- Email: [email protected]
Influence of Mo addition on the microstructure and magnetic properties of Sm2(Fe, Al, Mo)17C1.5 alloys
Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
Influence of Mo addition on the microstructure and magnetic properties of Sm (Fe, Al, Mo) C alloys 2 17 1.5 W. Tang!,*, Z.Q. Jin!, J.H. Yin!, J.R. Zhang!, S.L. Tang!, Y.W. Du!, F.L. Wei", Z. Yang" ! Department of Physics and National Laboratory of Solid State Microstructure, Nanjing University, Nanjing, 210093, China " Magnetics Laboratory, Lanzhou University, State Education Commission, Lanzhou, 730001, China Received 5 November 1997; received in revised form 20 January 1998
Abstract The microstructure and magnetic properties of Sm Fe Mo Al C alloys with Mo additions have been 2 (15.5~x) x 1.5 1.5 investigated by means of XRD, SEM, TEM, and magnetic measurements. The as-cast alloys have a multiphase structure consisting of a 2/17-type carbide, a considerable amount of a-Fe, and a SmFe phase. Through rapid quenching, the 2 SmFe phase is removed and the amount of a-Fe greatly reduced in the quenched ribbons. It is found that the addition of 2 Mo can produce fine grains and simultaneously decrease the amorphous tendency of the ribbons. The magnetic hardening of Mo-containing alloys can be achieved by direct quenching. With increasing Mo content within a wide range, the coercivity of the ribbon gradually increases while its remanence remains approximately constant. An optimal coercivity exceeding 1.35 T is obtained for the Sm Fe Mo Al C ribbon spun at 40 m s~1. The reduced 2 14.9 0.6 1.5 1.5 remanence of the ribbon is beyond 0.7. These results reveal that the addition of Mo is very effective on improving the coercivity and enhancing the exchange coupling between the 2/17-type and a-Fe phases. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 75.30.!m; 75.50.Bb Keywords: Rare-earth alloys; Addition effects; Melt-spinning; Microstructure; Magnetic properties
1. Introduction Rare-earth-iron intermetallic compounds based on the 2/17 structure have been attracting much attention as potential permanent magnet materials.
* Corresponding author. Fax: #86 25 332 6028; e-mail: [email protected]
It was found that R Fe C compounds prepared 2 17 Y by substituting with other elements, such as Ga, Cr, Al, or Si for Fe [1—5] have improved magnetic properties and temperature stability. The magnetic hardening of the compounds could be achieved by the substitution of Ga, Cr, Al for Fe followed by melt-spinning. Recently, it was found that other refractory elements, such as Ti, V, Mo, Zr, and Nb [6,7] have some possibilities of improving the
0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 0 6 0 - 2
116
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
coercivity of R Fe C compounds. However, the 2 17 Y effects of these elements on the structure and magnetic properties have not been systematically studied. The purposes of adding these elements are to obtain nanocrystalline isotropic alloys. Due to the effect of exchange coupling between the grains of nanostructured alloys the remanence can be enhanced above one-half of the spontaneous polarization [8,9]. On the condition of the nanostructure, a further improvement of the remanence can be achieved by adding a second ferromagnetic phase with a larger polarization (e.g. a-Fe). To obtain such enhanced remanence composite magnets with high coercivity, the average grain size of the soft magnetic phase has to be smaller than the Bloch wall width of the hard magnetic phase [10,11]. Therefore it is a key that the alloys first have a nanostructure. More recently, our research [12] showed that the addition of Zr to Sm (Fe, Zr, 2 Al) C alloys results in the precipitation of a ZrC 17 Y compound within intergranular regions. It has been found that the ZrC precipitates are effective on inhibiting grain growth during annealing and thus improving the coercivity and exchange coupling at interphase boundaries. The annealed Zr-containing ribbons attain higher coercivity and remanence, which are attributed to the addition of Zr. In this paper we have extended our study to the influences of Mo addition on the structure and magnetic properties of Sm Fe Mo Al C alloys 2 (15.5~x) x 1.5 1.5 with x ranging from 0 to 0.8. It is found that the behavior of Mo is quite different from that of Zr in the alloys. By direct quenching, the Mo-containing alloys may achieve a higher coercivity.
wheel was 40 m s~1. The as-quenched ribbons were annealed at 750°C for 20 min under argon atmosphere. The phase structures were determined by X-ray diffraction (XRD) with Cu K radiation. The rooma temperature magnetic properties were measured using a vibrating sample magnetometer with a maximum applied field of 2.0 T. In order to decrease the influence of demagnetization factor, the powder samples for the measurement of VSM were fixed into sphericity by molten wax. The microstructures were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The composition distributions were determined using a scanning electron microscope equipped with an energy-dispersive X-ray analysis (EDAX) unit.
