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Thermomagnetic behaviour of mechanically alloyed Fe-Si
Journal of Magnetism and Magnetic Materials lOl(l991) North-Holland
Thermomagnetic F. Carmona,
behaviour of mechanically alloyed Fe-Si
J.M. Gonzalez,
Institute de Ciencia de Materiales,
119-121
A. Martin
and V.E. Martin
Serrano 144, 28006 Madrid, Spain
A. Garcia-Escorial CENIM-CSIC,
Auda. Gregorio del Amo 8, 28040 Madrid
Spain
Results are reported on the temperature dependence of the magnetization of mechanically alloyed Fe,%, _-x samples with 0.25 5 x I 0.95. From the magnetic data the final compositions obtained after ball milling followed by heat treatment are inferred. An estimate is also made of the mass fraction of amorphous phase in the as-milled material.
1. Introduction Mechanical alloying has motivated a great deal of research during the last few years, basically due to the wide range of phases achievable by means of this outof-equilibrium preparation technique [l]. This range includes amorphous phases and supersaturated solid solutions. Also, the technique allows very fine (nanocrystalline) microstructures to be produced. These characteristics are especially important when the technique is used to obtain magnetic materials. In this work, complementing a previous report [2], we present an analysis, based on the thermomagnetic behavior, of the phases present in mechanically alloyed Fe,Si,_. with 0.25 _
350
T (*C)
700
Fig. 2. Heating (H) and cooling (C) curves of the starting composition Fe&G,,, mechanical alloy show that irreversible changes take place on heating.
2. Experimental techniques Mixtures of pure iron and silicon were ball milled for 60 h in an inert atmosphere with a planetary ball mill. The iron content in the starting mixture ranged from 25 to 95 at%. In order to improve diffusion, silicon was given a previous milling and reduced to a powder
I
375
T (-0
750
Fig. 1. Mechanically alloyed Fe-Si. Thermal demagnetization curves for different starting compositions (Fe,%, _,). 0312~8853/91/$03.50
of average particle size 1 pm. The size of the iron powder was 40 pm. After milling, the powders were characterized by XRD, DSC and SEM combined with EDX. The results corresponding to this characterizations have already been reported [2], and the most meaningful feature was the observation in DSC thermograms of exothermic phenomena that were interpreted as due to the crystallization of amorphous phases. Magnetic measurements were made on pressed powder cylinders with a vibrating sample magnetometer, linked to a computerized data acquisition device with which the applied magnetic field (or temperature) was also registered. The magnetometer was calibrated with a nickel cylinder of the same shape as the samples. Magnetic fields were measured with a calibrated Halleffect probe. Magnetization curves were traced of samples of as-milled material, go that the specific saturation magnetization (os) was determined, and this provided a means for measuring the quantity of magnetic phase formed in the milling process. Measurements were then made of the magnetization at a fixed field while heating, in an inert atmosphere, past the Curie temperature. This provided information about the transformations taking place during the heating process and, therefore, clues
0 1991 - Elsevier Science Publishers B.V. All rights reserved
F. Carmona et al. / Mechanically alloyed Fe-Si
120
Table 1 Mechanically alloyed Fe-Si. Atomic fraction of iron in the nominal composition (x). Specific saturation magnetization before (aa) and after ( eA) heat treatment. Curie temperatures of the final solid solution (T,,) and of the amorphous phase present in the as-milled material (T,,). Inferred from the magnetic data are a (composition of the final solid solution Fe,Si,_,) and j,, (mass fraction of amorphous material) x
0.95 0.75 0.625 0.55 0.50 0.25
06
*A
(emu/g)
(emu/g)
206.5 145.7 101.0 80.8 78.6 14.0
205.7 143.3 113.0 98.1 95.8 11.8
561 529 510 508 495
about the nature of the products involved. Finally, magnetization curves were traced after cooling down, so as to measure us once more and thus determine the net gain in magnetic phase(s) brought about by the heating process. 3. Results and discussion Thermal demagnetization curves ranged from almost featureless to those in which destruction and generation of magnetic phases were evident (fig. 1). Relevant magnetic data are summarized in table 1. Two different transitions were generally observed. Those at higher temperatures (T,,) can be assigned to solid solutions of Si in Fe [3]; and those at lower temperatures (T,,) to amorphous phases, as discussed in ref. [2]. This is confirmed by the fact that, upon cooling down to room temperature, no traces of T,, were observed (fig. 2). Also noteworthy in this context are the differences between specific saturation magnetizations before (es) and after (O~) heating (table 1).
m
I%------4r
fcr
vv
T Fig. 3. Relative amounts of amorphous and crystalline phases are assumed to be proportional to earnj,, and a,,j,,, where earn and Ok, are specific magnetizations, and j,, and j,, are mass fractions of the respective phases. or, is the specific saturation magnetization before heat treatment. If all the amorphous phase transforms into crystalline phase, e,_rcan be assimilated to e* (specific saturation magnetization ajter heat treatment), and j,, can be estimated.
