb, Si, and B, because most of the B atoms are thought to make a transition into this region, without the nearest neighbor of Si atoms. Thus, the dierence in the peak morphology determined between Figs 1 and 2 is rationally interpreted within the framework of the structural model proposed by Dubois et al. [16±18]. 4.3. Aging behavior of the high-frequency magnetic properties In comparison between Figs 6 and 7, we can see a notable dierence in the aging temperature dependence of me. These results also suggest that the structural change in the amorphous Fe84 ÿ xSi6B10Nbx alloys occurs in the vicinity of 2± 3 at.% Nb, because the dierence in the soft magnetic properties closely re¯ects their amorphous structures. A signi®cant me decrease determined in the former results from formation of a large amount of a-Fe particles, as recently reported by the author [13]; therefore, such thermal instability should originate from the amorphous structure illustrated on the left side of Fig. 9. It is important to notice that the a-Fe phase formation corresponds
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
to the crystallization of the AFe region, suggesting that the alloys containing less than 2 at.% Nb possess an unstable amorphous structure. In the alloys containing more than 3 at.% Nb, the low-temperature aging causes an increase in me, in spite of the fact that the amorphous phase remains unchanged even after prolonged aging for 0.6 Ms. Thus, the high thermal stability and peculiar me behavior of these alloys appear to be associated with the amorphous structure illustrated on the right side of Fig. 9. It is inferred that the addition of a large concentration of Nb atoms results in a marked thermal stabilization of the amorphous phase, because the AFe region is crowded with metallic Fe and Nb atoms as well as metalloid Si and B atoms. It seems likely that the me increase determined upon prolonged aging is attributable to the phase separation to the Fe- and Nb-enriched areas in the AFe region; however, a more detailed investigation is necessary to con®rm the validity of this interpretation. In a previous paper [11], the reason why the formation of the ultra®ne a-Fe particles causes an improvement in soft magnetic properties has been attributed to the large reduction in the magnetocrystalline anisotropy as previously mentioned. However, particular attention should be paid to the results exhibited in Figs 6 and 7 in order to completely understand the mechanism of nanocrystallization. Apparently, the origin of the me enhancement is essentially associated with the change in the atomic con®guration of the amorphous phase. 4.4. Mechanism of the nanocrystalline a-Fe phase formation It is obvious from Figs 4 and 5 that the dierence in the Nb content has a close relationship with the microstructural morphology of the a-Fe particles. As seen in the former, the aged alloys with a Nb content of less than 2 at.% consist of a mixed structure of the a-Fe phase and an amorphous matrix. The grain size of the dendritic a-Fe particles tends to slightly decrease with increasing Nb content. However, Nb addition of more than 3 at.% causes a morphological change in the a-Fe particles. They possess spherical morphology and their average grain size drastically decreases with the higher Nb content. It is concluded from these data that the crystallization-induced a-Fe phase with the ultra®ne grains is obtained even through the sole addition of Nb, suggesting that the existence of a large concentration of Nb atoms plays a key role in the nanocrystallization. Using atom probe ®eld ion microanalysis (APFIM), Hono et al. [9, 10] have recently reported the role of metallic elements in the nanocrystallization process for an amorphous Fe73.5Si13.5B9Nb3Cu1 alloy. According to their postulation, the additional Cu causes a chemical inhomogeneity of the amorphous matrix through cluster formation at the incipient stage of aging,
403
and it results in a signi®cantly increased number of nucleation sites for the a-Fe phase formation. As the aging proceeds to the later stage, the Nb atoms are excluded from the crystallized region and are enriched in the remaining amorphous phase, because they possess little solubility in the a-Fe phase. Thus, the additional Nb has been believed to act so as to stabilize the amorphous phase and concurrently to suppress the grain growth of the a-Fe phase. Their data obtained using APFIM appear to explain the nanocrystallization process in the amorphous Fe±Si±B alloys containing Nb and Cu simultaneously; however, no compositional eect of Nb has been taken into consideration. Based on the present investigation, it should be emphasized that there are two conditions to achieve the nanocrystallization of the amorphous Fe84 ÿ xSi6B10Nbx alloys. The ®rst is to obtain the amorphous structure illustrated on the right side of Fig. 9, through Nb addition of more than 3 at.%. As is presumed from Fig. 5, the second is to pack a large concentration of Nb atoms into the AFe region. The role of Cu in the nanocrystallization of the amorphous Fe±Si±B alloys is discussed elsewhere [19]. The changes in the rate of crystal growth and the frequency of homogeneous nucleation with the degree of supercooling (DT) are illustrated in Fig. 10. In general, a crystal has a larger grain size when the DT value is small leading to a smaller number of nucleation sites. On the contrary, a greater DT causes a marked increase in the frequency of homogeneous nucleation and a resultant smaller grain size, because the mobility of atoms is extremely reduced under such a condition. Thus, the structural change in the amorphous phase illustrated in Fig. 9 is closely related to the dierence in DT, which is indicated by the arrows in Fig. 10. The amorphous structure exhibited on the left side
Fig. 10. Schematic diagram showing the changes in the rate of crystal growth and the frequency of homogeneous nucleation as a function of the degree of supercooling.
