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Microstructures and magnetic properties of cast alnico 8 permanent magnets under various heat treatment conditions
Physica B: Condensed Matter 552 (2019) 136–141
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Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Microstructures and magnetic properties of cast alnico 8 permanent magnets under various heat treatment conditions
T
Sajjad Ur Rehmana, Qingzheng Jianga, Weikai Leia, Kai Liua, Liangliang Zenga, Mahpara Ghazanfarb, Tahir Ahmadc, Renhui Liua, Shengcan Maa, Zhenchen Zhonga,∗ a
Jiangxi Key Lab for Rare Earth Magnetic Materials and Devices, Institute for Rare Earth Magnetic Materials and Devices (IREMMD), JXUST, Ganzhou, China Department of Physics, Riphah International University, Islamabad, Pakistan c Department of Metallurgy and Materials Engineering, CEET, University of the Punjab, Lahore, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Alnico alloys Magnetic properties Heat treatment Microstructure Spinodal decomposition
There is a close relationship between the magnetic properties of alnico permanent magnetic alloys and the nanoscaled spinodally decomposed structure which results from various heat treatment cycles. In this paper, different heat treatment cycles are employed to cast alnico 8 alloys with composition 32.3Fe-37.5Co-13.7Ni-6.2Al-5.8Ti3.4Cu-0.2Zr-0.1S-0.8Nb. It is observed that magnetic field treatment at high temperature for specific time is the most effective method to obtain better microstructure and magnetic properties. The mosaic structures consisting of Fe-Co phase and Al-Ni rich phase are best separated during thermomagnetic treatment for 4–5 min and refined during low temperature treatments. The bias growth of the ferromagnetic phase does not develop in absence of magnetic field, and hence the phases are not refined and separated completely. This produces isotropic alnico alloys with low magnetic properties. However, continuous cooling of the alloys in magnetic field followed by isothermal treatment without magnetic field provides moderate magnetic properties. Treatment at high temperatures in field and without field for longer time leads to the coarsening of spinodal phases and poor magnetic properties. The optimum magnetic properties of Hcj = 1.7 kOe, Br = 8.0 kGs and (BH)max = 5.02 MGOe are attributed to the refined microstructure of the thermomagnetically treated alloys.
1. Introduction Alnico alloys consisting of transition metals are important permanent magnetic materials which have the capability to operate at temperatures above 500 °C [1]. Alnico alloys were extensively researched and produced during 1930s–1960s until they were dropped out of favor due to the discovery of high coercivity Rare Earth (RE) based SmCo and then NdFeB magnets. The high prices and supply restrictions of rare earth (RE) ignited renewed interest in alnico alloys during the last few years [2]. The world wide availability of the main constituent elements of alnico (Al, Ni, Co and Fe) negates monopoly over production. The magnetic properties of alnico alloys are associated with the nanostructured ferromagnetic Fe-Co rich phase implanted in paramagnetic Al-Ni matrix phase. The Fe-Co rich phase is called α1 while Al-Ni rich phase is called α2 phase. This two phased structure is formed by spinodal decomposition (SD) process [3,4]. The constituent elements of alnico alloys coexist at high temperature above 1250 °C. However, a miscibility gap develops between the constituent elements when the molten alloy is cooled below 1250 °C. This miscibility gap results in
∗
paramagnetic Al-Ni phase and ferromagnetic Fe-Co phases, and the process of splitting is termed as spinodal decomposition [5,6]. These phases are further refined by different heat treatments at various temperatures depending on composition [6]. The chemistry, structure, size and orientation of these two phases play important roles in inculcating magnetic properties in the alloys. Various dopant elements have been added to improve the microstructure and magnetic properties of alnico alloys [7–15]. Different processing conditions, i.e. cooling rate, magnetic field during cooling, holding time in magnetic field, and magnetic field strength have important roles in obtaining magnetic properties in the alloys [8,16]. The heat treatment cycles for alnico 8 include homogenization at around 1250 °C for 30–60 min. Homogenization modifies and fixes the intermetallic structures present at the interfaces, removes the stresses of as-cast alloys, and develops a steady state of chemistry in the alloys [17]. At 1250 °C the homogenized samples exhibit a bcc structure which is locked in the alloys by quenching in water, oil or air. Fast quenching bypasses the formation of deleterious effects of γ (soft magnetic) phase [6]. The homogenized and quenched samples are further treated either with magnetic field or without
Corresponding author. E-mail address: [email protected] (Z. Zhong).
https://doi.org/10.1016/j.physb.2018.10.007 Received 12 September 2018; Received in revised form 2 October 2018; Accepted 3 October 2018 Available online 03 October 2018 0921-4526/ © 2018 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 552 (2019) 136–141
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magnetic field. Magnetic heat treatment results in anisotropic alnico magnets while heat treatment without magnetic field results in isotropic alnico magnets. The magnetic heat treatment develops spinodal phases by the textured preferential growth of ferromagnetic phase parallel to the direction of the applied field [18]. Cooling in magnetic field helps to separate ferromagnetic and paramagnetic phases. Different alloy compositions have different suitable temperatures for evolution of desirable microstructures and magnetic properties. Nevertheless, many reports agree that magnetic field treatment for alnico alloys lies between 790 and 860 °C [7,8,16,19]. After the high temperature (1250 °C and above 790 °C) treatments, two low temperature treatments are applied to refine the spinodal phases. These treatments include step aging I at around 650 °C, and step aging II at around 550 °C. The atomic migration is completed during the low temperate treatments through the migration of Fe-Co atoms towards α1 phase and Al-Ni and other elements toward α2 phase. The higher aspect ratio (diameter to length ratio) of the ferromagnetic phase during step aging I results in the increment of coercivity. The packing density (the density of Fe-Co rich rods) also increases during aging treatments [20,21]. Both aging treatments are reported to reduce the TC of Al-Ni rich phase by purifying it from Fe content, so that it behaves as paramagnetic phase at room temperature [11]. Over aging has been reported to have detrimental effects on magnetic properties of alnico alloys [22]. Zhou et al. [16] studied the effect of different heat treatments on alnico 8 alloys produced by powder metallurgy. However, the powder metallurgy, involving close-coupled gas-atomization and complicated hot isostatic pressing (HIP), is a difficult and expensive fabrication technique. Furthermore, complex shaped alnico magnets (end products) can be best made by casting techniques. In this paper, we report the effects of different heat treatments on the microstructure and magnetic properties of alnico 8 alloys produced by a rather simple method, i.e., the traditional casting and simplified but effective heat treatment techniques. The effects of different heat treatments (with and without magnetic field) on microstructure and magnetic properties are studied and compared. We have tried to establish a close relationship between nano-scaled spinodally decomposed structure and magnetic properties of cast alnico 8. Discussed also are the effects of different inclusions and their possible roles in the alloy.
