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Microstructure and magnetic domains in sintered NdFeB magnets made by hydrogen decrepitation and conventional techniques
Journal of the Less-Common
Metals, 167( 1990) 153-I 60
153
~ICROSTRU~U~ AND ~G~TI~ DO~INS IN SI~E~D Nd-Fe-B MAGNETS MADE BY HYDROGEN DECREPITATION AND CONVENTIONAL TECHNIQUES 1. S. MOOSA, G. W. JOHNSON and J. NUTTING* ~~~~sio~of MetaI~~~w, School of ~~at~rials, U~i~,ersi~
ofLeeds,Leeds
(U.K.)
(Received June 22, 1990)
Summary The microstructur~s of magnets fabricated by the hydrogen decrepitation process and a conventional route have been compared using transmission electron microscopy. The results reveal that magnets produced by the conventional technique contain particles of neodymium-rich and boron-rich phases in the hard magnetic Nd,Fe,,B grains, whilst this matrix phase is found to be almost free of neodymium-rich particles in the magnets produced by the hydrogen decrepitation route, but the boron-rich particles still exist within the matrix phase. In addition, in both routes the microstructure showed that the Nd,Fe,,B grains have a clean and faultless structure. Magnetic domain walls have been observed within the Nd*Fe,,B phase using Lorentz electron microscopy with no special stage. The result shows that all the domain walls end, and are impeded, at the neodymium-rich regions around the grain boundaries, confirming that HCi in the sintered Nd-Fe-B magnets is nucleation-controlled rather than pinning-controlled inside the matrix phase.
1. Intr~uction In earlier papers the present authors have reported upon the microstructure of Nd-Fe-B permanent magnet alloys. The materials had been produced by two fabrication routes: (i) a conventional casting method followed by crushing and milling to give a powder which was then compacted and sintered to give the finished shape; and (ii} after casting, the alloy was broken into pieces and then reduced to powder by hydrogen decrepitation (HD), which was followed by milling before compaction and sintering as before. In the present investigation the transmission electron microscope (TEM) has been used to augment the earlier work carried out with the scanning electron microscope (SEM), but as before the EDAX system has been used to determine the chemical composition of the phases present in the alloys. *Emeritus Professor, University of Leeds. 0 Elsevier Sequoia/Printed
in The Netherlands
154
The advantage of using the TEM is that much higher resolution can be obtained than with the SEM and hence the possibility arises of assessing more accurately the morphology of the dispersed phases. Also, by using the Lorentz imaging mode in the TEM, it becomes possible to observe directly magnetic domains present in the materials. However, in order to examine the structure in the TEM it is necessary to prepare thin foils, and a description is given of a technique which has been developed successfully for this purpose. 2. Experimental
method
Cast ingots of a Nd,,Fe,,B, alloy were obtained from Johnson Matthey Rare Earth Products (U.K.). Two routes were then used to fabricate magnets as follows. 2.1. Conventional method Lumps of the as-cast ingot were crushed between hard steel plattens under cyclohexane. The crushed lumps were then ball-milled using a planetary ball mill. After magnetic alignment, the particles were compacted and sintered in a high purity argon gas atmosphere. 2.2. Hydrogen decrepitation route
Lumps of the as-cast ingot were exposed to hydrogen at a pressure of 4 bar at room temperature in a previously evacuated stainless steel vessel. The decrepitated hydride powder was placed in a flexible bag, magnetically aligned and isostatically compacted at a pressure of 180-200 MPa. The green compact obtained was sintered at 1030 “C for 1 h and slowly cooled to room temperature. Some specimens were given a post-sintering anneal at 640 “C for 1 h and then rapidly cooled. Details of the methods of producing the magnets by the two routes have been reported previously [ 11. Magnetization of the as-sintered compacts was carried out using a pulse magnetizer prior to the measurement of the magnetic properties. Prior to preparing thin foils, both types of sintered magnets were thermally demagnetized by heating to just above the Curie point (T, = 312 “C). Slices some 0.3-0.5 mm thick were then cut using an Isomet low speed saw. The discs were then thinned further by grinding with 600 grade emery paper and a final smooth surface was obtained with 1200 grade paper to give foils 50-60 pm in thickness. These foils were sandwiched between copper specimen grids having a 1.5 mm hole and were readily handleable to enable the final ion beam thinning to be carried out. The foils so produced were examined in a JEOL 200CX electron microscope fitted with an EDAX system. For high resolution work, a Philips 300 kV instrument was used. 3. Results
Significant structural differences were found between the magnets made by the conventional route and the HD route.
