Mechanical alloying of Co-Si-B based amorphous magnetic alloys

Transcription

1 Intl. Conf. on Materials Processing for Properties and Performance, vol. 3, K.A. Khor, R.V. Ramanujan, C.P. Ooi and J.H. Zhao (eds.), Singapore, p (2004). Mechanical alloying of Co-Si-B based amorphous magnetic alloys H.F. Li and R.V. Ramanujan * School of Materials Engg., Nanyang Technological University Singapore , Singapore * ramanujan@ntu.edu.sg ABSTRACT Mechanical alloying (MA) is potentially useful in the processing of amorphous soft magnetic alloys. MA of multi-component amorphous Co 65 Si 15 B 14 Fe 4 Ni 2 alloy from elemental powders was investigated in this work. The as milled powders were characterized using SEM, XRD, DSC and VSM. The alloying process was rapid. XRD and DSC results indicated an amorphous phase formation after the milling of the elemental powders. Amorphization occurred directly from the reaction at the interface of metal and metalloid elemental powders. Dynamic heating induced crystallization events at temperatures of 657 o C, 537 o C, 524 o C and 511 o C for the 150 min, 20 h, 40 h and 80 h milled powders respectively. The early formed amorphous phase was inferred to be boron rich from its high crystallization temperature. The saturation magnetization of the as milled powders decreased continuously with increase of milling time, indicating a continuous transformation process. The large stress in the 150 min as milled powders led to a peak of coercivity in the Hc v/s milling time curve. Due to the heterogeneous crystallization induced by mechanical milling, further high energy milling after 20 h was not suitable to prepare homogenous amorphous phase. Annealing of the as milled powders cause deterioration of the soft magnetic properties due to the crystallization, except for the 350 o C and 400 o C annealing of 20 h milled powders. The properties of the MA amorphous Co 65 Si 15 B 14 Fe 4 Ni 2 alloy were comparable to those of melt-spun amorphous alloy. Keywords: magnetic properties. mechanical alloying, amorphous magnetic alloy, Co based alloy, crystallization,

2 1. INTRODUCTION Co-Si-B based amorphous magnetic alloys have superior soft magnetic properties due to their near zero magnetostriction characteristics [1-5]. These magnetic alloys are generally prepared by rapid solidification process; they are usually prepared in wire or ribbon form with small dimensions which limits their applications. Mechanical alloying (MA) is a versatile technique in processing nanostructured and amorphous materials [6-7]; it can overcome the limitations of rapid solidification technique in producing large dimension and complex shaped parts. MA also has other advantages such as low cost, materials in a large composition range can be prepared, large scale processing, etc. MA is potentially useful in processing amorphous soft magnetic materials [8]. MA has been carried out to prepare M-Si-B (M=Fe, Co, Ni) amorphous alloy powders [9-13]. The starting materials used in their investigation included annealed amorphous ribbons, crystalline ribbon or elemental powders. In the milling of partially amorphous Fe 78 Si 12 B 10 alloy, Wang [9] reported that an amorphization reaction occurred and the crystal size of primary crystalline a-(fe,si) phase decreased to the nanometer range. The addition of small amount of Ni powders destabilized the initial amorphous matrix, led to the crystallization of the matrix into Fe 2 B and?-(fe, Ni) phase. The high energy milling of Ni 78 Si 10 B 12 amorphous alloy ribbon led to partial or total crystallization of the powders [10]. The crystallization was attributed to the large comminution energy and heating; surface active additions were suggested to obtain fully amorphous alloy powders. Matyja [11] compared the millings from elemental powders and crystallized amorphous ribbons in a large composition range; his investigation showed that nanocrystalline structures was formed after milling of elemental mixtures and Fe-rich crystallized amorphous alloys while the amorphous phase was formed after milling of Co-rich crystallized amorphous alloys. Pekala [12] investigated the effect of milling on the magnetic moment in amorphous and crystallized ribbons as well as mixtures of elemental powders; he showed that milling of amorphous and crystallized ribbons resulted in an increase in magnetic moment while milling of elemental powders led to a decrease. Kaczmarek [13] investigated the milling of crystallized Co 70.3 Si 15 B 10 Fe 4.7 ribbons and obtained similar properties in the as milled and annealed powders of the amorphous and crystallized ribbons; his investigation suggested that the coercivity of the powders increased with the decrease of powder size during prolonged milling. In this work, MA of Co 65 Si 15 B 14 Fe 4 Ni 2 amorphous powders from elemental powders was carried out. The alloying process, magnetic properties of as milled powders and the effect of annealing on the magnetic properties is reported in this paper. A comparison of MA powders to the amorphous and crystallized ribbons is also included. 2. EXPERIMENTAL PROCEDURE The starting material is a mixture of elemental powders Co, Si, B, Fe and Ni. The MA was conducted in a planetary ball miller (Fritsch Pulverisette, model F5) using hardened steel balls and vials. Ar gas was used to protect the powders from oxidation during the milling. The milling