Fig. 1. X-ray diffraction patterns of alnico alloys at different stages of heat treatment.
analysis. The crystalline structure was determined by X-ray diffractometer using Cu kα radiation. 3. Results and discussion Fig. 1 shows the XRD patterns of the designated alnico alloys at different stages of heat treatment. The peaks observed in the patterns are compared with PDF card no. 06–0696. The as-cast and homogenized alloys exhibit bcc structure with peaks at 44.3 and 82.2° 2θ corresponding to (110) and (211) indices. The pattern of the cost alloy has sharp peaks while the peaks in homogenized alloy are broadened, which indicates that the spinodal phases have started to separate during quenching process. The peaks of the fully heat treated (HT-II) alloy have split into two parts, which indicates that the bcc α phase has separated to form α1 and α2 phases during heat treatment. The lattice parameters are slightly deviated from bcc towards tetragonal structure (a/c = 0.99 and c/a = 1.007) due to higher content of Co. These results are in agreement with the findings of Liu et al. [11] who showed that the bcc structure of alnico becomes slightly tetragonal due to high content of Co. The magnetic properties of alnico 8 alloys at different stages of heat treatment are given in Table 1. The as-cast alloys have low intrinsic coercivity, Hcj, while partial decomposition has taken place during air quenching. The Hcj of the alloy is enhanced when cooled from 855 °C to 810 °C at the rate 5 °C/min and annealed at 810 °C for 30 min. The cooling rate and annealing temperature have important effects on the magnetic properties of the alloys. Cooling from 855 °C to 840 °C and annealing at 840 °C does not result in good magnetic properties. The magnetic properties were improved when the cooling was performed in magnetic field and treated at 840 °C and 810 °C. Continuous cooling in magnetic field is usually performed for commercial alnico 5 alloys [21]. However, it is also useful in alnico 8 alloys for economic considerations. It is reported that treatment above TC leads to the growth of soft magnetic phases in the alloys, while activation energy of atoms is not enough for migration of atoms below 790 °C. This makes spinodal decomposition process sensitive to both lower and upper temperatures [9]. To further improve the magnetic properties, the samples were treated in magnetic field for various times at 840 °C and 810 °C. The treatment at 840 °C was found out to be the optimized temperature for improving magnetic properties. Annealing time is also critical to enhance magnetic properties. The magnetic properties increase initially as a function of magnetic annealing time, reach a saturation point and finally decrease. For the current investigated alloys, the best annealing condition is found to be 4–5 min in magnetic field at 840 °C, followed by HT-I and HT-II. The magnetic field annealing at 810 °C also yields good magnetic properties, but it takes longer time as compared to
2. Experimental details Alnico alloys were produced by melting in electric arc furnace equipped with magnetic stirrer and casting technique using pure elements (99.99 wt%) of nominal composition 32.3Fe-37.5Co-13.7Ni6.2Al-5.8Ti-3.4Cu-0.2Zr-0.1S-0.8Nb. After melting the alloys composition was verified by XRF analysis. Cylindrical samples of 5 mm in diameter and 5 mm in length were obtained from the cast ingots by electric discharge machining. Various heat treatments were applied after homogenization at 1250 °C for 35 min. In the first experiment, samples were furnace cooled from 855 °C to 810 °C at the rate of 5 °C/min and treated at 810 °C for 30 min in Ar. atmosphere. The samples were further cooled down to 650 °C at the rate of 8 °C and treated 5 h at 650 °C (here after HT-I) and 15 h at 540 °C (here after HT-II). Another group of samples were furnace cooled from 855 °C to 840 °C and treated for 30 min at 840 °C followed by HT-I and HT-II. In the second experiment samples were air cooled (∼60 °C/second) in magnetic field (0.6 T) from 1250 °C to 810 °C and treated at 810 °C for 30 min without magnetic field followed by HT-I and HT-II. Another group of samples were air cooled down from 1250 °C to 840 °C in magnetic field and treated at 840 °C for 30 min without field, followed by HT-I and HT-II. In the last experiment samples were treated at 840 °C for longer time (2–30 min) in magnetic field followed by HT-I and HT-II. The samples were etched in 7 ml HNO3 and 100 ml ethanol solution for 40–50 s and studied under Olympus measuring microscope, STM 7 and field emission scanning electron microscope (FE-SEM, MLA650F) for microstructural 137
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Table 1 Magnetic properties of selected alnico magnets after different heat treatments. Process
1250 °C
(≥810 °C)
HT- I
HT- II
Hcj [kOe]
Br [kGs]
(BH)max [MGOe]
Homogenized Cooled without field Cooled in field Isothermal field treatment
Yes Yes Yes Yes
No Yes Yes Yes
No Yes Yes Yes
No Yes Yes Yes
0.063 1.29 1.46 1.70
5.4 5.3 5.8 8.0
0.05 1.80 2.48 5.02