3. Results and discussion 3.1. Formation and structure of phase The XRD patterns for induction melted Sm Fe Mo Al C alloys with different 2 (15.5~x) x 1.5 1.5 Mo content are shown in Fig. 1. It can be seen from Fig. 1 that the alloys consist mainly of a Th Zn 2 17 type phase and a considerable amount of a-Fe. No
2. Experiment Fe, Sm, Fe—C, and Fe—Mo alloys with at least 99.8% purity were levitation melted in a watercooled copper crucible under a purified argon atmosphere. An excess of 15% Sm was added to compensate for the evaporation loss of Sm during melting and melt-spinning processes. The ingots were turned over and remelted four times to ensure homogeneity. The ingots were melt-spun in a standard melt-spinner with a single copper wheel of 200 mm diameter. The surface velocity of the Cu
Fig. 1. XRD patterns of Sm Fe Mo Al C alloys: 2 (15.5~x) x 1.5 1.5 (a) x"0, (b) x"0.2, and (c) x"0.6.
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
117
Table 1 Influence of Mo content x on the lattice parameters and saturation magnetization for as-cast Sm Fe Mo Al C 2 (15.5~x) x 1.5 1.5 samples x (at%)
a (nm)
c (nm)
» (nm3)
p (emu/g) 4
0 0.2 0.4 0.6 0.8
0.862 0.863 0.864 0.865 0.865
1.250 1.251 1.252 1.252 1.252
0.804 0.807 0.809 0.811 0.811
104.6 98.4 94.0 90.2 86.5
compounds based on Mo are found in the Moadded alloys. Because of a small excess of Sm compared to the 2/17 stoichiometry, all the samples also contain SmFe as an impurity phase. With 2 increasing Mo content, the amount of a-Fe slowly decreases, and the lattice parameters and unit-cell volume for the alloys slightly increase (listed in Table 1). It suggests that some Mo atoms substitute into the Fe sites in the 2/17 phase. As a result of the antiferromagnetic coupling of Mo with Fe moments, the introduction of Mo lowers the saturation magnetization from 104.6 emu/g for x"0 to 86.5 emu/g for x"0.8. Fig. 2 shows the XRD patterns of the ribbons quenched at 40 m s~1. For the as-spun ribbon without Mo, a line broadening of diffraction peaks corresponding to the 2/17 phase is observed, revealing a mixture structure of amorphous and nanocrystalline phases. But for the as-spun ribbon with Mo, it is still a completely crystalline 2/17-type structure, indicating that the addition of Mo may obviously reduce the amorphousizing tendency of the ribbons. Moreover, it may be noticed that the SmFe phase disappears, and the amount of a-Fe 2 greatly decreases in the as-spun ribbon, indicating a nonequilibrium nature of the rapid quenching process. According to the samarium—iron phase diagram, the SmFe phase is formed by a series of 2 peritectic reactions from a liquid phase and a a-Fe phase. In this case, the rapid quenching process can prohibit the transformation from a high-temperature peritectic phase Sm Fe to the low-temper2 17 ature peritectic phase SmFe and a-Fe phase, and 2 thus stabilize 2/17-type structure and reduce the formation of a-Fe.
Fig. 2. XRD patterns of Sm Fe Mo Al C ribbons 2 (15.5~x) x 1.5 1.5 quenched at 40 m s~1: (a) x"0, (b) x"0.6.
Fig. 3. XRD patterns of Sm Fe Mo Al C ribbons 2 (15.5~x) x 1.5 1.5 annealed at 750°C for 20 min: (a) x"0, (b) x"0.6.
