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The dynamic and in situ observation of the permanent magnet Sm(Co, Cu, Fe, Zr)7.4
154
Journal
THE DYNAMIC AND IN SITU OBSERVATION Sm(Co, Cu, Fe, Zr)7.4 PAN Shuming
a and ZHAO
of Magnetism
and Magnetic
OF THE PERMANENT
Materials 75 (1988) 154-158 North-Holland, Amsterdam
MAGNET
Zhibo b
0 General Research Institute of Non-Ferrous Metals CNNC, Beijing, P.R. China b Department Received
of Materials Physics, Beijing University of Iron and Steel Technology, Beijing, P.R. China
26 April 1988
The dynamic and in situ observation of the precipitation hardened 2: 17 rare earth cobalt magnet been carried out by means of HVEM. The fine dispersed cellular structures begin to appear at 460 morphology forms until 700 o C. The whole process of formation and growth of thin lamellae has indicate that the high coercivity and the fact.that the shape of the virgin magnetization curve is not not induced by the appearance of the thin lamellae directly. The coercivity mechanism should factors.
1. Introduction The 2 : 17 rare earth cobalt magnets have been studied intensively in recent years. One of the reasons for the increasing interest for such magnets is that their magnetic properties are superior to those of 1: 5 magnets. The 2 : 17 magnets can be generally represented as Sm(Co, Fe, Cu, M),,, with M = Zr, Ti and Hf, and exhibit the coercivity in the range of 5-20 kOe and energy product as high as 33 MGOe [l]. These magnets belong to the precipitation hardened family whose microstructures have been reported by many investigators (e.g. see refs. [2-71). It is well known that the coercive force is strongly influenced by the shape and size of the cellular structures, whereas the latter is closely related to the composition and heat treatment. In the case of relatively low coercivity (< 7 kOe), the typical microstructure is that the Sm,Co,, cells are surrounded by the cell boundaries 1 : 5 phase. However, the rnicrostructure of the high coercivity magnet consists of thin lamellae superimposing on the cellular morphology. Meanwhile, it was also reported that the shape of the virgin magnetization curve for high coercivity magnet is not in agreement with the typical pinning mechanism [8]. Since the coercivity of the precipitation hardened magnet at certain composition is strongly dependent on the heat 0304-8853/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Sm(Co, Cu, Fe, Zr),,, has o C and the perfect cellular been exhibited. The results the typical pinning one are be considered from other
treatment [5], it is useful to investigate how the microstructures are influenced by the aging temperature and the holding time at certain temperature. As we know the dynamic and in situ observations have so far not been reported in great detail. In the present work, the observation on the microstructure at different temperatures (room temperature to 850” C) has been undertaken. It is unfortunate that further holding at 85O’C and observation at higher temperatures are not possible due to experimental limitations of the apparatus, therefore, simple discussion is made according to the presently available results.
2. Experimental The alloy was prepared by arc-melting of the constituents: 26.01% Sm, 51.38% Co, 4.36% Cu, 15.19% Fe and 2.99% Zr in a cooled copper crucible under argon atmosphere. The ingot was pulverized into powders with a few km in size. After sintering, the samples were homogenized at 1180 o C for half an hour and then quenched. The discs with thickness of 0.25 mm were cut down perpendicular to the c axis. The thin film was obtained after mechanical polishing and ion thinning. The observation was carried out on the JEM-1000X equipped with a heating apparatus, B.V.
S. Pan, Z. Zhao / Permanent magnet Sm(Co, Cu, Fe, Zr),,
Fig. 1. The temperature-time
curve showing
whereas the microstructures exhibited heating process were viedotaped. The thermal profile is represented ture-time curve in fig. 1.
the thermal
profile, the letters represent
the spots at which a micrograph
155
is taken.
in the whole by tempera-
3. Results Fig. 2 is the micrograph of the magnet at room temperature. The contrast and the electron diffraction readily confirm that the microstructure is single phase of solid solution. As the temperature is further increased, no remarkable change of microstructure can be found, and no precipitation
Fig. 2. The micrograph of the magnet homogenized for 0.5 hour at room temperature, 1OOOOOX , bright corresponding to a in fig. 1.
at 1180 o C field image,
Fig. 3. The micrograph of ihe magnet at 460 0 C, ~OOOOOX, corresponding to b in fig. 1.
