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The influence of niobium on the magnetic properties of the columnar alnico 5 permanent magnets
Journal of Magnetism and Magnetic Materials 24 (1981) 285-287 North-Holland Publishing Company
285
THE INFLUENCE OF NIOBIUM ON THE MAGNETIC PROPERTIES OF THE COLUMNAR ALNICO 5 PERMANENT MAGNETS S. SZYMURA Institute of Physics, Technical University o f Cz{stochowa, Poland and
S. GOLBA Steel Works "Baildon ", Katowice, Poland
Received 1 May 1981
An investigation of the influence of Nb addition (0-1.24%) on Alnico 5 columnar alloy's magnetic properties showed that Nb content up to 0.5% slightly decrease remanent induction while at the same time increasing coercivity and improving the rectangularity of the demagnetization curve. The addition of niobium above 0.5% causes a further increase in coercivity simultaneously decreasing magnetic induction in every point of the demagnetization curve.
1. Introduction Many problems are solved concerning remagnetization theory in the magnetically hard materials of the Alnico type, and basic composition and microaddition influence of some elements on the material's magnetic properties have been established. Nevertheless magnetic properties of the Alnico type alloys obtained in practice are far from those theoretically predicted [1 ]. There is the still unsolved problem of how to reach higher coercive force and magnetic induction values - closer to the theoretical ones and how to maximize magnetic induction with the highest coercive force existing at the same time. Therefore investigations on the thermal treatment [2,3] and chemical composition [4,5] optimization are still carried on with the aim of ensuring the existence of conditions for obtaining the ideal shape of the fine dispersive ferromagnetic particles during the alloy's spinodal decomposition process ensuring magnetization by coherent rotations. The magnetic induction of the Alnico 5 alloy containing 8% A1, 14% Ni, 24% Co, 3% Cu, 51% Fe was increased by forming columnar crystals in the mate03044~853/81/0000--0000/$02.75 © 1981 North-Holland
rial [6-8]. Its coercive force was increased by Ti addition [9,10]. Though a titanium addition results in considerable magnetic hardening it also leads to an appreciable fall in the induction values at all points on the demagnetization curve. Simultaneously obtaining high induction and coercivity can only be reached by additional element alloying or by replacing Ti with another element. It seems necessary to consider more seriously the possibility of controlling magnetic properties by Nb addition alloying. Investigations [11,12] .ofNb influence on the magnetic properties of Alnico type alloys containing 34% Co and 4.5-8% Ti showed that the addition of 0.5% Nb caused a moderate reduction in the remanent induction with a simultaneous substantial increase of coercivity and consequently an improved maximum energy product has been achieved. In spite of the several works done [13-15] the influence of Nb addition on the magnetic properties of Alnico alloys Ti-free has not yet been established. It therefore appeared to be interest to study systematically the influence of Nb on the magnetic properties of titanium-free Alnico 5 alloy with columnar crystals.
286
S. Szymura, S. Golba / Nb and the properties of Alnico 5 permanent magnets The specimens were ground after heat treating so as to eliminate the influence of surface oxidized layers on the magnetic properties.
2. E x p e r i m e n t a l
2.1. Preparation o f the specimens The composition of the tested alloys are shown in table 1. The alloys were melted in an induction furnace from standard, commercial-grade starting materials: iron, electrolytic copper, electrolytic cobalt, carbonyl-nickel and pure aluminium offcuts. The niobium was added as ferro-niobium containing 65% Nb. The columnar structure of the specimens was achieved by ceramic mould casting method [16].