Annealed at 750°C for 20 min, as shown in Fig. 3, the Mo-free ribbon exhibits a completely crystalline 2/17-type structure along with a-Fe phase, whereas the Mo-containing one still remains the same structure as the as-spun. Thus, the annealing does not affect the phase structure for the Mocontaining ribbons. 3.2. SEM and TEM image analysis The SEM micrographs of the as-cast alloys with x"0 and 0.6 are shown in Fig. 4. It can be seen that the alloys consist of a white phase and a gray
118
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
Fig. 4. SEM micrographs of as-cast Sm Fe Mo Al C alloys with different x values: (a) x"0, (b) x"0.6. 2 (15.5~x) x 1.5 1.5
phase. The EDAX analysis determines that the white and gray are 2/17 phase and dendritic a-Fe phase, respectively. The a-Fe dendrite is embedded in the 2/17 matrix. With increasing Mo content, the dendritic size obviously decreases. This behavior of Mo inhibiting grain growth is consistent with that of Mo preventing the formation of amorphous phase. In addition, EDAX analysis on the Moadded samples reveals that the distribution of Mo content in the matrix and around the grain boundary is nearly equal, indicating the nonexistence of Mo-rich segregation or a Mo-rich phase around grain boundaries. This result is coincident with the XRD measurements. Fig. 5 shows the dark-field electron micrograph and selected area diffraction pattern of the as-spun ribbon with Mo addition. The micrograph confirms that the ribbon has a two-phase structure consisting of the 2/17 phase (gray) and a-Fe phase (white) with an average grain size of 35—50 nm. The selected area diffraction pattern indicates that the microstructure is crystallographically isotropic. The fine-grained microstructure is necessary to obtain a high coercivity and a remanence enhancement. However, the grain size of a-Fe is much larger than that predicted by theoretical models for the optimal hardening [10,11]. According to the SEM analysis and the XRD measurements, it can
Fig. 5. TEM dark-field microgrph and selected area diffraction pattern of as-spun Sm Fe Mo Al C ribbon with 2 (15.5~x) x 1.5 1.5 x"0.6.
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
119
be concluded that there is no Mo-rich segregation in the as-spun ribbons. Although there is not an additional Mo-rich phase formed during crystallizing and quenching, the addition of Mo can still affect the crystallization characteristic of the alloys. The refractory Mo atoms which substitute into the Fe sites in the 2/17 phase may constitute a localized order structure of the 2/17 phase together with other atoms, and subsequently provide a large amount of crystal nuclei in the liquid. The existence of the nuclei helps obtain a fine-grained structure and meanwhile reduce the amorphousizing tendency of the ribbons. Therefore, rapidly quenched, the ribbons with Mo addition obtain a nanocrystalline structure. 3.3. Magnetic properties The magnetic hardening of the alloys is obtained through rapid quenching and subsequent annealing. Fig. 6 shows the Mo content dependence of the magnetic properties of the as-spun and annealed ribbons. The as-spun ribbons without Mo addition are magnetically soft with very low coercivity H and remanence p . It is obviously due to the # 3 existence of the mixture of fine-grained and amorphous phases. But both the H and p are # 3 remarkably enhanced after adding Mo. With increasing Mo content, the H and p increase # 3 to a certain extent. When x"0.2 and 0.6, the p and H achieve the maximum values of up to 3 # 50.6 emu/g and 1.37 T, respectively. Upon annealing at 750°C, as shown in Fig. 6, the ribbons with and without Mo additions exhibit a different change in magnetic properties. For the Mo-free ribbons, the annealing makes the H and # p to be drastically increased. Through annealing, 3 the ribbons composed of partially amorphous phase attain a fine-grained structure with the 2/17 carbide and a-Fe phases and thus higher H and p . # 3 However, for the Mo-containing ribbons, which have been an entirely crystalline structure, annealing leads to grain coarsing and thus the reduction of H and p . It can be seen from Fig. 6 that all the # 3 H and p of the ribbons with different Mo content # 3 decrease to a certain extent after annealing. Consequently, the Mo-containing ribbons are possible to obtain an optimal structure by direct quenching.
Fig. 6. Dependence of the magnetic properties of as-spun and annealed ribbons on Mo content.
