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Microstructure of melt-spun NdFeB magnet
Volume 7, number
MATERIALS
1,2
MICROSTRUCTURE
OF MELT-SPUN
August
LETTERS
1988
NdFeB MAGNET
H.C. HUA, G.Y. WANG, C.H. ZHENG Institute ofMetallurgy,Chinese Academy of Sciences, 865 Chang Ning Road, Shanghai 200050, China
Shanghai
G.X. HUANG,
Q.Z. XU, L.H. WU and S.Y. SHI
Shanghai
Iron and Steel Research
Received
24 May 1988
Institute, 1001 Taihe Road,
Wusong, Shanghai,
China
A HREM investigation of melt-spun Nd,3.5Fe *I 74B4 76 magnet revealed that the optimally quenched Nd,Fe,,B grains and the crystal is nearly perfect. Intergranular phases are not observed in the quenched must be strongly related to the fine and almost perfect NdzFe14B grains.
1. Introduction High magnetic energy NdFeB permanent magnets are produced by either powder metallurgy techniques (sintered magnet) or rapid quenching (meltspun) [ 1,2]. The microstructure and magnetic properties of sintered NdFeB alloys have been investigated, and it has been found that the coercivity may be attributed to domain wall pinning at an intergranular second phase [ 3-6 1. The microstructure of melt-spun alloys is characterized by a very fine grain size [ 7-91, amorphous film and boundary phase surrounding the NdzFel,B grains were also reported [lO,ll]. In this work, transmission electron microscopy and MGssbauer spectroscopy are used to study the microstructure of melt-spun NdFeB alloy.
2. Experiment Ribbons used in this work are 2.2 mm wide and 30 pm thick. Ingots melted in an induction furnace were melt-spun in an argon atmosphere at wheel speeds of 22-25 m/s. The composition of melt-spun ribbon is analysed as Nd13.5Fe8L.74B4.76. Electron transparent foils were prepared directly from ribbon fragment. Samples were ion milled at liquid-nitrogen temperature. The processed samples were examined 0167-577x/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
NdFeB ribbon ribbon.
has fine
High coercivity
in a JEOL 4000EX electron microscope at 400 kV. Magnetic properties were measured by a vibrating sample magnetometer with a maximum magnetizing field of 17 kOe. The ribbon sample was employed as the absorbor of 57Fe Mijssbauer spectroscope. The source was 57Co (Pd) and kept at room temperature.
3. Results and discussion The magnetic properties of ribbons of wheel speeds 22-25 m/s are listed in table 1. The energy product of the resin bonded magnet of the V= 24 m/s ribbon can reach 8 MG Oe. The optimal ribbon quenched at V=24 m/s is studied in detail. Its cross section is observed by SEM, which consists of equi-axis grains, amorphous regions cannot be seen. The TEM image of NdzFeIqB grains is shown in fig. 1, grain sizes are not uniform, at a range of 20-300 nm and average 60-80 nm. A Table 1 Magnetic
properties
of melt-spun
NdFeB alloys
V (m/s)
H,, (Oe) 4xMs (G)
B.V.
22
23
24
25
11000 9940
13000 8400
12000 10400
10000 10130
65
Volume 7, number I,2
MATERIALS LETTERS
Fig. 1.TEM photograph and diffraction pattern from a thin foil of optimally quenched ( V= 24 m/s) NdFeB.
two-dimensional lattice image of (110) Nd2Fe,,B can be seen in fig. 2, the crystal is free of defects and nearly perfect. Fig. 3 shows the image of grain boundaries. Moire strings appear in some boundary area, but neither trace of amorphous layer nor intergranular phase is found. These results are confirmed by taking a series of images from various samples prepared in the same way. The absence of a second phase in melt-spun material is further supported by X-ray diffraction and Mossbauer spectroscopy. The Miissbauer spectra (fig. 4) do not show any notable paramagnetic Nd rich and/or B rich phase. When the optimally quenched ribbons are annealed at 300°C and 600°C for 30 min, an intergranular phase occurs. Fig. 5 shows the thin layer ( 10 nm) of intergranular phase. It must be pointed out
Fig. 2. HREM lattice image of (110) Nd,Fe,,B, showing lattices in [OOI], [llO].
that the boundary phase usually appears on boundaries of extremely big grains, while the line grains, the majority of the sample, are held in single phase state even after heat-treatment. This is in agreement with the opinion that there is a thin Nd rich layer along the NdzFeldB grain boundary area [ 8,9], and the intergranular phase is enriched in Nd [ 3,7,12 1. Since the second phase can only be found at boundaries of extremely big grains in annealed samples, it can be inferred that the Nd enrichment is more serious in big grains. From all these observations it is concluded that in this work, the optimally quenched NdFeB alloy is single phased and Nd2Fei4B grain is nearly perfect, amorphous layer and intergranular phase have not been observed. Since the NdzFe14B grain is smaller than the single domain particle size (DC=260 nm)
Fig. 3. Lattice image of Nd,Fe,,B grains and boundaries.
66
August 1988
Volume 7. number
MATERIALS
I,?
