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Dependence of the anisotropic permanent magnet properties of Fe-Al-C (Cu) on the Cu concentration
Journal of Magnetism North-Holland
and Magnetic
Materials
265
83 (1990) 265-266
DEPENDENCE OF THE ANISOTROPIC PERMANENT OF Fe-Al-C (Cu) ON THE Cu CONCENTRATION J.J. WYSLOCKI Institute
MAGNET
PROPERTIES
and S. SZYMURA
of Physics, Technical University of Czptochowa,
Zawadzkiego
19, 42-200 Czgtochowa,
Magnetic properties of the anisotropic Fe-Al-C (Cu) magnet have been investigated It has been shown that the remanence B, decreases as the Cu concentration increases rises. Torque curves and microstructure of this alloy has been investigated.
1. Introduction The magnetically hard alloy of the Fe-Al-C system proposed by Mishima and Makino [l] has again raised the interest of researchers after nearly 30 years [2-61. Metallurgical investigations [3,4] have shown that this alloy, within the composition range (in wtW): 89.3-90.7 Fe, 7.5-8.5 Al and 1.8-2.2 C, has its optimum magnetic properties (H, = 13.7 kA/m, (B . H),,, = 5 kJ/m3 [4]) when it consists of two phases, the ferrite a-Fe(A1) and carbide Fe,AlC phase, formed during quenching of the fee high-temperature austenite phase y and subsequent low-temperature tempering [4]. Addition of a small amount of Cu to the basic ternary alloy causes an increase of the coercive force and simultaneous decrease of the remanence [l]. Application of a magnetic field during tempering of the alloy induces a uniaxial magnetic anisotropy with the easy magnetization direction parallel to the direction of the applied field [5]. So far the effect of the alloying elements, e.g. Cu, on the magnetic properties of the new anisotropic Fe-Al-C magnet has not been investigated. Therefore these investigations are the main aim of the present paper.
2. Material and experimental methods The investigations were performed on the anisotropic magnet (90.04 - x)Fe-7.86Al-2.10C-xCu, where
Poland
as a function of the Cu concentration. and at the same time the coercivity H,
x = 0.18, 0.75, 2.05, 3.55 (all in wtW). In order to induce a uniaxial magnetic anisotropy in this alloy both homogenization and tempering were carried out in a magnetic field H, = 270 kA/m. The torque intensity T as a function of the angle B between the applied magnetic field direction and easy magnetization axis of the sample was measured at room temperature using a torque magnetometer in a magnetic field up to 2030 kA/m. The magnetic properties of the samples were determined from the demagnetization curves by means of the permeameter. The microstructural features were observed with the use of the Philips EM 301 transmission electron microscope. 3. Results and discussion The variation of the coercive force H, and residual induction B, with Cu concentration for an anisotropic Fe-Al-C(Cu) magnet is presented in fig. 1. From curve 1 in fig. la it follows that the coercive force H, increases with Cu concentration from 13.9 to 21.9 kA/m. For comparison the variation of H, with Cu concentration in isotropic Fe-Al-C magnets (curve 2, from ref. [l]) is also shown in fig. la. Thus modification of the alloy composition by addition of Cu as alloying element appears to be useful, as regarding to the coercive force. However, the residual induction B, decreases as the Cu concentration increases (fig.lb). It should be pointed
I2 a 0
1
2
&%1
4
0
1
2
3
4
Fig. 1. The influence of the Cu concentration on the magnetic properties of the Fe-Al-C(Cu) alloy: (a) coercive force H, (curve 1) and (b) residual induction B, (curve 1). For the comparison the changes in magnetic properties of isotropic alloys are also shown (curves 2 from ref. [l]). 0304-8853/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
J.J. l+stocki.
266
out that centration
anisotropic (0.18
samples
wt%) possess
with
the
a higher
S. Sz.ymura / Anmtropic lowest
value
properties of Fe-Al-C(Cu)
Cu con-
of B, = 720
mT than isotropic samples (B, = 510 mT) as seen from fig. lb. Therefore this anisotropic alloy is a suitable replacement for some cobalt steels and other low (B H),,, materials and may also be of use in situations where easy reversal of polarity is required. Changes in the alloy composition could possibly increase the difference in magnetization between the two principal phases - Fe,AlC (fig. 2. dark spots) and a-Fe(A1) (fig. 2, bright spots) and hence the coercivity of the Fe-Al-C (Cu) magnet, Fig. 2 shows a comparison of the microstructure of this alloy with 2.05 wt’% Cu, taken from specimen surface parallel (fig. 2a) and perpendicular (fig. 2b) to the easy direction of magnetization. In the case of the parallel sample (fig. 2a) one can see an ordered structure with elongated grains of the OLphase oriented in a parallel direction to each other. On the other hand, the perpendicular sample (fig. 2b) exhibits a regular net of rectangular Fe,AlC carbides. The anisotropic properties of the Fe-Al-C(Cu) alloy are also
Fig.
