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Effect of annealing on structural and magnetic properties of Al substituted nanocrystalline Fe–Si–Co alloy powders
Journal of Magnetism and Magnetic Materials 417 (2016) 62–68
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Effect of annealing on structural and magnetic properties of Al substituted nanocrystalline Fe–Si–Co alloy powders P.C. Shyni, Perumal Alagarsamy n Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2016 Received in revised form 29 March 2016 Accepted 18 May 2016 Available online 19 May 2016
We report effects of annealing and substitution of Al on structural and magnetic properties of nanocrystalline Fe80–xAlxCo5Si15 (x ¼0–10) alloy powders prepared by mechanical alloying process using a planetary ball mill technique. All the as-milled powders exhibit non-equilibrium solid solution of α-Fe (Si,Co,Al). While the average size of crystals decreases, the lattice constant and dislocation density increase with increasing Al content. On the other hand, the annealing at elevated temperatures increases the size of the crystals and decreases the dislocation density. In addition, the substitution of Al in FeAlCoSi alloy powders controls growth of the crystals during annealing. As a result, coercivity (HC) of the annealed powders decreases considerably. However, the variation in HC is dominated by the dislocation density. Fe70Al10Co5Si15 powder annealed at 900 °C exhibits improved magnetic properties (HC 14 Oe and moderate magnetization of 160 emu/g) due to optimum nanocrystalline microstructure with fine nanocrystals ( 18 nm) and reduced dislocation density. Systematic correlations observed between structural and magnetic properties for Fe80 xAlxCo5Si15 powders reveal a promising approach to control the growth of the crystals in the annealed nanocrystalline alloys and to improve the magnetic properties of mechanically alloyed Fe–Si based nanocrystalline alloys by adding suitable substituting elements. & 2016 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline materials Annealing Coercivity X-ray techniques Dislocation density Crystal growth
1. Introduction Nanostructured materials have received considerable attention among scientists due to their unique electrical, magnetic, mechanical and optical properties, and potential applications in a wide variety of technological areas. In particular, nanocrystalline magnetic materials with high saturation magnetization, low coercivity, high permeability and low core loss have been experiencing a rapid development over last few decades for saving energy in electric power transmission, transformers, magnetic sensors, electromagnetic noise suppression and automotive applications [1–5]. With this connection, Fe–Si based nanocrystalline alloys were studied extensively because of their remarkable properties, which are sensitive to the nanocrystalline microstructure and considerably different from the microcrystalline counterpart [6–8]. It is well established that these materials display improved soft magnetic properties when the average size of the crystals is less than ferromagnetic exchange correlation length, which is typically a few tens of nanometers. Among various preparation methods, mechanical alloying has been proven as a simple and an inexpensive method to synthesize a wide variety of n
Corresponding author. E-mail address: [email protected] (P. Alagarsamy).
http://dx.doi.org/10.1016/j.jmmm.2016.05.058 0304-8853/& 2016 Elsevier B.V. All rights reserved.
nanostructured materials [9,10] on a large scale with controlled magnetic properties. In this process, the pure elemental mixtures undergo repeated cold welding and fracturing, which lead to the development of nanocrystalline alloys with highly refined crystals and tunable grain boundary regions. Note that such heavy cold formation yields a large amount of internal strain in the nanocrystalline material due to dislocations and results in a high coercivity (HC). Shen et al. reported that HC of mechanically alloyed sample depends on both crystal size and dislocation density [11]. In order to relieve the induced strain in the as-milled powders, the process of annealing at elevated temperatures is inevitable. Furthermore, the as-milled powders have to be consolidated at high temperatures to produce high density components in different shapes and dimensions. It may be noted that such annealing process is not only expected to improve soft magnetic properties [12,13] due to the relaxation of strain, but also induces additional crystal growth [14] and structural modifications. However, the formation of new and undesired crystalline phases [15,16] may degrade the magnetic properties. Therefore, the optimization of the annealing process for the as-milled powders at selected temperatures is quite essential to understand the correlation between modification of microstructure with annealing and the resulting magnetic properties. Extensive studies on the effects of annealing on the structural and magnetic properties of various Fe–Si based alloys prepared by mechanical alloying have been reported [14–
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17]. Nevertheless, there have been continuous attempts to improve the soft magnetic properties of the nanocrystalline alloys by tailoring alloy compositions [18,19], changing substituting elements [20–22], various preparation parameters, optimizing nanostructure and changing annealing conditions. With this connection, in this work, we report the effects of optimized annealing and Al substitution on the nanocrystalline microstructure and the resulting magnetic properties of Fe80 xAlxCo5Si15 alloy powders, which are prepared by mechanical alloying process using a highenergy planetary ball mill and annealing at different temperatures.
2. Experimental details Weighed quantities of high purity ( 499.9%) Fe, Co, Si and Al elemental powders with different particle sizes of 60–100 μm, 2– 4 μm, 5–30 μm and 8–13 μm, respectively corresponding to the composition of Fe80 xAlxCo5Si15 with x ¼0, 2, 5 and 10 were milled in a hardened steel bowl together with 8 mm diameter hardened steel milling balls for 40 h of dry milling at 600 rpm milling speed in a planetary ball mill under argon atmosphere. The ball-topowder weight ratio was maintained at 10:1. All the as-milled powders exhibiting non-equilibrium solid solution of α-Fe (Co,Si, Al) were flame sealed in fused silica ampoules at a pressure of 10 4 Pa created using diffusion and rotary pumps combination and then annealed at different temperatures (TA) up to 900 °C for 5 h durations in a resistive tubular furnace (Okay, India). The annealing temperature in the furnace could be controlled within 72 °C to the set temperature using temperature controller. The annealing temperature and annealing time were optimized by monitoring the formation of nanocrystalline microstructure and the resulting properties of the annealed powders. The structural changes occurred due to the heat treatment were investigated by X-ray diffraction (XRD) obtained from high-power (18 kW) X-Ray diffractometer (Rigaku TTRAX III, Japan) using Cu–Kα radiation (λ ¼ 1.541 Å). XRD data were collected at a slow scan rate of 0.005°/ s to facilitate proper estimation of lattice parameter, the average size of the crystals and dislocation density of the as-milled and annealed powders. The microstructure was characterized by using transmission electron microscope (TEM, JEOL 2100). The overall composition of the powders was analyzed using energy-dispersive X-ray spectroscopy (EDS) attached to the scanning electron microscope unit. Room temperature magnetic properties were measured using a vibrating sample magnetometer (VSM, LakeShore Model 7410, USA) with the maximum applied field of 712 kOe.