Fig. 7. Room-temperature hysteresis loop Sm Fe Mo Al C ribbon spun at 40 m s~1. 2 14.9 0.6 1.5 1.5
of
Fig. 7 shows the hysteresis loop of the as-spun Sm Fe Mo Al C ribbons with x"0.6. 2 (15.5~x) x 1.5 1.5 Only minor loops can be attained in a maximum applied field of 2.0 T. As expected in the isotropic uniaxial magnetic materials, the ratio of p to p is 3 H
120
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
greater than 0.7 (The p is the magnetization meaH sured at the maximum applied field of 2.0 T). The optimal coercivity (1.37 T) is higher than that of Sm Fe Zr Al C ribbons (1.16 T) [12]. It 2 15.3 0.2 1.5 1.5 indicates that the addition of Mo is more effective on improving the coercivity. It may be noticed that the demagnetization curve exhibits a shouldershaped step. This characteristic could be caused by the presence of a certain amount of coarse-grained a-Fe, which can be observed from Fig. 5. The a-Fe and 2/17 phases lead to a weak exchange coupling and reverse incongruously to produce a shouldered hysteresis loop and a lower remanence. Moreover, the existence of the coarse-grained a-Fe could increase the reversed component of magnetization at a given reverse field and lead to low coercive forces. Therefore it is possible that the remanence and coercivity can be further improved by controlling the microstructure to obtain a homogeneous nanocrystalline structure. Such a study is now in progress.
4. Conclusions (1) The addition of Mo to the Sm Fe 2 (15.5~x) Mo Al C alloys results in the reduction of x 1.5 1.5 saturation magnetization. During crystallizing and quenching, the addition of Mo can lead to fine grains, and at the same time decline the amorphous tendency of ribbons. (2) The quenching process can stabilize the 2/17 phase structure, and inhibit the formation of free iron dendrites. (3) By direct quenching at 40 m s~1, the ribbons with Mo addition attain a nanocrystalline structure, and thus magnetic hardening. An optimal coercivity of up to 1.37 T along with a remanence of up to 49.6 emu/g is obtained for the as-spun
Sm Fe Mo Al C ribbon. In contrast to 2 14.9 0.6 1.5 1.5 Zr, Mo is more effective on improving the coercivity for the studied alloys.
Acknowledgements We are grateful to Professor M. Lu for the assistance in the magnetic measurements. This work was supported by grant projects NMS and NSFC, and Jiang Su Province under contract No. BJ97043, People’s Republic of China.
References [1] B.G. Shen, L.S. Kong, F.W. Wang, L. Cao, J. Appl. Phys. 75 (1994) 6253. [2] Z.H. Cheng, B.G. Shen, F.W. Wang, J.X. Zhang, J. Phys.: Condens. Matter 6 (1994) L185. [3] S. Sugimoto, K. Kurihara, H. Nakamura, M. Okda, M. Homma, Mater. Trans. JIM 33 (1992) 146. [4] Y.H. Zheng, A.S. Murthy, F.M. Yang, G.C. Hadjipanayis, J. Magn. Magn. Mater. 140—144 (1995) 1081. [5] G.C. Hadjipanayis, Y.H. Zheng, A.S. Murthy, W. Gong, F.M. Yang, J. Alloys Compounds 222 (1995) 49. [6] H. Horiuchi, U. Koike, H. Kaneko, T. Kurino, H. Uchida, J. Alloys Compounds 222 (1995) 127. [7] L.J. Fang, L. Bo, Y. Xiaojun, P. Wei, D. Linzhi, Proc. 7th Int. Symp. on Magnetic Anisotropy and Coercivity in RE-TM Alloys, Canberra, July 1992, Hi-Perm Laboratory, Research Center for Advanced Mineral and Materials Processing, University of West Australia, Nedlands, 1992, p. 372. [8] T. Schrefl, J. Fidler, H. Kronmu¨ller, J. Magn. Magn. Mater. 138 (1994) 291. [9] A. Manaf, R.A. Buckley, H.A. Davis, M. Leonowiez, J, Magn. Magn. Mater. 101 (1990) 360. [10] E.F. Kneller, R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [11] T. Schrefl, H. Kronmu¨ller, J. Fidler, Phys. Rev. B 49 (1994) 6100. [12] W. Tang, Z.Q. Jin, J.R. Zhang, L.Y. Lu, S.Y. Zhang, Y.W. Du, J. Appl. Phys. 82 (1997) 6351.
