Mechanical Alloying of FeCo Nanocrystalline Magnetic Powders

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1 Journal of ELECTRONIC MATERIALS, Vol. 33, No. 11, 2004 Special Issue Paper Mechanical Alloying of FeCo Nanocrystalline Magnetic Powders H.F. LI 1 and R.V. RAMANUJAN 1,2 1. School of Materials Engineering, Nanyang Technological University, Singapore ramanujan@ntu.edu.sg Mechanical alloying of the Fe 50 Co 50 equiatomic-magnetic alloy from elemental powders has been studied. Two milling speeds of 200 rpm and 300 rpm were used to process these powders. The as-milled powders were characterized using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM). The mixing of Fe and Co was completed in 200 min at a milling speed of 300 rpm; however, an increase in saturation magnetization was observed up to 10-h milling, indicating an increase in compositional homogeneity as a function of milling time. These findings were also reflected in the XRD results. During the milling of Fe and Co at 300 rpm, an increase of powder size was observed after 100-min milling. Further milling at 300 rpm led to a reduction in powder size; the decrease of powder size was more effective when milling was conducted at 200 rpm. This was attributed to a difference in the milling mechanisms dominating at these two speeds. The TEM observation showed that a banded microstructure was observed in the as-milled powders. The banded structure consists of grains, many of which show texture effects. After further milling, the banded structure became finer, then randomly arranged, and finally disappeared. Key words: Mechanical alloying, magnetic material, nanomaterial, transmission electron microscopy (TEM) INTRODUCTION Mechanical alloying is a versatile technique and has been successful to process a variety of commercially useful and scientifically interesting materials, such as intermetallics and amorphous, nanocrystalline, and nanocomposite materials. 1 5 The FeCobased magnetic alloys with compositions of at.% Co in Fe have the highest moment per unit mass or volume of any magnetic material; thus, they have attracted considerable research effort. 6 It is well known that good, soft magnetic properties have been obtained in HiTperm alloys, which contain equiatomic Fe and Co as well as other metalloid elements. 7 The HiTperm alloys were prepared by crystallization from melt-spun amorphous precursors and have a thermally stable microstructure of highdensity nanoprecipitates embedded in amorphous (Received April 7, 2004; accepted June 18, 2004) matrix. Mechanical alloying of FeCo-based alloys from elemental powders can simplify the processing of FeCo-based alloys and lead to the formation of nanostructures; it also has the potential advantage of being amenable to processing of complicated shapes and large-sized electronic devices. Therefore, mechanical alloying of FeCo-based alloys is potentially useful in the processing of magnetic materials. 6,8 Previous investigations have reported mechanically alloyed Fe-Co systems The effect of milling media on the phases obtained in the Fe-Co equiatomic mixture of mechanically alloyed elemental powders was significant, as discussed by Gonzalez et al. 9 Two milling methods, cyclic and conventional operation, were tested to optimize the milling process; and the cyclic milling method was found to be more effective to obtain a smaller-grained structure. 10 The mixing of Fe and Co by ball milling was considered to be the result of Co diffusion into the α-fe while Fe did not diffuse into the ε-co Generally for long enough 1289