3 parameters are listed in Table 1. For each group of milling, the started ball to powder weight ratio was 10:1; small samples were retrieved in the middle of milling for characterization. For example, in group 1, a small of sample was retrieved after milling for 50min and 100min respectively. The as milled powders were characterized using SEM (scanning electron microscope, JEOL SEM 6360), XRD (X-ray diffractometer, Shimadzu 6000 lab X-ray diffractometer, Cu target?= å), TEM (transmission electron microscope, JEOL 200kV TEM), DSC (differential scanning calorimeter, NETZCH 400DSC, vacuum) and VSM (vibrating sample magnetometer, LakeShore VSM 736). Annealing of the as milled powders was carried out in a vacuum tube furnace (vacuum 10-5 Torr). Selected samples were annealed at 350 o C, 400 o C, 450 o C and 500 o C for 2h. The heating rate was 5 o C/min to reach these temperatures and the samples were furnace cooled after holding at these temperatures for 2h. The magnetic properties of the annealed samples were tested using VSM. 3. RESULTS Fig. 1 shows the powder morphological evolution of the powders with milling time: the mixture of elemental powders first formed large powders, lots of fine powders embedded in each large powder can be clearly observed (Fig. 1 a); after a short time of further milling, the surface of the powders became smooth, the powders were more uniform in size (Fig. 1 b); sharp sheared surfaces can then be observed in some of the powders, at this stage most of the powders had a size less than 10 µm and powder agglomeration occurred (Fig. 1 c); at a later stage of 80h of milling time, the powders were layered and a number of holes were observed in these powders (Fig. 1 d). During the retrieval of the powders, it was found that sticking of the powders to the vials was serious after 20 h of milling. The XRD results of the as milled powders show a broad peak of 2? in the range of 40~50 degree (Fig. 2). The broad crystalline peak corresponding to the Co phase can be also seen at about 44 o of 2? in the XRD result of sample after 5h of milling; after 10 h of milling, no crystalline peak was discernible. Fig. 3 shows the dynamic DSC results of as milled powders at a heating rate of 20 o C/min. For the powders after milling for 150 min, a large exothermic hump was observed during heating; a small exothermic peak at 657 o C could also be seen. There was an exothermic peak at 537 o C, 524 o C and 511 o C respectively for the powders after 20 h, 40 h and 80 h of milling; besides the exothermic peak, a small exothermic hump was observed in the 350 o C~500 o C temperature range. The saturation magnetization M s of as milled powders decreased with milling time; the coercivity Hc of as milled powders increased initially with milling time then decreased (Fig. 4). Hc showed a peak of 150 Oer after 150 min of milling (Fig. 4). After annealing, Ms decreased further and Hc increased considerably (Fig. 5). The Ms decreased with increased annealing temperature for all the samples; for 20h and 80h milled powders Hc showed a maximum after 450 o C annealing for 2h (Fig. 5). 4. DISCUSSION 4.1 The alloying process and amorphization of Co 65 Si 15 B 14 Fe 4 Ni 2.