Fig. 1. Microstructure of a conventionally produced magnet: (a) some neodymium-rich particles inside the Nd2Fe,,B grain (magnified 58 x 10”); (b) boron-rich particles in the Nd,Fe,,B phase (magnified 23 x 10”); (c) neodymium-rich particle within the boron-rich grain (magnified 47 x 10”).
3.1. General structural features The microstructure of conventionally produced magnets showed spherical particles within a matrix phase which was identified as Nd,Fe,,B. Using the STEM mode and EDAX, the particles were found to be rich in neodymium, but a few were rich in boron. Weizhong et al. [2] have also identified neodymium-rich particles in the matrix phase of conventionally fabricated magnets. The boron-rich particles could be identified by the presence of structural defects visible within the microstructure. Micrographs illustrating the two structural types are given in Figs. 1(a) and 1(b). There was some evidence that duplex particles were present, consisting primarily of the boron-rich phase in which were embedded small neodymiumrich regions. These particles are illustrated in Fig. l(c). The microstructures of the magnets produced by the HD route showed the presence of some boron-rich particles in the Nd,Fe,,B matrix phase. However, when a HD pressure of 10 bar, or greater, was used, no neodymium-rich particles were found in the matrix. Characteristic dislocation structures within the boronrich phase are shown in Figs. 2(a) and (b). Apart from the interesting dislocation shown in Fig. 2(b), spiral dislocations, dissociated dislocations and stacking fault bands with dislocation nodes were found, as shown in Figs. 3(a), (b), (c) and (d).
Fig. 2. Spherical boron-rich Darticles in the matrix phase of a magnet made by the HD process jmag&cation’: (a) 22 x lo”, (b) 73.X lo”).
Fig. 3. Different types of dislocation within the boron-rich grains ((a)-(c)) and a stacking fault band (d) (magnification:(a)54~ 103,(b)50X 10J,(c)57x IOX,(d) 103X 103).
Other workers have reported similar structural defects [3,4]. The grain boundaries are characterized by the presence of the neodymium-rich phase in the form of films varying from very thin to almost 60 nm thick. In general, the boundary phase widths were more uniform at about 20 run in the case of the magnets prepared by the HD route. Some examples of the different types of boundary films are given in Figs. 4(a), (b) and (c).
Fig. 4. Grain boundary widths for magnets produced by the conventional ((a) and (b)) and the HD ((c) and(d))routes(magnification:(a)gOx 10’,(b) 54X IO”,(c) 106X lo”,(d)45 X 10’).
Fig. 5. Needle-shaped precipitates at the grain boundaries of magnets produced slowly cooled after sintering (magnification: (a) 56 X 1O’, (b) 5 1 X 10”).
by the HD route,
At the grain boundary triple points, large particles of the neodymium-rich phase were present in the magnets prepared by each of the two routes. The microstructure of the magnets which had been slowly cooled after sintering showed needle-shaped precipitates at the grain boundaries (see Figs. 5(a) and (b)). Similar structures have been reported by Sagawa [5] and are believed to be b.c.c. structures with a similar neodymium to iron compositional ratio to that of Nd?Fe,,B. When
Fig. 6. High resolution micrographs with fringe contrast corresponding to c-crystal lattice parameters of: (a) 2.55 nm (undefected region, magnified 470 X 103), (b) 3.25 nm (defected gram, magnified
Fig. 7. Lorentz micrographs showing domain walls in the hard magnetic phase parallel to the easy direction ((a) and (b)) and perpendicular to the easy direction (c). (Magnification: (a) 22X lo”, (h) 30 x lo”, (c) 18 x 10’).
the magnets were given a post-sintering heat treatment at 640 “C for 1 h, the needle-shaped precipitates disappeared and this was accompanied by an increase in the intrinsic coercivity of about 50 kA m- ’ above that of the as-sintered slowly cooled magnets.