-5
-4
-3
-2
-1 mm
Fig. 4. “Fe Mdssbauer
August
LETTERS
1
0 set
spectra of melt-spun
2
3
4
1988
5
-1
NdFeB ribbon
( V= 24 m/s).
Acknowledgement
The authors wish to thank Professor Xun-yi Zeng for his help and valuable suggestions. References [ I] M. Sagawa, F. Fujimura
Fig. 5. Intergranular phase in between extremely bon annealed at 300°C for 30 min.
big grains of rib-
[ 13,14 1, the melt-spun NdFeB magnet can be considered as an assembly of nearly perfect structured single domain particles. It is proposed that the high coercivity in melt-spun NdFeB ribbons is strongly related to the fine and almost perfect Nd2FeL4B grains, the effect of domain wall pinning at intergranular phase may be not as important as that in sintered magnets.
and N. Tagawa, Appl. Phys. Letters 55 (1984) 2083. [ 2 ] J.J. Croat, J.F. Herbst and R.W. Lee, Appl. Phys. Letters 44 (1984) 148. [3] J. Fidler. Proceedings of the 9th International Workshop on RE Magnets and Their Applications, 3 1 August-2 September 1987, Bad Soden, FRG, p. 233. [4] J. Fidler, IEEE Trans. Mag. MAC-21 (1985) 1955. [S] J.F. Herbst, J.J. Croat and F.E. Pinkerton, Dhys. Rev. B 29 (1984) 4176. [6] K. Hiraga, Japan. J. Appt. Phys. 24 ( 1985) L30. [ 7 ] Y.L. Chen, IEEE Trans. Mag. MAG-2 1 ( 1985) 9. [S] B.W. Corb, Appl. Phys. Letters SO (1987) 353. 191 A. Hutten and J. Wecker, Proceedings of the 9th Intemational Workshop on RE Magnets and Their Applications, 31 August-2 September 1987, Bad Soden, FRG, p. 233. [ lo] PK. Mishra, J. Mag. Mag. Mater. 54/57 (1986) 450. [Ii] M. Sagawa, iEEE Trans. Mag. MAG-22 (1986) 910. [ 121 P. Schrey, IEEETrans. Mag. MAC-22 (1986) 913. [ 133 M. Sagawa, Proceedings of the 8th International Workshop on RE Magnets and Their Applications, 1-8 May 1985, Dayton, Ohio, US4, p. 587. [ 141 J.D. Livingston, Proceedings ofthe 8th International Workshop on RE Magnets and Their Applications, l-8 May 1985, Dayton, Ohio. USA. p, 42.
67
MATERIALS
1,2
MICROSTRUCTURE
OF MELT-SPUN
August
LETTERS
1988
NdFeB MAGNET
H.C. HUA, G.Y. WANG, C.H. ZHENG Institute ofMetallurgy,Chinese Academy of Sciences, 865 Chang Ning Road, Shanghai 200050, China
Shanghai
G.X. HUANG,
Q.Z. XU, L.H. WU and S.Y. SHI
Shanghai
Iron and Steel Research
Received
24 May 1988
Institute, 1001 Taihe Road,
Wusong, Shanghai,
China
A HREM investigation of melt-spun Nd,3.5Fe *I 74B4 76 magnet revealed that the optimally quenched Nd,Fe,,B grains and the crystal is nearly perfect. Intergranular phases are not observed in the quenched must be strongly related to the fine and almost perfect NdzFe14B grains.
1. Introduction High magnetic energy NdFeB permanent magnets are produced by either powder metallurgy techniques (sintered magnet) or rapid quenching (meltspun) [ 1,2]. The microstructure and magnetic properties of sintered NdFeB alloys have been investigated, and it has been found that the coercivity may be attributed to domain wall pinning at an intergranular second phase [ 3-6 1. The microstructure of melt-spun alloys is characterized by a very fine grain size [ 7-91, amorphous film and boundary phase surrounding the NdzFel,B grains were also reported [lO,ll]. In this work, transmission electron microscopy and MGssbauer spectroscopy are used to study the microstructure of melt-spun NdFeB alloy.
2. Experiment Ribbons used in this work are 2.2 mm wide and 30 pm thick. Ingots melted in an induction furnace were melt-spun in an argon atmosphere at wheel speeds of 22-25 m/s. The composition of melt-spun ribbon is analysed as Nd13.5Fe8L.74B4.76. Electron transparent foils were prepared directly from ribbon fragment. Samples were ion milled at liquid-nitrogen temperature. The processed samples were examined 0167-577x/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
NdFeB ribbon ribbon.
has fine
High coercivity
in a JEOL 4000EX electron microscope at 400 kV. Magnetic properties were measured by a vibrating sample magnetometer with a maximum magnetizing field of 17 kOe. The ribbon sample was employed as the absorbor of 57Fe Mijssbauer spectroscope. The source was 57Co (Pd) and kept at room temperature.