3. Torque
curves for the Fe-AI-C(Cu) different Cu concentration.
magnets
with
confirmed by the type of torque curves. Fig. 3 shows the comparison of torque curves measured in magnetic field H = 2000 kA/m for different Cu concentrations. It is seen from the figure that the torque curve for the lowest Cu addition is described by the function of the type sin 28 + sin 46’ (curve 1). whereas for the higher Cu concentrations torque curves are gradually changed (curve 2) and for the highest Cu concentration torque curve is described by the sin 28 function (curve 3). The changes in shape anisotropy of the ferrite grains due to Cu additions, confirmed in fig. 3 by the variation of the torque curves, could be a contributing factor to the increase of coercive force. It seems also likely that changes in alloy composition produce an increase in the number of regions which cause domain wall pinning. However, the reason for the decrease of the residual induction due to addition of Cu is not yet clear. This work has been subsidized by the Institute of Physics, Polish Academy of Sciences, Warsaw, Poland. References
Fig. 2. Comparison ot the electron micrographs of the Fe-AI-C-2.05 wt% Cu alloy taken from the specimens which are parallel (a) and perpendicular (b) to the easy direction of the magnetization. Bright spots: a-Fe(A1) phase: dark spots: Fe,AlC carbide.
[I] T. Mishima and N. Makino. J. Iron Steel Inst. (Japan) 43 (1957) 556. [2] 1. Briggs and A.G. Clegg, IEEE Trans. Magn. MAG-20 (1984) 1628. [3] S.F.H. Parker, P.J. Grundy and G.A. Jones, IEEE Trans. Magn. MAC-20 (1984) 1630. [4] S. Szymura, J. Less-Common Met. 128 (1987) 313. [5] S. Szymura, J. Less-Common Met. 136 (1987) 161. [6] J.J. Wyslocki and S. Szymura, J. de Phys. 49 (1988) CX-671.
and Magnetic
Materials
265
83 (1990) 265-266
DEPENDENCE OF THE ANISOTROPIC PERMANENT OF Fe-Al-C (Cu) ON THE Cu CONCENTRATION J.J. WYSLOCKI Institute
MAGNET
PROPERTIES
and S. SZYMURA
of Physics, Technical University of Czptochowa,
Zawadzkiego
19, 42-200 Czgtochowa,
Magnetic properties of the anisotropic Fe-Al-C (Cu) magnet have been investigated It has been shown that the remanence B, decreases as the Cu concentration increases rises. Torque curves and microstructure of this alloy has been investigated.
1. Introduction The magnetically hard alloy of the Fe-Al-C system proposed by Mishima and Makino [l] has again raised the interest of researchers after nearly 30 years [2-61. Metallurgical investigations [3,4] have shown that this alloy, within the composition range (in wtW): 89.3-90.7 Fe, 7.5-8.5 Al and 1.8-2.2 C, has its optimum magnetic properties (H, = 13.7 kA/m, (B . H),,, = 5 kJ/m3 [4]) when it consists of two phases, the ferrite a-Fe(A1) and carbide Fe,AlC phase, formed during quenching of the fee high-temperature austenite phase y and subsequent low-temperature tempering [4]. Addition of a small amount of Cu to the basic ternary alloy causes an increase of the coercive force and simultaneous decrease of the remanence [l]. Application of a magnetic field during tempering of the alloy induces a uniaxial magnetic anisotropy with the easy magnetization direction parallel to the direction of the applied field [5]. So far the effect of the alloying elements, e.g. Cu, on the magnetic properties of the new anisotropic Fe-Al-C magnet has not been investigated. Therefore these investigations are the main aim of the present paper.