Fig. 4. The micrograph of the magnet at 500 “C,’ 1OOOOOX, correspondng to c in fig. 1.
156
S. Pan, 2. Zhao / Permanent magnet Sm(Co, Cu, Fe, Zr),.,
Fig. 5. The micrograph of the magnet at 700 o C, 100000X, corresponding to d in fig. 1. *
Fig. 6. The micrograph of the magnet at 780° C, 100000X, corresponding to e in fig. 1. *
Fig. 7. The micrograph of the magnet at 800° C, 100000X, corresponding to f in fig. 1. *
Fig. 8. The micrograph of the magnet at 820 0 C, 100000X, corresponding to g in fig. 1.
phases can be resolved until the temperature is up to 460°C. It is clearly shown in figs. 3 and 4 that the embryonic form of the cellular structure appears dispersely and the cellular structure grows in size continuously. Perfectly cellular morphology forms at about 700° C and the micrograph is given in fig. 5. However, the cellular morphology in this case is not completely the same as the previously reported one (e.g. see ref. [5]) taken from the aged magnet at room temperature. The difference may be due to the fact that the former is observed at the high temperature state. When temperature is further raised to 780°C the thin lamellae superimposing on the cellular morphology begin to be observed. Such lamellae are temperature dependent both in size and in quantity in the temperature range 780-840 o C (see figs. 6-9). Comparing the micrographs in fig. 9 and in fig. 10, it can be noted that the situation of thin lamellae are also related to the holding time at a certain temperature (e.g. 840°C here). As the sample is held at 840 o C for half an hour, the thin lamellae are already coarse enough to cross the cells.
* The pictures are reproduced from the videotape because of the difficulty in the actual controlling of temperature during the experiment.
S. Pan, Z. Zhao / Permanent magnet Sm(Co, Cu, Fe, Zr),,,
Fig. 9. The micrograph
of the magnet
at 840 o C corresponding
4. Discussion The dynamic and in situ observation of the formation of cellular structures indicates the importance of the temperature. In practice, the effect of aging temperature has been well noted in the preparation of 2 : 17 rare earth cobalt magnets. For low coercivity magnets, an example of the relationship between the coercivity and the aging temperature was given in ref. [9]. These results can be explained according to the presented dynamic and in situ observation on microstructure of the magnets. In our observation, no precipitation phases can be found at low temperature (< 460°C) even though they appear, the cellular
Fig. 10. The micrograph of the magnet at 840°C for half an hour, corresponding to i in fig. 1, 1000@4 x
157
structures are small in size and they are not large enough to touch one another during the initial stage. The growth of the embryos to the perfect cellular morphology needs further increasing of the temperature to 7Od” C. The influence of the microstructure on coercivity has been elaborated by Fidler et al. on various 2 : 17 systems with different compositions [5]. It has been shown that the cellular morphology is responsible to the coercivity of the magnets. The relatively low coercivity is attributed to the domain wall pinning at the cell boundary. As a consequence, the aging temperature in practical preparation of the magnet must be higher than 700°C for forming a perfect cellular morphology, i.e. the effect of the cellular morphology on coercivity is embodied through the aging temperature. As previously reported, the microstructure of high coercivity magnets are the mixtures of cellular and thin lamellae. Electron diffraction studies suggest that thin lamellae are 1: 3 type phase [4], whereas the lattice image indicate a 2 : 17 hexagonal structure [lO,ll]. Superficially it may lead to the conclusion that high coercivity is induced by the appearance of thin lamellae. However, the dynamic and in situ observation do not support it. From fig. 10, it can be observed that thin lamellae are already coarse and cell across when the specimen is held at 840 o C for half an hour. If the thin lamellae are main reason for high coercivity, about half an hour is enough for holding at the peak aging temperature. Actually, the coercivity is strongly dependent on the holding time [12]. It has been shown that lo-20 hours are needed at aging temperature for coercivity increasing with large scale. Therefore, we believe that high coercivity is not induced by the appearance of thin lamellae directly, other factors should be considered. It should be noted that Zr element has the significant effects on the cellular morphology as well as thin lamellae [l]. Although thin lamellae can not lead to the immediate enhancement of coercivity once they appear, they may behave as the diffusion channels. For the short time aged treatment, the cellular morphology superimposed by thin lamellae suffer the less composition fluctuation, the diffusion of Zr and Cu elements can be readily carried out through the channels as
S. Pan, Z. Zhoo / Permanent magnet Sm(Co, Cu, Fe, 2r),,4
158
the magnets are long time aged. If so, the difference of the compositions between the cell interior and the cell boundary could be facilitated and hence, the coercivity can be enhanced due to the higher energy barrier at pinning sites. In the case of coexistence of thin lamellae and cellular morphology, the pinning sites may redistribute with respect to that of cellular morphology alone. It can be suggested that the domain walls situate at the places with different pinning ability, which can explain the fact that the shape of the virgin magnetization curve is not the typical pinning one. Further research is needed to clarify the exact knowledge about the high coercivity mechanism of 2 : 17 magnets.
