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Oxidation-controlled aging of SmCo5 magnets
Journal of the Less-Common
Metals, 77 (1981)
OXIDATION-CONTROLLED
221
221
- 226
AGING OF SmCo5 MAGNETS
P. J. JORGENSEN
SRI International, Menlo Park, CA 94025 (Received
(U.S.A.)
May 19,198O)
Summary The minimum irreversible magnetic aging characteristics of SmCoS magnets can be calculated based on the kinetics of internal oxidation of SmCoe. ~perimen~ly measured open-circuit remanent induction losses determined as a function of time showed excellent correlations with the calculated curves for temperatures up to 200 “C. Above 200 “C deviations occurred from the calculated aging curves based on short-term oxidation kinetics because of coalescence of the oxide fibers in the subscale. Measurement of internal oxidation kinetics at 300 “C for times comparable with the magnetic aging times again allowed accurate computation of the irreversible magnetic losses.
1. Introduction Internal oxidation in SmCo, is extremely rapid. Because the magnetic properties of the alloy are destroyed in the oxidized region, it is of interest to calculate the magnetic aging ch~cte~stics of SmCo, magnets at low temperatures based on internal oxidation kinetics. The calculated aging curves should correspond to the minimum irreversible non-recoverable characteristics that can be achieved in unprotected or non-coated SmCoB permanent magnets. SmCoS exhibits an unusual mode of internal oxidation [l] in which the subscale that is formed consists of a composite microstructure containing samarium oxide fibers and &Co. Scanning electron micrographs taken parallel and pe~endicul~ to the growth direction of the subscale are shown in Fig. 1. The orientation of the oxide fibers is generally perpendicular to the subscale-alloy interface. This unusual oxidation behavior is caused by the high concentration of samarium in the intermetallic compound and the low solubility of samarium in the metallic cobalt phase. The growth of the subscale for relatively short times (10 h) follows the parabolic oxidation law; the rate of oxidation is 0022-5088/81/0000-0000/$02.50
0 Eisevier
Sequoia/Printed
in The Netherlands
(a)
(bf
Fig. 1. Scanning electron micrographs of the oxide-cobalt (a) transverse; (b) longitudinal.
subscale on SmCo5 magnets:
Fig. 2. Internal oxidation of SmCoa as a function of reciprocal temperature.
plotted in Fig. 2 as a function of reciprocal temperature. The kinetics of internal oxidation have been measured between 100 and 1125 “C [1],and extrapoIation of the line shown in Fig. 2 to 100 “C indicates agreement with the experimen~y detained internal oxidation rates [l] . The kinetics of oxidation are too fast to be explained by lattice diffusion in either the oxide or the cobalt phases. Experiments have also shown [l] that the subscale grows at the subscale-alloy interface and that oxygen diffusion down the oxide fiber-cobalt interface is the most probable transport mechanism for the internal oxidation of SmCoS. An interface transport mechanism can also account for the apparent change in activation at the elevated temperature shown in Fig. 2. The oxide fibers increase in size with increasing temperature, beginning at about 750 “C. The increased fiber size decreases the interfacial area per unit volume of subscale, which decreases the oxidation rate. An increase in oxide fiber
223
size should also be expected at temperatures lower than 750 “C for long-term exposures to the oxidizing environment.