272 233 208 202
a
f am
0.76 0.73 0.71 0.71 0.70
0.11 0.36 0.78 0.74 0.40
What is expected from the milling process followed by a heat treatment is not a single substance. The Curie temperature T,, would thus account for a sort of average end product rather than the end product of the process. This reservation notwithstanding, we have used the results of other authors [3] to assign, to each Curie temperature, a value of a in Fe,Si,_, and, to each a, a value of the specific saturation magnetization 0,. Now, if the overall reaction that takes place is assumed to be xFe + (1 - x)Si -+ AFe,Si, _-a + BFe + CSi, where BFe and CSi are “excess” amounts of these elements (which need not be in the form of elements), magnetic measurements and identification of coefficients lead us to A = qn,/o,m,; B = x - Aa; C = (1 - x) - A(1 - a), where a, is the specific saturation referred to the mass of the sample, and mX and m, are the molecular weights of Fe&, _-x and Fe& _-ol.Values of A, B and C are coherent with the conclusions that can be drawn from the differences between a, and ea. Namely: - For x > 0.75 we basically obtain the solid solution given by the nominal composition. Our values of a, are coincidental, within experimental error, with those given in ref. [3] (207.5 and 138.9 for x = 0.95 and 0.75 respectively). a,-~, differences are small. - For intermediate values we obtain Fe,Si,_, compounds, with a close to 0.75. There is very little iron in excess (small B) and there is an excess in Si which gets higher as x gets smaller. uA-ua differences are very big (up to 22% in the case of x = 0.50) and can be explained on the basis of the crystallization of an amorphous phase. - For x = 0.25 there is definitely too much Si (large C). uA-us is also large, but negative. It seems reasonable to assume that, in this case, heating will favor the formation of compounds which are not ferromagnetic (FeSi or FeSi,).
F. Carmona et al. / Mechanically alloyed Fe-$
In the cases in which an amorphous phase was assumed to transform into a crystalline phase, an estimation has been made of the mass fraction of amorphous phase in the as-milled material, according to the sketch shown in fig. 3. Values, shown in table 1 under the heading f,,, show a maximum around x = 0.5, as would be generally expected [l]. References [l] L. Schultz, Proc. MRS Europe Meeting on Amorphous Metals and Non-Equilibrium Processing, ed. M. von Allmen, (Editions de Physique, les Ulis, 1984) P. 135.
121
[2] A. Garcia Escorial, P. Adeva, M.C. Cristina, A. Martin, F. Carmona, F. Cebollada, V.E. Martin, M Leonato and J.M. GonzAlez, Mater. Sci. Eng. A 134 (1991) 1394. [3] A.E. Berkowitz and E. Rneller, Magnetism and Metallurgy (Academic Press, 1969) pp. 336-343.
Thermomagnetic F. Carmona,
behaviour of mechanically alloyed Fe-Si
J.M. Gonzalez,
Institute de Ciencia de Materiales,
119-121
A. Martin
and V.E. Martin
Serrano 144, 28006 Madrid, Spain
A. Garcia-Escorial CENIM-CSIC,
Auda. Gregorio del Amo 8, 28040 Madrid
Spain
Results are reported on the temperature dependence of the magnetization of mechanically alloyed Fe,%, _-x samples with 0.25 5 x I 0.95. From the magnetic data the final compositions obtained after ball milling followed by heat treatment are inferred. An estimate is also made of the mass fraction of amorphous phase in the as-milled material.
1. Introduction Mechanical alloying has motivated a great deal of research during the last few years, basically due to the wide range of phases achievable by means of this outof-equilibrium preparation technique [l]. This range includes amorphous phases and supersaturated solid solutions. Also, the technique allows very fine (nanocrystalline) microstructures to be produced. These characteristics are especially important when the technique is used to obtain magnetic materials. In this work, complementing a previous report [2], we present an analysis, based on the thermomagnetic behavior, of the phases present in mechanically alloyed Fe,Si,_. with 0.25 _
350
T (*C)
700
Fig. 2. Heating (H) and cooling (C) curves of the starting composition Fe&G,,, mechanical alloy show that irreversible changes take place on heating.