Influence of Mo addition on the microstructure and magnetic properties of Sm (Fe, Al, Mo) C alloys 2 17 1.5 W. Tang!,*, Z.Q. Jin!, J.H. Yin!, J.R. Zhang!, S.L. Tang!, Y.W. Du!, F.L. Wei", Z. Yang" ! Department of Physics and National Laboratory of Solid State Microstructure, Nanjing University, Nanjing, 210093, China " Magnetics Laboratory, Lanzhou University, State Education Commission, Lanzhou, 730001, China Received 5 November 1997; received in revised form 20 January 1998
Abstract The microstructure and magnetic properties of Sm Fe Mo Al C alloys with Mo additions have been 2 (15.5~x) x 1.5 1.5 investigated by means of XRD, SEM, TEM, and magnetic measurements. The as-cast alloys have a multiphase structure consisting of a 2/17-type carbide, a considerable amount of a-Fe, and a SmFe phase. Through rapid quenching, the 2 SmFe phase is removed and the amount of a-Fe greatly reduced in the quenched ribbons. It is found that the addition of 2 Mo can produce fine grains and simultaneously decrease the amorphous tendency of the ribbons. The magnetic hardening of Mo-containing alloys can be achieved by direct quenching. With increasing Mo content within a wide range, the coercivity of the ribbon gradually increases while its remanence remains approximately constant. An optimal coercivity exceeding 1.35 T is obtained for the Sm Fe Mo Al C ribbon spun at 40 m s~1. The reduced 2 14.9 0.6 1.5 1.5 remanence of the ribbon is beyond 0.7. These results reveal that the addition of Mo is very effective on improving the coercivity and enhancing the exchange coupling between the 2/17-type and a-Fe phases. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 75.30.!m; 75.50.Bb Keywords: Rare-earth alloys; Addition effects; Melt-spinning; Microstructure; Magnetic properties
1. Introduction Rare-earth-iron intermetallic compounds based on the 2/17 structure have been attracting much attention as potential permanent magnet materials.
* Corresponding author. Fax: #86 25 332 6028; e-mail: [email protected]
It was found that R Fe C compounds prepared 2 17 Y by substituting with other elements, such as Ga, Cr, Al, or Si for Fe [1—5] have improved magnetic properties and temperature stability. The magnetic hardening of the compounds could be achieved by the substitution of Ga, Cr, Al for Fe followed by melt-spinning. Recently, it was found that other refractory elements, such as Ti, V, Mo, Zr, and Nb [6,7] have some possibilities of improving the
0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 0 6 0 - 2
116
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
coercivity of R Fe C compounds. However, the 2 17 Y effects of these elements on the structure and magnetic properties have not been systematically studied. The purposes of adding these elements are to obtain nanocrystalline isotropic alloys. Due to the effect of exchange coupling between the grains of nanostructured alloys the remanence can be enhanced above one-half of the spontaneous polarization [8,9]. On the condition of the nanostructure, a further improvement of the remanence can be achieved by adding a second ferromagnetic phase with a larger polarization (e.g. a-Fe). To obtain such enhanced remanence composite magnets with high coercivity, the average grain size of the soft magnetic phase has to be smaller than the Bloch wall width of the hard magnetic phase [10,11]. Therefore it is a key that the alloys first have a nanostructure. More recently, our research [12] showed that the addition of Zr to Sm (Fe, Zr, 2 Al) C alloys results in the precipitation of a ZrC 17 Y compound within intergranular regions. It has been found that the ZrC precipitates are effective on inhibiting grain growth during annealing and thus improving the coercivity and exchange coupling at interphase boundaries. The annealed Zr-containing ribbons attain higher coercivity and remanence, which are attributed to the addition of Zr. In this paper we have extended our study to the influences of Mo addition on the structure and magnetic properties of Sm Fe Mo Al C alloys 2 (15.5~x) x 1.5 1.5 with x ranging from 0 to 0.8. It is found that the behavior of Mo is quite different from that of Zr in the alloys. By direct quenching, the Mo-containing alloys may achieve a higher coercivity.
wheel was 40 m s~1. The as-quenched ribbons were annealed at 750°C for 20 min under argon atmosphere. The phase structures were determined by X-ray diffraction (XRD) with Cu K radiation. The rooma temperature magnetic properties were measured using a vibrating sample magnetometer with a maximum applied field of 2.0 T. In order to decrease the influence of demagnetization factor, the powder samples for the measurement of VSM were fixed into sphericity by molten wax. The microstructures were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The composition distributions were determined using a scanning electron microscope equipped with an energy-dispersive X-ray analysis (EDAX) unit.