159
3.2. Lattice images Using high resolution transmission microscopy, it is possibL to prod& lattice images from the boron-rich phase within the grains. It has been reported by many workers that this phase has a tetragonal structure with different lattice parameters [4, 6, 71. The lattice images obtained in the present work gave two different d-spacing values. Figures 6(a) and (b) show high resolution micrographs with fringe contrast corresponding to c-crystal lattice parameters of 2.55 and 3.25 nm.
3.3. Magnetic domains One of the advantages of using the transmission electron microscope to examine thin foils of ferromagnetic materials is that with the aid of the Lorentz technique it is possible to obtain images indicating the presence of domain walls. In the present study it was found that with an accelerating voltage of 200 kV, magnetic domains in the matrix phase could readily be observed without using a special Lorentz microscopy stage. This was possible because the high coercivity of the alloy meant that the magnetic field of the objective lens did not saturate the specimen. Examples of the domain patterns are given in Figs. 7(a), (b) and (c). It was found that the domain size was much smaller than the gram size, domain widths being found to range from OS-l.2 pm, while the matrix gram size was about 4-7 pm. A further interesting feature was that the domain walls did not cross the grain boundaries as is usual in many ferromagnetic materials, but were isolated from the neighbouring grain by the neodymium-rich layer present at the boundaries.
4. Conclusians The rn~crost~c~res of sintered magnets made by the KD and the cunventionaf routes show significant differences. With the HD route, the grain boundary width is less than that in specimens produced by the conventional process. This is because of the production of very fine neodymium-rich particles during the HD process, which leads to a very uniform distribution of the neodymium-rich phase in the gram boundaries and hence to an improvement in H,i. Also, the spherical neodymium-rich particles within the matrix phase of the cast material disappear because of the reactivity of the neodymium with hydrogen during the HD process. However9 with the conventional technique all the spherical particles remain within the matrix phase, which may lead to a decrease in the volume fraction of the neodymium-rich phase in the grain boundaries, Magnetic domain walls were observed using Lorentz electron microscopy without a special stage. ft is conchrded that all the wafts are pinned at neodymiumrich regions around the grain boundaries, which supports the view that the coercivity in the sintered Nd-Fe-B magnets is nucleation-controlled rather than due to pinning in the matrix phase.
160
Acknowledgments The authors greatly appreciate Professor I. R. Harris of the School of Materials, University of Birmingham, for helpful discussions and the provision of facilities for making the magnetic measurements.
References I. S. Moosa, G. W. Johnson and J. Nutting, J. Less-Common Met, 158 (1990) 5 l-58. T. Weizhong, Z. Shouzeng and W. Run, J. Less-Common Met., 141( 1988) 2 17-223. J. Fidler, IEEE Trans. Magn., 21(1985) 1955-1957. G. C. Hadijpanayis, K. R. Lawless and R. C. Dickerson, J. Appl. Phys., 257( 1985) 4097-4099. 5 M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura, S. Hirosawa and K. Hiraga, in K. J. Strnat (ed.), 1 2 3 4
Proc. 8th Int. Workshop on Rare Earth Magnets and their Applications, and 4th Int. Symp. on Magnetic Anisotropy and Coercivity in Rare Earth Transition Metal Alloys, University of Dayton, Dayton, OH, 1985, pp. 587-6 11. 6 J. Fidler and L. Yang, in K. J. Strnat (ed.), Proc. 8th Int. Workshop on Rare Earth Magnets and their Applications, and 4th Int. Symp. on Magnetic Anisotropy and Coercivity in Rare Earth Transition Metal Alloys, University of Dayton, Dayton, OH, 1985, pp. 647-656. 7 Y. C. Chuang, C. H. Wu, Y. C. Chang and L. Kao, J. Less-Common Met., 147(1989) 113-120.