2. Calculated aging behavior Rare earth-Co (R-Co) compounds of the CaCu, type are metastable at low temperatures [2 - 41 and decompose into the compounds R&o, and This eutectoid decomposition occurs in SmCoB at approximately R&017. 600 “C [4] ; however, below about 500 “C! the decomposition kinetics are so slow that microstructural changes cannot be observed. In calculating the aging curves for SmCos magnets it was assumed that long-term exposures at temperatures at or below 300 “C do not result in a loss of magnetic properties due to eutectoid decomposition. The calculated aging curves are based on microstructural changes and are therefore related to the total remanent magnetization after subtracting the recoverable magnetization losses due to heating to the elevated temperatures. In SmCos the recoverable magnetic losses, i.e. those magnetic losses that can be recovered by remagnetization after cooling to room temperature, occur relatively quickly, i.e. within the first 1 or 2 h at room temperature [ 51. Therefore the irreversible magnetic losses can be determined in principle by normalizing the magnetic loss data using the remanent magnetization that remains after the initial recoverable losses have occurred. The calculated magnetic aging characteristics presented in this work are based on the assumption that the magnetic properties of the unoxidized alloy remain constant with time after the initial recoverable losses have occurred. The oxidation kinetics of the outer mixed cobalt oxide-samarium oxide scale on SmCo5 have not been measured, but it is reasonable to assume that these kinetics are essentially similar to the oxidation kinetics of cobalt [6], since the oxidation occurring at the outer scale is the oxidation of B-Co. If we assume that the outer-scale oxidation kinetics are exactly equal to the oxidation kinetics of cobalt, and if we neglect the outer-scale oxidation in computing the total oxide scale thickness, the error at 300 “C is equal to 0.4%. The thickness of the outer scale was therefore neglected in computing the volume of the oxidized layer that builds up during aging of SmCos magnets. The inner oxide scale (Sm,O, + /~-CO)formed on SmCor, has a slightly larger volume than the original SmCos volume. If this increase in volume is neglected in computing the amount or thickness of SmCo, that has been oxidized, it will result in an error of 1.6%. This error is in the opposite direction to the error caused by neglecting the outer-scale oxidation. Combining the two errors would result in an error at 300 “C of 1.2%. Since the magnitude of this error is within the experimental error of the oxidation and magnetic aging measurements, it has been neglected in calculating the magnetic aging curves.
224
The internal oxidation kinetics [l] were determined using times of less than 10 h. In calculating the extent of oxidation occurring during magnetic aging it has been assumed that the microst~c~re of the internal oxide scale does not change. The data presented in this work indicate that this assumption is reasonable at 150 “C but that it is a poor assumption at 300 “C. The magnetization or the magnetic moment of the entire SmCoe magnet is a function of the volume of the magnet; therefore the variation with time of the m~etization divided by the value of the m~etization after the initial recoverable losses have occurred should be equal to the volume of the unoxidized SmCos divided by the original volume of the magnet. For the purposes of computation in this work the magnets were assumed to have a cylindrical geometry without internal cracks or open porosity.
3. Comp~~son of c~culat~
and expe~men~
aging behavior
The best magnetic aging data currently available are the data of Mildrum and coworkers [ 5,7] , who measured the open-circuit remanent induction (OCRI) of many SmCo, magnets as a function of time. They defined the initial magnetic losses as those losses that occurred during the first 15 min of aging, and they used the flux value after 15 min of exposure at the testing temperatu~ to normalize their msgnetic aging data. If we compare these data with the volume of the unoxidized SmCoS divided by the original volume (V/V,), then we are assuming the following. (1) The magnet in the remanent state is uniformly magnetized. (2) The local intrinsic coercive force is greater than the local demagnetizing field. (3) The demagnetizing field is uniform. The last ~sumption is strictly true only for ellipsoidal shapes and the first two assumptions are true for a magnet that has an ideally square M uersus H curve, Therefore we shall compare the norm~iz~ OCR1 data of Mild~m and Strnat [5] with V/V, for the meets with a geometry corresponding to B/H = 1.0, recognizing that the comparison is idealized. Figure 3 is a comparison of normalized OCR1 data with calculated V/V,, data for “as-received” (no prior thermal knockdown) magnets. The full lines are the normalized volume changes calculated from the internal oxidation kinetics and the data points are the best aging data obtained by ~ild~rn and Strnat [ 5] at the ~mperat~es indicated. A similar set of curves is shown in Fig, 4 for magnets that were prestabilized by heating for 2 h at a temperature 50 “C above the testing temperature before measuring the magnetic losses. The oxide film formed during the prestabilization heat treatment was taken into account in calculating the V/V, curves. A comparison of the experimental points with the calculated curves in Figs. 3 and 4 shows excellent agreement at 150 and 200 “C but rather poor
225
Fig. 3. Comparison of normalized OCR1 aging data of “as-received” magnets with calculated normalized volume changes.