2 1290 Li and Ramanujan milling time, high-energy milling could process the nanocrystalline Fe-Co alloy However, the magnetic properties of these nanocrystalline Fe-Co alloys were not soft; the reason was considered to be the internal strain induced by milling. Based on previous work, 9 16 to process FeCo alloys with good, soft magnetic properties, the following points are considered to be important: (a) the microstructure of mechanically alloyed powders and its effects on the magnetic properties; (b) strain relaxation and microstructural evolution after heat treatment; and (c) the effects of elemental additions on microstructures. In this paper, the microstructural evolution of as-milled Fe 50 Co 50 powders will be described. The alloying process and magnetic properties will be discussed. EXPERIMENTAL Starting from elemental powders Fe (99.9%, 22 mesh) and Co (99.9%, 100 mesh), mechanical alloying was conducted in hardened steel vials with hardened steel balls (about 10 mm in diameter) using an F5 planetary ball miller (Fritsch Pulverisette, Germany, model F5). Argon gas was used to minimize the oxidation during milling. The speed was initially set to be 300 rpm; after 5-h milling, two speeds 300 rpm and 200 rpm were selected for further milling and for comparison purposes. Reverse mode after every 5-min milling was used during the milling process to decrease the sticking of powders to the balls and vials and also to make the milling more effective. Four groups of milling were selected. The milling time and speeds are listed in Table I. The weight ratio of balls to powders was set to be 10:1 for the beginning of each group of samples. A small sample of powders was retrieved after milling for different periods of time before reaching the longest milling time. For example, in the first group, elemental powders were filled into the vial with the weight ratio of balls to powder 10:1; after milling for 25 min, a small amount of powders was retrieved, but the remaining powder underwent further milling; then after milling for 50 min, powders were again retrieved. The as-milled powder was then characterized using scanning electron microscopy (SEM, JEOL scanning electron microscope 5410, Japan Electron Optics, Tokyo), x-ray diffraction (XRD, SHIMADZU 6000 Lab X diffractometer, Cu target, Kyoto, Japan), transmission electron microscopy (TEM, JEOL 200 kv transmission electron microscopy), x-ray energy-dispersive spectroscopy (EDX, OXFORD, Oxon, UK), and vibrating sample magnetometry (VSM, vibrating sample magnetometer 736, Lake Shore Cryotronics, Westerville, OH). For the TEM sample preparation, the as-milled powders were mixed with epoxy and filled into a copper tube; then, the copper tube was cut into disks; the disks were then ground slightly, dimpled, and ionmilled. For the magnetic properties testing, the as-milled powders were filled into a small holder (VSM attachment); the magnetic hysteresis loops were obtained under a maximum field of 10 k Oer ( A/m). The powder size was directly determined by measuring the diameter of powders from representative SEM micrographs; generally, a size of more than 200 powders was measured using Analytical Imaging Station image analysis software (Image Research, Inc., Brock University, Canada). RESULTS The morphology of as-milled powders after shorttime milling is shown in Fig. 1. From Fig. 1, a significant increase of powder size after milling for 100 min was observed. Figure 2 shows the elemental mapping of the as-milled powders. The maps of the elements Fe and Co were well separated for powders milled for less than 100 min, but very well correlated for powders milled for 200 min. Figure 3a e shows the morphologies of as-milled powders for long-time milling at a speed of 300 rpm; the morphologies of powders for an initial milling at 300 rpm for 5 h followed by 200-rpm milling are shown in Fig. 3f h. From Fig. 3, it can be seen that there is a decrease in powder size for longer milling time. The shape of powders also appeared to be more equiaxed, although the surface features of the powders did not change significantly. For the powders milled at 200 rpm after 5-h milling at 300 rpm (Fig. 3f h), the decrease in powder size was significant; the powders were nearly equiaxed after 10-h milling at 200 rpm (Fig. 3g). The size distributions of as-milled powders obtained from the measurement of more than 200 powders observed in SEM micrographs are shown in Fig. 4. The mean powder size calculated from the powder-size analysis is listed in Table II. A significant narrowing of the powder-size distribution range was observed for longer milling time; the mean powder size decreased sharply from 5-h to 15-h milling, but decreased slowly on further milling. The phases, as determined using XRD of powders milled for different milling time, are shown in Fig. 5. From Fig. 5, it can be seen when the milling time is less than 100 min, the Fe and Co peaks can be distinguished; but after 200-min milling, only one set of Table I. Milling Parameters Group Composition Milling Speed Milling Time 1 Fe 50 Co rpm 25 min, 50 min, 100 min, 200 min rpm 5 h, 10 h, 15 h, 20 h rpm 30 h, 40 h, 50 h, 70 h rpm (5 h) 200 rpm 5 h, 10 h, 15 h