more compact grain boundaries with fewer inclusions. To analyze different phases in alnico alloys during different stages of heat treatment, two samples were fractured by hammering. The fractured surfaces of cast alloys provide useful information because there is no effect of etchant on fractured surface. The SEM micrographs of the fractured as-cast and homogenized samples are presented in Fig. 3 (a) and (b). The compositions of the selected areas are given in Table 2. Fig. 3 (b) shows the grains of the fractured sample after homogenization. It is evident that the grains have well defined boundaries and there are white portions on the grains boundaries. These white portions are rich in high melting point elements like Zr, Ti, and Nb. The sharp boundaries indicate that deleterious γ phase has been effectively suppressed during quenching. The α phase is the main phase of alnico type alloys which decomposes into α1 and α2 phases. There are other phases and structures in alnico alloys in addition to the α phase. Fig. 4 displays the phases studied at different stages of heat treatment. Fig. 4 (a) shows elongated Ti-S rich rod. Ti is the most reactive element and forms sulfides, carbides and nitrides reacting with sulfur, oxygen and nitrogen respectively. Ti mostly distributes in the Al-Ni rich phase and narrows the miscibility gap between the two SD phases [15]. Iwama reported that Ti reduces the precipitation of deleterious soft magnetic phases (γ) in the alnico alloys by narrowing the γ phase formation temperature [10] In this study, we found a number of Ti-S rich phases in the alloys which elongate across α (α1+α2) phase structure and sometimes across the grain boundaries. The amount of these phases is less when S is not added into the alloy [6], however, Ti-S rods are found in relatively high quantity when S is added into the alloys. The shape of these rods remains unchanged by changing content of S, but the quantity does change. The Ti-S rich phases elongate inside the grain as shown in Fig. 4 (a), and sometimes across the grain boundary. It is speculated that such elongated Ti-S rich phases help in reducing the brittleness of alnico alloys. The composition of this phase determined by EDXS is Ti = 31.52, S = 10.2, C = 5.08, Al = 4 .19, Fe = 19.0, Co = 19.9, Ni = 8.0 and Cu = 2.1. Carbon is reported to deteriorate the magnetic properties of alnico [23]. Thus Ti-S rich rods make the main phases (α1 and α2) relatively free from C content and thus contribute in enhancing magnetic properties of the alnico 8 alloys. Small amount of S is
annealing at 840 °C. The best magnetic properties obtained through each heat treatment cycle are depicted in Table 1. It is obvious that short time isothermal field treatment, followed by long time low temperature treatment, is the most effective heat treatment cycle for alnico 8 alloys. There was no obvious change in properties for samples treated at the same temperature for longer times (7, 10, 15 min), however the magnetic properties deteriorated when the samples were treated for 20 min and 30 min, which suggests that the coarsening of phases occurs during long time magnetic annealing. Isothermal field treatment is not an easy process due for economic reasons, and has processing difficulties. Therefore, it is concluded that isothermal heat treatment for 4–5 min followed by HT-I and HT-II is the most feasible heat treatment cycle. Furthermore, it is observed that the Hcj of the alloys increase by 15–30% after the low temperature heat treatments (HT-I and HT-II). This indicates that the atomic migration still takes place during HT-I and HT-II. The change in magnetic properties of isotropic alloys is found to be more prominent than magnetically treated anisotropic magnetic alloys. The optical microstructures of the alloy after casting, homogenization and HT-II are depicted in Fig. 2 (a)–(c). All grains are polycrystalline having equi-axed orientations with no discernible change in the dimension attributable to heat treatment. The grain size varies from 50 to 130 μm. Fig. 2 (a) shows optical micrograph of as-cast alloy. Many inclusions are seen in the form of dark spots. These inclusions are speculated to be oxides and sulfides. Some of these inclusions dissolve during homogenization treatment. The sulfides like iron sulfides dissolve during homogenization and sulfur combines with Ti to form Ti-S rich rods which in turn become permanent part of the alnico cast alloys. Alnico alloys produced by induction melting at industrial scale have fewer inclusions because inclusions are effectively removed in induction melting process. In arc melting small samples are fabricated and it is hard to effectively reduce the inclusions. It is, therefore, expected that alnico magnetic alloys produced through induction casting exhibit better magnetic properties as compared to the alloys fabricated through electric arc melting. The micrograph of the homogenized alloy in Fig. 2(b) has an improved microstructure with fewer inclusions, and the grain boundaries are clearly visible after being etched. The optical micrograph in Fig. 2 (c) is obtained after HT-II. This micrograph has
(a)
(b)
(c)
Inclusions
Grain boundary Inclusions
Fig. 2. Optical micrographs of alnico alloys (a) as-cast, (b) homogenized, (c) fully heat treated. 138
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E
(a) (a)
(b)
B
A
Al-rich phase
F
C
Ti-Zr-Nb rich
D
Fig. 3. SEM micrographs of fractured samples, (a) as-cast, (b) homogenized.