2. Experimental techniques Mixtures of pure iron and silicon were ball milled for 60 h in an inert atmosphere with a planetary ball mill. The iron content in the starting mixture ranged from 25 to 95 at%. In order to improve diffusion, silicon was given a previous milling and reduced to a powder
I
375
T (-0
750
Fig. 1. Mechanically alloyed Fe-Si. Thermal demagnetization curves for different starting compositions (Fe,%, _,). 0312~8853/91/$03.50
of average particle size 1 pm. The size of the iron powder was 40 pm. After milling, the powders were characterized by XRD, DSC and SEM combined with EDX. The results corresponding to this characterizations have already been reported [2], and the most meaningful feature was the observation in DSC thermograms of exothermic phenomena that were interpreted as due to the crystallization of amorphous phases. Magnetic measurements were made on pressed powder cylinders with a vibrating sample magnetometer, linked to a computerized data acquisition device with which the applied magnetic field (or temperature) was also registered. The magnetometer was calibrated with a nickel cylinder of the same shape as the samples. Magnetic fields were measured with a calibrated Halleffect probe. Magnetization curves were traced of samples of as-milled material, go that the specific saturation magnetization (os) was determined, and this provided a means for measuring the quantity of magnetic phase formed in the milling process. Measurements were then made of the magnetization at a fixed field while heating, in an inert atmosphere, past the Curie temperature. This provided information about the transformations taking place during the heating process and, therefore, clues
0 1991 - Elsevier Science Publishers B.V. All rights reserved
F. Carmona et al. / Mechanically alloyed Fe-Si
120
Table 1 Mechanically alloyed Fe-Si. Atomic fraction of iron in the nominal composition (x). Specific saturation magnetization before (aa) and after ( eA) heat treatment. Curie temperatures of the final solid solution (T,,) and of the amorphous phase present in the as-milled material (T,,). Inferred from the magnetic data are a (composition of the final solid solution Fe,Si,_,) and j,, (mass fraction of amorphous material) x
0.95 0.75 0.625 0.55 0.50 0.25
06
*A
(emu/g)
(emu/g)
206.5 145.7 101.0 80.8 78.6 14.0
205.7 143.3 113.0 98.1 95.8 11.8
561 529 510 508 495
about the nature of the products involved. Finally, magnetization curves were traced after cooling down, so as to measure us once more and thus determine the net gain in magnetic phase(s) brought about by the heating process. 3. Results and discussion Thermal demagnetization curves ranged from almost featureless to those in which destruction and generation of magnetic phases were evident (fig. 1). Relevant magnetic data are summarized in table 1. Two different transitions were generally observed. Those at higher temperatures (T,,) can be assigned to solid solutions of Si in Fe [3]; and those at lower temperatures (T,,) to amorphous phases, as discussed in ref. [2]. This is confirmed by the fact that, upon cooling down to room temperature, no traces of T,, were observed (fig. 2). Also noteworthy in this context are the differences between specific saturation magnetizations before (es) and after (O~) heating (table 1).
m
I%------4r
fcr
vv
T Fig. 3. Relative amounts of amorphous and crystalline phases are assumed to be proportional to earnj,, and a,,j,,, where earn and Ok, are specific magnetizations, and j,, and j,, are mass fractions of the respective phases. or, is the specific saturation magnetization before heat treatment. If all the amorphous phase transforms into crystalline phase, e,_rcan be assimilated to e* (specific saturation magnetization ajter heat treatment), and j,, can be estimated.
272 233 208 202
a
f am
0.76 0.73 0.71 0.71 0.70
0.11 0.36 0.78 0.74 0.40
What is expected from the milling process followed by a heat treatment is not a single substance. The Curie temperature T,, would thus account for a sort of average end product rather than the end product of the process. This reservation notwithstanding, we have used the results of other authors [3] to assign, to each Curie temperature, a value of a in Fe,Si,_, and, to each a, a value of the specific saturation magnetization 0,. Now, if the overall reaction that takes place is assumed to be xFe + (1 - x)Si -+ AFe,Si, _-a + BFe + CSi, where BFe and CSi are “excess” amounts of these elements (which need not be in the form of elements), magnetic measurements and identification of coefficients lead us to A = qn,/o,m,; B = x - Aa; C = (1 - x) - A(1 - a), where a, is the specific saturation referred to the mass of the sample, and mX and m, are the molecular weights of Fe&, _-x and Fe& _-ol.Values of A, B and C are coherent with the conclusions that can be drawn from the differences between a, and ea. Namely: - For x > 0.75 we basically obtain the solid solution given by the nominal composition. Our values of a, are coincidental, within experimental error, with those given in ref. [3] (207.5 and 138.9 for x = 0.95 and 0.75 respectively). a,-~, differences are small. - For intermediate values we obtain Fe,Si,_, compounds, with a close to 0.75. There is very little iron in excess (small B) and there is an excess in Si which gets higher as x gets smaller. uA-ua differences are very big (up to 22% in the case of x = 0.50) and can be explained on the basis of the crystallization of an amorphous phase. - For x = 0.25 there is definitely too much Si (large C). uA-us is also large, but negative. It seems reasonable to assume that, in this case, heating will favor the formation of compounds which are not ferromagnetic (FeSi or FeSi,).
F. Carmona et al. / Mechanically alloyed Fe-$
In the cases in which an amorphous phase was assumed to transform into a crystalline phase, an estimation has been made of the mass fraction of amorphous phase in the as-milled material, according to the sketch shown in fig. 3. Values, shown in table 1 under the heading f,,, show a maximum around x = 0.5, as would be generally expected [l]. References [l] L. Schultz, Proc. MRS Europe Meeting on Amorphous Metals and Non-Equilibrium Processing, ed. M. von Allmen, (Editions de Physique, les Ulis, 1984) P. 135.
121
[2] A. Garcia Escorial, P. Adeva, M.C. Cristina, A. Martin, F. Carmona, F. Cebollada, V.E. Martin, M Leonato and J.M. GonzAlez, Mater. Sci. Eng. A 134 (1991) 1394. [3] A.E. Berkowitz and E. Rneller, Magnetism and Metallurgy (Academic Press, 1969) pp. 336-343.