3. Results
Demagnetization curves of columnar Alnico 5 alloys containing 0 to 1.24% Nb are shown in fig. 1. As can be deduced from the diagram niobium addition leads to significant improvement in the coercive force, which increases from 56.5 kA/m in the niobium free alloy to 65.5 kA/m in the 1.24% Nb alloy. In the 0-0.75% Nb interval, the remanent induction (Br) decreases only 0.04 T; when Nb addition is above 0.75% these changes are significantly greater, for instance for 1.24% Nb the reduction of Br in relation to the Nb free alloy is 0.14 T. The analysis of magnetic induction BA and magnetic field H A at the point of maximum magnetic energy (BH)max shows that their values increase up to 0.5% Nb content only, similar to demagnetization curve's rectangularity coefficient 3'. Increasing of Nb content over 0.5% decreases BA as well as B r significantly due to the magnetic induction decreasing in every point of demagnetization curve. It also causes a decrease of the demagnetization curve's rectangularity coefficient; 3' falls from 0.87% down to 0.47% with Nb addition from 0.5% to 1.24%, respectively. Magnetic properties changes observed can be explained as due to macrostructural changes, because our investigations showed that macrostructure of the columnar Alnico 5 is not affected with the Nb addition. The lack of the Nb addition influence on Alnico 5 alloy's structure is unexpected as this addition increases oxygen solubility in the alloy (see table 1)
2.2. Heat treatment Homogenization of the specimens was carried out by heating at 1523 K for 25 rain. Solutioning at high temperature (1523 K) was followed by cooling at a rate which was sufficiently high to prevent precipitation of the ct3, phase, since this phase has a deterimental effect on magnetic properties of the material. Controlled cooling (15 K/s) in a magnetic field (240 kA/m), during which decomposition of the a phase occurs, was carried out from 1523 to 873 K, after which the magnetic field is again removed and cooling is allowed to proceed freely down to room temperature. After this step, the specimens were subjected to double-tempering treatments: 2 h at 773 K + 6 h at 833 K. 2.3. Measurements o f magnetic properties The various magnetic properties were obtained by means of an MJ-55 type AEG permeameter with an accuracy of the order of 1%.
Table 1 Chemical composition of the alloy (wt%) and lattice constant of a-phase (A) Nb 1
2 3 4 5 6
-
0.25 0.49 0.75 1.00 1.24
02
A1
Ni
Co
Cu
C
Mn
Si
S
Fe
a
0.0058 0.0065 0.0071 0.0074 0.0086 0.0093
7.98 8.00 8.05 7.92 7.97 8.10
15.42 15.18 14.93 15.20 15.08 15.12
25.13 25.21 25.18 25.06 25.30 25.10
3.25 3.15 3.08 3.21 3.05 3.10
0.02 0.02 0.03 0.02 0.03 0.03
0.04 0.03 0.04 0.02 0.02 0.02
0.15 0.10 0.15 0.13 0.16 0.12
0.04 0.05 0.05 0.03 0.04 0.04
rest
2.8728 2.8740 2.8749 2.8786 2.8801 2.8817
S. Szymura, S. Golba / Nb and the properties of Alnico 5permanent magnets
287
with columnar structure distincty increases coercivity and improves rectangularity of the demagnetization curve. Thanks to it, in spite of slight decrease of remanent induction, the specific energy ( B n ) m a x increases. So it has been showed that Nb influences the magnetic properties in the similar way as it was observed in alloy with high Ti (up to 8%) and Co (~>34%) contents. 8m ~ 08~
References
64
48 32 Hc [ k A / m ]
16
2
0.4
Fig. 1. The influence of Nb on the demagnetization curve of the columnar Alnico 5 alloy; B r - the remanent induction, H c - the coercive force, B A - the magnetic induction and H A - the magnetic field in the point of maximum magnetic energy (BH) max-
which according to ref. [17] should cause a change in the breadth of columnar crystals. Increased oxygen contents in the alloy is due to the distinct increase of the alloy's lattice constant caused by Nb addition (see table 1). The effect of niobium on the microstructure of Alnico 5 alloys has not yet been ascertained. However, it should be expected that like in the case of Ti addition [18], the magnetic induction of the alloy decreases with the nonmetallic inclusions increase due to Nb content increase. The coercivity of the alloy is increased due to the single-domain ferromagnetic al-phase precipitation's shape improvement like in Alnico-type alloys with higher contents of Co (40%) under Ti addition it was observed [19].