for α2 phase. The coarsened spatial dimension is one reason, in addition to the random orientation, for the low magnetic properties of the alloys. The coarsened and randomly oriented phases confirm the notion that magnetic field treatment at high temperatures is necessary for bias growth of ferromagnetic phase. Fig. 5 (b) shows samples annealed at 840 °C after cooling from 855 °C. The SD phases have further coarsened and the magnetic properties are reduced. The reduction in the Hcj is more prominent which reduces from 1.29 kOe to 1.18 kOe but the reduction in the Br value is only nominal resulting in moderate reduction in energy density (BH)max value. The average dimension of the Fe-Co rich phase is found to be 50 ± 5 nm. Some abnormal growth of SD phases is also observed in the alloys treated at 840 °C for 30 min. Another heat treatment, the continuous cooling in magnetic field, was applied to analyze the effect of magnetic field on SD structure and magnetic properties. The samples were cooled down from 1250 °C to 900 °C without magnetic field, and then magnetic field was applied and sample was allowed to cool down to 810 °C. This sample was initially annealed at 810 °C for 30 min without magnetic field, and further heattreated at 650 °C for 4 h and 560 °C for 15 h. The HRSEM micrograph is given in Fig. 6 (a) where two phase spinodally decomposed structure is clearly visible. The diameter of α1 phase is found out to be 28 ± 2 nm while α2 phase has a diameter of 25 ± 2 nm. The effective temperature range of spinodal decomposition is between 790 and 865 °C [9]. Hence the sample underwent ∼10 s annealing under magnetic field in the effective temperature range of SD. The micrograph in Fig. 6 (b) shows SD structure of sample cooled from 1250 °C to 900 °C, continuously cooled from 900 °C to 840 °C and annealed at 840 °C for 30 min. The phases, especially the Fe-Co rich phase has coarsened as compared to the phases in micrograph in Fig. 6 (a) with diameters increasing from
Table 2 Composition of different areas of the cast alnico 8 alloys. Spectrum
Al
Ti
Fe
Co
Ni
Cu
Nb
Zr
S
A B C D E F
8.58 10.34 5.72 7.34 6.03 6.81
7.02 6.20 7.80 6.32 6.67 6.52
31.73 30.44 32.41 32.28 33.87 33.04
37.09 36.76 37.59 37.97 37.30 37.67
12.43 13.36 13.13 12.80 12.88 12.73
2.62 2.60 2.85 2.79 2.75 2.63
0.45 0.90 0.09 0.10 0.20 0.20
0.1 0.2 0.2 0.3 0.1 0.2
0.1 0.2 0.2 0.2 0.1 0.2
sometimes added to alnico alloys in industry to improve the wetability of the alloys, which eases the casting process. High melting point elements preferably accommodate at grain boundaries and triple junctions of α-grains. Fig. 4 (b) shows Nb-Ti rich phase. While Fig. 4(c) depicts Zr-Nb-Ti rich phase at a triple junction. The composition of this phase is Zr = 21.8, Nb = 4.2, Ti = 6.6, Al = 3.1, Co = 28.1, Fe = 20.2, Ni = 13.0, Cu = 3.02. Micrographs in Fig. 3 (a) and (c) are taken after heat treatment HT-II. Hence two phase structure in the main grains can be observed. Different heat treatment cycles left no obvious impact on the structure and composition of the phases shown in Fig. 4 (a)–(c), which is an indication that high melting point elements do not easily migrate during heat treatment process. Fig. 5 shows HRSEM micrographs of isotropic alnico 8 alloys annealed without applying magnetic field. The spinodally decomposed (SD) structure is seen in the micrograph in Fig. 5 (a). The SD structure depicts a random orientation of the ferromagnetic α1 phase. The moderate magnetic properties of isotropic alloys, shown in Table 1, are attributed to this randomly oriented SD structure. The average diameters of these phases are found to be 40 ± 5 nm for α1 and 35 ± 5 nm
(c)
(b)
(a)
Zr-Nb-Ti rich phase Ti-S rich phase
Nb-Ti rich phase
Fig. 4. SEM micrographs of high melting points elements, (a) Ti-S rich rods, (b) Ti-Nb rich phase, (c) Zr-Nb-Ti rich phase. 139
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(a)
(b) 2
1
2
1
Fig. 5. HRSEM micrographs of isotropic alnico alloys treated at: (a) 810 °C, (b) 840 °C.
28 ± 2 nm to 34 ± 2 nm. No obvious change in magnetic properties was observed in alloys annealed at 810 °C and 840 °C. From these experiments it is apparent that temperature has critical role in forming and coarsening spinodal phases. SD phases grow more rapidly at high temperature, but there is no obvious evidence of further splitting of FeCo rich phases. In the last experiment the samples were treated at 810 °C and 840 °C for various times followed by low temperature annealing at 650 °C for 4 h and 560 °C for 15 h. The alloys treated at 840 °C resulted in better magnetic properties. Fig. 7 (a) shows the HRSEM micrograph of the alloy treated at 840 °C for 5 min. The refined Fe-Co rich α1 and Al-Ni rich α2 phases are visible. Due to relatively long time of magnetic heat treatment below Curie temperature, the ferromagnetic phase has responded to the magnetic field and has elongated parallel to the direction of the field. This biased growth of the ferromagnetic phase is the reason for the better magnetic properties of the alloys. This sample has not been treated for long time at high temperature (> 800 °C), so the spinodal phases are not coarsened. The diameter of the α1 phase is calculated to be 20 ± 3 nm while the α2 phase has an average diameter of 18 ± 3 nm. At this low spatial dimension the ferromagnetic phases behave as single domains [8] which give better magnetic properties. Reducing the dimensions of spinodal phases below 20 nm is practically not viable in bulk alnico alloys, however, in our recent work we reported spatial dimension of the order of 5 nm in alnico nano ribbons [24]. We also find that there is a potential problem with low dimensional SD phases. The SD phases cannot be purified when their dimension is very low [24]. In addition to elongated phase, some
(a)
(b) Branching
2 1
1
2
Splitting
Fig. 7. HRSEM micrographs of alnico alloys treated isothermally in magnetic field, (a) 5 min, (b) 30 min.