0951 200’ 0
’ 400
’
’
6W
BW
1 Iax TIME,
~~
izw
1 14w
’ 1f.w
1
’
18W
2cm
howl
Fig. 4. Comparison of normalized OCR1 aging data for prestabilized magnets with calculated normalized volume changes.
agreement at 250 and 300 “C. The experimental data indicate that the aging at 300 OC,for example, does not occur as fast in the best aging magnets as would be predicted by the internal oxidation kinetics. We therefore suspected that the assumption of a constant microstructure for the internal oxide scale is incorrect and that for long times at 250 and 300 “C! the samarium oxide fibers in the oxide+Co scale are coalescing. The driving force for the postulated coalescence is the reduction of the interfacial energy. The internal oxidation kinetics of SmCo5 were therefore measured in air at 300 “C for times comparable with the magnetic aging times using arcmelted alloys. The parabolic oxidation rate shown in Fig. 2 for 300 “C holds for approximately 200 h and then the oxidation deviates from the parabolic law to lower oxide thickness values. For example, at 900 h the initial oxidation kinetics would predict an oxide layer thickness of 49.2 pm, whereas the experimentally measured oxide thickness was 36 pm. Examination of the microstructure of the internal oxide scale after 905 h at 300 “C shows that the fibers have started to sinter together to form necks along the fiber axes and have broken into shorter lengths with a tendency toward spheroidization. A new V/V,, uersus time curve, shown in Fig. 5, was calculated based on the long-term oxidation experiments at 300 “C. Figure 5 also contains the best magnetic aging data obtained by Mildrum and Stmat [ 51 for comparison with the calculated curve. The agreement between the two sets of data is reasonable, with the internal oxidation kinetics predicting slightly better
226
Fig. 5. Comparison of normalized OCR1 aging data for “as-received” magnets at 300 “C with calculated normalized volume changes based on long-term oxidation data.
aging characteristics. The faster magnetic aging observed experimentally could be due to cracks or open porosity in the magnet. 4. Conclusions The internal oxidation kinetics in SmCoB are exceptionally fast and the extent of the structural changes caused by this oxidation can be used to predict the minimum irreversible magnetic losses occurring during long-term magnetic aging of SmCoB. The calculated oxidation-controlled aging curves at 150 - 300 “C are in agreement with the best experimentally measured magnetic aging curves and it is reasonable to conclude that the magnetic aging process is dominated by the internal oxidation of SmCo5. The internal oxidation kinetics measured for short times (less than 10 h) provide data whereby accurate magnetic aging curves can be calculated at 150 and 200 “C. Since the oxidation occurs by transport of oxygen along the cobalt-oxide fiber interface, we can conclude that the microstructure of the internal oxide scale remains essentially constant at 150 and 200 “C or that coalescence of the oxide fibers is not occurring at these temperatures; however, the kinetics of coalescence are fast enough at 300 “C to cause changes in the microstructure and these changes result in a reduction in the long-term oxidation kinetics. References R. X. Bartlett and P. J. Jorgensen, Metoll. Trans., 5 (1974) 355. F. J. A. den Broeder and K. H. J. Buschow, J. Less-Common Met., 29 (1972) 65. K. H. J. Buschow, J. Less-Common Met., 29 (1972) 283. K. H. J. Buschow, J. Less-Common Met., 37 (1974) 91. H. F. Mildrum and K. J. Strnat, Research to investigate the aging characteristics of samarium cobalt magnets, U.S. Air Force Materials Laboratory, Final Rep. AFMLTR-74-50, March 1974. E. A. Gulbransen and K. F. Andrew, J. Electrochem. Sot., 98 (1951) 241. H. F. Mildrum, M. F. Hartings, K. D. Wong and J. K. Strnat, IEEE Trans. Magn., 10 (1974) 723.