4. Conclusions The investigation results obtained unequivocally showed that 0.5% Nb addition to Ti free Alnico alloy
[1] S. Szymura and B. Wystocki, Elektronika 22 (1980) 14 (in Polish - with abstracts in English and Russian). [2] N. Miheyev and A.A. Prozorov, Izv. Severo - Kavkazkovo Nauchnovo Centra Vysshey Shkoli 2 (1976) 44 (in Russian). [3] G. Marcon, R. Peffen and H.L. Lemaire, IEEE Trans. Magn. MAG-14 (1978) 688. [4] Yu.I. Kozlov and R.T. Davidova, Met. Term. Obrob. Metall. No. 4 (1976) 49 (in Russian). [5] S. Szymura, B. Wystocki and J. Cis~o, Acta Phys. Pol. A52 (1977) 519. [6] Y. Shirakawa, T. Ohara and T. Abe, J. Japan. Inst. Metals 27 (1963) 130. [7] N. Makino, Y. Kimura and I. Yamaki, J. Japan. Inst. Metals 27 (1963) 582. [8] S. Szymura, Hutnik 34 (1967) 484 (in Polish - with abstracts in English and Russian). [9] Y. Kimura, N. Makino and I. Yamaki, J. Japan. Inst. Metals 29 (1965) 577. [10] E. Lassocirlska and S. Szymura, Prace Inst. Hutniczych 20 (1968) 97 (in Polish - with abstracts in English and Russian). [11] C. Bronner, J.P. Haberer, E. Planchard and J. Sauze, Cobalt 46 (1970) 15. [12] C. Bronner, Cobalt 1 (1973) 11. [13] D. Hadfield, Nature 162 (1948) 799. [14] V.M. Kuznecov, Ye.S. Lobeneev and A.M. Samarin, Fiziko-Chemicheskiye Osnovy Proizvodstva Stali (Nauka, Moskva, 1971) p. 96 (in Russian). [15] V.M. Kuzniecov, Izv. Akad. Nauk SSSR, Metally 1 (1977) 139 (in Russian). [16] Y. Kamata and N. Chikamatsu, Annual Meeting of Japan. Inst. Metals (1964). [17] S. Szymura, L. Sojka and A. Zawada, Mater. Sci. 5 (1979) 43. [18] Y. Iwama and M. Takeuchi, Conf. Intermag. Tokyo (1972).
285
THE INFLUENCE OF NIOBIUM ON THE MAGNETIC PROPERTIES OF THE COLUMNAR ALNICO 5 PERMANENT MAGNETS S. SZYMURA Institute of Physics, Technical University o f Cz{stochowa, Poland and
S. GOLBA Steel Works "Baildon ", Katowice, Poland
Received 1 May 1981
An investigation of the influence of Nb addition (0-1.24%) on Alnico 5 columnar alloy's magnetic properties showed that Nb content up to 0.5% slightly decrease remanent induction while at the same time increasing coercivity and improving the rectangularity of the demagnetization curve. The addition of niobium above 0.5% causes a further increase in coercivity simultaneously decreasing magnetic induction in every point of the demagnetization curve.
1. Introduction Many problems are solved concerning remagnetization theory in the magnetically hard materials of the Alnico type, and basic composition and microaddition influence of some elements on the material's magnetic properties have been established. Nevertheless magnetic properties of the Alnico type alloys obtained in practice are far from those theoretically predicted [1 ]. There is the still unsolved problem of how to reach higher coercive force and magnetic induction values - closer to the theoretical ones and how to maximize magnetic induction with the highest coercive force existing at the same time. Therefore investigations on the thermal treatment [2,3] and chemical composition [4,5] optimization are still carried on with the aim of ensuring the existence of conditions for obtaining the ideal shape of the fine dispersive ferromagnetic particles during the alloy's spinodal decomposition process ensuring magnetization by coherent rotations. The magnetic induction of the Alnico 5 alloy containing 8% A1, 14% Ni, 24% Co, 3% Cu, 51% Fe was increased by forming columnar crystals in the mate03044~853/81/0000--0000/$02.75 © 1981 North-Holland
rial [6-8]. Its coercive force was increased by Ti addition [9,10]. Though a titanium addition results in considerable magnetic hardening it also leads to an appreciable fall in the induction values at all points on the demagnetization curve. Simultaneously obtaining high induction and coercivity can only be reached by additional element alloying or by replacing Ti with another element. It seems necessary to consider more seriously the possibility of controlling magnetic properties by Nb addition alloying. Investigations [11,12] .ofNb influence on the magnetic properties of Alnico type alloys containing 34% Co and 4.5-8% Ti showed that the addition of 0.5% Nb caused a moderate reduction in the remanent induction with a simultaneous substantial increase of coercivity and consequently an improved maximum energy product has been achieved. In spite of the several works done [13-15] the influence of Nb addition on the magnetic properties of Alnico alloys Ti-free has not yet been established. It therefore appeared to be interest to study systematically the influence of Nb on the magnetic properties of titanium-free Alnico 5 alloy with columnar crystals.