3. Results and discussion 3.1. Formation and structure of phase The XRD patterns for induction melted Sm Fe Mo Al C alloys with different 2 (15.5~x) x 1.5 1.5 Mo content are shown in Fig. 1. It can be seen from Fig. 1 that the alloys consist mainly of a Th Zn 2 17 type phase and a considerable amount of a-Fe. No
2. Experiment Fe, Sm, Fe—C, and Fe—Mo alloys with at least 99.8% purity were levitation melted in a watercooled copper crucible under a purified argon atmosphere. An excess of 15% Sm was added to compensate for the evaporation loss of Sm during melting and melt-spinning processes. The ingots were turned over and remelted four times to ensure homogeneity. The ingots were melt-spun in a standard melt-spinner with a single copper wheel of 200 mm diameter. The surface velocity of the Cu
Fig. 1. XRD patterns of Sm Fe Mo Al C alloys: 2 (15.5~x) x 1.5 1.5 (a) x"0, (b) x"0.2, and (c) x"0.6.
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
117
Table 1 Influence of Mo content x on the lattice parameters and saturation magnetization for as-cast Sm Fe Mo Al C 2 (15.5~x) x 1.5 1.5 samples x (at%)
a (nm)
c (nm)
» (nm3)
p (emu/g) 4
0 0.2 0.4 0.6 0.8
0.862 0.863 0.864 0.865 0.865
1.250 1.251 1.252 1.252 1.252
0.804 0.807 0.809 0.811 0.811
104.6 98.4 94.0 90.2 86.5
compounds based on Mo are found in the Moadded alloys. Because of a small excess of Sm compared to the 2/17 stoichiometry, all the samples also contain SmFe as an impurity phase. With 2 increasing Mo content, the amount of a-Fe slowly decreases, and the lattice parameters and unit-cell volume for the alloys slightly increase (listed in Table 1). It suggests that some Mo atoms substitute into the Fe sites in the 2/17 phase. As a result of the antiferromagnetic coupling of Mo with Fe moments, the introduction of Mo lowers the saturation magnetization from 104.6 emu/g for x"0 to 86.5 emu/g for x"0.8. Fig. 2 shows the XRD patterns of the ribbons quenched at 40 m s~1. For the as-spun ribbon without Mo, a line broadening of diffraction peaks corresponding to the 2/17 phase is observed, revealing a mixture structure of amorphous and nanocrystalline phases. But for the as-spun ribbon with Mo, it is still a completely crystalline 2/17-type structure, indicating that the addition of Mo may obviously reduce the amorphousizing tendency of the ribbons. Moreover, it may be noticed that the SmFe phase disappears, and the amount of a-Fe 2 greatly decreases in the as-spun ribbon, indicating a nonequilibrium nature of the rapid quenching process. According to the samarium—iron phase diagram, the SmFe phase is formed by a series of 2 peritectic reactions from a liquid phase and a a-Fe phase. In this case, the rapid quenching process can prohibit the transformation from a high-temperature peritectic phase Sm Fe to the low-temper2 17 ature peritectic phase SmFe and a-Fe phase, and 2 thus stabilize 2/17-type structure and reduce the formation of a-Fe.
Fig. 2. XRD patterns of Sm Fe Mo Al C ribbons 2 (15.5~x) x 1.5 1.5 quenched at 40 m s~1: (a) x"0, (b) x"0.6.
Fig. 3. XRD patterns of Sm Fe Mo Al C ribbons 2 (15.5~x) x 1.5 1.5 annealed at 750°C for 20 min: (a) x"0, (b) x"0.6.