404
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
of Fig. 9 is thought to re¯ect a smaller DT; therefore, a small number of a-Fe particles are formed possessing a larger grain size during aging for crystallization. However, the AFe region presented on the right side of Fig. 9 is crowded with a variety of metallic and metalloid elements such as Fe, Nb, Si, and B as previously mentioned. Additionally, a large concentration of Nb atoms having a larger atomic size exist in this region. This amorphous structure seems to be re¯ected in a larger DT, due to the signi®cant diculty in the diusion of Fe and Si atoms. In this case, the aging treatment appears to cause an increased number of homogeneous a-Fe nuclei with increasing Nb content. Hence, it is appropriate to conclude that the formation of the a-Fe phase with nanoscale grains should originate from the amorphous structure illustrated on the right side of Fig. 9.
300, whereas the me value was 1450 after aging for 0.6 Ms at 523 K. On the contrary, the lowtemperature aging produced me enhancement for the latter alloys, and it reached a maximum value of 10,000 on aging the Fe79Si6B10Nb5 alloy for 0.6 Ms at 573 K.
AcknowledgementsÐThe author is grateful to Professor Emeritus T. Masumoto and Professor H. Fujimori of the Institute for Materials Research, Tohoku University, for their valuable discussions and continuous encouragement. Thanks are also due to Professor Emeritus O. Ochi for the generous permission to use the transmission electron microscope at the Advanced Instrumentation Center for Chemical Analysis, Ehime University. This work was supported by an ISIJ Research Promotion Grant from the Iron and Steel Institute of Japan.
5. CONCLUSIONS
REFERENCES
Amorphous Fe84 ÿ xSi6B10Nbx alloys with a Nb content ranging from 0 to 6 at.% were prepared by a rapid quenching technique, for the purpose of clarifying the compositional eects of Nb on the aging behavior and the mechanism of nanocrystallization. The main results obtained using DSC analysis, TEM observations, and high-frequency magnetic measurements are summarized as follows.
1. Hilzinger, H. R., I.E.E.E. Trans. Magn., 1985, 21, 2020. 2. Makino, Y., in Proc. 5th International Conference on Rapidly Quenched Metals, Vol. 2, ed. S. Steeb and H. Warlimont. North-Holland, Amsterdam 1985, p. 1699. 3. Yagielinski, T., Arai, K. I., Tsuya, N., Ohnuma, S. and Masumoto, T., I.E.E.E. Trans. Magn., 1977, 13, 1553. 4. Hasegawa, R. and O'Handley, R. C. J. appl. Phys., 1979, 50, 1551. 5. Hilzinger, H. R., J. Mag. Mag. Mater., 1990, 83, 370. 6. Yoshizawa, Y., Oguma, S. and Yamauchi, K., J. appl. Phys., 1988, 64, 6044. 7. Kataoka, N., Inoue, A., Masumoto, T., Yoshizawa, Y. and Yamauchi, K., Jap. J. appl. Phys., 1989, 28, L 1820. 8. Noh, T. H., Lee, M. B., Kim, H. J. and Kang, I. K., J. appl. Phys., 1990, 67, 5568. 9. Hono, K., Inoue, A. and Sakurai, T., Appl. Phys. Lett., 1991, 58, 2180. 10. Hono, K., Hiraga, K., Wang, Q., Inoue, A. and Sakurai, T., Acta metall. mater., 1992, 40, 2137. 11. Helzer, G, I.E.E.E. Trans. Magn., 1990, 26, 1397. 12. Naohara, T., Appl. Phys. Lett., 1996, 68, 1012. 13. Naohara, T., J. Appl. Phys., 1996, 79, 7926. 14. Inoue, A., Komuro, M. and Masumoto, T., J. Mater. Sci., 1984, 19, 4125. 15. Naohara, T., Metall. Mater. Trans. A, 1996, 27A, 2454. 16. Dubois, J. M. and Le Caer, G., Nucl. Instr. Meth., 1982, 199, 307. 17. Al Bijat, S., Iraldi, R., Dubois, J. M., Le Caer, G. and Tete, C., in Proc. 4th International Conference on Rapidly Quenched Metals, Vol. 1, ed. T. Masumoto and K. Suzuki, The Japan Institute of Metals, Sendai, 1982, p. 375. 18. Dubois, J. M. and Le Caer, G., Acta metall., 1983, 32, 2101. 19. Naohara, T., Metall. Mater. Trans. A, 1996, 27A, 3424.