branching of ferromagnetic phase is also observed in the micrograph Fig. 7(a). Such areas either undergo poor spinodal decomposition or do not decompose at all, and are called non spinodally decomposed structures. Though Ti and other high melting point elements play important role in heat treatment cycles and magnetic properties of the alloys [10], they hardly migrate through the phases due to large radius. They usually accumulate at grains boundaries, or form rod like structures as shown in Fig. 4(a–c). The high melting point elements are, therefore, speculated to have a role in the branching of spinodal phases. Thus the content of high melting point elements in the alloy is more critical compared to regular elements (Fe-Co-Ni-Al). Fig. 7 (b) shows micrograph of alnico alloy treated at 840 °C for 30 min. The SD phases have apparently coarsened. The magnetic properties are deteriorated due to the coarsening of the phases. It was observed that the optimum
Fig. 6. HRSEM micrographs of anisotropic alnico alloys continuously cooled in magnetic field and treated at: (a) 810 °C, (b) 840 °C for 30 min. 140
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magnetic properties are obtained by annealing in magnetic field for 5 min and no obvious change in magnetic properties was observed by annealing for 7, 10 and 15 min. The magnetic properties deteriorated when samples were annealed for more than 15 min in magnetic field. Heat treatment conditions like temperature range, holding time and cooling rate play significant role in the evolution of microstructures and magnetic properties of alnico magnetic alloys. Furthermore it is suggested, on the basis of different heat treatments, that the spinodal decomposition would not occur effectively in absence of magnetic field during high temperature treatment. Magnetic field has a decisive role in evolution of microstructure and hence in inculcating magnetic properties in the alloys. The spinodal decomposition and coarsening of phases happen during high temperature treatments. During the low temperature treatment only small migration of respective atoms to Fe-Co rich phase and Al-Ni rich phase occurs. As a result the intrinsic coercivity Hcj of the alloys increases during HT-I and HT-II. However, there is no obvious change in the microstructure before and after HT-I and HT-II. The splitting of SD phases and the coarsening of all phases imply that other mechanism like nucleation and growth may also play some role in the structural transformation of the phases besides spinodal decomposition [16]. The splitting shown in Fig. 7 (a) indicates that some elastic strain energy is introduced during coarsening and phases start splitting. It is reported that the splitting of phases occurs between 5 min and 10 min of isothermal annealing in magnetic field [20]. It is also known that splitting of phases cannot occur if SD is the only mechanism and that the phases should coarsen without splitting [20]. The splitting of phases was rarely observed in our investigated alloys, while the coarsening of phases was prominent in all alloys during various treatments depending both on temperature and time. Therefore, it is concluded that spinodal decomposition is the dominant mechanism for two phase formation in alnico alloys, while small effects of nucleation and growth of phases cannot be ruled out.
Foundation of China (Grant Nos. 51564037, 51661011 and 51671097). References [1] J.T. Zhao, Y.L. Sun, L. Liu, D. Lee, Z. Liu, X.C. Feng, A.R. Yan, Correlations of phase structure and thermal stability for Alnico 8 alloys, J. Magn. Magn Mater. 442 (2017) 208–211. [2] A. Palasyuk, E. Blomberg, R. Prozorov, L. Yue, M.J. Kramer, R.W. McCallum, I.E. Anderson, S. Constantinides, Advances in characterization of non-rare-earth permanent magnets: exploring commercial Alnico grades 5–7 and 9, JOM (J. Occup. Med.) 65 (2013) 862–869. [3] L. Zhou, M.K. Miller, P. Lu, L. Ke, R. Skomski, H. Dillon, Q. Xing, A. Palasyuk, M.R. McCartney, D.J. Smith, S. Constantinides, R.W. McCallum, I.E. Anderson, V. Antropov, M.J. Kramer, Architecture and magnetism of alnico, Acta Mater. 74 (2014) 224–233. [4] D.A. Laptei, V.E. Shcjherbakov, Magnetic structure of alnico alloy, Russ. Phys. J. 16 (1973) 567–569. [5] Y. Iwama, M. Takeuchi, Spinodal decomposition in Alnico 8 magnet alloy, Trans. Jpn. Inst. Metals 15 (1974) 371–377. [6] S.U. Rehman, Q. Jiang, Q. Ge, W. Lei, L. Zhang, Q. Zeng, A. ul Haq, R. Liu, Z.C. Zhong, Microstructure and magnetic properties of alnico permanent magnetic alloys with Zr-B additives, J. Magn. Magn Mater. 451 (2018) 243–247. [7] S.U. Rehman, Z. Ahmad, A. ul Haq, S. Akhtar, Effects of Zr alloying on the microstructure and magnetic properties of Alnico permanent magnets, J. Magn. Magn Mater. 442 (2017) 136–140. [8] R.A. McCurrie, The Structure and Properties of Alnico Permanent Magnet Alloys 3 (1982), pp. 107–188. [9] G. Albanese, G. Asti, R. Criscuoli, On the state and distribution of iron in the alnico 8 alloy, IEEE Trans. Magn. 6 (1970) 161–163. [10] Y. Iwama, M. Inagaki, T. Miyamoto, Effects of titanium in Alnico 8-type magnet alloys, Trans. Jpn. Inst. Metals 11 (1970) 268–274. [11] T. Liu, W. Li, M. Zhu, Z. Guo, Y. Li, Effect of Co on the thermal stability and magnetic properties of Alnco 8 alloys, J. Appl. Phys. 115 (2014) 17A751. [12] M. Fan, Y. Liu, R. Jha, G.S. Dulikravich, J. Schwartz, C.C. Koch, C. C, On the formation and evolution of Cu–Ni-Rich bridges of alnico alloys with thermomagnetic treatment, IEEE Trans. Magn. 52 (2016) 1–10. [13] C. Bronner, J.P. Haberer, E. Planchard, J. Sauze, Contribution to the study of influence of niobium on the magnetic properties of alnico alloys containing 4.5 to 6.5% titanium, Cobalt 46 (1970) 15–22. [14] Y. Kamata, T.K. Anbo, The effects of the duplex addition of sulfur and carbon on the columnar crystallization of high coercivity alnico alloys containing titanium and magnetic properties of some of these alloys, Nippon Kinzoku Gakkai-SI 31 (1967) 1053–1057. [15] S.M. Hao, K. Ishida, T. Nishizawa, Role of alloying elements in phase decomposition in Alnico magnet alloys, Metall. Trans. A 16 (1985) 179–185. [16] L. Zhou, W. Tang, L. Ke, W. Guo, J.D. Poplawsky, I.E. Anderson, M.J. Kramer, Microstructural and magnetic property evolution with different heat-treatment conditions in an alnico alloy, Acta Mater. 