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Phase formation and magnetic properties of annealed mechanical alloying Nd-Fe-V-Ti alloys and their nitrides
Journal of Alloys and Compounds 264 (1998) 240–243
L
Phase formation and magnetic properties of annealed mechanical alloying Nd-Fe-V-Ti alloys and their nitrides S.L. Tang
a ,b ,
*, C.H. Wu a ,c , X.M. Jin a , B.W. Wang a , G.S. Li a , B.Z. Ding a , Y.C. Chuang a a
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, P.R. China Department of Mechanical Engineering, Xiangtan University, Hunan 411105, P.R. China c International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110015, P.R. China b
Received 18 March 1997; received in revised form 13 May 1997
Abstract Phase formation in annealed mechanical alloyed NdFe 1 0 . 5 1 xV1 . 5 ( 1 2 x ) Tix , and magnetic properties of the consequent phases and their nitrides were investigated by using X-ray diffraction and magnetic measurement. From the X-ray diffraction patterns we have constructed a phase formation diagram as a function of Ti content x and annealing temperature. The demarcation curve between 1:7 and 1:12 phases moves to higher annealing temperatures with increasing Ti content. In the NdFe 11 Ti alloy the ThMn 12 structure is obtained by annealing the as-milled powder at temperatures above 950 8C. The nitrides NdFe 10.510.5xV1.5( 12x) Ti x N y retain the same structure as their parent compounds. The nitride with high V content is more stable than that with low V content. Upon nitrogenation, the Curie temperature is enhanced from 150 8C to 180 8C. In NdFe 10.510.5xV1.5( 12x) Ti x N y T c the values fall in the range 450|480 8C. The hard magnetic properties of NdFe 10.510.5xV1.5( 12x) Ti x N y magnets have been assessed. 1998 Elsevier Science S.A. Keywords: Mechanical alloying; ThMn 12 type structure compound; Permanent magnetic property
1. Introduction It has been discovered that the absorption of nitrogen in R(Fe,M) 12 compounds (R5rare earth, T5Ti, V, and Mo) with tetragonal ThMn 12 structure leads to strong increases in the Curie temperature, T C , and saturation magnetization, MS . The magnetocrystalline anisotropy is also significantly modified owing to the change of the sign of the second order crystal field parameter A 20 from negative to positive [1]. Worldwide efforts were carried out to investigate the magnetic properties of nitrogenated 1:12 compounds, intending to provide a new alloy for permanent magnet applications [2–5]. Magnetic harding of the V, and Mo materials to obtain substantial intrinsic coercivity have proven to be successful and to lead to coercivities of 6–9 kOe. Magnetic hardening is difficult for nitrides of NdFe 11 Ti, where coercivities of only 1.4–2.5 kOe have been reported when nitriding mechanically alloyed powders [6–8]. The low coercivity is due to formation of a phase having the disordered hexagonal TbCu 7 structure at low annealing temperature, which impedes the formation of NdFe 11 Ti characterized by the ThMn 12 structure (for*Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00250-8
mation only above 950 8C). The generation of high coercivity by the mechanical alloying technique relies on the formation of ThMn 12 -type material at low annealing temperatures prior to nitriding in order to obtain nanometre sized grains. In this article, phase formation in annealed mechanical alloyed NdFe 10.510.5xV1.5( 12x) Ti x alloys (0#x#1) is examined. The hard magnetic properties of NdFe 10.510.5xV1.5(12x) Ti x N y magnets are discussed.
2. Experimental techniques The starting materials used in this study were powders of Nd, V, Ti of 99.9%, Fe of 99.8% purity, their mixtures having the starting composition NdFe 10.510.5xV1.5(12x) Ti x (x50.2, 0.4, 0.6, 0.8, and 1). An extra 25 wt % Nd was added to compensate for losses due to oxidation. Mechanical alloying was carried out in a vibrating ball mill using a hardened steel vial and 12 mm diameter steel balls. A ball to powder mass of 10:1 was used, and the milling was carried out for 7 h. Loading and sealing of the vial and all subsequent powder handling were carried out in a glove box filled with the high purity argon.