4 Alloying by mechanical milling can be attributed to the repeatedly fracturing, welding and diffusion during milling of powders [6-7]. For the milling of ductile and brittle materials, the brittle powders broke up into smaller sized powder and trapped in the ductile powders; alloying was induced by short range diffusion after more energy and heat was introduced into the powders through milling. In this multi-component composition Co 65 Si 15 B 14 Fe 4 Ni 2, Co, Fe and Ni elemental powders behaved as ductile material and occluded the fragmented metalloid Si and B elemental powders in the initial stage of milling (Fig. 1 a). In Fig. 1 (a), a lot of fine powders were observed to be agglomerated into large powders. Further reaction was rapid: after just 50 min of further milling, powders with a smooth surface were obtained (Fig. 1 b). This stage mainly included fracture of the work hardened ductile powders, which led to a homogenization of metalloids Si and B in the metal matrix. At this stage, the alloying could initiate at the interface of the metal and the metalloid powders. The later stages of milling led to alloying and homogenization of the alloy. The as milled powders had numbers of holes (Fig. 1 c&d). This may be because that the alloy formed had good ductility but it was difficult to weld. The sharp sheared surface of the powders (Fig. 1 c) reflected the work hardening of the powders induced by milling. The XRD and DSC results indicated amorphous phase formation after milling. As shown in Fig. 2, below the broad peak (corresponding to the Co phase), a large hump can be observed in the 5 h as milled powders, suggesting the formation of the amorphous phase. The broad peak suggested the Co phase was highly faulted at this stage. The amorphization continued under further milling; only broad humps were observed in the XRD results of the powders as milled for 10 h and longer time (Fig. 2). The different behavior of 150min as milled and 20 h, 40 h, 80 h as milled powders during dynamic heating (Fig. 3) also suggested the formation of an amorphous phase: for the 150min as milled powders, a large exothermic hump was observed during the heating process, which corresponded to the stress relaxation process of as milled powders; for the 20 h, 40 h and 80 h as milled powders, exothermic peaks were observed at 537 o C, 524 o C and 511 o C respectively, corresponding to the crystallization of amorphous phase. In the DSC curve of 150 min milled powders, an exothermic peak was also observed at 657 o C, suggesting a crystallization process. The DSC results showed that crystallization temperatures of the as milled powders decreased with an increase of milling time. This can be due to the following reasons: (a) the early formed amorphous phase has a higher crystallization temperature, after the homogenization of the composition, the crystallization temperature decreased; (b) crystallization of the amorphous phase was induced during milling and the crystallized phase behaved as a nucleation site for the crystallization during heating; (c) more stress induced for the longer time of milling aided the crystallization during heating; and (d) contamination by the milling media introduced Fe, C and Cr etc. into the powders, which decreased the crystallization temperature of the amorphous phase formed. Amorphization is easy in this composition Co 65 Si 15 B 14 Fe 4 Ni 2 for milling from elemental powders. Only after milling for 5 h, a large portion of amorphous phase was formed (Fig. 2); even after 150 min, some amorphous phase was detected from DSC results (Fig. 3); after 10 h of milling, only the amorphous phase can be detected in XRD; and in the DSC, sharp exothermic peak

5 corresponding to the crystallization of amorphous was observed in the sample after 20h of milling. Compared to the amorphization process from crystallized amorphous ribbons with similar composition [11, 13], this process was even faster. In Kaczmarek s work 50h of milling was required to complete the reamorphization process of Co 70.3 Si 15 B 10 Fe 4.7 crystalline alloy and in Matyja s work more than 50 h was used to reamorphization of Co 66 Fe 12 Si 9 B 13 alloy. This indicated that the amorphization was not a subsequent reaction of formation crystalline phase from elemental powders but instead a direct process in Co 65 Si 15 B 14 Fe 4 Ni 2. The DSC result of the 150min as milled powders also reflected the direct amorphization process: an exothermic peak corresponding to the crystallization event was observed in the DSC result as well as a large exothermic hump induced by stress relaxation. The early formed amorphous phase is expected to be B rich since the temperature for crystallization into boride phase is usually high [14-15]. From the XRD result (Fig. 3), the center of the hump was at a higher angle for 5 h milled powders than for longer time milled powders, suggesting that the short range coordinated atoms had a smaller distance, the amorphous phase was thus possibly boron rich. This can be attributed to two reasons. One is that boron itself aids in amorphization; the other is that boron can diffusion readily. Boron is also an important element for the amorphization of Fe-Si-B alloys [9, 16-17]. 4.2 Magnetic properties of as milled and annealed powders. The saturation magnetization decreased continuously with the milling time in the as milled powders (Fig. 4), indicating a continuous transformation process. The starting material contains magnetic phases Co, Fe, Ni; the saturation magnetization is a simple weight ratio sum of these magnetic phases in the powder mixture and was calculated to be 132 emu/g. From Fig. 4, the Ms of 50 min as milled powders was close to this value; for the 100min and 150min of milling, the decrease of Ms was small; for longer time of milling, Ms decreased significantly; until after 32 h of milling, Ms was in a slowly decreasing region. The coercivity of the starting powder mixtures was determined by the Hc of Co (since it is the magnetically hardest phase in this mixture) and was about 170 Oer (measured in the same VSM we used for MA powders). It can be seen that this value is much larger than the 50 min and 100 min as milled powders. The decrease of Hc after this short time of milling was due to the phase transformation of hcp Co to fcc Co induced by the milling [18]. The peak Hc after 150 min milling was due to the large strain and stress in the as milled powders. The further milling after 150min of milling caused a sharp decrease of Hc, indicating a true amorphous formation process. It can be concluded that the formation of the amorphous phase reduced the stress; the DSC results also supported this conclusion by comparison the results of 150 min and 20 h milled powders. For longer time of milling than 20 h, the powders stuck to the vials and usually clump of powders formed. These clumps had much larger coercivity (the dashed line connected Hc in Fig. 4) than those powders that did not stick to the vials. This large coercivity was induced by the heterogeneous crystallization of amorphous powders by the vials. Therefore, after 20 h of milling, there was a competition between amorphization and crystallization process, which is undesirable. Annealing did not improve the soft magnetic properties of the as milled powders. In Fig. 5, it can be seen that Ms decreased and Hc increased after annealing, the only exception is the Hc of 20 h