133 (2017) 73–80. [17] S.U. Rehman, Q. Jiang, W. Lei, L. He, Q. Tan, Q. Quan, L. Wang, M. Zhong, S. Ma, A. ul Haq, Z.C. Zhong, Improved microstructure and magnetic properties of alnico 8 by B-doping, IEEE Trans. Magn. 99 (2018) 1–6. [18] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, John Wiley & Sons, 2011. [19] W.G. Chu, W.D. Fei, X.H. Li, D.Z. Yang, J.L. Wang, Evolution of Fe–Co rich particles in Alnico 8 alloy thermomagnetically treated at 800°C, Mater. Sci. Technol. 16 (9) (2000) 1023–1028. [20] W.G. Chu, W.D. Fei, X.H. Li, D.Z. Yang, A study on the microstructure of Alnico 8 alloy thermomagnetically treated at various temperatures, Mater. Chem. Phys. 73 (2002) 290–294. [21] V. Sergeyev, T. Bulygina, Magnetic properties of alnico 5 and alnico 8 phases at the sequential stages of heat treatment in a field, IEEE Trans. Magn. 6 (1970) 194–198. [22] A.G. Clegg, Spoiling characteristics of alloys of Fe Ni Al Co system, Zeitschrift fur angewandte phyisck 21 (1966) 77. [23] Y. Kamata, T. Anbo, The effects of duplex addition of sulfur and carbon on the columnar crystallization of high coercivity alnico alloys containing titanium and magnetic properties of some of these alloys, Nippon Kinzoku Gakkai-S 31 (9) (1967) 1053–1057. [24] S.U. Rehman, Q. Jiang, L. He, M. Ghazanfar, W. Lei, X. Hu, S.U. Awan, S. Ma, Z.C. Zhong, Synthesis, microstructures, magnetic properties and thermal stabilities of isotropic alnico ribbons, J. Magn. Magn Mater. 466 (2018) 277–282.
4. Conclusion The alnico 8 alloys were fabricated by electric arc melting and annealed by subsequent heat treatments. Investigated systematically are the effects of different heat treatment cycles on microstructures and magnetic properties of the alloys. It is discovered that the high temperature (840 °C) treatment under magnetic field is most effective for the designated composition. The best magnetic properties in the alloys are obtained by the thermomagnetic treatment for five minutes followed by two low temperature draw cycles. High temperature annealing for long time results in coarsening of spinodal phases to deteriorate the magnetic properties. An obvious improvement in Hcj values was observed which indicates that small scale uphill diffusion takes place during HT-I and HT-II, though heat treatment at low temperatures (HT-I and HT-II) has no obvious effects on the microstructure of alnico alloys. Continuous cooling in magnetic field leads to moderate magnetic properties in the alloys. Treatment without magnetic field inculcates isotropic properties in the alloys which are lower than the properties of anisotropic magnets treated in magnetic field. Acknowledgement This work was supported by the National Natural Science
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Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Microstructures and magnetic properties of cast alnico 8 permanent magnets under various heat treatment conditions
T
Sajjad Ur Rehmana, Qingzheng Jianga, Weikai Leia, Kai Liua, Liangliang Zenga, Mahpara Ghazanfarb, Tahir Ahmadc, Renhui Liua, Shengcan Maa, Zhenchen Zhonga,∗ a
Jiangxi Key Lab for Rare Earth Magnetic Materials and Devices, Institute for Rare Earth Magnetic Materials and Devices (IREMMD), JXUST, Ganzhou, China Department of Physics, Riphah International University, Islamabad, Pakistan c Department of Metallurgy and Materials Engineering, CEET, University of the Punjab, Lahore, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Alnico alloys Magnetic properties Heat treatment Microstructure Spinodal decomposition
There is a close relationship between the magnetic properties of alnico permanent magnetic alloys and the nanoscaled spinodally decomposed structure which results from various heat treatment cycles. In this paper, different heat treatment cycles are employed to cast alnico 8 alloys with composition 32.3Fe-37.5Co-13.7Ni-6.2Al-5.8Ti3.4Cu-0.2Zr-0.1S-0.8Nb. It is observed that magnetic field treatment at high temperature for specific time is the most effective method to obtain better microstructure and magnetic properties. The mosaic structures consisting of Fe-Co phase and Al-Ni rich phase are best separated during thermomagnetic treatment for 4–5 min and refined during low temperature treatments. The bias growth of the ferromagnetic phase does not develop in absence of magnetic field, and hence the phases are not refined and separated completely. This produces isotropic alnico alloys with low magnetic properties. However, continuous cooling of the alloys in magnetic field followed by isothermal treatment without magnetic field provides moderate magnetic properties. Treatment at high temperatures in field and without field for longer time leads to the coarsening of spinodal phases and poor magnetic properties. The optimum magnetic properties of Hcj = 1.7 kOe, Br = 8.0 kGs and (BH)max = 5.02 MGOe are attributed to the refined microstructure of the thermomagnetically treated alloys.
