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Influence of residual internal stresses on the coercive field of soft magnetic powders
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1875-1876
~i
Journalof renalnellsm magnetic ~1~ materials
ELSEVIER
Influence of residual internal stresses on the coercive field of soft magnetic powders I. C h i c i n a § *, N . J u m a t e , G h . M a t e i Department of Materials Science and Technology, Technical University, 103-105 Muneii Avenue, 3400 Cluj-Napoca, Romania
Abstract We have found evidence of the existence of second- and third-order internal stresses within Fe powder grains (affected zone size: second order -- crystalline grains size; third order = few interatomic distances) by means of X-ray diffraction. It is shown that the second-order internal stresses in powder grains induced during pulverization have qualitatively the same dependence on grain size and heat treatment as BHc .
The coercive field BHc of a ferromagnetic material generally depends very much on the internal stresses of the material [1]. In the case of magnetic powders, BHc depends on the internal stresses induced during pulverization and on the powder grain sizes [2]. For green compacts the coercive field depends on the porosity and specific surface of pores [2,3]. The heat treatments applied for the powders or compacts from powders also influence the coercive field [4]. This paper presents the influence of residual internal stresses on the coercive field of iron powders with low carbon content (0.04% C). This powder was obtained by water pulverization from the liquid phase of a steel with 0.37% C. Fractions of powders were subjected to a heat treatment for 2 h at 1050°C in an atmosphere of dissociated ammonium. The coercive fields were determined from the hysteresis loops, which were measured on green compacts [5]. It was observed that BHc decreases strongly with increasing powder grain sizes (Fig. 1). After heat treatment this decrease in the coercive field with increasing powder granulosity is much more reduced. This behaviour is connected with internal stresses induced during pulverization, caused by the rapid rate of cooling (104-106°C/s) of the powder grains. This rate is dependent on grain sizes. Complete diffractograms of the powder indicate the absence of macrostresses and suggest the presence of second- and third-order stresses, which were determined by a procedure described in [5,6]. The width of the X-ray diffraction lines depends on the mean sizes of crystalline grains as well as on the second-order internal stresses. In
o~ 3.z.
~ 3.0 _°2.6
;:o
22 1.8 1/.i
'
125
~
1
~o ~
'
'
315 400 GRAIN SIZES [)urn]
order to decide whether the additional widths of diffraction lines are due only to the crystalline grains size, or also to second-order internal residual stresses, we plotted their
8
t 6
5 ~4
2 F~i = f (tg O) i
0.~
i
08
F~i = f ( AI cos~) I
1.2
i
I
1.6
i
I
2.0
i
I
2A
i
21~B
Fig. 2. Total width of diffraction lines versus tg 0 (a, b) and versus A/cos 0 (c, d): (a, c) before, and (b, d) after heat treatment.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 1 3 9 2 - 6
'
100
Fig. 1. Variation of BHc versus grain sizes: (a) before, and (b) after heat treatment.
0 * Corresponding author. Fax: + 40-64-145887.
do
L Chicina~ et aL /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1875-1876
1876
~1.2
0.~
0! 0£ 50 .
. 63. 80 . . 100 .
~o 2~
125 160
GRAIN
3~5 4 ~o
SIZES
[IJm]
Fig. 3. Dependence of total width as a function of grain size: (a) before, and (b) after heat treatment. total widths as functions of A / c o s 0 and of tg0, respectively (Fig. 2). The linear dependence of the total width f i as a function of A / c o s 0 suggests the lack of second-order internal stresses, while the linear dependence on tg 0 shows the presence of such stresses [6]. The data plotted in Fig. 2 suggest the presence of second-order internal stresses in powder grains after pulverization. The total width for each granulosity class was calculated from diffraction maxima at 20 = 44.45 °. The dependence of the total width as a function of grain size and heat treatment (Fig. 3) suggests a decrease in second-order stresses for non-heat-treated powder, similar to that evidenced for the coercive field as function of grain size. We assume that this is due to the different cooling rates of powder grains as function of granulosity class. After heat treatment the internal residual stresses were removed, so that the total width of the diffraction lines no longer depended on grain size. The decrease in total width is influenced not only by the decreasing internal stresses, but also by the increase in crystalline grain sizes with heat treatment. This latter increase was confirmed by optical and electron microscopy studies [5]. The removal of internal stresses by heat treat-
,
63
~
~
,
L
125
1~0
t
2oo ~
,
3;~
GRAIN S/ZES lpm]
Fig. 5. Third-order internal stresses versus grain sizes.