1. A notable change in the peak morphology of the DSC curves was observed for the alloys containing more than 3 at.% Nb. The right side of the ®rst exothermic peak extended to the higher temperature side, corresponding to the marked decrease in the intensity of the second exothermic peak. As a result of the morphological change in the exothermic peaks, the DH1 and DH2 values exhibited an abrupt increase and decrease, respectively, in the vicinity of 2±3 at.% Nb. 2. Although all the alloys were composed of a mixed structure of a-Fe phase and an amorphous matrix after aging at Tp1 for 3.6 ks, the Nb content aected the morphology of the a-Fe phase. These particles possessed dendritic morphology with a grain size of 0.2±0.3 mm at a Nb content below 2 at.%; however, Nb additions of more than 3 at.% caused the formation of spherical aFe particles, which accompanied the remarkable grain re®nement with increasing Nb content. 3. The aging temperature dependence of me at 1 kHz was dierent between the alloys containing less than 2 at.% Nb and more than 3 at.% Nb. In the Fe82Si6B10Nb2 alloy, for instance, aging for 0.6 Ms at 573 K caused a decrease in me to
Acta mater. Vol. 46, No. 2, pp. 397±404, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 S1359-6454(97)00271-1
THE ROLE OF Nb IN THE NANOCRYSTALLIZATION OF AMORPHOUS Fe±Si±B±Nb ALLOYS T. NAOHARA Department of Materials Science and Engineering, Ehime University, Matsuyama 790, Japan (Received 12 June 1996; accepted 29 July 1997) AbstractÐThe aging behavior of amorphous Fe84 ÿ xSi6B10Nbx alloys was investigated using dierential scanning calorimetry (DSC), transmission electron microscopy, and high-frequency magnetic measurements. A marked change in the peak morphology was observed on the DSC curve for the alloys containing more than 3 at.% Nb. The right side of the ®rst exothermic peak extended to the higher temperature side, while the second exothermic peak was drastically reduced in its intensity. After aging for 3.6 ks in the ®rst exothermic temperature ranges, all the alloys with a Nb content up to 6 at.% were composed of a mixed structure of the a-Fe phase and an amorphous matrix; however, the addition of more than 3 at.% Nb caused a signi®cant grain re®nement of the crystallization-induced a-Fe particles. Furthermore, the lowtemperature aging produced a notable increase in the eective permeability only for the alloys containing more than 3 at.% Nb, even though the amorphous phase remained unchanged without crystallization. Based on these results, the mechanism of nanocrystallization was reasonably explained as being related to a change in the amorphous structure which occurs in the vicinity of 2±3 at.% Nb. # 1998 Acta Metallurgica Inc.
1. INTRODUCTION
As is generally known, the eective permeability (me) of Fe-based amorphous alloys is somewhat lower compared with that of Co-based amorphous alloys [1, 2], probably because of their large magnetostriction [3]. Less attention, therefore, has been paid to the practical use of high-frequency magnetic materials of the amorphous Fe±Si±B alloys, despite the fact that they possess a large saturation magnetization (Bs) greater than 1.50 T [4]. The development of soft magnetic materials with a large Bs as well as a high me is an important subject to meet the growing demands for higher eciency and resultant miniaturization of electronic components [5]; therefore, great eorts have been devoted to improvement in the soft magnetic properties of the amorphous Fe±Si±B alloys. Yoshizawa et al. [6] have recently determined that the crystallization of amorphous Fe±Si±B alloys containing Nb and Cu simultaneously causes the formation of a nanoscale a-Fe phase and a concurrent me enhancement. A number of research studies have already been performed to examine the nanocrystallization behavior of the amorphous Fe±Si±B±Nb±Cu alloys and its eects on the soft magnetic properties. According to previous reports [7, 8], the simultaneous addition of Nb and Cu is necessary for the appearance of the nanoscale a-Fe phase in the amorphous Fe±Si±B alloys. It has been explained that the existence of Cu assists in uniform nucleation of the a-Fe phase, while the addition of Nb suppresses grain growth [9, 10]. 397