1. Introduction Alnico alloys consisting of transition metals are important permanent magnetic materials which have the capability to operate at temperatures above 500 °C [1]. Alnico alloys were extensively researched and produced during 1930s–1960s until they were dropped out of favor due to the discovery of high coercivity Rare Earth (RE) based SmCo and then NdFeB magnets. The high prices and supply restrictions of rare earth (RE) ignited renewed interest in alnico alloys during the last few years [2]. The world wide availability of the main constituent elements of alnico (Al, Ni, Co and Fe) negates monopoly over production. The magnetic properties of alnico alloys are associated with the nanostructured ferromagnetic Fe-Co rich phase implanted in paramagnetic Al-Ni matrix phase. The Fe-Co rich phase is called α1 while Al-Ni rich phase is called α2 phase. This two phased structure is formed by spinodal decomposition (SD) process [3,4]. The constituent elements of alnico alloys coexist at high temperature above 1250 °C. However, a miscibility gap develops between the constituent elements when the molten alloy is cooled below 1250 °C. This miscibility gap results in
∗
paramagnetic Al-Ni phase and ferromagnetic Fe-Co phases, and the process of splitting is termed as spinodal decomposition [5,6]. These phases are further refined by different heat treatments at various temperatures depending on composition [6]. The chemistry, structure, size and orientation of these two phases play important roles in inculcating magnetic properties in the alloys. Various dopant elements have been added to improve the microstructure and magnetic properties of alnico alloys [7–15]. Different processing conditions, i.e. cooling rate, magnetic field during cooling, holding time in magnetic field, and magnetic field strength have important roles in obtaining magnetic properties in the alloys [8,16]. The heat treatment cycles for alnico 8 include homogenization at around 1250 °C for 30–60 min. Homogenization modifies and fixes the intermetallic structures present at the interfaces, removes the stresses of as-cast alloys, and develops a steady state of chemistry in the alloys [17]. At 1250 °C the homogenized samples exhibit a bcc structure which is locked in the alloys by quenching in water, oil or air. Fast quenching bypasses the formation of deleterious effects of γ (soft magnetic) phase [6]. The homogenized and quenched samples are further treated either with magnetic field or without
Corresponding author. E-mail address: [email protected] (Z. Zhong).
https://doi.org/10.1016/j.physb.2018.10.007 Received 12 September 2018; Received in revised form 2 October 2018; Accepted 3 October 2018 Available online 03 October 2018 0921-4526/ © 2018 Elsevier B.V. All rights reserved.
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magnetic field. Magnetic heat treatment results in anisotropic alnico magnets while heat treatment without magnetic field results in isotropic alnico magnets. The magnetic heat treatment develops spinodal phases by the textured preferential growth of ferromagnetic phase parallel to the direction of the applied field [18]. Cooling in magnetic field helps to separate ferromagnetic and paramagnetic phases. Different alloy compositions have different suitable temperatures for evolution of desirable microstructures and magnetic properties. Nevertheless, many reports agree that magnetic field treatment for alnico alloys lies between 790 and 860 °C [7,8,16,19]. After the high temperature (1250 °C and above 790 °C) treatments, two low temperature treatments are applied to refine the spinodal phases. These treatments include step aging I at around 650 °C, and step aging II at around 550 °C. The atomic migration is completed during the low temperate treatments through the migration of Fe-Co atoms towards α1 phase and Al-Ni and other elements toward α2 phase. The higher aspect ratio (diameter to length ratio) of the ferromagnetic phase during step aging I results in the increment of coercivity. The packing density (the density of Fe-Co rich rods) also increases during aging treatments [20,21]. Both aging treatments are reported to reduce the TC of Al-Ni rich phase by purifying it from Fe content, so that it behaves as paramagnetic phase at room temperature [11]. Over aging has been reported to have detrimental effects on magnetic properties of alnico alloys [22]. Zhou et al. [16] studied the effect of different heat treatments on alnico 8 alloys produced by powder metallurgy. However, the powder metallurgy, involving close-coupled gas-atomization and complicated hot isostatic pressing (HIP), is a difficult and expensive fabrication technique. Furthermore, complex shaped alnico magnets (end products) can be best made by casting techniques. In this paper, we report the effects of different heat treatments on the microstructure and magnetic properties of alnico 8 alloys produced by a rather simple method, i.e., the traditional casting and simplified but effective heat treatment techniques. The effects of different heat treatments (with and without magnetic field) on microstructure and magnetic properties are studied and compared. We have tried to establish a close relationship between nano-scaled spinodally decomposed structure and magnetic properties of cast alnico 8. Discussed also are the effects of different inclusions and their possible roles in the alloy.
Fig. 1. X-ray diffraction patterns of alnico alloys at different stages of heat treatment.
analysis. The crystalline structure was determined by X-ray diffractometer using Cu kα radiation. 3. Results and discussion Fig. 1 shows the XRD patterns of the designated alnico alloys at different stages of heat treatment. The peaks observed in the patterns are compared with PDF card no. 06–0696. The as-cast and homogenized alloys exhibit bcc structure with peaks at 44.3 and 82.2° 2θ corresponding to (110) and (211) indices. The pattern of the cost alloy has sharp peaks while the peaks in homogenized alloy are broadened, which indicates that the spinodal phases have started to separate during quenching process. The peaks of the fully heat treated (HT-II) alloy have split into two parts, which indicates that the bcc α phase has separated to form α1 and α2 phases during heat treatment. The lattice parameters are slightly deviated from bcc towards tetragonal structure (a/c = 0.99 and c/a = 1.007) due to higher content of Co. These results are in agreement with the findings of Liu et al. [11] who showed that the bcc structure of alnico becomes slightly tetragonal due to high content of Co. The magnetic properties of alnico 8 alloys at different stages of heat treatment are given in Table 1. The as-cast alloys have low intrinsic coercivity, Hcj, while partial decomposition has taken place during air quenching. The Hcj of the alloy is enhanced when cooled from 855 °C to 810 °C at the rate 5 °C/min and annealed at 810 °C for 30 min. The cooling rate and annealing temperature have important effects on the magnetic properties of the alloys. Cooling from 855 °C to 840 °C and annealing at 840 °C does not result in good magnetic properties. The