Metals, 167( 1990) 153-I 60
153
~ICROSTRU~U~ AND ~G~TI~ DO~INS IN SI~E~D Nd-Fe-B MAGNETS MADE BY HYDROGEN DECREPITATION AND CONVENTIONAL TECHNIQUES 1. S. MOOSA, G. W. JOHNSON and J. NUTTING* ~~~~sio~of MetaI~~~w, School of ~~at~rials, U~i~,ersi~
ofLeeds,Leeds
(U.K.)
(Received June 22, 1990)
Summary The microstructur~s of magnets fabricated by the hydrogen decrepitation process and a conventional route have been compared using transmission electron microscopy. The results reveal that magnets produced by the conventional technique contain particles of neodymium-rich and boron-rich phases in the hard magnetic Nd,Fe,,B grains, whilst this matrix phase is found to be almost free of neodymium-rich particles in the magnets produced by the hydrogen decrepitation route, but the boron-rich particles still exist within the matrix phase. In addition, in both routes the microstructure showed that the Nd,Fe,,B grains have a clean and faultless structure. Magnetic domain walls have been observed within the Nd*Fe,,B phase using Lorentz electron microscopy with no special stage. The result shows that all the domain walls end, and are impeded, at the neodymium-rich regions around the grain boundaries, confirming that HCi in the sintered Nd-Fe-B magnets is nucleation-controlled rather than pinning-controlled inside the matrix phase.
1. Intr~uction In earlier papers the present authors have reported upon the microstructure of Nd-Fe-B permanent magnet alloys. The materials had been produced by two fabrication routes: (i) a conventional casting method followed by crushing and milling to give a powder which was then compacted and sintered to give the finished shape; and (ii} after casting, the alloy was broken into pieces and then reduced to powder by hydrogen decrepitation (HD), which was followed by milling before compaction and sintering as before. In the present investigation the transmission electron microscope (TEM) has been used to augment the earlier work carried out with the scanning electron microscope (SEM), but as before the EDAX system has been used to determine the chemical composition of the phases present in the alloys. *Emeritus Professor, University of Leeds. 0 Elsevier Sequoia/Printed
in The Netherlands
154
The advantage of using the TEM is that much higher resolution can be obtained than with the SEM and hence the possibility arises of assessing more accurately the morphology of the dispersed phases. Also, by using the Lorentz imaging mode in the TEM, it becomes possible to observe directly magnetic domains present in the materials. However, in order to examine the structure in the TEM it is necessary to prepare thin foils, and a description is given of a technique which has been developed successfully for this purpose. 2. Experimental
method
Cast ingots of a Nd,,Fe,,B, alloy were obtained from Johnson Matthey Rare Earth Products (U.K.). Two routes were then used to fabricate magnets as follows. 2.1. Conventional method Lumps of the as-cast ingot were crushed between hard steel plattens under cyclohexane. The crushed lumps were then ball-milled using a planetary ball mill. After magnetic alignment, the particles were compacted and sintered in a high purity argon gas atmosphere. 2.2. Hydrogen decrepitation route
Lumps of the as-cast ingot were exposed to hydrogen at a pressure of 4 bar at room temperature in a previously evacuated stainless steel vessel. The decrepitated hydride powder was placed in a flexible bag, magnetically aligned and isostatically compacted at a pressure of 180-200 MPa. The green compact obtained was sintered at 1030 “C for 1 h and slowly cooled to room temperature. Some specimens were given a post-sintering anneal at 640 “C for 1 h and then rapidly cooled. Details of the methods of producing the magnets by the two routes have been reported previously [ 11. Magnetization of the as-sintered compacts was carried out using a pulse magnetizer prior to the measurement of the magnetic properties. Prior to preparing thin foils, both types of sintered magnets were thermally demagnetized by heating to just above the Curie point (T, = 312 “C). Slices some 0.3-0.5 mm thick were then cut using an Isomet low speed saw. The discs were then thinned further by grinding with 600 grade emery paper and a final smooth surface was obtained with 1200 grade paper to give foils 50-60 pm in thickness. These foils were sandwiched between copper specimen grids having a 1.5 mm hole and were readily handleable to enable the final ion beam thinning to be carried out. The foils so produced were examined in a JEOL 200CX electron microscope fitted with an EDAX system. For high resolution work, a Philips 300 kV instrument was used. 3. Results
Significant structural differences were found between the magnets made by the conventional route and the HD route.