ment and obtaining a grain size independent of particle size [5] should lead to values of BHc that are independent of granulosity. The slightly decreasing coercive field with increasing granulosity class for the heat-treated powder could be assigned to the different porosities of the green compacts. Fig. 4 shows the difference A fli = f l i N - fliT (where f i n and fiT are the total widths before and after heat treatment, respectively) as a function of grain size. Thus the value of f i contains only the additive effects of second-order internal stresses and of crystalline grain sizes. The presence of third-order internal stresses is confirmed by the strong decrease in the total intensities of the diffraction maxima for the non-heat-treated powders compared with those of heat-treated powders. This effect is illustrated by the I J l o ratio, where I d is the total intensity when internal stresses occur, and I o is the total intensity of the same maximum after the removal of internal stresses. This effect is shown in Fig. 5 as a function of grain size for the diffraction maxima at 20 = 44.45 °. This kind of dependence does not correspond with the variation of the coercive field with grain size. X-ray diffraction studies show that second-order residual internal stresses from grain powder were the main cause of a H c behaviour versus grain size and heat treatment.
References
~o~ A i .
i
i
i 250
J ~00
GRAIN SIZES[Hm]
Fig. 4. Variation of supplementary total width, due to internal
stresses and to crystalline grain sizes, versus grain sizes.
[1] C. Kittel, Rev. Mod. Phys. 21 (1941) 541. [2] G. Jangg, M. Drozda, G. Eder and H. Danninger, Powder Metall. Int. 16 (1984) 60. [3] G. Jangg, M. Drozda, H. Danninger and G. Eder, Powder Metall. Int. 16 (1984) 264. [4] G. Jangg, M. Drozda, H. Danninger and R.E. Nad, Powder Metall. Int. 15 (1983) 173 [5] Gh. Matei, I. Chicina§ and N. Jumate, Rom. Rep. Phys. 46 (1994) 245. [6] I. Chicina§, N. Jumate, Gh. Matei and E. Bicsak, in: Proc. 3rd Nat. Conf. on Powder Metall., vol. 2 (Technical University, Cluj-Napoca, Romania, 1988) p. 117 (in Romanian).
~i
Journalof renalnellsm magnetic ~1~ materials
ELSEVIER
Influence of residual internal stresses on the coercive field of soft magnetic powders I. C h i c i n a § *, N . J u m a t e , G h . M a t e i Department of Materials Science and Technology, Technical University, 103-105 Muneii Avenue, 3400 Cluj-Napoca, Romania
Abstract We have found evidence of the existence of second- and third-order internal stresses within Fe powder grains (affected zone size: second order -- crystalline grains size; third order = few interatomic distances) by means of X-ray diffraction. It is shown that the second-order internal stresses in powder grains induced during pulverization have qualitatively the same dependence on grain size and heat treatment as BHc .
The coercive field BHc of a ferromagnetic material generally depends very much on the internal stresses of the material [1]. In the case of magnetic powders, BHc depends on the internal stresses induced during pulverization and on the powder grain sizes [2]. For green compacts the coercive field depends on the porosity and specific surface of pores [2,3]. The heat treatments applied for the powders or compacts from powders also influence the coercive field [4]. This paper presents the influence of residual internal stresses on the coercive field of iron powders with low carbon content (0.04% C). This powder was obtained by water pulverization from the liquid phase of a steel with 0.37% C. Fractions of powders were subjected to a heat treatment for 2 h at 1050°C in an atmosphere of dissociated ammonium. The coercive fields were determined from the hysteresis loops, which were measured on green compacts [5]. It was observed that BHc decreases strongly with increasing powder grain sizes (Fig. 1). After heat treatment this decrease in the coercive field with increasing powder granulosity is much more reduced. This behaviour is connected with internal stresses induced during pulverization, caused by the rapid rate of cooling (104-106°C/s) of the powder grains. This rate is dependent on grain sizes. Complete diffractograms of the powder indicate the absence of macrostresses and suggest the presence of second- and third-order stresses, which were determined by a procedure described in [5,6]. The width of the X-ray diffraction lines depends on the mean sizes of crystalline grains as well as on the second-order internal stresses. In
o~ 3.z.