3. Results and discussion 3.1. Structural properties Fig. 1 depicts a typical comparison of XRD patterns of Fe70Al10Co5Si15 powders in the state of as-mixed, as-milled and annealed at 500 °C and 900 °C. It is clear from the figure that the as-mixed powders exhibit sharp characteristic Bragg reflections corresponding to all four constituting elements, i.e., Fe (body centered cubic), Co (face centered cubic and hexagonal closepacked structure), Si (diamond cubic) and Al (face centered cubic). However, the Bragg reflections corresponding to Si, Co and Al disappear after milling for 40 h. This can be attributed to diffusion of Si, Co and Al in Fe matrix, which results in the formation of nonequilibrium body centered cubic (bcc) solid solutions of α-Fe(Si,Co, Al). Moreover, no additional Bragg peaks corresponding to any other phases or compounds were observed within the resolution of high-power XRD system. But the XRD pattern of the as-milled
Fig. 1. Room temperature XRD patterns of as-mixed, as-milled and annealed Fe70Al10Co5Si15 alloy powders at different temperatures.
powders shows a large peak broadening possibly due to size refinement and induced strain. Similarly, the XRD pattern of the annealed powders at 500 °C exhibits almost the same nature of asmilled powders except with a slight reduction in the peak width. On further increasing TA to 900 °C, a large reduction in peak broadening is observed for all the powders. On the other hand, all the as-milled and annealed powders show a considerable shift in the peak position with increasing Al content. In order to understand the effect of Al substitution in Fe80 xAlxCo5Si15 powders annealed at different TA systematically, all the annealed powders were characterized by XRD and presented in Fig. 2. It is interesting to note that for the as-milled powders and for the powders annealed at a particular temperature, the width of the peaks increases noticeably with increasing Al content in FeAlCoSi powders. This confirms a possible refinement of nanocrystals in the as-milled powders or control of crystal growth in annealed powders with increasing Al content. While the as-milled powders display a slight shift in the peak position to lower angles, a considerable shift in the peak position is observed for the annealed powders to higher angles with increasing Al. To study the effects of annealing and Al substitution on the structural parameters such as lattice constant, average crystal size, and dislocation density, XRD patterns were analyzed using Williamson– Hall Plot (WHP) method [23] and modified Williamson–Hall Plot (MWHP) method [24,25]. According to MWHP method, the individual contribution to the broadening of XRD peaks can be expressed as 2
( ΔK )
⎛ 0.9 ⎞2 ⎛ πb2ρ ⎞ 2 ⎟ +⎜ ⎟K C =⎜ ⎝ D ⎠ ⎝ 2B ⎠
(1)
where ΔK [ ¼(2cosθB ΔθB)/λ], ΔθB is full width at half maximum (FWHM) of the Bragg reflections (in radian) after correcting instrumental broadening, λ is wavelength of the X-ray, K ¼2sinθB/λ, b is modulus of Burgers vector of dislocations taken as b¼ 3 /2 a ,
(
)
D is average size of the crystals, a is lattice constant, B is a constant (taken as 10 for a wide range of dislocation distributions [24]), ρ is average dislocation density, C [ = Chkl = Ch00(1 − qH2)] is dislocation
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Fig. 3. The applicability of (a) WHP and (b) MWHP methods for the analysis of XRD data typically for as-milled Fe75Al5Co5Si15 alloy powders. The determined values of intercept and slope with errors are shown in inset of the figure.
that dislocations are main contributors to the strain. Similarly, the individual contribution to the broadening of XRD peaks in WHP method [23] can be defined as,
ΔθB cos θB =
Fig. 2. Room temperature XRD patterns of (a) as-milled and annealed Fe80 xAlxCo5Si15 alloy powders with different Al content at different temperatures: (b) 500 °C and (c) 900 °C.
contrast factor introduced to take care of elastically anisotropic materials, where the residual strains affect some Bragg reflections
(
)(
)
more than the others, H2 = h2k 2 + k 2l 2 + l 2h2 / h2 + k 2 + l 2 for a cubical system and q is a constant. To correct the instrumental broadening in XRD, the starting pure iron powder annealed at 1273 K for 24 h was used as the reference powder. The instrumental broadening corrected line profile breadth (ΔθB), in radians, of each reflection was calculated from the parabolic approximation correction [14], as given by, 2
( ΔθB)
⎛ B2 ⎞ = A2 ⎜ 1 − 2 ⎟ ⎝ A ⎠
(2)
where A and B are the breadths of the same Bragg peak (in radians) from the XRD scans of the experimental and reference powders, respectively. The values of A and B were determined by the Origin software as the FWHM after automatic background removal. In the MWHP formulation, Ungar et al. [24,25] considered
kλ + 4e sin θ D
(3)
where k is a constant taken as 0.9 by assuming spherical nature of particles, and e is the effective strain. Fig. 3 demonstrates the applicability of both WHP and MWHP methods for Fe75Al5Co5Si15 milled for 40 h. It is clear from the figure that the quality of the fit fails to obey WHP formalism (see Fig. 3a). This can be attributed to anisotropic variation in the residual strain. On the other hand, the 2
plot of ( ΔK ) versus K2C depicts that all Bragg reflections fall on a straight line and the fitting of the data using a linear equation provides a preeminent fit with R-squared value of nearly 1. This clearly supports that the residual strain affects the Bragg reflections differently. The values of average size of the crystals and dislocation density were calculated from intercept and slope of the fitted straight line, respectively. For all the presently investigated alloys, the values of q were ranged between 1.7 and 2.6. Fig. 4 depicts the variations of average crystal size, dislocation density and lattice constant as a function of Al content for asmilled and annealed Fe80 xAlxCo5Si15 powders. It is revealed that the substitution of Al in as-milled FeAlCoSi powders interestingly reduces the average crystal size from about 11 nm to about 6.5 nm and increases the dislocation density considerably from 4.2 1017 m 2 to 13.5 1017 m 2. This confirms the existence of almost 0.4 to 1.35 dislocations for every one nm2 area induced by the milling process. It is well-known that the mechanical alloying produces nanostructured materials at solid state and the rate of
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ρ=2 3
Fig. 4. Variations of average crystal size (D), dislocation density (ρ) and lattice constant (a) of as-milled and annealed Fe80–xAlxCo5Si15 alloy powders as a function of Al content. The representation of the symbols for different annealing temperatures is same in all figures.