3. Results and discussion The magnetic properties of ribbons of wheel speeds 22-25 m/s are listed in table 1. The energy product of the resin bonded magnet of the V= 24 m/s ribbon can reach 8 MG Oe. The optimal ribbon quenched at V=24 m/s is studied in detail. Its cross section is observed by SEM, which consists of equi-axis grains, amorphous regions cannot be seen. The TEM image of NdzFeIqB grains is shown in fig. 1, grain sizes are not uniform, at a range of 20-300 nm and average 60-80 nm. A Table 1 Magnetic
properties
of melt-spun
NdFeB alloys
V (m/s)
H,, (Oe) 4xMs (G)
B.V.
22
23
24
25
11000 9940
13000 8400
12000 10400
10000 10130
65
Volume 7, number I,2
MATERIALS LETTERS
Fig. 1.TEM photograph and diffraction pattern from a thin foil of optimally quenched ( V= 24 m/s) NdFeB.
two-dimensional lattice image of (110) Nd2Fe,,B can be seen in fig. 2, the crystal is free of defects and nearly perfect. Fig. 3 shows the image of grain boundaries. Moire strings appear in some boundary area, but neither trace of amorphous layer nor intergranular phase is found. These results are confirmed by taking a series of images from various samples prepared in the same way. The absence of a second phase in melt-spun material is further supported by X-ray diffraction and Mossbauer spectroscopy. The Miissbauer spectra (fig. 4) do not show any notable paramagnetic Nd rich and/or B rich phase. When the optimally quenched ribbons are annealed at 300°C and 600°C for 30 min, an intergranular phase occurs. Fig. 5 shows the thin layer ( 10 nm) of intergranular phase. It must be pointed out
Fig. 2. HREM lattice image of (110) Nd,Fe,,B, showing lattices in [OOI], [llO].
that the boundary phase usually appears on boundaries of extremely big grains, while the line grains, the majority of the sample, are held in single phase state even after heat-treatment. This is in agreement with the opinion that there is a thin Nd rich layer along the NdzFeldB grain boundary area [ 8,9], and the intergranular phase is enriched in Nd [ 3,7,12 1. Since the second phase can only be found at boundaries of extremely big grains in annealed samples, it can be inferred that the Nd enrichment is more serious in big grains. From all these observations it is concluded that in this work, the optimally quenched NdFeB alloy is single phased and Nd2Fei4B grain is nearly perfect, amorphous layer and intergranular phase have not been observed. Since the NdzFe14B grain is smaller than the single domain particle size (DC=260 nm)
Fig. 3. Lattice image of Nd,Fe,,B grains and boundaries.
66
August 1988
Volume 7. number
MATERIALS
I,?
-5
-4
-3
-2
-1 mm
Fig. 4. “Fe Mdssbauer
August
LETTERS
1
0 set
spectra of melt-spun
2
3
4
1988
5
-1
NdFeB ribbon
( V= 24 m/s).
Acknowledgement
The authors wish to thank Professor Xun-yi Zeng for his help and valuable suggestions. References [ I] M. Sagawa, F. Fujimura
Fig. 5. Intergranular phase in between extremely bon annealed at 300°C for 30 min.
big grains of rib-
[ 13,14 1, the melt-spun NdFeB magnet can be considered as an assembly of nearly perfect structured single domain particles. It is proposed that the high coercivity in melt-spun NdFeB ribbons is strongly related to the fine and almost perfect Nd2FeL4B grains, the effect of domain wall pinning at intergranular phase may be not as important as that in sintered magnets.
and N. Tagawa, Appl. Phys. Letters 55 (1984) 2083. [ 2 ] J.J. Croat, J.F. Herbst and R.W. Lee, Appl. Phys. Letters 44 (1984) 148. [3] J. Fidler. Proceedings of the 9th International Workshop on RE Magnets and Their Applications, 3 1 August-2 September 1987, Bad Soden, FRG, p. 233. [4] J. Fidler, IEEE Trans. Mag. MAC-21 (1985) 1955. [S] J.F. Herbst, J.J. Croat and F.E. Pinkerton, Dhys. Rev. B 29 (1984) 4176. [6] K. Hiraga, Japan. J. Appt. Phys. 24 ( 1985) L30. [ 7 ] Y.L. Chen, IEEE Trans. Mag. MAG-2 1 ( 1985) 9. [S] B.W. Corb, Appl. Phys. Letters SO (1987) 353. 191 A. Hutten and J. Wecker, Proceedings of the 9th Intemational Workshop on RE Magnets and Their Applications, 31 August-2 September 1987, Bad Soden, FRG, p. 233. [ lo] PK. Mishra, J. Mag. Mag. Mater. 54/57 (1986) 450. [Ii] M. Sagawa, iEEE Trans. Mag. MAG-22 (1986) 910. [ 121 P. Schrey, IEEETrans. Mag. MAC-22 (1986) 913. [ 133 M. Sagawa, Proceedings of the 8th International Workshop on RE Magnets and Their Applications, 1-8 May 1985, Dayton, Ohio, US4, p. 587. [ 141 J.D. Livingston, Proceedings ofthe 8th International Workshop on RE Magnets and Their Applications, l-8 May 1985, Dayton, Ohio. USA. p, 42.
67