3 Mechanical Alloying of FeCo Nanocrystalline Magnetic Powders 1291 a b c d e f Fig. 1. Morphologies of short-time as-milled powders. (a) Pure Fe (unmilled), (b) pure Co (unmilled), (c) 25 min., (d) 50 min., (e) 100 min., and (f) 200 min. peaks corresponding to the formation of the FeCo compound was observed. Figure 6a d shows brightfield TEM micrographs of powders as milled at 300 rpm for various milling times. The early stage of alloying shows banded structure formation; for longer milling time, the banded structure became more random and finer. The selected area diffraction pattern (SADP) and dark-field image in Fig. 6e shows that the formation of the FeCo phase occurred in 5 h of milling, and the banded microstructure consisted of small grains showing texture effects. Using the EDX attachment of TEM, the compositions between two neighboring bands were determined and were found to be similar (Table III). The magnetic properties of the as-milled powders determined using VSM are shown in Fig. 7. An increase in magnetization was observed for powders milled at 300 rpm for milling time less than 10 h. Further milling did not change the saturation magnetization much. However, the coercivity appeared to increase significantly with milling time. DISCUSSION The Alloying Process During the high-energy milling of ductile materials, the powders are repeatedly flattened, coldwelded, fractured, and re-welded. 1 The starting elemental powders Fe and Co have different shapes, i.e., Fe powders have an irregular shape (Fig. 1a) and Co have a spheroidal shape (Fig. 1b). After 25-min milling with a speed of 300 rpm (Fig. 1c), it is difficult to distinguish these two elemental powders directly from the SEM morphological observation; the size of the powders also decreased significantly. From Fig. 2a, it is clear that Fe and Co had not yet mixed because they had quite different elemental maps. Thus, at this stage, the cold working and fracture were dominant compared to the alloying. After milling at 300 rpm for 50 min, an increase of powder size was observed, and a high volume fraction of powders with large size (about 20 µm) was observed (Fig. 1d). The elemental mapping in Fig. 2b showed that there was some overlapping in the elemental distribution of Fe and Co, which indicated that the mixing of Fe and Co had proceeded to some extent after 50-min milling. Each individual powder consisted of two regions: one was Fe rich and one was Co rich. After milling at 300 rpm for 100 min (Fig. 1e), most powders had a large size (more than 20 µm), and the surface of the powders seems to be less rough than after 25-min and 50-min milling. The mapping of Fe and Co was quite similar (Fig. 2c); no obvious Fe- or Co-rich region was observed. Therefore, from min, cold welding and fracture were dominant, which led to the mixing and alloying of Fe and Co. After milling at 300 rpm for 200 min, the powder size decreased again; the mapping of Fe and Co was almost the same, which indicated the mixing of Fe and Co was completed for 200-min milling. At this stage of milling, the fracture of powders was dominant. The XRD result of powders, which had undergone short periods of milling (Fig. 5b e), also reflected this alloying process. In Fig. 5b, which shows the XRD result of powders that underwent 25-min milling, the bcc Fe and hcp Co peaks could be distinguished, and the bcc Fe was sharp, but compared to the peaks of the premixing powders, the Fe peaks were relatively

4 1292 Li and Ramanujan Fig. 2. (a) (d) X-ray elemental mapping of short-time as-milled powders. broad for 25-min milling; the intensity of both Fe and Co peaks also decreased significantly. This means that the milling had introduced defects, but the alloying had not yet begun. For 50-min and 100-min further milling at 300 rpm (Fig. 5c and d), broadening of original Fe peaks was observed, and there was a decrease in intensity of both Fe and Co peaks. For the powders that underwent 100-min milling, the Co peaks almost disappeared. A deviation from the Fe peaks was also observed for powders that underwent 100-min and 200-min milling; thus, the XRD results indicated that an alloying process had begun for 100-min and 200-min milling. From the preceding analysis, the alloying process can be simply described as (1) flattening and fracturing of elemental powders; (2) cold-welding and mixing of Fe and Co powders; and (3) homogenization of composition, alloying, and fracture of alloyed powders. The Powder Size For longer periods of milling at 300 rpm, the powder size decreased with the milling time (Fig. 3a e). However, the morphology of the powders did not change much during this reduction of powder size. The powder-size distribution (Fig. 4) indicated an effective reduction of powder size during milling, especially after 10-h and 15-h milling. As listed in Table II, after 5-h milling, the powder size has a wide distribution range from 5 µm to more than 30 µm. After 15-h milling, the powder-size distribution range was much smaller, i.e., from 2 µm to 16 µm; for h milling, the powder-size distribution range became narrower. The mean powder size calculated from the average diameter of the powders shows a decrease in powder size with milling for the milling time 5 70 h (Table II). As in the case of the powder-size distribution range,

5 Mechanical Alloying of FeCo Nanocrystalline Magnetic Powders 1293 Fig. 3. (a) (h) Morphologies of powders as milled for long times. the decrease in mean powder size was significant for 10-h and 15-h milling. The mean powder size was 19.6 µm, 12.3 µm, and 5 µm for 5-h, 10-h, and 15-h milling at 300 rpm, respectively. Further milling at 300 rpm did not decrease the mean powder size much; even up to 70-h milling, the powder size was 3.3 µm. An increase in powder size from h milling was due to the effect of the increase of ball-to-powder weight ratio by retrieving of powders. Thus, the ball-to-weight ratio may also be an important factor for mechanical alloying of Fe and Co. The broadening of the XRD peaks is mainly related to the size of grains within the powders. Because the grain size is of the order of nanometers, its effect on peak broadening obscures the much smaller broadening effect because of the powder size, which is in the micron range. Hence, XRD broadening was not used to measure the powder