Metals, 77 (1981)
OXIDATION-CONTROLLED
221
221
- 226
AGING OF SmCo5 MAGNETS
P. J. JORGENSEN
SRI International, Menlo Park, CA 94025 (Received
(U.S.A.)
May 19,198O)
Summary The minimum irreversible magnetic aging characteristics of SmCoS magnets can be calculated based on the kinetics of internal oxidation of SmCoe. ~perimen~ly measured open-circuit remanent induction losses determined as a function of time showed excellent correlations with the calculated curves for temperatures up to 200 “C. Above 200 “C deviations occurred from the calculated aging curves based on short-term oxidation kinetics because of coalescence of the oxide fibers in the subscale. Measurement of internal oxidation kinetics at 300 “C for times comparable with the magnetic aging times again allowed accurate computation of the irreversible magnetic losses.
1. Introduction Internal oxidation in SmCo, is extremely rapid. Because the magnetic properties of the alloy are destroyed in the oxidized region, it is of interest to calculate the magnetic aging ch~cte~stics of SmCo, magnets at low temperatures based on internal oxidation kinetics. The calculated aging curves should correspond to the minimum irreversible non-recoverable characteristics that can be achieved in unprotected or non-coated SmCoB permanent magnets. SmCoS exhibits an unusual mode of internal oxidation [l] in which the subscale that is formed consists of a composite microstructure containing samarium oxide fibers and &Co. Scanning electron micrographs taken parallel and pe~endicul~ to the growth direction of the subscale are shown in Fig. 1. The orientation of the oxide fibers is generally perpendicular to the subscale-alloy interface. This unusual oxidation behavior is caused by the high concentration of samarium in the intermetallic compound and the low solubility of samarium in the metallic cobalt phase. The growth of the subscale for relatively short times (10 h) follows the parabolic oxidation law; the rate of oxidation is 0022-5088/81/0000-0000/$02.50
0 Eisevier
Sequoia/Printed
in The Netherlands
(a)
(bf
Fig. 1. Scanning electron micrographs of the oxide-cobalt (a) transverse; (b) longitudinal.
subscale on SmCo5 magnets:
Fig. 2. Internal oxidation of SmCoa as a function of reciprocal temperature.
plotted in Fig. 2 as a function of reciprocal temperature. The kinetics of internal oxidation have been measured between 100 and 1125 “C [1],and extrapoIation of the line shown in Fig. 2 to 100 “C indicates agreement with the experimen~y detained internal oxidation rates [l] . The kinetics of oxidation are too fast to be explained by lattice diffusion in either the oxide or the cobalt phases. Experiments have also shown [l] that the subscale grows at the subscale-alloy interface and that oxygen diffusion down the oxide fiber-cobalt interface is the most probable transport mechanism for the internal oxidation of SmCoS. An interface transport mechanism can also account for the apparent change in activation at the elevated temperature shown in Fig. 2. The oxide fibers increase in size with increasing temperature, beginning at about 750 “C. The increased fiber size decreases the interfacial area per unit volume of subscale, which decreases the oxidation rate. An increase in oxide fiber
223
size should also be expected at temperatures lower than 750 “C for long-term exposures to the oxidizing environment.