dissolution of the powders could be related to the dislocation density introduced by comminution [26]. Thus, the dislocation density, considered as one of the major structural parameters influencing the final nanocrystalline microstructure, is correlated (see Eq. (4)) directly to the mean square lattice strain and inversely to the size of the nanocrystal [27].
65
ηrms Db
(4)
where o ηrms 4 is root mean square lattice strain. This suggests that the development of a large density of dislocations increases the probability of forming highly refined nanocrystals, as evident from Fig. 4. With increasing Al, the rate of decrease in the average crystal size decreases. This indicates that further addition of Al may not produce more dislocations due to difficulty in generating dislocations at smaller crystals, which results in a saturation like behavior of ρ with increasing Al [28]. On the other hand, it is observed from the annealing data that (i) while the values of crystal size increase slightly for the powders annealed at 500 °C, the powders annealed at 900 °C show a large increase in the crystal size up to a maximum of 30 nm. Interestingly, the average size of the crystals decreases with increasing Al content for the samples annealed at a particular TA. This supports that Al plays a crucial role in controlling the growth of the crystals. (ii) In contrast, the dislocation density decreases largely with increasing TA, which could be attributed to the strain relaxation and growth of the crystals. The variations of crystal size and dislocation density with Al substitution in Fe80 xAlxCo5Si15 powders depict a close correlation between them [27]. To understand the solid solubility level and the effect of annealing on the formation of nanocrystalline microstructure, the lattice constant was calculated from the peak positions for the as-milled and annealed powders. It is observed that the lattice constant increases from 2.8658 7 0.0004 Å to 2.8698 7 0.0006 Å with increasing Al content in milled FeAlCoSi powders. This suggests that a maximum lattice constant change of 0.0004 Å per at% Al occurred by the substitution of Al. Although the observed results are in close agreement with the earlier reports on similar systems [29,30], the increase in lattice constant for the presently investigated alloys is slightly lower than those reported for typical binary Fe–Al alloys. This could be attributed to the change in the dissolution process in the multicomponent materials as compared to the binary alloys. On the other hand, the lattice constant in the annealed powders decreases with increasing Al. This might be correlated to the reduction in the dislocation density by the annealing process, which causes the relaxation of lattice strain [14] and change in atomic ordering, which leads to a modification of microstructure after the annealing [17]. These results reveal that the variation in structural parameters strongly
Fig. 5. Bright-field TEM micrographs and selected area electron diffraction patterns for as-milled Fe75Al5Co5Si15 alloy powder and annealed Fe70Al10Co5Si15 alloy powder at 900 °C.
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depends on TA and Al substitution in Fe–Al–Si–Co alloy. In order to confirm changes in the microstructure due to annealing, TEM micrographs were obtained for as-milled and annealed powders. Fig. 5 depicts typical bright-field TEM (BF-TEM) micrographs and selected area electron diffraction (SAED) patterns for as-milled Fe75Al5Co5Si15 powders and Fe70Al10Co5Si15 powders annealed at 900 °C. BF-TEM micrograph of as-milled powder confirms the existence of fine-grain structure with the average size of about 7.5 nm having non-uniform contrast inside each grain and along grain boundaries. This could be attributed to a large strain in the as-milled powders. Similarly, BF-TEM micrograph of annealed sample confirms the nanocrystalline microstructure with a large size of the crystals, i.e., the average crystal size of about 18 nm for Fe70Al10Co5Si15 sample annealed at 900 °C. On the other hand, the rings present in SAED pattern reveal the nanocrystalline microstructure and match with the Bragg reflections in the XRD patterns. The values of average crystal size and lattice parameters calculated from TEM analysis show a close agreement with those obtained from the XRD data. Composition analysis performed using EDS for the milled Fe80 xAlxCo5Si15 with x¼0, 2, 5 and 10 showed the overall compositions of the milled powders as Fe80.8Co4.6Si14.6, Fe79Al1.8Co4.8Si14.4, Fe75.9Al4.7Co4.7Si14.7 and Fe71.1Al9.8Co4.5Si14.6, respectively. While these results evidently support the presence of Si, Co and Al in the form of solid solution in α-Fe, the Fe content in the final powder is observed to be slightly more than the actual one. This might be aroused due to contamination from the grinding media, which is the typical results in mechanical alloying process. Nevertheless, these observed results clearly confirm that (i) the substitution of Al in Fe80 xAlxCo5Si15 powders not only helps reducing the average crystal size during milling, but also controls the growth of the crystals efficiently during the annealing process. (ii) Annealing at moderate temperature (500 °C) reduces the strain largely without much growth of the crystals. (iii) On the other hand, annealing at higher temperature (900 °C) not only induces the growth of the crystals substantially, but also reduces the strain largely in the asmilled powders. These two processes are expected to show sizeable changes in the magnetic properties of the annealed powders. 3.2. Magnetic properties To understand changes in the magnetic properties, room temperature M–H loops were measured using VSM. Fig. 6 depicts room temperature M–H loops of as-milled and annealed Fe80 xAlxCo5Si15 alloy powders. The expanded versions of the M–H loops close to origin are shown as insets. These M–H loops show the typical nature of soft magnetic materials. To study the effect of annealing on the magnetic parameters, we have extracted HC from the M–H loops and determined the values of saturation magnetization (MS) from the fitting of the data to the power series,