Fig. 1. Microstructure of a conventionally produced magnet: (a) some neodymium-rich particles inside the Nd2Fe,,B grain (magnified 58 x 10”); (b) boron-rich particles in the Nd,Fe,,B phase (magnified 23 x 10”); (c) neodymium-rich particle within the boron-rich grain (magnified 47 x 10”).
3.1. General structural features The microstructure of conventionally produced magnets showed spherical particles within a matrix phase which was identified as Nd,Fe,,B. Using the STEM mode and EDAX, the particles were found to be rich in neodymium, but a few were rich in boron. Weizhong et al. [2] have also identified neodymium-rich particles in the matrix phase of conventionally fabricated magnets. The boron-rich particles could be identified by the presence of structural defects visible within the microstructure. Micrographs illustrating the two structural types are given in Figs. 1(a) and 1(b). There was some evidence that duplex particles were present, consisting primarily of the boron-rich phase in which were embedded small neodymiumrich regions. These particles are illustrated in Fig. l(c). The microstructures of the magnets produced by the HD route showed the presence of some boron-rich particles in the Nd,Fe,,B matrix phase. However, when a HD pressure of 10 bar, or greater, was used, no neodymium-rich particles were found in the matrix. Characteristic dislocation structures within the boronrich phase are shown in Figs. 2(a) and (b). Apart from the interesting dislocation shown in Fig. 2(b), spiral dislocations, dissociated dislocations and stacking fault bands with dislocation nodes were found, as shown in Figs. 3(a), (b), (c) and (d).
Fig. 2. Spherical boron-rich Darticles in the matrix phase of a magnet made by the HD process jmag&cation’: (a) 22 x lo”, (b) 73.X lo”).
Fig. 3. Different types of dislocation within the boron-rich grains ((a)-(c)) and a stacking fault band (d) (magnification:(a)54~ 103,(b)50X 10J,(c)57x IOX,(d) 103X 103).
Other workers have reported similar structural defects [3,4]. The grain boundaries are characterized by the presence of the neodymium-rich phase in the form of films varying from very thin to almost 60 nm thick. In general, the boundary phase widths were more uniform at about 20 run in the case of the magnets prepared by the HD route. Some examples of the different types of boundary films are given in Figs. 4(a), (b) and (c).
Fig. 4. Grain boundary widths for magnets produced by the conventional ((a) and (b)) and the HD ((c) and(d))routes(magnification:(a)gOx 10’,(b) 54X IO”,(c) 106X lo”,(d)45 X 10’).
Fig. 5. Needle-shaped precipitates at the grain boundaries of magnets produced slowly cooled after sintering (magnification: (a) 56 X 1O’, (b) 5 1 X 10”).
by the HD route,
At the grain boundary triple points, large particles of the neodymium-rich phase were present in the magnets prepared by each of the two routes. The microstructure of the magnets which had been slowly cooled after sintering showed needle-shaped precipitates at the grain boundaries (see Figs. 5(a) and (b)). Similar structures have been reported by Sagawa [5] and are believed to be b.c.c. structures with a similar neodymium to iron compositional ratio to that of Nd?Fe,,B. When
Fig. 6. High resolution micrographs with fringe contrast corresponding to c-crystal lattice parameters of: (a) 2.55 nm (undefected region, magnified 470 X 103), (b) 3.25 nm (defected gram, magnified
Fig. 7. Lorentz micrographs showing domain walls in the hard magnetic phase parallel to the easy direction ((a) and (b)) and perpendicular to the easy direction (c). (Magnification: (a) 22X lo”, (h) 30 x lo”, (c) 18 x 10’).
the magnets were given a post-sintering heat treatment at 640 “C for 1 h, the needle-shaped precipitates disappeared and this was accompanied by an increase in the intrinsic coercivity of about 50 kA m- ’ above that of the as-sintered slowly cooled magnets.