S.L. Tang et al. / Journal of Alloys and Compounds 264 (1998) 240 – 243
241
X-ray diffraction measurements showed that the all as-milled samples consisted of a mixture of a bcc solid solution of V and Ti in a-Fe and an amorphous phase (Fig. 1a). The phase formation in mechanical alloyed NdFe 10.510.5xV1.5(12x) Ti x is varied significantly with different heat treatment temperature and Ti content. The ThMn 12 structure is readily formed in mechanical alloyed NdFe 11 Ti when the annealing temperature is higher than 950 8C. When annealing at temperatures lower than 850 8C, the mechanical alloy transforms into the disordered TbCu 7 type structures. When substituting V for Ti, the ThMn 12 structure is formed at lower temperatures. As an example, the X-ray patterns of the NdFe 10.9 V0.3 Ti 0.8 alloy annealed at 750 8C, 800 8C and 850 8C, respectively, are shown in Fig. 1 b–d. The sample annealed at 750 8C has a clear disordered TbCu 7 diffraction pattern except for small impurity peaks of a-Fe(V,Ti) and a Nd-rich phase (Fig. 1b). The 1:7 phase has a Curie temperature T c 5205 8C which has been determined by an ac susceptibility vs temperature measurement. When the annealing temperature
is increased to 800 8C, the diffraction pattern begins to transform into that characteristic of the ThMn 12 structure, and hence it represents a partially transformed material in this stage (Fig. 1c). It may be supposed that this diffraction pattern consists of the 1:7 phase and the 1:12 phase which would explain the reflection line broadening. However, this supposition is in conflict with the fact that only one Curie temperature (T C 5288 8C) was observed, which is higher than that of the 1:7 compound and lower than that of the 1:12 compound. When the annealing temperature is in creased to 850 8C, the transformation into the ThMn 12 structure is essentially complete (Fig. 1d), its Curie temperature is 313 8C. From the X-ray diffraction patterns we construct in Fig. 2 the phase formation diagram as a function of composition x (abscissa) and annealing temperature (ordinate). The filled circles represent sample which fall clearly within the ThMn 12 structure type, whereas the open circles represent samples having the TbCu 7 -type structure. The open triangles denote samples which do not fall into either one of the two structure types, and represent in most cases partially transformed material. The demarcation between two structure types moves to significantly higher annealing temperatures with increasing Ti content, indicating that the formation of the ThMn 12 structure is much more favourable at low Ti content. Fig. 3 shows the XRD patterns of the nitrides (x50, 0.2, 0.6 and 1) obtained by heating at 450 8C for 15 h in highly pure nitrogen gas. The results show that all XRD lines of NdFe 10.5 V1.5 N y except for Nd-rich phase could be indexed as a ThMn 12 -type tetragonal structure (Fig. 3a). When substituting the smaller Ti for V, the 1:12 nitrides start to decompose and a small amount of a-Fe(V,Ti) precipitates. For x51, a large amount of the a-Fe (Ti) phase precipitates. The Nd(Fe, V, Ti) 12 N y nitride of higher V content is more stable. Nitriding resulted in an expansion of the Nd(Fe, V, Ti) 12 lattice. The lattice parameters a and c do not vary very much with increasing V content, the
Fig. 1. X-ray diffraction pattern of mechanical alloyed NdFe 0.9 V0.3 Ti 0.8 (a) as milled; (b) annealed at 750 8C for 30 min; (c) annealed at 800 8C for 30 min; (d) annealed at 850 8C for 30 min.
Fig. 2. Phase formation diagram of mechanically alloyed NdFe 10.510.5xV1.5( 12 x) Ti x alloys as a function of composition and temperature. (d) ThMn 12 , (s) TbCu 7 structure. (,)the partially transformed material.
After milling the powder it was annealed under a vacuum of 1310 25 Torr for 30 min at temperatures in the range from 650 8C to 1000 8C. Nitrogenation of the annealed powder specimens was carried out at 350 8C| 450 8C for 15 h in a highly pure nitrogen atmosphere. The samples were examined by X-ray diffraction using a D/ max-rA diffractometer equipped with a pyrolytic graphite monochromator and using Cu Ka radiation. The Curie temperature was determined by measuring the temperature dependence of ac initial susceptibility. The magnetic measurements were made with a pulse magnetometer in the magnetic field up to 8 T at room temperature.
3. Results and discussion
3.1. Phase formation and structure
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S.L. Tang et al. / Journal of Alloys and Compounds 264 (1998) 240 – 243
Fig. 3. X-ray diffraction pattern of NdFe 10.510.5xV1.5( 12x) Ti x N y compound, nitrated at 450 8C for 15 h. (a) x50, (b) x50.2, (c) x50.6, (d) x51.
values are around 0.859 and 0.479 nm for Nd(Fe, V, Ti) 12 and 0.871 and 0.490 nm for the nitrides. The volume expansion is about 5%.