2. Calculated aging behavior Rare earth-Co (R-Co) compounds of the CaCu, type are metastable at low temperatures [2 - 41 and decompose into the compounds R&o, and This eutectoid decomposition occurs in SmCoB at approximately R&017. 600 “C [4] ; however, below about 500 “C! the decomposition kinetics are so slow that microstructural changes cannot be observed. In calculating the aging curves for SmCos magnets it was assumed that long-term exposures at temperatures at or below 300 “C do not result in a loss of magnetic properties due to eutectoid decomposition. The calculated aging curves are based on microstructural changes and are therefore related to the total remanent magnetization after subtracting the recoverable magnetization losses due to heating to the elevated temperatures. In SmCos the recoverable magnetic losses, i.e. those magnetic losses that can be recovered by remagnetization after cooling to room temperature, occur relatively quickly, i.e. within the first 1 or 2 h at room temperature [ 51. Therefore the irreversible magnetic losses can be determined in principle by normalizing the magnetic loss data using the remanent magnetization that remains after the initial recoverable losses have occurred. The calculated magnetic aging characteristics presented in this work are based on the assumption that the magnetic properties of the unoxidized alloy remain constant with time after the initial recoverable losses have occurred. The oxidation kinetics of the outer mixed cobalt oxide-samarium oxide scale on SmCo5 have not been measured, but it is reasonable to assume that these kinetics are essentially similar to the oxidation kinetics of cobalt [6], since the oxidation occurring at the outer scale is the oxidation of B-Co. If we assume that the outer-scale oxidation kinetics are exactly equal to the oxidation kinetics of cobalt, and if we neglect the outer-scale oxidation in computing the total oxide scale thickness, the error at 300 “C is equal to 0.4%. The thickness of the outer scale was therefore neglected in computing the volume of the oxidized layer that builds up during aging of SmCos magnets. The inner oxide scale (Sm,O, + /~-CO)formed on SmCor, has a slightly larger volume than the original SmCos volume. If this increase in volume is neglected in computing the amount or thickness of SmCo, that has been oxidized, it will result in an error of 1.6%. This error is in the opposite direction to the error caused by neglecting the outer-scale oxidation. Combining the two errors would result in an error at 300 “C of 1.2%. Since the magnitude of this error is within the experimental error of the oxidation and magnetic aging measurements, it has been neglected in calculating the magnetic aging curves.
224
The internal oxidation kinetics [l] were determined using times of less than 10 h. In calculating the extent of oxidation occurring during magnetic aging it has been assumed that the microst~c~re of the internal oxide scale does not change. The data presented in this work indicate that this assumption is reasonable at 150 “C but that it is a poor assumption at 300 “C. The magnetization or the magnetic moment of the entire SmCoe magnet is a function of the volume of the magnet; therefore the variation with time of the m~etization divided by the value of the m~etization after the initial recoverable losses have occurred should be equal to the volume of the unoxidized SmCos divided by the original volume of the magnet. For the purposes of computation in this work the magnets were assumed to have a cylindrical geometry without internal cracks or open porosity.
3. Comp~~son of c~culat~
and expe~men~
aging behavior
The best magnetic aging data currently available are the data of Mildrum and coworkers [ 5,7] , who measured the open-circuit remanent induction (OCRI) of many SmCo, magnets as a function of time. They defined the initial magnetic losses as those losses that occurred during the first 15 min of aging, and they used the flux value after 15 min of exposure at the testing temperatu~ to normalize their msgnetic aging data. If we compare these data with the volume of the unoxidized SmCoS divided by the original volume (V/V,), then we are assuming the following. (1) The magnet in the remanent state is uniformly magnetized. (2) The local intrinsic coercive force is greater than the local demagnetizing field. (3) The demagnetizing field is uniform. The last ~sumption is strictly true only for ellipsoidal shapes and the first two assumptions are true for a magnet that has an ideally square M uersus H curve, Therefore we shall compare the norm~iz~ OCR1 data of Mild~m and Strnat [5] with V/V, for the meets with a geometry corresponding to B/H = 1.0, recognizing that the comparison is idealized. Figure 3 is a comparison of normalized OCR1 data with calculated V/V,, data for “as-received” (no prior thermal knockdown) magnets. The full lines are the normalized volume changes calculated from the internal oxidation kinetics and the data points are the best aging data obtained by ~ild~rn and Strnat [ 5] at the ~mperat~es indicated. A similar set of curves is shown in Fig, 4 for magnets that were prestabilized by heating for 2 h at a temperature 50 “C above the testing temperature before measuring the magnetic losses. The oxide film formed during the prestabilization heat treatment was taken into account in calculating the V/V, curves. A comparison of the experimental points with the calculated curves in Figs. 3 and 4 shows excellent agreement at 150 and 200 “C but rather poor
225
Fig. 3. Comparison of normalized OCR1 aging data of “as-received” magnets with calculated normalized volume changes.