⎛ ⎞ a a M (H ) = MS ⎜ 1 − 1 − 22 + …⎟ ⎝ ⎠ H H
(5)
where M(H) is the magnetization in an applied magnetic field H and, a1 and a2 are constant coefficients [31]. Fig. 7 displays the variations of MS and HC as a function of Al content for all Fe80 xAlxCo5Si15 alloy powders. It is observed that (i) MS decreases from 205 emu/g to 167 emu/g with increasing Al content up to 10 at% in the as-milled powders. (ii) Upon annealing at 500 °C, MS shows almost no change as compared to the as-milled powders. This may be due to the release of strain accumulated in the asmilled powders without much crystal growth as depicted in Fig. 4. (iii) On the other hand, the powders annealed at 900 °C exhibit a slight decrease in magnetization for all the compositions. This might be correlated to the occurrence of atomic ordering
Fig. 6. Room temperature M–H loops of (a) as-milled and annealed Fe80 xAlxCo5Si15 alloy powders at different temperatures: (b) 500 °C and (c) 900 °C. The representation of the symbols for different annealing temperatures is same in all figures. The expanded versions of the loops close to the origin are shown as insets.
[28,32,33]. Nevertheless, HC of as-milled powders is ranged between 90 Oe and 56 Oe. Such high HC is mainly due to different contributions such as dislocation density, refinement of crystals and compositional dependent grain boundary formation. With increasing TA, a significant decrease in HC has been observed for all the samples. However, the relative decrease in HC with increasing TA strongly depends on the Al content in Fe80 xAlxCo5Si15 annealed powders. In order to understand variation of HC with TA, HC has been plotted as a function of average ρ in Fig. 8 for Fe80 xAlxCo5Si15 alloy powders. According to Neel's theory [34], HC of the mechanically alloyed powders is expected to be proportional to ρ when magnetostatic energy is higher than anisotropy energy. However, we have not observed a linear variation of HC in the presently investigated samples. There exists a compositional dependent non-linear variation of HC with ρ. While the annealed FeCoSi powders exhibit a large variation of HC with ρ, the relative decrease in HC with ρ in Al substituted Fe80 xAlxCo5Si15 powders is reduced significantly. This could be attributed to the difference in the strain relaxation in refined crystals due to annealing and enhancement in the crystal growth, which reduces the density of dislocations in the annealed samples. It may be noted that amongst all samples, Fe70Al10Co5Si15 powders annealed at 900 °C yielded the lowest value of HC of 14 Oe and moderate MS of 160 emu/g. Sunol et al. [18] reported that FeNbSiB powders milled at different milling periods and annealed at different temperatures show HC in the range of 40–80 Oe and MS between 140 emu/g and 180 emu/g. The values of MS and HC increase significantly with increasing TA. Yousefi et al. [19] reported that HC of the as-milled (Fe65Co25)100 xSix powders with x¼ 0–20 has been in the range of 20–50 Oe and MS decreases largely from 190 emu/g to 125 emu/g
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the value of MS increases largely to 160 emu/g. This reveals that the choice of the substitution element is very much important in tuning the soft magnetic properties of the Fe–Si based nanocrystalline alloys prepared by mechanical alloying process.
4. Conclusions
Fig. 7. Variations of (a) MS and (b) HC of as-milled and annealed Fe80 xAlxCo5Si15 alloy powders at different temperatures. The representation of the symbols for different annealing temperatures is same in all figures.
We have studied the effects of annealing and Al substitution on the structural and magnetic properties of nanocrystalline Fe80 xAlxCo5Si15 alloy powders prepared by mechanical alloying process in a high-energy planetary ball mill. All the as-milled powders exhibit non-equilibrium solid solution of α-Fe(Si,Co,Al). Annealing at elevated temperatures has significant effects on their structure and magnetic properties. The average crystal size increases and dislocation density decreases with increasing annealing temperatures. The substitution of Al in Fe80Co5Si15 powders helps reducing the average crystal size in as-milled state and controls the crystal growth effectively during annealing. The average magnetization decreases slightly after annealing which could be attributed to the atomic ordering. Coercivity of the samples has been observed to be dominated by the dislocation density. Although annealing results in a considerable improvement in the soft magnetic properties of the Fe80 xAlxCo5Si15 powders, the residual defects in the annealed powders causes the coercivity in the range between 14 Oe and 40 Oe. Systematic correlations between structural and magnetic properties were observed for these annealed Fe80 xAlxCo5Si15 powders. However, the structural and magnetic parameters are strongly dependent on the alloy composition. Fe70Al10Co5Si15 powders annealed at 900 °C exhibited improved magnetic properties. This was correlated to optimum nanocrystalline microstructure with fine crystals of about 18 nm with reduced dislocation density by the annealing process. The present study revealed a possible way to control the growth of the crystal and improve the magnetic properties by the proper choice of substituting elements in annealed nanocrystalline Fe–Si based alloy powders prepared by mechanical alloying process.
Acknowledgments The authors acknowledge the infrastructural facilities provided by Department of Science and Technology, India vide project No: [SR/S2/CMP-19/2006, SR/FST/PII-020/2009].