3.2. Magnetic properties The Ti concentration dependence of the Curie temperature T c for the annealed mechanical alloyed NdFe 10.510.5xV1.5(12x) Ti x of ThMn 12 structure and the corresponding nitrides are shown in Fig. 4. Upon nitrogenation, the Curie temperatures enhanced from 150 8C to 180 8C. The T C of NdFe 10.510.5xV1.5(12 x) Ti x N y is 450| 480 8C. The values of T c decrease by 60 8C for NdFe 10.510.5xV1.5(12 x) Ti x and by only 30 8C for NdFe 10.510.5xV1.5(12x) Ti x N y when x varies from 0 to 1. Fig. 5 shows the annealing temperature dependence of the intrinsic coercivity for the nitrided samples. It is seen that the intrinsic coercivity increases, reaches a maximum and then decreases with increasing annealing temperature for all samples. The temperatures at which the intrinsic coercivity reaches the maximum are different for different Ti concentrations (850 8C for x#0.5, 900 8C for x$0.75).
Fig. 4. The Ti concentration dependence of the Curie temperature T c of NdFe 10.510.5xV1.5( 12x) Ti x phase with ThMn 12 structure and their nitrides.
Fig. 5. The annealing temperature dependence of the intrinsic coercivity for NdFe 10.510.5xV1.5( 12x) Ti x N y .
The reason for this behaviour is that too low annealing temperatures hinder complete solid state reaction for forming a perfect ThMn 12 -type phase which is responsible for the magnetic hardening, and too high annealing temperatures cause excessive grain growth which decrease the intrinsic coercivity. On the one hand it is evident that the lower Ti concentration is favourable for the formation of a perfect Nd(Fe, V, Ti) 12 phase at lower annealing temperature, implying maximum coercivity at the lower annealing temperature. On the other hand, the Ti content also affects the coercivity which strongly decreases with increasing Ti content. The highest coercivity obtained for NdFe 11 TiN y is only i Hc 52.3 kOe. Compared to the NdFe 10.5 V1.5 N y compound, the lower coercivity may originate from the high annealing temperature (about 950 8C) heeded to stabilize 1:12 phase and the unavoidable precipitation of a relatively abundant soft magnetic a-Fe(Ti) during the nitriding process. Hysteresis loops of the partially magnetic isotropic resin-bonded NdFe 10.510.5xV1.5( 12x) Ti x N y (x50 and 1) magnets prepared from annealed mechanical alloyed powders are shown in Fig. 6. It can be seen that the virgin
Fig. 6. Hysteresis loop of alloys NdFe 10.5 V1.5 N y (x50) annealed at 850 8C and nitrided at 450 8C for 15 h, and NdFe 11 TiN y (x51) annealed at 950 8C for 30 min and nitrided at 400 8C for 15 h.
S.L. Tang et al. / Journal of Alloys and Compounds 264 (1998) 240 – 243
magnetization curve (x50) is smooth, which implies no pinning sites in the magnet and the coercivity should be determined by reversed domain nucleation. This is different from the virgin magnetization curves of annealed mechanical alloyed NdFe 10.5 Mo 1.5 N x [9] and PrFe 10.5 Mo 1.5 N x [10], in which there are jumps in magnetization fields equal to their coercivities, which means that domain wall pinning determines the coercivities in these compounds. With increasing Ti content, HC and (BH ) max remarkably decrease (Fig. 6, x51). The magnetic properties of a magnet prepared from mechanical alloyed NdFe 10.5 V1.5 N y are Br 55.6 kGs, i Hc 58.8 kOe and (BH ) max 55.8 MGOe.
243
coercivity strongly decreases with increasing Ti content. The lower coercivity for NdFe 11 TiN y may result from the required high annealing temperature and the precipitation of a relatively abundant soft magnetic a-Fe(Ti) phase during the nitriding process.
Acknowledgements This work was supported by the National Natural Science Foundation of China.