Moreover, the reason for such an improvement in the soft magnetic properties has been attributed to the large reduction in the magnetocrystalline anisotropy, resulting from an ultra®ne equiaxed microstructure [11]. However, recent research by the author [12, 13] has revealed that the crystallization-induced a-Fe phase with a grain size of 030 nm appears on aging of an amorphous Cu-free Fe79Si6B10Nb5 alloy, suggesting that the nanocrystallization occurs due solely to addition of Nb. It has also been determined that a marked me enhancement is achieved in this amorphous alloy upon prolonged aging below 623 K, which precedes the nanocrystallization. Taking these data into consideration, a more detailed investigation from dierent viewpoints is indispensable for completely understanding the mechanism of nanocrystallization. In the present work, we have attempted to examine the aging behavior of the amorphous Fe±Si±B±Nb alloys from the viewpoint of the compositional eect of Nb. 2. EXPERIMENTAL PROCEDURE
The specimens used in the present work were Febased alloys containing 6 at.% Si, 10 at.% B, and Nb ranging from 0 to 6 at.%. Mixtures of pure metals and metalloids were melted under an argon atmosphere in an induction furnace. The melts were sucked into a quartz tube with an inner diameter of 5 mm and then allowed to solidify in air. Continuous ribbon-shaped specimens 020 mm thick
398
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
Fig. 1. DSC curves of amorphous Fe84 ÿ xSi6B10Nbx alloys with lower Nb content: (a) X = 0; (b) X = 1; (c) X = 2.
and 2 mm wide were prepared from these master alloys using a melt-spinning apparatus having a steel roller with a diameter of 200 mm. The crystallization behavior upon heating from room temperature to 1073 K was examined using dierential scanning calorimetry (DSC) at a heating rate of 0.17 K/s. Subsequently, the crystallization temperatures (Tp1 and Tp2) were determined from the top temperatures of the ®rst and second exothermic peaks, while the crystallization enthalpies DHTotal (=DH1+DH2) were evaluated from the areas of these exothermic peaks. The rapidly quenched specimens were subjected to two kinds of aging treatments. They were aged for 0.6 Ms at low temperatures below 623 K, and the high-temperature aging was carried out for 3.6 ks at Tp1 and Tp2. The microstructure of the specimens after aging was investigated using transmission electron microscopy (TEM). The me was measured at 1 kHz with a vector impedance analyzer in a driving ®eld of 0.8 Am/m.
Fig. 2. DSC curves of amorphous Fe84 ÿ xSi6B10Nbx alloys with higher Nb content: (a) X = 3; (b) X = 4; (c) X = 5; (d) X = 6.
the other hand, the right side of the ®rst exothermic peak extends to the higher temperature side, accompanying a drastic decrease in the intensity of the second exothermic peak. It is apparent from these results that the peak morphology of the DSC curves changes in the vicinity of 2±3 at.% Nb. Figure 3 shows the compositional dependence of the DH values evaluated from the previously
3. RESULTS
3.1. Thermal analysis Figures 1 and 2 show the DSC curves of the rapidly quenched Fe84 ÿ xSi6B10Nbx alloys containing less than 2 at.% Nb and more than 3 at.% Nb, respectively. In comparing these ®gures, we notice that there is a distinct dierence in the peak morphology of the DSC curves. The former curves exhibit two exothermic peaks with broad and sharp shapes, and they shift to a higher temperature side with increasing Nb content. In the latter curves, on
Fig. 3. Changes in the DHTotal, DH1, and DH2 as a function of Nb content for rapidly quenched Fe84 ÿ xSi6B10Nbx alloys.
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
399
Fig. 4. Bright ®eld electron micrographs and selected area diraction patterns of Fe84 ÿ xSi6B10Nbx alloys aged for 3.6 ks at Tp1 after rapid quenching: (a),(b) X = 0; (c),(d) X = 1; (e),(f) X = 2.