286
S. Szymura, S. Golba / Nb and the properties of Alnico 5 permanent magnets The specimens were ground after heat treating so as to eliminate the influence of surface oxidized layers on the magnetic properties.
2. E x p e r i m e n t a l
2.1. Preparation o f the specimens The composition of the tested alloys are shown in table 1. The alloys were melted in an induction furnace from standard, commercial-grade starting materials: iron, electrolytic copper, electrolytic cobalt, carbonyl-nickel and pure aluminium offcuts. The niobium was added as ferro-niobium containing 65% Nb. The columnar structure of the specimens was achieved by ceramic mould casting method [16].
3. Results
Demagnetization curves of columnar Alnico 5 alloys containing 0 to 1.24% Nb are shown in fig. 1. As can be deduced from the diagram niobium addition leads to significant improvement in the coercive force, which increases from 56.5 kA/m in the niobium free alloy to 65.5 kA/m in the 1.24% Nb alloy. In the 0-0.75% Nb interval, the remanent induction (Br) decreases only 0.04 T; when Nb addition is above 0.75% these changes are significantly greater, for instance for 1.24% Nb the reduction of Br in relation to the Nb free alloy is 0.14 T. The analysis of magnetic induction BA and magnetic field H A at the point of maximum magnetic energy (BH)max shows that their values increase up to 0.5% Nb content only, similar to demagnetization curve's rectangularity coefficient 3'. Increasing of Nb content over 0.5% decreases BA as well as B r significantly due to the magnetic induction decreasing in every point of demagnetization curve. It also causes a decrease of the demagnetization curve's rectangularity coefficient; 3' falls from 0.87% down to 0.47% with Nb addition from 0.5% to 1.24%, respectively. Magnetic properties changes observed can be explained as due to macrostructural changes, because our investigations showed that macrostructure of the columnar Alnico 5 is not affected with the Nb addition. The lack of the Nb addition influence on Alnico 5 alloy's structure is unexpected as this addition increases oxygen solubility in the alloy (see table 1)
2.2. Heat treatment Homogenization of the specimens was carried out by heating at 1523 K for 25 rain. Solutioning at high temperature (1523 K) was followed by cooling at a rate which was sufficiently high to prevent precipitation of the ct3, phase, since this phase has a deterimental effect on magnetic properties of the material. Controlled cooling (15 K/s) in a magnetic field (240 kA/m), during which decomposition of the a phase occurs, was carried out from 1523 to 873 K, after which the magnetic field is again removed and cooling is allowed to proceed freely down to room temperature. After this step, the specimens were subjected to double-tempering treatments: 2 h at 773 K + 6 h at 833 K. 2.3. Measurements o f magnetic properties The various magnetic properties were obtained by means of an MJ-55 type AEG permeameter with an accuracy of the order of 1%.