References 4. Conclusions We have established the phase the phase formation diagram of annealed mechanical alloyed NdFe 10.510.5xV1.5(12x) Ti x as a function of composition and shown that it changes smoothly between NdFe 10.5 V1.5 and NdFe 11 Ti (abscissa). When the annealing temperature was varied from 650 8C to 1000 8C (ordinate), the demarcation between the 1:7 and 1:12 phases moves to higher annealing temperatures with increasing Ti content. The NdFe 11 Ti compound with ThMn 12 structure is obtained by annealing the as-milled powder at temperatures above 950 8C. For NdFe 10.510.5xV1.5(12x) Ti x the Curie temperatures T c decreases with increasing Ti content. The NdFe 10.510.5xV1.5(12x) Ti x N y alloys have T c values between about 450 8C and |480 8C. The best result ( i Hc 58.8 kOe) was obtained for a NdFe 10.5 V1.5 N y magnet. The intrinsic
[1] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan, S.L. Ge, Solid State Commun. 78 (1991) 317. [2] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan, S.L. Ge, Appl. Phys. Lett. 58 (1991) 2042. [3] Y.C. Yang, Q. Pan, X.D. Zhang, J. Yang, M.H. Zhang, S.L. Ge, Appl. Phys. Lett. 61 (1992) 2723. [4] M. Anagnostou, C. Christides, M. Pissas, D. Niarchos, J. Appl. Phys. 70 (1991) 6012. [5] B.-P. Hu, Y.-Z. Wang, K.-Y. Wang, G.-C. Liu, W.-Y. Lai, J. Magn. Magn. Mater. 140-144 (1995) 1023. [6] G. Wei, G.C. Hadjipanayis, IEEE Trans. Magn., MAG- 28 (1992) 2563. [7] M. Endoh, K. Nakamura, H. Mikami, IEEE Trans. Magn., MAG- 28 (1992) 2560. [8] Z.Q. Jin, X.K. Sun, W. Liu, X.G. Zhao, Q.F. Xiao, Y.C. Sui, Z.D. Zhang, J. Appl. Phys. 79 (1996) 5525. [9] K. Schnitzke, L. Schulz, J. Wecker, M. Katter, Appl. Phys. Lett. 57 (1990) 57. [10] Y.-C. Yang, Q. Pan, B.-P. Cheng, X.-D. Zhang, Z.-X. Liu, Y.-X. Sun, S.-L. Ge, J. Appl. Phys. 76 (1994) 6752.
L
Phase formation and magnetic properties of annealed mechanical alloying Nd-Fe-V-Ti alloys and their nitrides S.L. Tang
a ,b ,
*, C.H. Wu a ,c , X.M. Jin a , B.W. Wang a , G.S. Li a , B.Z. Ding a , Y.C. Chuang a a
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, P.R. China Department of Mechanical Engineering, Xiangtan University, Hunan 411105, P.R. China c International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110015, P.R. China b
Received 18 March 1997; received in revised form 13 May 1997
Abstract Phase formation in annealed mechanical alloyed NdFe 1 0 . 5 1 xV1 . 5 ( 1 2 x ) Tix , and magnetic properties of the consequent phases and their nitrides were investigated by using X-ray diffraction and magnetic measurement. From the X-ray diffraction patterns we have constructed a phase formation diagram as a function of Ti content x and annealing temperature. The demarcation curve between 1:7 and 1:12 phases moves to higher annealing temperatures with increasing Ti content. In the NdFe 11 Ti alloy the ThMn 12 structure is obtained by annealing the as-milled powder at temperatures above 950 8C. The nitrides NdFe 10.510.5xV1.5( 12x) Ti x N y retain the same structure as their parent compounds. The nitride with high V content is more stable than that with low V content. Upon nitrogenation, the Curie temperature is enhanced from 150 8C to 180 8C. In NdFe 10.510.5xV1.5( 12x) Ti x N y T c the values fall in the range 450|480 8C. The hard magnetic properties of NdFe 10.510.5xV1.5( 12x) Ti x N y magnets have been assessed. 1998 Elsevier Science S.A. Keywords: Mechanical alloying; ThMn 12 type structure compound; Permanent magnetic property
1. Introduction It has been discovered that the absorption of nitrogen in R(Fe,M) 12 compounds (R5rare earth, T5Ti, V, and Mo) with tetragonal ThMn 12 structure leads to strong increases in the Curie temperature, T C , and saturation magnetization, MS . The magnetocrystalline anisotropy is also significantly modified owing to the change of the sign of the second order crystal field parameter A 20 from negative to positive [1]. Worldwide efforts were carried out to investigate the magnetic properties of nitrogenated 1:12 compounds, intending to provide a new alloy for permanent magnet applications [2–5]. Magnetic harding of the V, and Mo materials to obtain substantial intrinsic coercivity have proven to be successful and to lead to coercivities of 6–9 kOe. Magnetic hardening is difficult for nitrides of NdFe 11 Ti, where coercivities of only 1.4–2.5 kOe have been reported when nitriding mechanically alloyed powders [6–8]. The low coercivity is due to formation of a phase having the disordered hexagonal TbCu 7 structure at low annealing temperature, which impedes the formation of NdFe 11 Ti characterized by the ThMn 12 structure (for*Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00250-8
mation only above 950 8C). The generation of high coercivity by the mechanical alloying technique relies on the formation of ThMn 12 -type material at low annealing temperatures prior to nitriding in order to obtain nanometre sized grains. In this article, phase formation in annealed mechanical alloyed NdFe 10.510.5xV1.5( 12x) Ti x alloys (0#x#1) is examined. The hard magnetic properties of NdFe 10.510.5xV1.5(12x) Ti x N y magnets are discussed.