References [l] G.C. Hadjipanayis, REPM (1983) 483.
in: Proc.
7th Intern.
Workshop
on
PI J.D. Livingston
and D.L. Martin,
J. Appl. Phys. 48 (1977)
1350. A. Fukuno and 131 R.K. Mishra, G. Thomas, T. Yoneyama, T. Ojima, J. Appl. Phys. 52 (1981) 2517. R.K. Mishra and G. Thomas, J. Appl. [41 L. Rabenberg, Phys. 53 (1982) 2389. 151J. Fidler and P. Skalicky, J. Magn. Magn. Mat. 27 (1982) 127. in: Proc. 7th PI J. Fidler, P. Skalicky and F. Rothwarf, Intern. Workshop on REPM (1983) 319. E.J. Yadlowsky and S.H. Wollins, J. 171 G.C. Hadjipanayis, Appl. Phys. 53 (1982) 2386. WI Sun Tian duo, Gui Rongli and Zhao Hui, in: Proc. 7th Intern. Workshop on REPM (1983) 329. (1979). 191 Sun Daku and Liu Yujin, in: Proc. Intermag-MMM 1101J. Fidler, P. Skahcky and F. Rothwarf, in: Intermag Conf. Philadelphia, USA (1983). and K.R. Lawless, Electron Mi[illG.C. Hadjipanayis croscopy Society of America Meeting, Phoenix, Arizona (1983). WI Sun Daku and Liu Yujin, in: Proc. 6th Intern. Workshop on REPM (1982) 709.
Journal
THE DYNAMIC AND IN SITU OBSERVATION Sm(Co, Cu, Fe, Zr)7.4 PAN Shuming
a and ZHAO
of Magnetism
and Magnetic
OF THE PERMANENT
Materials 75 (1988) 154-158 North-Holland, Amsterdam
MAGNET
Zhibo b
0 General Research Institute of Non-Ferrous Metals CNNC, Beijing, P.R. China b Department Received
of Materials Physics, Beijing University of Iron and Steel Technology, Beijing, P.R. China
26 April 1988
The dynamic and in situ observation of the precipitation hardened 2: 17 rare earth cobalt magnet been carried out by means of HVEM. The fine dispersed cellular structures begin to appear at 460 morphology forms until 700 o C. The whole process of formation and growth of thin lamellae has indicate that the high coercivity and the fact.that the shape of the virgin magnetization curve is not not induced by the appearance of the thin lamellae directly. The coercivity mechanism should factors.
1. Introduction The 2 : 17 rare earth cobalt magnets have been studied intensively in recent years. One of the reasons for the increasing interest for such magnets is that their magnetic properties are superior to those of 1: 5 magnets. The 2 : 17 magnets can be generally represented as Sm(Co, Fe, Cu, M),,, with M = Zr, Ti and Hf, and exhibit the coercivity in the range of 5-20 kOe and energy product as high as 33 MGOe [l]. These magnets belong to the precipitation hardened family whose microstructures have been reported by many investigators (e.g. see refs. [2-71). It is well known that the coercive force is strongly influenced by the shape and size of the cellular structures, whereas the latter is closely related to the composition and heat treatment. In the case of relatively low coercivity (< 7 kOe), the typical microstructure is that the Sm,Co,, cells are surrounded by the cell boundaries 1 : 5 phase. However, the rnicrostructure of the high coercivity magnet consists of thin lamellae superimposing on the cellular morphology. Meanwhile, it was also reported that the shape of the virgin magnetization curve for high coercivity magnet is not in agreement with the typical pinning mechanism [8]. Since the coercivity of the precipitation hardened magnet at certain composition is strongly dependent on the heat 0304-8853/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Sm(Co, Cu, Fe, Zr),,, has o C and the perfect cellular been exhibited. The results the typical pinning one are be considered from other
treatment [5], it is useful to investigate how the microstructures are influenced by the aging temperature and the holding time at certain temperature. As we know the dynamic and in situ observations have so far not been reported in great detail. In the present work, the observation on the microstructure at different temperatures (room temperature to 850” C) has been undertaken. It is unfortunate that further holding at 85O’C and observation at higher temperatures are not possible due to experimental limitations of the apparatus, therefore, simple discussion is made according to the presently available results.