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Fig. 8. Variations of HC as a function of ρ for as-milled and annealed Fe80 xAlxCo5Si15 alloy powders.
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Effect of annealing on structural and magnetic properties of Al substituted nanocrystalline Fe–Si–Co alloy powders P.C. Shyni, Perumal Alagarsamy n Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2016 Received in revised form 29 March 2016 Accepted 18 May 2016 Available online 19 May 2016
We report effects of annealing and substitution of Al on structural and magnetic properties of nanocrystalline Fe80–xAlxCo5Si15 (x ¼0–10) alloy powders prepared by mechanical alloying process using a planetary ball mill technique. All the as-milled powders exhibit non-equilibrium solid solution of α-Fe (Si,Co,Al). While the average size of crystals decreases, the lattice constant and dislocation density increase with increasing Al content. On the other hand, the annealing at elevated temperatures increases the size of the crystals and decreases the dislocation density. In addition, the substitution of Al in FeAlCoSi alloy powders controls growth of the crystals during annealing. As a result, coercivity (HC) of the annealed powders decreases considerably. However, the variation in HC is dominated by the dislocation density. Fe70Al10Co5Si15 powder annealed at 900 °C exhibits improved magnetic properties (HC 14 Oe and moderate magnetization of 160 emu/g) due to optimum nanocrystalline microstructure with fine nanocrystals ( 18 nm) and reduced dislocation density. Systematic correlations observed between structural and magnetic properties for Fe80 xAlxCo5Si15 powders reveal a promising approach to control the growth of the crystals in the annealed nanocrystalline alloys and to improve the magnetic properties of mechanically alloyed Fe–Si based nanocrystalline alloys by adding suitable substituting elements. & 2016 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline materials Annealing Coercivity X-ray techniques Dislocation density Crystal growth
1. Introduction Nanostructured materials have received considerable attention among scientists due to their unique electrical, magnetic, mechanical and optical properties, and potential applications in a wide variety of technological areas. In particular, nanocrystalline magnetic materials with high saturation magnetization, low coercivity, high permeability and low core loss have been experiencing a rapid development over last few decades for saving energy in electric power transmission, transformers, magnetic sensors, electromagnetic noise suppression and automotive applications [1–5]. With this connection, Fe–Si based nanocrystalline alloys were studied extensively because of their remarkable properties, which are sensitive to the nanocrystalline microstructure and considerably different from the microcrystalline counterpart [6–8]. It is well established that these materials display improved soft magnetic properties when the average size of the crystals is less than ferromagnetic exchange correlation length, which is typically a few tens of nanometers. Among various preparation methods, mechanical alloying has been proven as a simple and an inexpensive method to synthesize a wide variety of n
Corresponding author. E-mail address: [email protected] (P. Alagarsamy).
http://dx.doi.org/10.1016/j.jmmm.2016.05.058 0304-8853/& 2016 Elsevier B.V. All rights reserved.
nanostructured materials [9,10] on a large scale with controlled magnetic properties. In this process, the pure elemental mixtures undergo repeated cold welding and fracturing, which lead to the development of nanocrystalline alloys with highly refined crystals and tunable grain boundary regions. Note that such heavy cold formation yields a large amount of internal strain in the nanocrystalline material due to dislocations and results in a high coercivity (HC). Shen et al. reported that HC of mechanically alloyed sample depends on both crystal size and dislocation density [11]. In order to relieve the induced strain in the as-milled powders, the process of annealing at elevated temperatures is inevitable. Furthermore, the as-milled powders have to be consolidated at high temperatures to produce high density components in different shapes and dimensions. It may be noted that such annealing process is not only expected to improve soft magnetic properties [12,13] due to the relaxation of strain, but also induces additional crystal growth [14] and structural modifications. However, the formation of new and undesired crystalline phases [15,16] may degrade the magnetic properties. Therefore, the optimization of the annealing process for the as-milled powders at selected temperatures is quite essential to understand the correlation between modification of microstructure with annealing and the resulting magnetic properties. Extensive studies on the effects of annealing on the structural and magnetic properties of various Fe–Si based alloys prepared by mechanical alloying have been reported [14–
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17]. Nevertheless, there have been continuous attempts to improve the soft magnetic properties of the nanocrystalline alloys by tailoring alloy compositions [18,19], changing substituting elements [20–22], various preparation parameters, optimizing nanostructure and changing annealing conditions. With this connection, in this work, we report the effects of optimized annealing and Al substitution on the nanocrystalline microstructure and the resulting magnetic properties of Fe80 xAlxCo5Si15 alloy powders, which are prepared by mechanical alloying process using a highenergy planetary ball mill and annealing at different temperatures.
2. Experimental details Weighed quantities of high purity ( 499.9%) Fe, Co, Si and Al elemental powders with different particle sizes of 60–100 μm, 2– 4 μm, 5–30 μm and 8–13 μm, respectively corresponding to the composition of Fe80 xAlxCo5Si15 with x ¼0, 2, 5 and 10 were milled in a hardened steel bowl together with 8 mm diameter hardened steel milling balls for 40 h of dry milling at 600 rpm milling speed in a planetary ball mill under argon atmosphere. The ball-topowder weight ratio was maintained at 10:1. All the as-milled powders exhibiting non-equilibrium solid solution of α-Fe (Co,Si, Al) were flame sealed in fused silica ampoules at a pressure of 10 4 Pa created using diffusion and rotary pumps combination and then annealed at different temperatures (TA) up to 900 °C for 5 h durations in a resistive tubular furnace (Okay, India). The annealing temperature in the furnace could be controlled within 72 °C to the set temperature using temperature controller. The annealing temperature and annealing time were optimized by monitoring the formation of nanocrystalline microstructure and the resulting properties of the annealed powders. The structural changes occurred due to the heat treatment were investigated by X-ray diffraction (XRD) obtained from high-power (18 kW) X-Ray diffractometer (Rigaku TTRAX III, Japan) using Cu–Kα radiation (λ ¼ 1.541 Å). XRD data were collected at a slow scan rate of 0.005°/ s to facilitate proper estimation of lattice parameter, the average size of the crystals and dislocation density of the as-milled and annealed powders. The microstructure was characterized by using transmission electron microscope (TEM, JEOL 2100). The overall composition of the powders was analyzed using energy-dispersive X-ray spectroscopy (EDS) attached to the scanning electron microscope unit. Room temperature magnetic properties were measured using a vibrating sample magnetometer (VSM, LakeShore Model 7410, USA) with the maximum applied field of 712 kOe.