2. Experimental techniques The starting materials used in this study were powders of Nd, V, Ti of 99.9%, Fe of 99.8% purity, their mixtures having the starting composition NdFe 10.510.5xV1.5(12x) Ti x (x50.2, 0.4, 0.6, 0.8, and 1). An extra 25 wt % Nd was added to compensate for losses due to oxidation. Mechanical alloying was carried out in a vibrating ball mill using a hardened steel vial and 12 mm diameter steel balls. A ball to powder mass of 10:1 was used, and the milling was carried out for 7 h. Loading and sealing of the vial and all subsequent powder handling were carried out in a glove box filled with the high purity argon.
S.L. Tang et al. / Journal of Alloys and Compounds 264 (1998) 240 – 243
241
X-ray diffraction measurements showed that the all as-milled samples consisted of a mixture of a bcc solid solution of V and Ti in a-Fe and an amorphous phase (Fig. 1a). The phase formation in mechanical alloyed NdFe 10.510.5xV1.5(12x) Ti x is varied significantly with different heat treatment temperature and Ti content. The ThMn 12 structure is readily formed in mechanical alloyed NdFe 11 Ti when the annealing temperature is higher than 950 8C. When annealing at temperatures lower than 850 8C, the mechanical alloy transforms into the disordered TbCu 7 type structures. When substituting V for Ti, the ThMn 12 structure is formed at lower temperatures. As an example, the X-ray patterns of the NdFe 10.9 V0.3 Ti 0.8 alloy annealed at 750 8C, 800 8C and 850 8C, respectively, are shown in Fig. 1 b–d. The sample annealed at 750 8C has a clear disordered TbCu 7 diffraction pattern except for small impurity peaks of a-Fe(V,Ti) and a Nd-rich phase (Fig. 1b). The 1:7 phase has a Curie temperature T c 5205 8C which has been determined by an ac susceptibility vs temperature measurement. When the annealing temperature
is increased to 800 8C, the diffraction pattern begins to transform into that characteristic of the ThMn 12 structure, and hence it represents a partially transformed material in this stage (Fig. 1c). It may be supposed that this diffraction pattern consists of the 1:7 phase and the 1:12 phase which would explain the reflection line broadening. However, this supposition is in conflict with the fact that only one Curie temperature (T C 5288 8C) was observed, which is higher than that of the 1:7 compound and lower than that of the 1:12 compound. When the annealing temperature is in creased to 850 8C, the transformation into the ThMn 12 structure is essentially complete (Fig. 1d), its Curie temperature is 313 8C. From the X-ray diffraction patterns we construct in Fig. 2 the phase formation diagram as a function of composition x (abscissa) and annealing temperature (ordinate). The filled circles represent sample which fall clearly within the ThMn 12 structure type, whereas the open circles represent samples having the TbCu 7 -type structure. The open triangles denote samples which do not fall into either one of the two structure types, and represent in most cases partially transformed material. The demarcation between two structure types moves to significantly higher annealing temperatures with increasing Ti content, indicating that the formation of the ThMn 12 structure is much more favourable at low Ti content. Fig. 3 shows the XRD patterns of the nitrides (x50, 0.2, 0.6 and 1) obtained by heating at 450 8C for 15 h in highly pure nitrogen gas. The results show that all XRD lines of NdFe 10.5 V1.5 N y except for Nd-rich phase could be indexed as a ThMn 12 -type tetragonal structure (Fig. 3a). When substituting the smaller Ti for V, the 1:12 nitrides start to decompose and a small amount of a-Fe(V,Ti) precipitates. For x51, a large amount of the a-Fe (Ti) phase precipitates. The Nd(Fe, V, Ti) 12 N y nitride of higher V content is more stable. Nitriding resulted in an expansion of the Nd(Fe, V, Ti) 12 lattice. The lattice parameters a and c do not vary very much with increasing V content, the
Fig. 1. X-ray diffraction pattern of mechanical alloyed NdFe 0.9 V0.3 Ti 0.8 (a) as milled; (b) annealed at 750 8C for 30 min; (c) annealed at 800 8C for 30 min; (d) annealed at 850 8C for 30 min.