Annealed at 750°C for 20 min, as shown in Fig. 3, the Mo-free ribbon exhibits a completely crystalline 2/17-type structure along with a-Fe phase, whereas the Mo-containing one still remains the same structure as the as-spun. Thus, the annealing does not affect the phase structure for the Mocontaining ribbons. 3.2. SEM and TEM image analysis The SEM micrographs of the as-cast alloys with x"0 and 0.6 are shown in Fig. 4. It can be seen that the alloys consist of a white phase and a gray
118
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
Fig. 4. SEM micrographs of as-cast Sm Fe Mo Al C alloys with different x values: (a) x"0, (b) x"0.6. 2 (15.5~x) x 1.5 1.5
phase. The EDAX analysis determines that the white and gray are 2/17 phase and dendritic a-Fe phase, respectively. The a-Fe dendrite is embedded in the 2/17 matrix. With increasing Mo content, the dendritic size obviously decreases. This behavior of Mo inhibiting grain growth is consistent with that of Mo preventing the formation of amorphous phase. In addition, EDAX analysis on the Moadded samples reveals that the distribution of Mo content in the matrix and around the grain boundary is nearly equal, indicating the nonexistence of Mo-rich segregation or a Mo-rich phase around grain boundaries. This result is coincident with the XRD measurements. Fig. 5 shows the dark-field electron micrograph and selected area diffraction pattern of the as-spun ribbon with Mo addition. The micrograph confirms that the ribbon has a two-phase structure consisting of the 2/17 phase (gray) and a-Fe phase (white) with an average grain size of 35—50 nm. The selected area diffraction pattern indicates that the microstructure is crystallographically isotropic. The fine-grained microstructure is necessary to obtain a high coercivity and a remanence enhancement. However, the grain size of a-Fe is much larger than that predicted by theoretical models for the optimal hardening [10,11]. According to the SEM analysis and the XRD measurements, it can
Fig. 5. TEM dark-field microgrph and selected area diffraction pattern of as-spun Sm Fe Mo Al C ribbon with 2 (15.5~x) x 1.5 1.5 x"0.6.
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
119
be concluded that there is no Mo-rich segregation in the as-spun ribbons. Although there is not an additional Mo-rich phase formed during crystallizing and quenching, the addition of Mo can still affect the crystallization characteristic of the alloys. The refractory Mo atoms which substitute into the Fe sites in the 2/17 phase may constitute a localized order structure of the 2/17 phase together with other atoms, and subsequently provide a large amount of crystal nuclei in the liquid. The existence of the nuclei helps obtain a fine-grained structure and meanwhile reduce the amorphousizing tendency of the ribbons. Therefore, rapidly quenched, the ribbons with Mo addition obtain a nanocrystalline structure. 3.3. Magnetic properties The magnetic hardening of the alloys is obtained through rapid quenching and subsequent annealing. Fig. 6 shows the Mo content dependence of the magnetic properties of the as-spun and annealed ribbons. The as-spun ribbons without Mo addition are magnetically soft with very low coercivity H and remanence p . It is obviously due to the # 3 existence of the mixture of fine-grained and amorphous phases. But both the H and p are # 3 remarkably enhanced after adding Mo. With increasing Mo content, the H and p increase # 3 to a certain extent. When x"0.2 and 0.6, the p and H achieve the maximum values of up to 3 # 50.6 emu/g and 1.37 T, respectively. Upon annealing at 750°C, as shown in Fig. 6, the ribbons with and without Mo additions exhibit a different change in magnetic properties. For the Mo-free ribbons, the annealing makes the H and # p to be drastically increased. Through annealing, 3 the ribbons composed of partially amorphous phase attain a fine-grained structure with the 2/17 carbide and a-Fe phases and thus higher H and p . # 3 However, for the Mo-containing ribbons, which have been an entirely crystalline structure, annealing leads to grain coarsing and thus the reduction of H and p . It can be seen from Fig. 6 that all the # 3 H and p of the ribbons with different Mo content # 3 decrease to a certain extent after annealing. Consequently, the Mo-containing ribbons are possible to obtain an optimal structure by direct quenching.
Fig. 6. Dependence of the magnetic properties of as-spun and annealed ribbons on Mo content.