6 milled powders annealed at 350 o C and 400 o C for 2h, which decreased a little compared to the as milled powders. The deterioration of the magnetic properties after annealing was due to the crystallization of the MA amorphous phase. Comparison of the variation of magnetic properties with annealing temperature for 20 h, 40 h and 80 h milled powders showed that the crystallization process was very complicated. For 350 o C annealing of 20 h and 40 h milled powders, it mainly involved stress relaxation. This stress relaxation included a homogenization in composition of as milled alloys; it thus induced a large decrease in Ms and had no significant effect on Hc. 400 o C annealing may have included a crystallization event in 40 h and 80 h milled powders because of the decrease of Ms and increase of Hc; but in 20 h milled powders both Ms and Hc decreased, suggesting a stress relaxation process for annealing at this temperature. For the 450 o C and 500 o C annealing, in all the three times milled powders, crystallization occurred, however, the crystallization products must be different for different time milled powders because of their large difference of Ms and Hc. From the above analysis, it can be concluded that further high energy milling after 20 h did not help the amorphization to obtain good soft magnetic properties. 4.3 Comparison of the MA and melt-spun amorphous phases. From this investigation, it was found that the amorphous phase could be yielded by MA from elemental powders. The amorphous alloy Co 65 Si 15 B 14 Fe 4 Ni 2 prepared by MA can be compared with melt-spun amorphous alloy [19-20]: firstly, the XRD results for amorphous phase prepared by this two technique were similar, both showed a broad hump in the range of 40~50 degree; secondly the crystallization event of amorphous phase prepared by 20 h mechanical milling occurred at a temperature close to melt-spun Co 65 Si 15 B 14 Fe 4 Ni 2 amorphous alloy, both showed sharp exothermic peak; thirdly although the soft magnetic properties was inferior in MA amorphous compared to the melt spun, the Ms of 20h milled powders after 400 o C annealing is close to that of melt spun amorphous alloy. It thus seems that a little shorter time than 20 h of milling has higher opportunity to prepare Co 65 Si 15 B 14 Fe 4 Ni 2 amorphous alloy with similar properties to melt-spun amorphous alloy. The larger coercivity in MA amorphous alloys was mainly due to crystallization induced by mechanical milling and the morphology of MA powders. It has been suggested that overheating [7] and heterogeneous nucleation [13] were the reason of crystallization. It would be better if low energy milling after amorphization could be selected to homogenize the composition and a surface active surfactant could be used to avoid sticking of the powders to the vials. Due to the fine powder morphology and the small holes inside each powder, the domain wall was pinned, thus a large coercivity was observed in MA powders. From the DSC results, it appeared that the stresses did not play an important role in the large coercivity of MA amorphous powders. The stresses could be relaxed for lower temperature annealing before crystallization, for example in the 20 h milled powders, a decrease of Hc was observed after 400 o C annealing for 2 h. The Hc in MA amorphous powders was comparable to the Fe powder and much smaller than the Ni bulk sample, suggesting the prepared powders was magnetically soft. Kollár [21] compacted 4h milled amorphous powders from crystallized Co 70.3 Fe 4.7 Si 10 B 15 melt-spun ribbons and obtained bulk metallic glass with good soft magnetic properties. Bulk metallic