3. Results and discussion 3.1. Structural properties Fig. 1 depicts a typical comparison of XRD patterns of Fe70Al10Co5Si15 powders in the state of as-mixed, as-milled and annealed at 500 °C and 900 °C. It is clear from the figure that the as-mixed powders exhibit sharp characteristic Bragg reflections corresponding to all four constituting elements, i.e., Fe (body centered cubic), Co (face centered cubic and hexagonal closepacked structure), Si (diamond cubic) and Al (face centered cubic). However, the Bragg reflections corresponding to Si, Co and Al disappear after milling for 40 h. This can be attributed to diffusion of Si, Co and Al in Fe matrix, which results in the formation of nonequilibrium body centered cubic (bcc) solid solutions of α-Fe(Si,Co, Al). Moreover, no additional Bragg peaks corresponding to any other phases or compounds were observed within the resolution of high-power XRD system. But the XRD pattern of the as-milled
Fig. 1. Room temperature XRD patterns of as-mixed, as-milled and annealed Fe70Al10Co5Si15 alloy powders at different temperatures.
powders shows a large peak broadening possibly due to size refinement and induced strain. Similarly, the XRD pattern of the annealed powders at 500 °C exhibits almost the same nature of asmilled powders except with a slight reduction in the peak width. On further increasing TA to 900 °C, a large reduction in peak broadening is observed for all the powders. On the other hand, all the as-milled and annealed powders show a considerable shift in the peak position with increasing Al content. In order to understand the effect of Al substitution in Fe80 xAlxCo5Si15 powders annealed at different TA systematically, all the annealed powders were characterized by XRD and presented in Fig. 2. It is interesting to note that for the as-milled powders and for the powders annealed at a particular temperature, the width of the peaks increases noticeably with increasing Al content in FeAlCoSi powders. This confirms a possible refinement of nanocrystals in the as-milled powders or control of crystal growth in annealed powders with increasing Al content. While the as-milled powders display a slight shift in the peak position to lower angles, a considerable shift in the peak position is observed for the annealed powders to higher angles with increasing Al. To study the effects of annealing and Al substitution on the structural parameters such as lattice constant, average crystal size, and dislocation density, XRD patterns were analyzed using Williamson– Hall Plot (WHP) method [23] and modified Williamson–Hall Plot (MWHP) method [24,25]. According to MWHP method, the individual contribution to the broadening of XRD peaks can be expressed as 2
( ΔK )
⎛ 0.9 ⎞2 ⎛ πb2ρ ⎞ 2 ⎟ +⎜ ⎟K C =⎜ ⎝ D ⎠ ⎝ 2B ⎠
(1)
where ΔK [ ¼(2cosθB ΔθB)/λ], ΔθB is full width at half maximum (FWHM) of the Bragg reflections (in radian) after correcting instrumental broadening, λ is wavelength of the X-ray, K ¼2sinθB/λ, b is modulus of Burgers vector of dislocations taken as b¼ 3 /2 a ,
(
)
D is average size of the crystals, a is lattice constant, B is a constant (taken as 10 for a wide range of dislocation distributions [24]), ρ is average dislocation density, C [ = Chkl = Ch00(1 − qH2)] is dislocation
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Fig. 3. The applicability of (a) WHP and (b) MWHP methods for the analysis of XRD data typically for as-milled Fe75Al5Co5Si15 alloy powders. The determined values of intercept and slope with errors are shown in inset of the figure.
that dislocations are main contributors to the strain. Similarly, the individual contribution to the broadening of XRD peaks in WHP method [23] can be defined as,
ΔθB cos θB =
Fig. 2. Room temperature XRD patterns of (a) as-milled and annealed Fe80 xAlxCo5Si15 alloy powders with different Al content at different temperatures: (b) 500 °C and (c) 900 °C.
contrast factor introduced to take care of elastically anisotropic materials, where the residual strains affect some Bragg reflections
(
)(
)
more than the others, H2 = h2k 2 + k 2l 2 + l 2h2 / h2 + k 2 + l 2 for a cubical system and q is a constant. To correct the instrumental broadening in XRD, the starting pure iron powder annealed at 1273 K for 24 h was used as the reference powder. The instrumental broadening corrected line profile breadth (ΔθB), in radians, of each reflection was calculated from the parabolic approximation correction [14], as given by, 2
( ΔθB)
⎛ B2 ⎞ = A2 ⎜ 1 − 2 ⎟ ⎝ A ⎠
(2)
where A and B are the breadths of the same Bragg peak (in radians) from the XRD scans of the experimental and reference powders, respectively. The values of A and B were determined by the Origin software as the FWHM after automatic background removal. In the MWHP formulation, Ungar et al. [24,25] considered
kλ + 4e sin θ D
(3)
where k is a constant taken as 0.9 by assuming spherical nature of particles, and e is the effective strain. Fig. 3 demonstrates the applicability of both WHP and MWHP methods for Fe75Al5Co5Si15 milled for 40 h. It is clear from the figure that the quality of the fit fails to obey WHP formalism (see Fig. 3a). This can be attributed to anisotropic variation in the residual strain. On the other hand, the 2
plot of ( ΔK ) versus K2C depicts that all Bragg reflections fall on a straight line and the fitting of the data using a linear equation provides a preeminent fit with R-squared value of nearly 1. This clearly supports that the residual strain affects the Bragg reflections differently. The values of average size of the crystals and dislocation density were calculated from intercept and slope of the fitted straight line, respectively. For all the presently investigated alloys, the values of q were ranged between 1.7 and 2.6. Fig. 4 depicts the variations of average crystal size, dislocation density and lattice constant as a function of Al content for asmilled and annealed Fe80 xAlxCo5Si15 powders. It is revealed that the substitution of Al in as-milled FeAlCoSi powders interestingly reduces the average crystal size from about 11 nm to about 6.5 nm and increases the dislocation density considerably from 4.2 1017 m 2 to 13.5 1017 m 2. This confirms the existence of almost 0.4 to 1.35 dislocations for every one nm2 area induced by the milling process. It is well-known that the mechanical alloying produces nanostructured materials at solid state and the rate of
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ρ=2 3
Fig. 4. Variations of average crystal size (D), dislocation density (ρ) and lattice constant (a) of as-milled and annealed Fe80–xAlxCo5Si15 alloy powders as a function of Al content. The representation of the symbols for different annealing temperatures is same in all figures.