mentioned DSC curves. While the change in DHTotal is presented as a function of Nb content in Fig. 3(a), those in each DH1 and DH2 value are displayed in Fig. 3(b). It is seen in the former that the DHTotal tends to gradually decrease with increasing Nb content up to 4 at.%, and then it slightly increases through the further addition of Nb. In the latter, the DH1 and DH2 values decrease in parallel at a Nb content of below 2 at.%. However, the addition of 3 at.% Nb produces a marked increase in DH1 up to ÿ5.6 kJ/ mol, corresponding to a decrease in DH2 to ÿ0.2 kJ/mol. Moreover, the DH1 and DH2 exhibit symmetrical behavior at Nb contents ranging from 3 to 6 at.%. Consequently, the compositional dependence of the DH1 and DH2 found in the alloys with the lower Nb content is dierent from those with the higher Nb content. 3.2. TEM observations Figures 4 and 5 show the bright ®eld electron micrographs and selected area diraction patterns of the Fe84 ÿ xSi6B10Nbx alloys aged for 3.6 ks at Tp1 after rapid quenching. The microstructure after aging consists of the a-Fe particles and an amorphous matrix for all the alloys; however, a
notable dierence in the a-Fe phase morphology is determined between these ®gures. Their particles possess dendritic morphology with a grain size of 0.2±0.3 mm in the alloys containing less than 2 at.% Nb. As seen in the latter, the aged alloys containing more than 3 at.% Nb have a-Fe particles with spherical morphology, which indicates that the increase in the Nb content causes a drastic grain re®nement. Correspondingly, a number of ring-like diraction spots appear in the selected area diraction patterns of Fig. 5(d), 5(f), and 5(h). The average grain size of the crystallization-induced a-Fe particles reaches an extremely small value of 20 nm at 6 at.% Nb as demonstrated in Fig. 5(g). 3.3. High-frequency magnetic properties Figures 6 and 7 show the changes in me at 1 kHz as a function of aging temperature for the rapidly quenched Fe84 ÿ xSi6B10Nbx alloys with Nb contents of less than 2 at.% and more than 3 at.%, respectively. It is apparent from these ®gures that the dierence in the Nb content considerably aects the aging behavior of me. It is noted in the former that the me behavior is almost the same for all the alloys over a wide temperature range from 423 K to Tp2. As shown by the open circle in
400
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
Fig. 5. Bright ®eld electron micrographs and selected area diraction patterns of Fe84 ÿ xSi6B10Nbx alloys aged for 3.6 ks at Tp1 after rapid quenching : (a),(b) X = 3; (c),(d) X = 4; (e),(f) X = 5; (g),(h) X = 6.
Fig. 6(a) the me value of the Nb-free Fe84Si6B10 alloy is 1800 in the as-quenched state. No appreciable change in me value is observed upon prolonged aging at 423 and 473 K, maintaining the me values of 2200 and 2150, respectively; however, aging at 523 K causes a decrease in me to 250. The me value is as small as 100 after aging over temperature ranges from 573 K to Tp2. In addition, an abrupt decrease in me occurs at higher temperatures between 523 and 573 K for the alloys containing 1 and 2 at.% Nb, as presented in Fig. 6(b) and (c). In the latter, the low-temperature aging produces a peculiar me behavior for the alloys containing more than 3 at.% Nb. The me value of these alloys
is approximately 3000 in the as-quenched state; however, a notable me increase is determined upon prolonged aging below 623 K. It is obvious from Fig. 7(a) that the me value of the Fe81Si6B10Nb3 alloy increases up to 5500 through aging at 573 K. As seen in Fig. 7(b) and 7(c), the aging at 623 K causes a me enhancement, which reaches a large value of 9200 for the Fe80Si6B10Nb4 alloy, while the Fe79Si6B10Nb5 alloy aged at 573 K possesses a maximum me value of 10,000. A steep decrease in me, however, is found to occur on the high-temperature aging of the alloys with higher Nb content. For instance, the me values of the Fe80Si6B10Nb4 alloy aged at Tp1 and Tp2 are at most 1000 and 90, respectively.
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
Fig. 6. Aging temperature dependence of me at 1 kHz in rapidly quenching Fe84 ÿ xSi6B10Nbx alloys: (a) X = 0; (b) X = 1; (c) X = 2. 4. DISCUSSION
4.1. Structural model of the amorphous Fe±Si±B alloys Figure 8(a) shows the compositional region of an amorphous phase formation in the Fe±Si±B ternary alloys [14]. The amorphous phase is obtained by rapid quenching over a wide compositional region, and it can be divided into two ranges possessing dierent amorphous structures as labelled in the ®gure [15]. According to the structural model originally proposed by Dubois et al. [16±18], the amorphous structures of [Range I] and [Range II] are illustrated in Fig. 8(b). Based on the principle that no metalloid atom is allowed to be the nearest neighbor of an other metalloid atom, the [Range I] is composed of both the AFe and AB regions. The former consists of Fe and Si atoms and the latter is occupied by Fe and B atoms. On the other hand, the [Range II] is composed of solely the AB region as a result of the structural change in the amorphous phase, leading to the further dissolution of Si atoms. It is appropriate to consider that new B-free environments are locally created in the AB region, in order to carry on the substitution of Si atoms. Thus, the AB region of [Range II] appears to include the Fe, Si, and B atoms, avoiding the nearest neighbor between the metalloid atoms. Due to recent research by the author [15], it is particularly emphasized that the eects of low-temperature aging on the microstructure and soft magnetic
401
Fig. 7. Aging temperature dependence of me at 1 kHz in rapidly quenching Fe84 ÿ xSi6B10Nbx alloys: (a) X = 3; (b) X = 4; (c) X = 5.