2. Material and experimental methods The investigations were performed on the anisotropic magnet (90.04 - x)Fe-7.86Al-2.10C-xCu, where
Poland
as a function of the Cu concentration. and at the same time the coercivity H,
x = 0.18, 0.75, 2.05, 3.55 (all in wtW). In order to induce a uniaxial magnetic anisotropy in this alloy both homogenization and tempering were carried out in a magnetic field H, = 270 kA/m. The torque intensity T as a function of the angle B between the applied magnetic field direction and easy magnetization axis of the sample was measured at room temperature using a torque magnetometer in a magnetic field up to 2030 kA/m. The magnetic properties of the samples were determined from the demagnetization curves by means of the permeameter. The microstructural features were observed with the use of the Philips EM 301 transmission electron microscope. 3. Results and discussion The variation of the coercive force H, and residual induction B, with Cu concentration for an anisotropic Fe-Al-C(Cu) magnet is presented in fig. 1. From curve 1 in fig. la it follows that the coercive force H, increases with Cu concentration from 13.9 to 21.9 kA/m. For comparison the variation of H, with Cu concentration in isotropic Fe-Al-C magnets (curve 2, from ref. [l]) is also shown in fig. la. Thus modification of the alloy composition by addition of Cu as alloying element appears to be useful, as regarding to the coercive force. However, the residual induction B, decreases as the Cu concentration increases (fig.lb). It should be pointed
I2 a 0
1
2
&%1
4
0
1
2
3
4
Fig. 1. The influence of the Cu concentration on the magnetic properties of the Fe-Al-C(Cu) alloy: (a) coercive force H, (curve 1) and (b) residual induction B, (curve 1). For the comparison the changes in magnetic properties of isotropic alloys are also shown (curves 2 from ref. [l]). 0304-8853/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
J.J. l+stocki.
266
out that centration
anisotropic (0.18
samples
wt%) possess
with
the
a higher
S. Sz.ymura / Anmtropic lowest
value
properties of Fe-Al-C(Cu)
Cu con-
of B, = 720
mT than isotropic samples (B, = 510 mT) as seen from fig. lb. Therefore this anisotropic alloy is a suitable replacement for some cobalt steels and other low (B H),,, materials and may also be of use in situations where easy reversal of polarity is required. Changes in the alloy composition could possibly increase the difference in magnetization between the two principal phases - Fe,AlC (fig. 2. dark spots) and a-Fe(A1) (fig. 2, bright spots) and hence the coercivity of the Fe-Al-C (Cu) magnet, Fig. 2 shows a comparison of the microstructure of this alloy with 2.05 wt’% Cu, taken from specimen surface parallel (fig. 2a) and perpendicular (fig. 2b) to the easy direction of magnetization. In the case of the parallel sample (fig. 2a) one can see an ordered structure with elongated grains of the OLphase oriented in a parallel direction to each other. On the other hand, the perpendicular sample (fig. 2b) exhibits a regular net of rectangular Fe,AlC carbides. The anisotropic properties of the Fe-Al-C(Cu) alloy are also
Fig.
3. Torque
curves for the Fe-AI-C(Cu) different Cu concentration.
magnets
with
confirmed by the type of torque curves. Fig. 3 shows the comparison of torque curves measured in magnetic field H = 2000 kA/m for different Cu concentrations. It is seen from the figure that the torque curve for the lowest Cu addition is described by the function of the type sin 28 + sin 46’ (curve 1). whereas for the higher Cu concentrations torque curves are gradually changed (curve 2) and for the highest Cu concentration torque curve is described by the sin 28 function (curve 3). The changes in shape anisotropy of the ferrite grains due to Cu additions, confirmed in fig. 3 by the variation of the torque curves, could be a contributing factor to the increase of coercive force. It seems also likely that changes in alloy composition produce an increase in the number of regions which cause domain wall pinning. However, the reason for the decrease of the residual induction due to addition of Cu is not yet clear. This work has been subsidized by the Institute of Physics, Polish Academy of Sciences, Warsaw, Poland. References
Fig. 2. Comparison ot the electron micrographs of the Fe-AI-C-2.05 wt% Cu alloy taken from the specimens which are parallel (a) and perpendicular (b) to the easy direction of the magnetization. Bright spots: a-Fe(A1) phase: dark spots: Fe,AlC carbide.
[I] T. Mishima and N. Makino. J. Iron Steel Inst. (Japan) 43 (1957) 556. [2] 1. Briggs and A.G. Clegg, IEEE Trans. Magn. MAG-20 (1984) 1628. [3] S.F.H. Parker, P.J. Grundy and G.A. Jones, IEEE Trans. Magn. MAC-20 (1984) 1630. [4] S. Szymura, J. Less-Common Met. 128 (1987) 313. [5] S. Szymura, J. Less-Common Met. 136 (1987) 161. [6] J.J. Wyslocki and S. Szymura, J. de Phys. 49 (1988) CX-671.