0951 200’ 0
’ 400
’
’
6W
BW
1 Iax TIME,
~~
izw
1 14w
’ 1f.w
1
’
18W
2cm
howl
Fig. 4. Comparison of normalized OCR1 aging data for prestabilized magnets with calculated normalized volume changes.
agreement at 250 and 300 “C. The experimental data indicate that the aging at 300 OC,for example, does not occur as fast in the best aging magnets as would be predicted by the internal oxidation kinetics. We therefore suspected that the assumption of a constant microstructure for the internal oxide scale is incorrect and that for long times at 250 and 300 “C! the samarium oxide fibers in the oxide+Co scale are coalescing. The driving force for the postulated coalescence is the reduction of the interfacial energy. The internal oxidation kinetics of SmCo5 were therefore measured in air at 300 “C for times comparable with the magnetic aging times using arcmelted alloys. The parabolic oxidation rate shown in Fig. 2 for 300 “C holds for approximately 200 h and then the oxidation deviates from the parabolic law to lower oxide thickness values. For example, at 900 h the initial oxidation kinetics would predict an oxide layer thickness of 49.2 pm, whereas the experimentally measured oxide thickness was 36 pm. Examination of the microstructure of the internal oxide scale after 905 h at 300 “C shows that the fibers have started to sinter together to form necks along the fiber axes and have broken into shorter lengths with a tendency toward spheroidization. A new V/V,, uersus time curve, shown in Fig. 5, was calculated based on the long-term oxidation experiments at 300 “C. Figure 5 also contains the best magnetic aging data obtained by Mildrum and Stmat [ 51 for comparison with the calculated curve. The agreement between the two sets of data is reasonable, with the internal oxidation kinetics predicting slightly better
226
Fig. 5. Comparison of normalized OCR1 aging data for “as-received” magnets at 300 “C with calculated normalized volume changes based on long-term oxidation data.
aging characteristics. The faster magnetic aging observed experimentally could be due to cracks or open porosity in the magnet. 4. Conclusions The internal oxidation kinetics in SmCoB are exceptionally fast and the extent of the structural changes caused by this oxidation can be used to predict the minimum irreversible magnetic losses occurring during long-term magnetic aging of SmCoB. The calculated oxidation-controlled aging curves at 150 - 300 “C are in agreement with the best experimentally measured magnetic aging curves and it is reasonable to conclude that the magnetic aging process is dominated by the internal oxidation of SmCo5. The internal oxidation kinetics measured for short times (less than 10 h) provide data whereby accurate magnetic aging curves can be calculated at 150 and 200 “C. Since the oxidation occurs by transport of oxygen along the cobalt-oxide fiber interface, we can conclude that the microstructure of the internal oxide scale remains essentially constant at 150 and 200 “C or that coalescence of the oxide fibers is not occurring at these temperatures; however, the kinetics of coalescence are fast enough at 300 “C to cause changes in the microstructure and these changes result in a reduction in the long-term oxidation kinetics. References R. X. Bartlett and P. J. Jorgensen, Metoll. Trans., 5 (1974) 355. F. J. A. den Broeder and K. H. J. Buschow, J. Less-Common Met., 29 (1972) 65. K. H. J. Buschow, J. Less-Common Met., 29 (1972) 283. K. H. J. Buschow, J. Less-Common Met., 37 (1974) 91. H. F. Mildrum and K. J. Strnat, Research to investigate the aging characteristics of samarium cobalt magnets, U.S. Air Force Materials Laboratory, Final Rep. AFMLTR-74-50, March 1974. E. A. Gulbransen and K. F. Andrew, J. Electrochem. Sot., 98 (1951) 241. H. F. Mildrum, M. F. Hartings, K. D. Wong and J. K. Strnat, IEEE Trans. Magn., 10 (1974) 723.