properties of the amorphous Fe88 ÿ xSi12Bx alloys are reasonably explained in terms of the previously mentioned structural model. As marked with an arrow in Fig. 8(a), the alloy composition, Fe84Si6B10,
Fig. 8. Compositional range for amorphous formation (a) and schematic representation of the amorphous structure (b) in Fe±Si±B ternary alloys.
402
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
Fig. 9. Schematic representation of the change in the amorphous structure for rapidly quenched Fe84 ÿ xSi6B10Nbx alloys.
employed in the present study is located near the edge of [Range I]; therefore, this alloy seems to originally possess an amorphous structure consisting of the AFe and AB regions. This strongly suggests that the ®rst and second exothermic reactions found in Fig. 1(a) correspond to the crystallization of the AFe and AB regions, respectively. 4.2. Eect of additional Nb on the amorphous structure The comparison of Fig. 1 with Fig. 2 clearly reveals that the Nb addition has a substantial eect on the peak morphology in the DSC curves. From the data exhibited in Fig. 1, the amorphous structure of the alloys containing 1 and 2 at.% Nb is thought to be essentially the same as that of the Nb-free Fe84Si6B10 alloy. However, a marked change in the peak morphology is observed in Fig. 2. While the ®rst exothermic peak extends to the high temperature side losing its symmetry, the second exothermic peak is drastically reduced in its intensity. As is obvious from Fig. 3(b), the compositional dependence of DH1 and DH2 exhibits such an anomalous behavior that a Nb addition of more than 3 at.% causes an abrupt increase in DH1, accompanying a decrease in DH2. These results allow us to consider that the structural change in the amorphous phase, which is illustrated in Fig. 9, occurs in the vicinity of 2±3 at.% Nb for the rapidly quenched Fe84 ÿ xSi6B10Nbx alloys. As described in the upper part of the ®gure, the metallic Nb atoms simultaneously penetrate into both the AFe and AB regions, because they should secure their sites by substituting those of Fe atoms. The
addition of more than 3 at.% Nb, however, seems to cause a remarkable extension in the AFe region, as illustrated on the right side of Fig. 9 [13]. It is presumed that the preservation of the amorphous structure illustrated on the left side of Fig. 9 becomes impossible with the addition of a large concentration of Nb atoms, probably due to their large size. Furthermore, it is emphasized that the AFe region in the alloys containing more than 3 at.% Nb includes a variety of elements such as Fe, Nb, Si, and B, because most of the B atoms are thought to make a transition into this region, without the nearest neighbor of Si atoms. Thus, the dierence in the peak morphology determined between Figs 1 and 2 is rationally interpreted within the framework of the structural model proposed by Dubois et al. [16±18]. 4.3. Aging behavior of the high-frequency magnetic properties In comparison between Figs 6 and 7, we can see a notable dierence in the aging temperature dependence of me. These results also suggest that the structural change in the amorphous Fe84 ÿ xSi6B10Nbx alloys occurs in the vicinity of 2± 3 at.% Nb, because the dierence in the soft magnetic properties closely re¯ects their amorphous structures. A signi®cant me decrease determined in the former results from formation of a large amount of a-Fe particles, as recently reported by the author [13]; therefore, such thermal instability should originate from the amorphous structure illustrated on the left side of Fig. 9. It is important to notice that the a-Fe phase formation corresponds
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
to the crystallization of the AFe region, suggesting that the alloys containing less than 2 at.% Nb possess an unstable amorphous structure. In the alloys containing more than 3 at.% Nb, the low-temperature aging causes an increase in me, in spite of the fact that the amorphous phase remains unchanged even after prolonged aging for 0.6 Ms. Thus, the high thermal stability and peculiar me behavior of these alloys appear to be associated with the amorphous structure illustrated on the right side of Fig. 9. It is inferred that the addition of a large concentration of Nb atoms results in a marked thermal stabilization of the amorphous phase, because the AFe region is crowded with metallic Fe and Nb atoms as well as metalloid Si and B atoms. It seems likely that the me increase determined upon prolonged aging is attributable to the phase separation to the Fe- and Nb-enriched areas in the AFe region; however, a more detailed investigation is necessary to con®rm the validity of this interpretation. In a previous paper [11], the reason why the formation of the ultra®ne a-Fe particles causes an improvement in soft magnetic properties has been attributed to the large reduction in the magnetocrystalline anisotropy as previously mentioned. However, particular attention should be paid to the results exhibited in Figs 6 and 7 in order to completely understand the mechanism of nanocrystallization. Apparently, the origin of the me enhancement is essentially associated with the change in the atomic con®guration of the amorphous phase. 