Fig. 2. Phase formation diagram of mechanically alloyed NdFe 10.510.5xV1.5( 12 x) Ti x alloys as a function of composition and temperature. (d) ThMn 12 , (s) TbCu 7 structure. (,)the partially transformed material.
After milling the powder it was annealed under a vacuum of 1310 25 Torr for 30 min at temperatures in the range from 650 8C to 1000 8C. Nitrogenation of the annealed powder specimens was carried out at 350 8C| 450 8C for 15 h in a highly pure nitrogen atmosphere. The samples were examined by X-ray diffraction using a D/ max-rA diffractometer equipped with a pyrolytic graphite monochromator and using Cu Ka radiation. The Curie temperature was determined by measuring the temperature dependence of ac initial susceptibility. The magnetic measurements were made with a pulse magnetometer in the magnetic field up to 8 T at room temperature.
3. Results and discussion
3.1. Phase formation and structure
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S.L. Tang et al. / Journal of Alloys and Compounds 264 (1998) 240 – 243
Fig. 3. X-ray diffraction pattern of NdFe 10.510.5xV1.5( 12x) Ti x N y compound, nitrated at 450 8C for 15 h. (a) x50, (b) x50.2, (c) x50.6, (d) x51.
values are around 0.859 and 0.479 nm for Nd(Fe, V, Ti) 12 and 0.871 and 0.490 nm for the nitrides. The volume expansion is about 5%.
3.2. Magnetic properties The Ti concentration dependence of the Curie temperature T c for the annealed mechanical alloyed NdFe 10.510.5xV1.5(12x) Ti x of ThMn 12 structure and the corresponding nitrides are shown in Fig. 4. Upon nitrogenation, the Curie temperatures enhanced from 150 8C to 180 8C. The T C of NdFe 10.510.5xV1.5(12 x) Ti x N y is 450| 480 8C. The values of T c decrease by 60 8C for NdFe 10.510.5xV1.5(12 x) Ti x and by only 30 8C for NdFe 10.510.5xV1.5(12x) Ti x N y when x varies from 0 to 1. Fig. 5 shows the annealing temperature dependence of the intrinsic coercivity for the nitrided samples. It is seen that the intrinsic coercivity increases, reaches a maximum and then decreases with increasing annealing temperature for all samples. The temperatures at which the intrinsic coercivity reaches the maximum are different for different Ti concentrations (850 8C for x#0.5, 900 8C for x$0.75).
Fig. 4. The Ti concentration dependence of the Curie temperature T c of NdFe 10.510.5xV1.5( 12x) Ti x phase with ThMn 12 structure and their nitrides.
Fig. 5. The annealing temperature dependence of the intrinsic coercivity for NdFe 10.510.5xV1.5( 12x) Ti x N y .