dissolution of the powders could be related to the dislocation density introduced by comminution [26]. Thus, the dislocation density, considered as one of the major structural parameters influencing the final nanocrystalline microstructure, is correlated (see Eq. (4)) directly to the mean square lattice strain and inversely to the size of the nanocrystal [27].
65
ηrms Db
(4)
where o ηrms 4 is root mean square lattice strain. This suggests that the development of a large density of dislocations increases the probability of forming highly refined nanocrystals, as evident from Fig. 4. With increasing Al, the rate of decrease in the average crystal size decreases. This indicates that further addition of Al may not produce more dislocations due to difficulty in generating dislocations at smaller crystals, which results in a saturation like behavior of ρ with increasing Al [28]. On the other hand, it is observed from the annealing data that (i) while the values of crystal size increase slightly for the powders annealed at 500 °C, the powders annealed at 900 °C show a large increase in the crystal size up to a maximum of 30 nm. Interestingly, the average size of the crystals decreases with increasing Al content for the samples annealed at a particular TA. This supports that Al plays a crucial role in controlling the growth of the crystals. (ii) In contrast, the dislocation density decreases largely with increasing TA, which could be attributed to the strain relaxation and growth of the crystals. The variations of crystal size and dislocation density with Al substitution in Fe80 xAlxCo5Si15 powders depict a close correlation between them [27]. To understand the solid solubility level and the effect of annealing on the formation of nanocrystalline microstructure, the lattice constant was calculated from the peak positions for the as-milled and annealed powders. It is observed that the lattice constant increases from 2.8658 7 0.0004 Å to 2.8698 7 0.0006 Å with increasing Al content in milled FeAlCoSi powders. This suggests that a maximum lattice constant change of 0.0004 Å per at% Al occurred by the substitution of Al. Although the observed results are in close agreement with the earlier reports on similar systems [29,30], the increase in lattice constant for the presently investigated alloys is slightly lower than those reported for typical binary Fe–Al alloys. This could be attributed to the change in the dissolution process in the multicomponent materials as compared to the binary alloys. On the other hand, the lattice constant in the annealed powders decreases with increasing Al. This might be correlated to the reduction in the dislocation density by the annealing process, which causes the relaxation of lattice strain [14] and change in atomic ordering, which leads to a modification of microstructure after the annealing [17]. These results reveal that the variation in structural parameters strongly
Fig. 5. Bright-field TEM micrographs and selected area electron diffraction patterns for as-milled Fe75Al5Co5Si15 alloy powder and annealed Fe70Al10Co5Si15 alloy powder at 900 °C.
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depends on TA and Al substitution in Fe–Al–Si–Co alloy. In order to confirm changes in the microstructure due to annealing, TEM micrographs were obtained for as-milled and annealed powders. Fig. 5 depicts typical bright-field TEM (BF-TEM) micrographs and selected area electron diffraction (SAED) patterns for as-milled Fe75Al5Co5Si15 powders and Fe70Al10Co5Si15 powders annealed at 900 °C. BF-TEM micrograph of as-milled powder confirms the existence of fine-grain structure with the average size of about 7.5 nm having non-uniform contrast inside each grain and along grain boundaries. This could be attributed to a large strain in the as-milled powders. Similarly, BF-TEM micrograph of annealed sample confirms the nanocrystalline microstructure with a large size of the crystals, i.e., the average crystal size of about 18 nm for Fe70Al10Co5Si15 sample annealed at 900 °C. On the other hand, the rings present in SAED pattern reveal the nanocrystalline microstructure and match with the Bragg reflections in the XRD patterns. The values of average crystal size and lattice parameters calculated from TEM analysis show a close agreement with those obtained from the XRD data. Composition analysis performed using EDS for the milled Fe80 xAlxCo5Si15 with x¼0, 2, 5 and 10 showed the overall compositions of the milled powders as Fe80.8Co4.6Si14.6, Fe79Al1.8Co4.8Si14.4, Fe75.9Al4.7Co4.7Si14.7 and Fe71.1Al9.8Co4.5Si14.6, respectively. While these results evidently support the presence of Si, Co and Al in the form of solid solution in α-Fe, the Fe content in the final powder is observed to be slightly more than the actual one. This might be aroused due to contamination from the grinding media, which is the typical results in mechanical alloying process. Nevertheless, these observed results clearly confirm that (i) the substitution of Al in Fe80 xAlxCo5Si15 powders not only helps reducing the average crystal size during milling, but also controls the growth of the crystals efficiently during the annealing process. (ii) Annealing at moderate temperature (500 °C) reduces the strain largely without much growth of the crystals. (iii) On the other hand, annealing at higher temperature (900 °C) not only induces the growth of the crystals substantially, but also reduces the strain largely in the asmilled powders. These two processes are expected to show sizeable changes in the magnetic properties of the annealed powders. 3.2. Magnetic properties To understand changes in the magnetic properties, room temperature M–H loops were measured using VSM. Fig. 6 depicts room temperature M–H loops of as-milled and annealed Fe80 xAlxCo5Si15 alloy powders. The expanded versions of the M–H loops close to origin are shown as insets. These M–H loops show the typical nature of soft magnetic materials. To study the effect of annealing on the magnetic parameters, we have extracted HC from the M–H loops and determined the values of saturation magnetization (MS) from the fitting of the data to the power series,