7 glass could be also possibly prepared by compaction of properly mechanical alloyed amorphous alloy powders since compaction of the as milled powders may be able to reduce Hc. 5. CONCLUSION Mechanical alloying of Co 65 Si 15 B 14 Fe 4 Ni 2 from elemental powders yielded an amorphous phase. The as milled powders were characterized using SEM, XRD, DSC and VSM. Annealing was selectively carried out. The conclusions are: (a) Alloying from elemental powders was realized by fracture of metalloid powders, trapping of the metalloid powders in the metal powders and diffusion. (b) The formation of the amorphous phase was by direct reaction of elemental powders, it was not after crystalline phase formation. (c) The early formed amorphous phase was boron rich, which had a higher crystallization temperature and smaller distance of short range order coordination (d) The Ms decreased with longer milling time of the as milled powders, indicating a continuous transformation process. The Hc of the as milled powders as a function of milling time showed a peak, followed by a decrease. The peak Hc corresponded to the large stress in the MA powders; the decrease of Hc after further milling suggested that an amorphous phase formed and the stress was released following amorphization. (e) The magnetic properties of as milled powders deteriorated after annealing; however, annealing of 20 h milled powders at 350 o C and 400 o C decreased Hc. The deterioration of magnetic properties was due to crystallization. (f) The MA and melt-spun amorphous alloys showed a number of similarities, especially in the case of the 20 h mechanical alloyed amorphous alloy. References 1. C. Gomez-polo, E. Pulido, G. Rivero and A. Hernando, J. Appl. Phys. 67(9), (1990). 2. F.E. Luborsky, in Ferromagnetic materials (Vol. 1), E.P. Wohlfarth, ed. (North-Holland Publishing Company, 1980), pp C.K. Kim and R.C. O Handley, Metall. Mater. Trans. A 28A, (1997). 4. M. Coisson, P. Tiberto, F. Vinai, J. Magn. Magn. Mater , (2002). 5. S.H. Lim, Y.S. Choi, T.H. Noh and I.K. Kang, J. Appl. Phys. 75 (10), (1994). 6. J.S. Benjamin, Mater. Sci. Forum 88-90, 1-18 (1992). 7. C. Suryanarayana, Prog. Mater. Sci. 46, (2001). 8. A. Arrott, Nanostruct. Mate. 12, (1999). 9. K.Y. Wang, J.T. Wang, M.X. Quan and W.D. Wei, Mater. Sci. Forum 88-90, (1992). 10. I.A. Tomilin, T.J. Mochalova and S.D. Kaloshkin, Mater. Sci. Forum, 88-90, (1992). 11. H. Matyja, D. Oleszak and J. Latuch, Mater. Sci. Forum 88-90, (1992). 12. M. Pekala, M. Jachimowicz, V.I. Fadeeva, H. Matyja, J. Non-Cryst. Solids 287, (2001).

8 13. W.A. Kaczmarek, A. Calka and B.W. Ninham, IEEE Tran. Magn. 29(6), (1993). 14. L. Fernandez Barquin, J.M. Barandiaran, I. Telleria, J.C. Gomez Sal, Phys. Stat. Sol. (A) 155, (1996). 15. Y. Takahara, H. Matsuda, Mater. Trans., JIM 36(7), (1995). 16. T. Ogasawara, A. Inoue and T. Masumoto, Mater. Sci. Eng. A134, (1991). 17. A.F. Filho, C. Bolfarini, Y. Xu and C.S. Kiminami, Scripta Mater. 42, (2000). 18. J.Y. Huang, Y.K. Wu, H.Q. Ye and K. Lu, Nanostruct. Mater. 6, (1995). 19. H.F. Li, R.V. Ramanujan, Mater. Sci. Eng. A , (2004). 20. H.F. Li, R.V. Ramanujan, Intermetallics 12(7-9), (2004). 21. P. Kollár, J. Bednarcik, S. Roth, H. Grahl, J. Eckert, J. Magn. Magn. Mater. 278(3), (2004). Table 1 Milling parameters. Group Composition (at%) Starting materials Milling speed Milling time 1 300rpm 50, 100, 150min 2 Co 65 Si 15 B 14 Elemental Co, Si, B, Fe 300rpm 5, 10, 20 h 3 Fe 4 Ni 2 and Ni powders 300rpm 32 h, 40 h 4 300rpm 80 h

9 (a) 100min (b) 150min (c) 10h (d) 80h Fig. 1 Morphology of as milled Co65 Si15 B14 Fe4 Ni 2 powders. 150min 20h 40 h 40h 80h Intensity (Arb. Un.) 32 h o exothermal 537 C 20 h o 657 C 10 h o 524 C o 511 C 5h Theta (degree) 60 Fig. 2 XRD results of as milled powders o Temperature ( C) 700 Fig. 3 DSC results of as milled powders.

10 Saturation magnetization (emu/g) Ms Hc Coercivity (Oer) Saturation magnetization (emu/g) h (Ms) 40h (Ms) 80h (Ms) 20h (Hc) 40h (Hc) 80h (Hc) Coercivity (Oer) Milling time (min) Annealing temperature ( o C) Fig. 4 Magnetic properties of as milled powders. Fig. 5 Magnetic properties of annealed powders.