The reason for this behaviour is that too low annealing temperatures hinder complete solid state reaction for forming a perfect ThMn 12 -type phase which is responsible for the magnetic hardening, and too high annealing temperatures cause excessive grain growth which decrease the intrinsic coercivity. On the one hand it is evident that the lower Ti concentration is favourable for the formation of a perfect Nd(Fe, V, Ti) 12 phase at lower annealing temperature, implying maximum coercivity at the lower annealing temperature. On the other hand, the Ti content also affects the coercivity which strongly decreases with increasing Ti content. The highest coercivity obtained for NdFe 11 TiN y is only i Hc 52.3 kOe. Compared to the NdFe 10.5 V1.5 N y compound, the lower coercivity may originate from the high annealing temperature (about 950 8C) heeded to stabilize 1:12 phase and the unavoidable precipitation of a relatively abundant soft magnetic a-Fe(Ti) during the nitriding process. Hysteresis loops of the partially magnetic isotropic resin-bonded NdFe 10.510.5xV1.5( 12x) Ti x N y (x50 and 1) magnets prepared from annealed mechanical alloyed powders are shown in Fig. 6. It can be seen that the virgin
Fig. 6. Hysteresis loop of alloys NdFe 10.5 V1.5 N y (x50) annealed at 850 8C and nitrided at 450 8C for 15 h, and NdFe 11 TiN y (x51) annealed at 950 8C for 30 min and nitrided at 400 8C for 15 h.
S.L. Tang et al. / Journal of Alloys and Compounds 264 (1998) 240 – 243
magnetization curve (x50) is smooth, which implies no pinning sites in the magnet and the coercivity should be determined by reversed domain nucleation. This is different from the virgin magnetization curves of annealed mechanical alloyed NdFe 10.5 Mo 1.5 N x [9] and PrFe 10.5 Mo 1.5 N x [10], in which there are jumps in magnetization fields equal to their coercivities, which means that domain wall pinning determines the coercivities in these compounds. With increasing Ti content, HC and (BH ) max remarkably decrease (Fig. 6, x51). The magnetic properties of a magnet prepared from mechanical alloyed NdFe 10.5 V1.5 N y are Br 55.6 kGs, i Hc 58.8 kOe and (BH ) max 55.8 MGOe.
243
coercivity strongly decreases with increasing Ti content. The lower coercivity for NdFe 11 TiN y may result from the required high annealing temperature and the precipitation of a relatively abundant soft magnetic a-Fe(Ti) phase during the nitriding process.
Acknowledgements This work was supported by the National Natural Science Foundation of China.
References 4. Conclusions We have established the phase the phase formation diagram of annealed mechanical alloyed NdFe 10.510.5xV1.5(12x) Ti x as a function of composition and shown that it changes smoothly between NdFe 10.5 V1.5 and NdFe 11 Ti (abscissa). When the annealing temperature was varied from 650 8C to 1000 8C (ordinate), the demarcation between the 1:7 and 1:12 phases moves to higher annealing temperatures with increasing Ti content. The NdFe 11 Ti compound with ThMn 12 structure is obtained by annealing the as-milled powder at temperatures above 950 8C. For NdFe 10.510.5xV1.5(12x) Ti x the Curie temperatures T c decreases with increasing Ti content. The NdFe 10.510.5xV1.5(12x) Ti x N y alloys have T c values between about 450 8C and |480 8C. The best result ( i Hc 58.8 kOe) was obtained for a NdFe 10.5 V1.5 N y magnet. The intrinsic
[1] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan, S.L. Ge, Solid State Commun. 78 (1991) 317. [2] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan, S.L. Ge, Appl. Phys. Lett. 58 (1991) 2042. [3] Y.C. Yang, Q. Pan, X.D. Zhang, J. Yang, M.H. Zhang, S.L. Ge, Appl. Phys. Lett. 61 (1992) 2723. [4] M. Anagnostou, C. Christides, M. Pissas, D. Niarchos, J. Appl. Phys. 70 (1991) 6012. [5] B.-P. Hu, Y.-Z. Wang, K.-Y. Wang, G.-C. Liu, W.-Y. Lai, J. Magn. Magn. Mater. 140-144 (1995) 1023. [6] G. Wei, G.C. Hadjipanayis, IEEE Trans. Magn., MAG- 28 (1992) 2563. [7] M. Endoh, K. Nakamura, H. Mikami, IEEE Trans. Magn., MAG- 28 (1992) 2560. [8] Z.Q. Jin, X.K. Sun, W. Liu, X.G. Zhao, Q.F. Xiao, Y.C. Sui, Z.D. Zhang, J. Appl. Phys. 79 (1996) 5525. [9] K. Schnitzke, L. Schulz, J. Wecker, M. Katter, Appl. Phys. Lett. 57 (1990) 57. [10] Y.-C. Yang, Q. Pan, B.-P. Cheng, X.-D. Zhang, Z.-X. Liu, Y.-X. Sun, S.-L. Ge, J. Appl. Phys. 76 (1994) 6752.