159
3.2. Lattice images Using high resolution transmission microscopy, it is possibL to prod& lattice images from the boron-rich phase within the grains. It has been reported by many workers that this phase has a tetragonal structure with different lattice parameters [4, 6, 71. The lattice images obtained in the present work gave two different d-spacing values. Figures 6(a) and (b) show high resolution micrographs with fringe contrast corresponding to c-crystal lattice parameters of 2.55 and 3.25 nm.
3.3. Magnetic domains One of the advantages of using the transmission electron microscope to examine thin foils of ferromagnetic materials is that with the aid of the Lorentz technique it is possible to obtain images indicating the presence of domain walls. In the present study it was found that with an accelerating voltage of 200 kV, magnetic domains in the matrix phase could readily be observed without using a special Lorentz microscopy stage. This was possible because the high coercivity of the alloy meant that the magnetic field of the objective lens did not saturate the specimen. Examples of the domain patterns are given in Figs. 7(a), (b) and (c). It was found that the domain size was much smaller than the gram size, domain widths being found to range from OS-l.2 pm, while the matrix gram size was about 4-7 pm. A further interesting feature was that the domain walls did not cross the grain boundaries as is usual in many ferromagnetic materials, but were isolated from the neighbouring grain by the neodymium-rich layer present at the boundaries.
4. Conclusians The rn~crost~c~res of sintered magnets made by the KD and the cunventionaf routes show significant differences. With the HD route, the grain boundary width is less than that in specimens produced by the conventional process. This is because of the production of very fine neodymium-rich particles during the HD process, which leads to a very uniform distribution of the neodymium-rich phase in the gram boundaries and hence to an improvement in H,i. Also, the spherical neodymium-rich particles within the matrix phase of the cast material disappear because of the reactivity of the neodymium with hydrogen during the HD process. However9 with the conventional technique all the spherical particles remain within the matrix phase, which may lead to a decrease in the volume fraction of the neodymium-rich phase in the grain boundaries, Magnetic domain walls were observed using Lorentz electron microscopy without a special stage. ft is conchrded that all the wafts are pinned at neodymiumrich regions around the grain boundaries, which supports the view that the coercivity in the sintered Nd-Fe-B magnets is nucleation-controlled rather than due to pinning in the matrix phase.
160
Acknowledgments The authors greatly appreciate Professor I. R. Harris of the School of Materials, University of Birmingham, for helpful discussions and the provision of facilities for making the magnetic measurements.
References I. S. Moosa, G. W. Johnson and J. Nutting, J. Less-Common Met, 158 (1990) 5 l-58. T. Weizhong, Z. Shouzeng and W. Run, J. Less-Common Met., 141( 1988) 2 17-223. J. Fidler, IEEE Trans. Magn., 21(1985) 1955-1957. G. C. Hadijpanayis, K. R. Lawless and R. C. Dickerson, J. Appl. Phys., 257( 1985) 4097-4099. 5 M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura, S. Hirosawa and K. Hiraga, in K. J. Strnat (ed.), 1 2 3 4
Proc. 8th Int. Workshop on Rare Earth Magnets and their Applications, and 4th Int. Symp. on Magnetic Anisotropy and Coercivity in Rare Earth Transition Metal Alloys, University of Dayton, Dayton, OH, 1985, pp. 587-6 11. 6 J. Fidler and L. Yang, in K. J. Strnat (ed.), Proc. 8th Int. Workshop on Rare Earth Magnets and their Applications, and 4th Int. Symp. on Magnetic Anisotropy and Coercivity in Rare Earth Transition Metal Alloys, University of Dayton, Dayton, OH, 1985, pp. 647-656. 7 Y. C. Chuang, C. H. Wu, Y. C. Chang and L. Kao, J. Less-Common Met., 147(1989) 113-120.