2. Experimental The alloy was prepared by arc-melting of the constituents: 26.01% Sm, 51.38% Co, 4.36% Cu, 15.19% Fe and 2.99% Zr in a cooled copper crucible under argon atmosphere. The ingot was pulverized into powders with a few km in size. After sintering, the samples were homogenized at 1180 o C for half an hour and then quenched. The discs with thickness of 0.25 mm were cut down perpendicular to the c axis. The thin film was obtained after mechanical polishing and ion thinning. The observation was carried out on the JEM-1000X equipped with a heating apparatus, B.V.
S. Pan, Z. Zhao / Permanent magnet Sm(Co, Cu, Fe, Zr),,
Fig. 1. The temperature-time
curve showing
whereas the microstructures exhibited heating process were viedotaped. The thermal profile is represented ture-time curve in fig. 1.
the thermal
profile, the letters represent
the spots at which a micrograph
155
is taken.
in the whole by tempera-
3. Results Fig. 2 is the micrograph of the magnet at room temperature. The contrast and the electron diffraction readily confirm that the microstructure is single phase of solid solution. As the temperature is further increased, no remarkable change of microstructure can be found, and no precipitation
Fig. 2. The micrograph of the magnet homogenized for 0.5 hour at room temperature, 1OOOOOX , bright corresponding to a in fig. 1.
at 1180 o C field image,
Fig. 3. The micrograph of ihe magnet at 460 0 C, ~OOOOOX, corresponding to b in fig. 1.
Fig. 4. The micrograph of the magnet at 500 “C,’ 1OOOOOX, correspondng to c in fig. 1.
156
S. Pan, 2. Zhao / Permanent magnet Sm(Co, Cu, Fe, Zr),.,
Fig. 5. The micrograph of the magnet at 700 o C, 100000X, corresponding to d in fig. 1. *
Fig. 6. The micrograph of the magnet at 780° C, 100000X, corresponding to e in fig. 1. *
Fig. 7. The micrograph of the magnet at 800° C, 100000X, corresponding to f in fig. 1. *
Fig. 8. The micrograph of the magnet at 820 0 C, 100000X, corresponding to g in fig. 1.
phases can be resolved until the temperature is up to 460°C. It is clearly shown in figs. 3 and 4 that the embryonic form of the cellular structure appears dispersely and the cellular structure grows in size continuously. Perfectly cellular morphology forms at about 700° C and the micrograph is given in fig. 5. However, the cellular morphology in this case is not completely the same as the previously reported one (e.g. see ref. [5]) taken from the aged magnet at room temperature. The difference may be due to the fact that the former is observed at the high temperature state. When temperature is further raised to 780°C the thin lamellae superimposing on the cellular morphology begin to be observed. Such lamellae are temperature dependent both in size and in quantity in the temperature range 780-840 o C (see figs. 6-9). Comparing the micrographs in fig. 9 and in fig. 10, it can be noted that the situation of thin lamellae are also related to the holding time at a certain temperature (e.g. 840°C here). As the sample is held at 840 o C for half an hour, the thin lamellae are already coarse enough to cross the cells.
* The pictures are reproduced from the videotape because of the difficulty in the actual controlling of temperature during the experiment.