6 1294 Li and Ramanujan a Fig. 4. Size distribution of powders as milled at 300 rpm. size; instead, direct measurements were made from SEM graphs as described earlier. The reduction in peak intensity after milling for 20 h may be due to the large strain induced by the increased ballto-powder weight ratio. From this powder size and distribution range analysis, it was concluded that on further milling after alloying, fracture was dominant, and cold welding was not effective. This may due to the fact that the alloyed phase was not ductile. As the powder size is reduced, further fracturing will need higher energy of milling; therefore, the reduction of powder size was not fast in the later stage of milling. Interestingly, the powder size of alloyed powders (5-h milling at 300 rpm) decreased more sharply when milled at 200 rpm than after milling at 300 rpm (Fig. 3e f). After further milling for 5 h at 200 rpm, the powder size was comparable with the powders for 50-h milling at 300 rpm. This greater effectiveness in reducing the powder size is due to the milling mechanism at low milling speed. At low speed, shearing of powders was the dominant milling mechanism, but at high speed, impaction was dominant. Therefore, to obtain smaller powder size and narrow powder-size distribution, low-speed or high-energy milling for a long time can be selected. Microstructural Observations The TEM observation of the microstructural evolution of the powders revealed the refinement of the microstructure caused by long-milling time. For 5-h and Table II. Powder-Size Distribution Range and Mean Powder Size of the As-Milled Powders at 300 rpm for Different Milling Times Milling Size Distribution Mean Times Range Powder Size 5 h 5 30 µm 19.6 µm 10 h 5 25 µm 12.3 µm 15 h 2 16 µm 5 µm 20 h 2 16 µm 4.6 µm 30 h 2 18 µm 6.9 µm 40 h 1 13 µm 5.5 µm 50 h 1 12 µm 4.3 µm 70 h 1 9 µm 3.3 µm b Fig. 5. (a) (l) The XRD results of powders as milled at 300 rpm for different milling times. 10-h milling at 300 rpm (Fig. 6a and b), a coarse banded structure was observed. Initially, the banded microstructure was regular, about 30 nm in wavelength with a length reaching the edge of the powder (Fig. 6a). However, when the powders underwent 20-h milling at 300 rpm (Fig. 6c), break up of the bands was observed; the bands were about 30 nm in wavelength and about 100 nm in length. These smaller bands were more randomly arranged than the initial bands of the powders milled for 5 h and 10 h. For 70-h milling (Fig. 6d), the banded microstructure was not observed. The break up of the banded structure was attributed to the introduction of more energy and defects into the powder during milling. From the dark-field image and SADP in Fig. 6e, the banded microstructure consists of small grains; many of which show texture effects. The banded microstructures have been generally observed in the mechanically milled ductile powders, e.g., the Ag-Cu system, and were considered to be a

7 Mechanical Alloying of FeCo Nanocrystalline Magnetic Powders 1295 a b c d e f Fig. 6. The TEM images of powders as milled at 300 rpm for different milling times. (a) 5 h, (b) 10 h, (c) 20 h, (d) 70 h, (e) SADP from a, and (f) Dark field image of a at the condition of e. composite lamellar structure of the constituent elements. 1 However, from the composition analysis of two neighboring bands (Table III), the bands are not composites of individual elements. Although the composition inhomogeneity does exist among the powders tested, neighboring bands have similar compositions. The SADP also indicates that the alloy has formed at this stage of milling. As discussed Table III. Compositions of Neighboring Bands in 5-h As-Milled Powders Determined Using the EDX Attachment in TEM Composition Neighboring Powder Bands Fe (at.%) Co (at.%) 1 A B C A B C A B C D earlier, the alloying has taken place after 50-min milling at 300 rpm, and powder size decreased for further milling after 100 min; the texture was considered to be induced by work hardening or shearing of alloyed powders during high-energy milling. As the powder size decreased, work hardening and shearing needed a greater amount of energy, and the banded structure broke up finally for the random impaction during milling. Textured microstructures were also previously observed in some ball-milled alloy powders, such as Fe-Ta 17 and Al-Mg-Sc; 18 the formation of a lamellar structure was reported to occur because of the cold working processes induced by milling, and it was an intermediate process in the refinement of microstructures. 18 The XRD results of 5 70-h milling at 300 rpm (Fig. 5) did not show any phase transformation during these periods of milling. Further, no obvious difference in peak broadening was observed. The XRD results can be correlated to TEM microstructural observations: when the banded structure was coarse, it did not consist of one grain but rather a number of grains with small orientation difference; when the banded structure became fine and randomly arranged as in Fig. 6c, refinement of the banded structure played a role in peak broadening.