4.4. Mechanism of the nanocrystalline a-Fe phase formation It is obvious from Figs 4 and 5 that the dierence in the Nb content has a close relationship with the microstructural morphology of the a-Fe particles. As seen in the former, the aged alloys with a Nb content of less than 2 at.% consist of a mixed structure of the a-Fe phase and an amorphous matrix. The grain size of the dendritic a-Fe particles tends to slightly decrease with increasing Nb content. However, Nb addition of more than 3 at.% causes a morphological change in the a-Fe particles. They possess spherical morphology and their average grain size drastically decreases with the higher Nb content. It is concluded from these data that the crystallization-induced a-Fe phase with the ultra®ne grains is obtained even through the sole addition of Nb, suggesting that the existence of a large concentration of Nb atoms plays a key role in the nanocrystallization. Using atom probe ®eld ion microanalysis (APFIM), Hono et al. [9, 10] have recently reported the role of metallic elements in the nanocrystallization process for an amorphous Fe73.5Si13.5B9Nb3Cu1 alloy. According to their postulation, the additional Cu causes a chemical inhomogeneity of the amorphous matrix through cluster formation at the incipient stage of aging,
403
and it results in a signi®cantly increased number of nucleation sites for the a-Fe phase formation. As the aging proceeds to the later stage, the Nb atoms are excluded from the crystallized region and are enriched in the remaining amorphous phase, because they possess little solubility in the a-Fe phase. Thus, the additional Nb has been believed to act so as to stabilize the amorphous phase and concurrently to suppress the grain growth of the a-Fe phase. Their data obtained using APFIM appear to explain the nanocrystallization process in the amorphous Fe±Si±B alloys containing Nb and Cu simultaneously; however, no compositional eect of Nb has been taken into consideration. Based on the present investigation, it should be emphasized that there are two conditions to achieve the nanocrystallization of the amorphous Fe84 ÿ xSi6B10Nbx alloys. The ®rst is to obtain the amorphous structure illustrated on the right side of Fig. 9, through Nb addition of more than 3 at.%. As is presumed from Fig. 5, the second is to pack a large concentration of Nb atoms into the AFe region. The role of Cu in the nanocrystallization of the amorphous Fe±Si±B alloys is discussed elsewhere [19]. The changes in the rate of crystal growth and the frequency of homogeneous nucleation with the degree of supercooling (DT) are illustrated in Fig. 10. In general, a crystal has a larger grain size when the DT value is small leading to a smaller number of nucleation sites. On the contrary, a greater DT causes a marked increase in the frequency of homogeneous nucleation and a resultant smaller grain size, because the mobility of atoms is extremely reduced under such a condition. Thus, the structural change in the amorphous phase illustrated in Fig. 9 is closely related to the dierence in DT, which is indicated by the arrows in Fig. 10. The amorphous structure exhibited on the left side
Fig. 10. Schematic diagram showing the changes in the rate of crystal growth and the frequency of homogeneous nucleation as a function of the degree of supercooling.
404
NAOHARA: AMORPHOUS Fe±Si±B±Nb ALLOYS
of Fig. 9 is thought to re¯ect a smaller DT; therefore, a small number of a-Fe particles are formed possessing a larger grain size during aging for crystallization. However, the AFe region presented on the right side of Fig. 9 is crowded with a variety of metallic and metalloid elements such as Fe, Nb, Si, and B as previously mentioned. Additionally, a large concentration of Nb atoms having a larger atomic size exist in this region. This amorphous structure seems to be re¯ected in a larger DT, due to the signi®cant diculty in the diusion of Fe and Si atoms. In this case, the aging treatment appears to cause an increased number of homogeneous a-Fe nuclei with increasing Nb content. Hence, it is appropriate to conclude that the formation of the a-Fe phase with nanoscale grains should originate from the amorphous structure illustrated on the right side of Fig. 9.
300, whereas the me value was 1450 after aging for 0.6 Ms at 523 K. On the contrary, the lowtemperature aging produced me enhancement for the latter alloys, and it reached a maximum value of 10,000 on aging the Fe79Si6B10Nb5 alloy for 0.6 Ms at 573 K.