S. Pan, Z. Zhao / Permanent magnet Sm(Co, Cu, Fe, Zr),,,
Fig. 9. The micrograph
of the magnet
at 840 o C corresponding
4. Discussion The dynamic and in situ observation of the formation of cellular structures indicates the importance of the temperature. In practice, the effect of aging temperature has been well noted in the preparation of 2 : 17 rare earth cobalt magnets. For low coercivity magnets, an example of the relationship between the coercivity and the aging temperature was given in ref. [9]. These results can be explained according to the presented dynamic and in situ observation on microstructure of the magnets. In our observation, no precipitation phases can be found at low temperature (< 460°C) even though they appear, the cellular
Fig. 10. The micrograph of the magnet at 840°C for half an hour, corresponding to i in fig. 1, 1000@4 x
157
structures are small in size and they are not large enough to touch one another during the initial stage. The growth of the embryos to the perfect cellular morphology needs further increasing of the temperature to 7Od” C. The influence of the microstructure on coercivity has been elaborated by Fidler et al. on various 2 : 17 systems with different compositions [5]. It has been shown that the cellular morphology is responsible to the coercivity of the magnets. The relatively low coercivity is attributed to the domain wall pinning at the cell boundary. As a consequence, the aging temperature in practical preparation of the magnet must be higher than 700°C for forming a perfect cellular morphology, i.e. the effect of the cellular morphology on coercivity is embodied through the aging temperature. As previously reported, the microstructure of high coercivity magnets are the mixtures of cellular and thin lamellae. Electron diffraction studies suggest that thin lamellae are 1: 3 type phase [4], whereas the lattice image indicate a 2 : 17 hexagonal structure [lO,ll]. Superficially it may lead to the conclusion that high coercivity is induced by the appearance of thin lamellae. However, the dynamic and in situ observation do not support it. From fig. 10, it can be observed that thin lamellae are already coarse and cell across when the specimen is held at 840 o C for half an hour. If the thin lamellae are main reason for high coercivity, about half an hour is enough for holding at the peak aging temperature. Actually, the coercivity is strongly dependent on the holding time [12]. It has been shown that lo-20 hours are needed at aging temperature for coercivity increasing with large scale. Therefore, we believe that high coercivity is not induced by the appearance of thin lamellae directly, other factors should be considered. It should be noted that Zr element has the significant effects on the cellular morphology as well as thin lamellae [l]. Although thin lamellae can not lead to the immediate enhancement of coercivity once they appear, they may behave as the diffusion channels. For the short time aged treatment, the cellular morphology superimposed by thin lamellae suffer the less composition fluctuation, the diffusion of Zr and Cu elements can be readily carried out through the channels as
S. Pan, Z. Zhoo / Permanent magnet Sm(Co, Cu, Fe, 2r),,4
158
the magnets are long time aged. If so, the difference of the compositions between the cell interior and the cell boundary could be facilitated and hence, the coercivity can be enhanced due to the higher energy barrier at pinning sites. In the case of coexistence of thin lamellae and cellular morphology, the pinning sites may redistribute with respect to that of cellular morphology alone. It can be suggested that the domain walls situate at the places with different pinning ability, which can explain the fact that the shape of the virgin magnetization curve is not the typical pinning one. Further research is needed to clarify the exact knowledge about the high coercivity mechanism of 2 : 17 magnets.
References [l] G.C. Hadjipanayis, REPM (1983) 483.
in: Proc.
7th Intern.
Workshop
on
PI J.D. Livingston
and D.L. Martin,
J. Appl. Phys. 48 (1977)
1350. A. Fukuno and 131 R.K. Mishra, G. Thomas, T. Yoneyama, T. Ojima, J. Appl. Phys. 52 (1981) 2517. R.K. Mishra and G. Thomas, J. Appl. [41 L. Rabenberg, Phys. 53 (1982) 2389. 151J. Fidler and P. Skalicky, J. Magn. Magn. Mat. 27 (1982) 127. in: Proc. 7th PI J. Fidler, P. Skalicky and F. Rothwarf, Intern. Workshop on REPM (1983) 319. E.J. Yadlowsky and S.H. Wollins, J. 171 G.C. Hadjipanayis, Appl. Phys. 53 (1982) 2386. WI Sun Tian duo, Gui Rongli and Zhao Hui, in: Proc. 7th Intern. Workshop on REPM (1983) 329. (1979). 191 Sun Daku and Liu Yujin, in: Proc. Intermag-MMM 1101J. Fidler, P. Skahcky and F. Rothwarf, in: Intermag Conf. Philadelphia, USA (1983). and K.R. Lawless, Electron Mi[illG.C. Hadjipanayis croscopy Society of America Meeting, Phoenix, Arizona (1983). WI Sun Daku and Liu Yujin, in: Proc. 6th Intern. Workshop on REPM (1982) 709.