8 1296 Li and Ramanujan Fig. 7. Saturation magnetization and coercivity of powders as milled at 300 rpm for different milling times. However, when the banded structure disappeared, grain boundaries led to peak broadening. Thus, the peak was relatively broad compared to the starting material, but no further change in broadening occurred at later times. Meanwhile, an increase in relative peak intensity of 15-h, 30-h, 40-h, and 70-h as-milled powders can be observed in XRD results, compared to 5-h and 10-h as-milled powders. This indicated that the lattice mismatch was reduced when milling was conducted after alloying of Fe and Co powders. Magnetic Properties The magnetic properties of the as-milled powders showed an increase in saturation magnetization; there was also a significant increase in coercivity with milling time (Fig. 7). For the premixed powders, the saturation magnetization was Am 2 /kg, which was very close to the weighted average of the saturation magnetization of pure Fe and Co powders. A small decrease of magnetization for powders milled for 25 min was due to the inhomogeneity in composition. A small increase was observed in the saturation magnetization for the powders milled for 50 min and 100 min. This indicated that alloying has occurred on the surface where Fe and Co impinged although they had not yet completely mixed. But from 100-min to 10-h milling, a large increase in saturation magnetization was observed, especially from 100-min to 200-min milling time; this reflected a true alloying process. The saturation magnetization reached its highest value for 10-h milling, which was 215 Am 2 /kg. This value is very close to the value reported by Lee et al. 11 The saturation magnetization did not vary much for h milling. From this point of view, the homogenization of the alloy compound was not completed for 200-min milling; although from the EDX elemental mapping, the two elements Fe and Co appeared to mix completely, the time required for the homogenization was 10 h of milling at 300 rpm. From the saturation-magnetization data and analysis, the alloying process can be further confirmed to be: premixing (flattening and fracture of elemental powders, 25-min milling); mixing and alloying ( min); and composition homogenization (after 5-h milling). As in Refs. 10 and 12 16, because of the large strains incorporated into the powders during milling, the as-milled powders have much larger coercivity compared to the commercial FeCo magnetic alloys (Fig. 7). For longer milling time, the coercivity increased significantly. This was because more defects, surface oxidation, and contamination were induced for longer milling time. These undesirable introductions were more serious when the ball-to-powder weight ratio was higher, as reflected by the abnormal increase of coercivity for 200-min milling and decrease for h milling. Meanwhile, the observations of the microstructures of the as-milled powders showed that large coercivity could also be caused by the formation of banded structures, which consists of grains showing texture effects. It is well known that the well-controlled texture in the cold-rolled silicon steel can lead to very low coercivity. However, the banded structure in these as-milled powders did not decrease the coercivity; on the contrary, from the Herzer mode, 19 at this size range, the coercivity may reach a maximum. From Fig. 7, it can be also seen that the coercivity decreased when the banded structure disappeared for 70-h milling. Therefore, to process the magnetic, soft FeCo alloy, the banded microstructure should be avoided by long-time milling. CONCLUSIONS Mechanical alloying from elemental Fe and Co powders was investigated by characterization of the as-milled powders using SEM, EDX mapping, XRD, TEM, and VSM. The mixing of Fe and Co powders was completed after 200-min milling at 300 rpm with an initial increase in powder size for 50-min and 100-min milling. The XRD results of the as-milled powder showed a deviation from the original bcc Fe peak, which was considered to be due to the alloying. The powder-size distribution range became narrower after milling times longer than 5 h at 300 rpm; the mean powder size also decreased. The reduction of powder size and narrowing of the powder-size distribution range were more effective when milled at a lower speed of 200 rpm. After only 5-h further milling at 200 rpm, the powder size was found to be comparable with 50-h milling at 300 rpm. This was considered to be due to the different mechanisms of ball milling at the two different milling speeds. A banded microstructure about 30 nm in wavelength was observed for powders milled at 300 rpm for 5 h and 10 h. The banded structure broke up on the further milling and disappeared after 70-h milling. This banded structure consisted of grains showing texture effects. The saturation magnetization of as-milled powders indicated a true alloying process. A significant increase in the saturation magnetization

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