⎛ ⎞ a a M (H ) = MS ⎜ 1 − 1 − 22 + …⎟ ⎝ ⎠ H H
(5)
where M(H) is the magnetization in an applied magnetic field H and, a1 and a2 are constant coefficients [31]. Fig. 7 displays the variations of MS and HC as a function of Al content for all Fe80 xAlxCo5Si15 alloy powders. It is observed that (i) MS decreases from 205 emu/g to 167 emu/g with increasing Al content up to 10 at% in the as-milled powders. (ii) Upon annealing at 500 °C, MS shows almost no change as compared to the as-milled powders. This may be due to the release of strain accumulated in the asmilled powders without much crystal growth as depicted in Fig. 4. (iii) On the other hand, the powders annealed at 900 °C exhibit a slight decrease in magnetization for all the compositions. This might be correlated to the occurrence of atomic ordering
Fig. 6. Room temperature M–H loops of (a) as-milled and annealed Fe80 xAlxCo5Si15 alloy powders at different temperatures: (b) 500 °C and (c) 900 °C. The representation of the symbols for different annealing temperatures is same in all figures. The expanded versions of the loops close to the origin are shown as insets.
[28,32,33]. Nevertheless, HC of as-milled powders is ranged between 90 Oe and 56 Oe. Such high HC is mainly due to different contributions such as dislocation density, refinement of crystals and compositional dependent grain boundary formation. With increasing TA, a significant decrease in HC has been observed for all the samples. However, the relative decrease in HC with increasing TA strongly depends on the Al content in Fe80 xAlxCo5Si15 annealed powders. In order to understand variation of HC with TA, HC has been plotted as a function of average ρ in Fig. 8 for Fe80 xAlxCo5Si15 alloy powders. According to Neel's theory [34], HC of the mechanically alloyed powders is expected to be proportional to ρ when magnetostatic energy is higher than anisotropy energy. However, we have not observed a linear variation of HC in the presently investigated samples. There exists a compositional dependent non-linear variation of HC with ρ. While the annealed FeCoSi powders exhibit a large variation of HC with ρ, the relative decrease in HC with ρ in Al substituted Fe80 xAlxCo5Si15 powders is reduced significantly. This could be attributed to the difference in the strain relaxation in refined crystals due to annealing and enhancement in the crystal growth, which reduces the density of dislocations in the annealed samples. It may be noted that amongst all samples, Fe70Al10Co5Si15 powders annealed at 900 °C yielded the lowest value of HC of 14 Oe and moderate MS of 160 emu/g. Sunol et al. [18] reported that FeNbSiB powders milled at different milling periods and annealed at different temperatures show HC in the range of 40–80 Oe and MS between 140 emu/g and 180 emu/g. The values of MS and HC increase significantly with increasing TA. Yousefi et al. [19] reported that HC of the as-milled (Fe65Co25)100 xSix powders with x¼ 0–20 has been in the range of 20–50 Oe and MS decreases largely from 190 emu/g to 125 emu/g
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the value of MS increases largely to 160 emu/g. This reveals that the choice of the substitution element is very much important in tuning the soft magnetic properties of the Fe–Si based nanocrystalline alloys prepared by mechanical alloying process.
4. Conclusions
Fig. 7. Variations of (a) MS and (b) HC of as-milled and annealed Fe80 xAlxCo5Si15 alloy powders at different temperatures. The representation of the symbols for different annealing temperatures is same in all figures.
We have studied the effects of annealing and Al substitution on the structural and magnetic properties of nanocrystalline Fe80 xAlxCo5Si15 alloy powders prepared by mechanical alloying process in a high-energy planetary ball mill. All the as-milled powders exhibit non-equilibrium solid solution of α-Fe(Si,Co,Al). Annealing at elevated temperatures has significant effects on their structure and magnetic properties. The average crystal size increases and dislocation density decreases with increasing annealing temperatures. The substitution of Al in Fe80Co5Si15 powders helps reducing the average crystal size in as-milled state and controls the crystal growth effectively during annealing. The average magnetization decreases slightly after annealing which could be attributed to the atomic ordering. Coercivity of the samples has been observed to be dominated by the dislocation density. Although annealing results in a considerable improvement in the soft magnetic properties of the Fe80 xAlxCo5Si15 powders, the residual defects in the annealed powders causes the coercivity in the range between 14 Oe and 40 Oe. Systematic correlations between structural and magnetic properties were observed for these annealed Fe80 xAlxCo5Si15 powders. However, the structural and magnetic parameters are strongly dependent on the alloy composition. Fe70Al10Co5Si15 powders annealed at 900 °C exhibited improved magnetic properties. This was correlated to optimum nanocrystalline microstructure with fine crystals of about 18 nm with reduced dislocation density by the annealing process. The present study revealed a possible way to control the growth of the crystal and improve the magnetic properties by the proper choice of substituting elements in annealed nanocrystalline Fe–Si based alloy powders prepared by mechanical alloying process.
Acknowledgments The authors acknowledge the infrastructural facilities provided by Department of Science and Technology, India vide project No: [SR/S2/CMP-19/2006, SR/FST/PII-020/2009].
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Fig. 8. Variations of HC as a function of ρ for as-milled and annealed Fe80 xAlxCo5Si15 alloy powders.
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