Fig. 7. Room-temperature hysteresis loop Sm Fe Mo Al C ribbon spun at 40 m s~1. 2 14.9 0.6 1.5 1.5
of
Fig. 7 shows the hysteresis loop of the as-spun Sm Fe Mo Al C ribbons with x"0.6. 2 (15.5~x) x 1.5 1.5 Only minor loops can be attained in a maximum applied field of 2.0 T. As expected in the isotropic uniaxial magnetic materials, the ratio of p to p is 3 H
120
W. Tang et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 115—120
greater than 0.7 (The p is the magnetization meaH sured at the maximum applied field of 2.0 T). The optimal coercivity (1.37 T) is higher than that of Sm Fe Zr Al C ribbons (1.16 T) [12]. It 2 15.3 0.2 1.5 1.5 indicates that the addition of Mo is more effective on improving the coercivity. It may be noticed that the demagnetization curve exhibits a shouldershaped step. This characteristic could be caused by the presence of a certain amount of coarse-grained a-Fe, which can be observed from Fig. 5. The a-Fe and 2/17 phases lead to a weak exchange coupling and reverse incongruously to produce a shouldered hysteresis loop and a lower remanence. Moreover, the existence of the coarse-grained a-Fe could increase the reversed component of magnetization at a given reverse field and lead to low coercive forces. Therefore it is possible that the remanence and coercivity can be further improved by controlling the microstructure to obtain a homogeneous nanocrystalline structure. Such a study is now in progress.
4. Conclusions (1) The addition of Mo to the Sm Fe 2 (15.5~x) Mo Al C alloys results in the reduction of x 1.5 1.5 saturation magnetization. During crystallizing and quenching, the addition of Mo can lead to fine grains, and at the same time decline the amorphous tendency of ribbons. (2) The quenching process can stabilize the 2/17 phase structure, and inhibit the formation of free iron dendrites. (3) By direct quenching at 40 m s~1, the ribbons with Mo addition attain a nanocrystalline structure, and thus magnetic hardening. An optimal coercivity of up to 1.37 T along with a remanence of up to 49.6 emu/g is obtained for the as-spun
Sm Fe Mo Al C ribbon. In contrast to 2 14.9 0.6 1.5 1.5 Zr, Mo is more effective on improving the coercivity for the studied alloys.
Acknowledgements We are grateful to Professor M. Lu for the assistance in the magnetic measurements. This work was supported by grant projects NMS and NSFC, and Jiang Su Province under contract No. BJ97043, People’s Republic of China.
References [1] B.G. Shen, L.S. Kong, F.W. Wang, L. Cao, J. Appl. Phys. 75 (1994) 6253. [2] Z.H. Cheng, B.G. Shen, F.W. Wang, J.X. Zhang, J. Phys.: Condens. Matter 6 (1994) L185. [3] S. Sugimoto, K. Kurihara, H. Nakamura, M. Okda, M. Homma, Mater. Trans. JIM 33 (1992) 146. [4] Y.H. Zheng, A.S. Murthy, F.M. Yang, G.C. Hadjipanayis, J. Magn. Magn. Mater. 140—144 (1995) 1081. [5] G.C. Hadjipanayis, Y.H. Zheng, A.S. Murthy, W. Gong, F.M. Yang, J. Alloys Compounds 222 (1995) 49. [6] H. Horiuchi, U. Koike, H. Kaneko, T. Kurino, H. Uchida, J. Alloys Compounds 222 (1995) 127. [7] L.J. Fang, L. Bo, Y. Xiaojun, P. Wei, D. Linzhi, Proc. 7th Int. Symp. on Magnetic Anisotropy and Coercivity in RE-TM Alloys, Canberra, July 1992, Hi-Perm Laboratory, Research Center for Advanced Mineral and Materials Processing, University of West Australia, Nedlands, 1992, p. 372. [8] T. Schrefl, J. Fidler, H. Kronmu¨ller, J. Magn. Magn. Mater. 138 (1994) 291. [9] A. Manaf, R.A. Buckley, H.A. Davis, M. Leonowiez, J, Magn. Magn. Mater. 101 (1990) 360. [10] E.F. Kneller, R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [11] T. Schrefl, H. Kronmu¨ller, J. Fidler, Phys. Rev. B 49 (1994) 6100. [12] W. Tang, Z.Q. Jin, J.R. Zhang, L.Y. Lu, S.Y. Zhang, Y.W. Du, J. Appl. Phys. 82 (1997) 6351.