Table 1 Chemical composition of the alloy (wt%) and lattice constant of a-phase (A) Nb 1
2 3 4 5 6
-
0.25 0.49 0.75 1.00 1.24
02
A1
Ni
Co
Cu
C
Mn
Si
S
Fe
a
0.0058 0.0065 0.0071 0.0074 0.0086 0.0093
7.98 8.00 8.05 7.92 7.97 8.10
15.42 15.18 14.93 15.20 15.08 15.12
25.13 25.21 25.18 25.06 25.30 25.10
3.25 3.15 3.08 3.21 3.05 3.10
0.02 0.02 0.03 0.02 0.03 0.03
0.04 0.03 0.04 0.02 0.02 0.02
0.15 0.10 0.15 0.13 0.16 0.12
0.04 0.05 0.05 0.03 0.04 0.04
rest
2.8728 2.8740 2.8749 2.8786 2.8801 2.8817
S. Szymura, S. Golba / Nb and the properties of Alnico 5permanent magnets
287
with columnar structure distincty increases coercivity and improves rectangularity of the demagnetization curve. Thanks to it, in spite of slight decrease of remanent induction, the specific energy ( B n ) m a x increases. So it has been showed that Nb influences the magnetic properties in the similar way as it was observed in alloy with high Ti (up to 8%) and Co (~>34%) contents. 8m ~ 08~
References
64
48 32 Hc [ k A / m ]
16
2
0.4
Fig. 1. The influence of Nb on the demagnetization curve of the columnar Alnico 5 alloy; B r - the remanent induction, H c - the coercive force, B A - the magnetic induction and H A - the magnetic field in the point of maximum magnetic energy (BH) max-
which according to ref. [17] should cause a change in the breadth of columnar crystals. Increased oxygen contents in the alloy is due to the distinct increase of the alloy's lattice constant caused by Nb addition (see table 1). The effect of niobium on the microstructure of Alnico 5 alloys has not yet been ascertained. However, it should be expected that like in the case of Ti addition [18], the magnetic induction of the alloy decreases with the nonmetallic inclusions increase due to Nb content increase. The coercivity of the alloy is increased due to the single-domain ferromagnetic al-phase precipitation's shape improvement like in Alnico-type alloys with higher contents of Co (40%) under Ti addition it was observed [19].
4. Conclusions The investigation results obtained unequivocally showed that 0.5% Nb addition to Ti free Alnico alloy
[1] S. Szymura and B. Wystocki, Elektronika 22 (1980) 14 (in Polish - with abstracts in English and Russian). [2] N. Miheyev and A.A. Prozorov, Izv. Severo - Kavkazkovo Nauchnovo Centra Vysshey Shkoli 2 (1976) 44 (in Russian). [3] G. Marcon, R. Peffen and H.L. Lemaire, IEEE Trans. Magn. MAG-14 (1978) 688. [4] Yu.I. Kozlov and R.T. Davidova, Met. Term. Obrob. Metall. No. 4 (1976) 49 (in Russian). [5] S. Szymura, B. Wystocki and J. Cis~o, Acta Phys. Pol. A52 (1977) 519. [6] Y. Shirakawa, T. Ohara and T. Abe, J. Japan. Inst. Metals 27 (1963) 130. [7] N. Makino, Y. Kimura and I. Yamaki, J. Japan. Inst. Metals 27 (1963) 582. [8] S. Szymura, Hutnik 34 (1967) 484 (in Polish - with abstracts in English and Russian). [9] Y. Kimura, N. Makino and I. Yamaki, J. Japan. Inst. Metals 29 (1965) 577. [10] E. Lassocirlska and S. Szymura, Prace Inst. Hutniczych 20 (1968) 97 (in Polish - with abstracts in English and Russian). [11] C. Bronner, J.P. Haberer, E. Planchard and J. Sauze, Cobalt 46 (1970) 15. [12] C. Bronner, Cobalt 1 (1973) 11. [13] D. Hadfield, Nature 162 (1948) 799. [14] V.M. Kuznecov, Ye.S. Lobeneev and A.M. Samarin, Fiziko-Chemicheskiye Osnovy Proizvodstva Stali (Nauka, Moskva, 1971) p. 96 (in Russian). [15] V.M. Kuzniecov, Izv. Akad. Nauk SSSR, Metally 1 (1977) 139 (in Russian). [16] Y. Kamata and N. Chikamatsu, Annual Meeting of Japan. Inst. Metals (1964). [17] S. Szymura, L. Sojka and A. Zawada, Mater. Sci. 5 (1979) 43. [18] Y. Iwama and M. Takeuchi, Conf. Intermag. Tokyo (1972).