~ 3.0 _°2.6
;:o
22 1.8 1/.i
'
125
~
1
~o ~
'
'
315 400 GRAIN SIZES [)urn]
order to decide whether the additional widths of diffraction lines are due only to the crystalline grains size, or also to second-order internal residual stresses, we plotted their
8
t 6
5 ~4
2 F~i = f (tg O) i
0.~
i
08
F~i = f ( AI cos~) I
1.2
i
I
1.6
i
I
2.0
i
I
2A
i
21~B
Fig. 2. Total width of diffraction lines versus tg 0 (a, b) and versus A/cos 0 (c, d): (a, c) before, and (b, d) after heat treatment.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 1 3 9 2 - 6
'
100
Fig. 1. Variation of BHc versus grain sizes: (a) before, and (b) after heat treatment.
0 * Corresponding author. Fax: + 40-64-145887.
do
L Chicina~ et aL /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1875-1876
1876
~1.2
0.~
0! 0£ 50 .
. 63. 80 . . 100 .
~o 2~
125 160
GRAIN
3~5 4 ~o
SIZES
[IJm]
Fig. 3. Dependence of total width as a function of grain size: (a) before, and (b) after heat treatment. total widths as functions of A / c o s 0 and of tg0, respectively (Fig. 2). The linear dependence of the total width f i as a function of A / c o s 0 suggests the lack of second-order internal stresses, while the linear dependence on tg 0 shows the presence of such stresses [6]. The data plotted in Fig. 2 suggest the presence of second-order internal stresses in powder grains after pulverization. The total width for each granulosity class was calculated from diffraction maxima at 20 = 44.45 °. The dependence of the total width as a function of grain size and heat treatment (Fig. 3) suggests a decrease in second-order stresses for non-heat-treated powder, similar to that evidenced for the coercive field as function of grain size. We assume that this is due to the different cooling rates of powder grains as function of granulosity class. After heat treatment the internal residual stresses were removed, so that the total width of the diffraction lines no longer depended on grain size. The decrease in total width is influenced not only by the decreasing internal stresses, but also by the increase in crystalline grain sizes with heat treatment. This latter increase was confirmed by optical and electron microscopy studies [5]. The removal of internal stresses by heat treat-
,
63
~
~
,
L
125
1~0
t
2oo ~
,
3;~
GRAIN S/ZES lpm]
Fig. 5. Third-order internal stresses versus grain sizes.
ment and obtaining a grain size independent of particle size [5] should lead to values of BHc that are independent of granulosity. The slightly decreasing coercive field with increasing granulosity class for the heat-treated powder could be assigned to the different porosities of the green compacts. Fig. 4 shows the difference A fli = f l i N - fliT (where f i n and fiT are the total widths before and after heat treatment, respectively) as a function of grain size. Thus the value of f i contains only the additive effects of second-order internal stresses and of crystalline grain sizes. The presence of third-order internal stresses is confirmed by the strong decrease in the total intensities of the diffraction maxima for the non-heat-treated powders compared with those of heat-treated powders. This effect is illustrated by the I J l o ratio, where I d is the total intensity when internal stresses occur, and I o is the total intensity of the same maximum after the removal of internal stresses. This effect is shown in Fig. 5 as a function of grain size for the diffraction maxima at 20 = 44.45 °. This kind of dependence does not correspond with the variation of the coercive field with grain size. X-ray diffraction studies show that second-order residual internal stresses from grain powder were the main cause of a H c behaviour versus grain size and heat treatment.
References
~o~ A i .
i
i
i 250
J ~00
GRAIN SIZES[Hm]
Fig. 4. Variation of supplementary total width, due to internal
stresses and to crystalline grain sizes, versus grain sizes.
[1] C. Kittel, Rev. Mod. Phys. 21 (1941) 541. [2] G. Jangg, M. Drozda, G. Eder and H. Danninger, Powder Metall. Int. 16 (1984) 60. [3] G. Jangg, M. Drozda, H. Danninger and G. Eder, Powder Metall. Int. 16 (1984) 264. [4] G. Jangg, M. Drozda, H. Danninger and R.E. Nad, Powder Metall. Int. 15 (1983) 173 [5] Gh. Matei, I. Chicina§ and N. Jumate, Rom. Rep. Phys. 46 (1994) 245. [6] I. Chicina§, N. Jumate, Gh. Matei and E. Bicsak, in: Proc. 3rd Nat. Conf. on Powder Metall., vol. 2 (Technical University, Cluj-Napoca, Romania, 1988) p. 117 (in Romanian).