AcknowledgementsÐThe author is grateful to Professor Emeritus T. Masumoto and Professor H. Fujimori of the Institute for Materials Research, Tohoku University, for their valuable discussions and continuous encouragement. Thanks are also due to Professor Emeritus O. Ochi for the generous permission to use the transmission electron microscope at the Advanced Instrumentation Center for Chemical Analysis, Ehime University. This work was supported by an ISIJ Research Promotion Grant from the Iron and Steel Institute of Japan.
5. CONCLUSIONS
REFERENCES
Amorphous Fe84 ÿ xSi6B10Nbx alloys with a Nb content ranging from 0 to 6 at.% were prepared by a rapid quenching technique, for the purpose of clarifying the compositional eects of Nb on the aging behavior and the mechanism of nanocrystallization. The main results obtained using DSC analysis, TEM observations, and high-frequency magnetic measurements are summarized as follows.
1. Hilzinger, H. R., I.E.E.E. Trans. Magn., 1985, 21, 2020. 2. Makino, Y., in Proc. 5th International Conference on Rapidly Quenched Metals, Vol. 2, ed. S. Steeb and H. Warlimont. North-Holland, Amsterdam 1985, p. 1699. 3. Yagielinski, T., Arai, K. I., Tsuya, N., Ohnuma, S. and Masumoto, T., I.E.E.E. Trans. Magn., 1977, 13, 1553. 4. Hasegawa, R. and O'Handley, R. C. J. appl. Phys., 1979, 50, 1551. 5. Hilzinger, H. R., J. Mag. Mag. Mater., 1990, 83, 370. 6. Yoshizawa, Y., Oguma, S. and Yamauchi, K., J. appl. Phys., 1988, 64, 6044. 7. Kataoka, N., Inoue, A., Masumoto, T., Yoshizawa, Y. and Yamauchi, K., Jap. J. appl. Phys., 1989, 28, L 1820. 8. Noh, T. H., Lee, M. B., Kim, H. J. and Kang, I. K., J. appl. Phys., 1990, 67, 5568. 9. Hono, K., Inoue, A. and Sakurai, T., Appl. Phys. Lett., 1991, 58, 2180. 10. Hono, K., Hiraga, K., Wang, Q., Inoue, A. and Sakurai, T., Acta metall. mater., 1992, 40, 2137. 11. Helzer, G, I.E.E.E. Trans. Magn., 1990, 26, 1397. 12. Naohara, T., Appl. Phys. Lett., 1996, 68, 1012. 13. Naohara, T., J. Appl. Phys., 1996, 79, 7926. 14. Inoue, A., Komuro, M. and Masumoto, T., J. Mater. Sci., 1984, 19, 4125. 15. Naohara, T., Metall. Mater. Trans. A, 1996, 27A, 2454. 16. Dubois, J. M. and Le Caer, G., Nucl. Instr. Meth., 1982, 199, 307. 17. Al Bijat, S., Iraldi, R., Dubois, J. M., Le Caer, G. and Tete, C., in Proc. 4th International Conference on Rapidly Quenched Metals, Vol. 1, ed. T. Masumoto and K. Suzuki, The Japan Institute of Metals, Sendai, 1982, p. 375. 18. Dubois, J. M. and Le Caer, G., Acta metall., 1983, 32, 2101. 19. Naohara, T., Metall. Mater. Trans. A, 1996, 27A, 3424.
1. A notable change in the peak morphology of the DSC curves was observed for the alloys containing more than 3 at.% Nb. The right side of the ®rst exothermic peak extended to the higher temperature side, corresponding to the marked decrease in the intensity of the second exothermic peak. As a result of the morphological change in the exothermic peaks, the DH1 and DH2 values exhibited an abrupt increase and decrease, respectively, in the vicinity of 2±3 at.% Nb. 2. Although all the alloys were composed of a mixed structure of a-Fe phase and an amorphous matrix after aging at Tp1 for 3.6 ks, the Nb content aected the morphology of the a-Fe phase. These particles possessed dendritic morphology with a grain size of 0.2±0.3 mm at a Nb content below 2 at.%; however, Nb additions of more than 3 at.% caused the formation of spherical aFe particles, which accompanied the remarkable grain re®nement with increasing Nb content. 3. The aging temperature dependence of me at 1 kHz was dierent between the alloys containing less than 2 at.% Nb and more than 3 at.% Nb. In the Fe82Si6B10Nb2 alloy, for instance, aging for 0.6 Ms at 573 K caused a decrease in me to