magnetic properties were improved when the cooling was performed in magnetic field and treated at 840 °C and 810 °C. Continuous cooling in magnetic field is usually performed for commercial alnico 5 alloys [21]. However, it is also useful in alnico 8 alloys for economic considerations. It is reported that treatment above TC leads to the growth of soft magnetic phases in the alloys, while activation energy of atoms is not enough for migration of atoms below 790 °C. This makes spinodal decomposition process sensitive to both lower and upper temperatures [9]. To further improve the magnetic properties, the samples were treated in magnetic field for various times at 840 °C and 810 °C. The treatment at 840 °C was found out to be the optimized temperature for improving magnetic properties. Annealing time is also critical to enhance magnetic properties. The magnetic properties increase initially as a function of magnetic annealing time, reach a saturation point and finally decrease. For the current investigated alloys, the best annealing condition is found to be 4–5 min in magnetic field at 840 °C, followed by HT-I and HT-II. The magnetic field annealing at 810 °C also yields good magnetic properties, but it takes longer time as compared to
2. Experimental details Alnico alloys were produced by melting in electric arc furnace equipped with magnetic stirrer and casting technique using pure elements (99.99 wt%) of nominal composition 32.3Fe-37.5Co-13.7Ni6.2Al-5.8Ti-3.4Cu-0.2Zr-0.1S-0.8Nb. After melting the alloys composition was verified by XRF analysis. Cylindrical samples of 5 mm in diameter and 5 mm in length were obtained from the cast ingots by electric discharge machining. Various heat treatments were applied after homogenization at 1250 °C for 35 min. In the first experiment, samples were furnace cooled from 855 °C to 810 °C at the rate of 5 °C/min and treated at 810 °C for 30 min in Ar. atmosphere. The samples were further cooled down to 650 °C at the rate of 8 °C and treated 5 h at 650 °C (here after HT-I) and 15 h at 540 °C (here after HT-II). Another group of samples were furnace cooled from 855 °C to 840 °C and treated for 30 min at 840 °C followed by HT-I and HT-II. In the second experiment samples were air cooled (∼60 °C/second) in magnetic field (0.6 T) from 1250 °C to 810 °C and treated at 810 °C for 30 min without magnetic field followed by HT-I and HT-II. Another group of samples were air cooled down from 1250 °C to 840 °C in magnetic field and treated at 840 °C for 30 min without field, followed by HT-I and HT-II. In the last experiment samples were treated at 840 °C for longer time (2–30 min) in magnetic field followed by HT-I and HT-II. The samples were etched in 7 ml HNO3 and 100 ml ethanol solution for 40–50 s and studied under Olympus measuring microscope, STM 7 and field emission scanning electron microscope (FE-SEM, MLA650F) for microstructural 137
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Table 1 Magnetic properties of selected alnico magnets after different heat treatments. Process
1250 °C
(≥810 °C)
HT- I
HT- II
Hcj [kOe]
Br [kGs]
(BH)max [MGOe]
Homogenized Cooled without field Cooled in field Isothermal field treatment
Yes Yes Yes Yes
No Yes Yes Yes
No Yes Yes Yes
No Yes Yes Yes
0.063 1.29 1.46 1.70
5.4 5.3 5.8 8.0
0.05 1.80 2.48 5.02
more compact grain boundaries with fewer inclusions. To analyze different phases in alnico alloys during different stages of heat treatment, two samples were fractured by hammering. The fractured surfaces of cast alloys provide useful information because there is no effect of etchant on fractured surface. The SEM micrographs of the fractured as-cast and homogenized samples are presented in Fig. 3 (a) and (b). The compositions of the selected areas are given in Table 2. Fig. 3 (b) shows the grains of the fractured sample after homogenization. It is evident that the grains have well defined boundaries and there are white portions on the grains boundaries. These white portions are rich in high melting point elements like Zr, Ti, and Nb. The sharp boundaries indicate that deleterious γ phase has been effectively suppressed during quenching. The α phase is the main phase of alnico type alloys which decomposes into α1 and α2 phases. There are other phases and structures in alnico alloys in addition to the α phase. Fig. 4 displays the phases studied at different stages of heat treatment. Fig. 4 (a) shows elongated Ti-S rich rod. Ti is the most reactive element and forms sulfides, carbides and nitrides reacting with sulfur, oxygen and nitrogen respectively. Ti mostly distributes in the Al-Ni rich phase and narrows the miscibility gap between the two SD phases [15]. Iwama reported that Ti reduces the precipitation of deleterious soft magnetic phases (γ) in the alnico alloys by narrowing the γ phase formation temperature [10] In this study, we found a number of Ti-S rich phases in the alloys which elongate across α (α1+α2) phase structure and sometimes across the grain boundaries. The amount of these phases is less when S is not added into the alloy [6], however, Ti-S rods are found in relatively high quantity when S is added into the alloys. The shape of these rods remains unchanged by changing content of S, but the quantity does change. The Ti-S rich phases elongate inside the grain as shown in Fig. 4 (a), and sometimes across the grain boundary. It is speculated that such elongated Ti-S rich phases help in reducing the brittleness of alnico alloys. The composition of this phase determined by EDXS is Ti = 31.52, S = 10.2, C = 5.08, Al = 4 .19, Fe = 19.0, Co = 19.9, Ni = 8.0 and Cu = 2.1. Carbon is reported to deteriorate the magnetic properties of alnico [23]. Thus Ti-S rich rods make the main phases (α1 and α2) relatively free from C content and thus contribute in enhancing magnetic properties of the alnico 8 alloys. Small amount of S is
annealing at 840 °C. The best magnetic properties obtained through each heat treatment cycle are depicted in Table 1. It is obvious that short time isothermal field treatment, followed by long time low temperature treatment, is the most effective heat treatment cycle for alnico 8 alloys. There was no obvious change in properties for samples treated at the same temperature for longer times (7, 10, 15 min), however the magnetic properties deteriorated when the samples were treated for 20 min and 30 min, which suggests that the coarsening of phases occurs during long time magnetic annealing. Isothermal field treatment is not an easy process due for economic reasons, and has processing difficulties. Therefore, it is concluded that isothermal heat treatment for 4–5 min followed by HT-I and HT-II is the most feasible heat treatment cycle. Furthermore, it is observed that the Hcj of the alloys increase by 15–30% after the low temperature heat treatments (HT-I and HT-II). This indicates that the atomic migration still takes place during HT-I and HT-II. The change in magnetic properties of isotropic alloys is found to be more prominent than magnetically treated anisotropic magnetic alloys. The optical microstructures of the alloy after casting, homogenization and HT-II are depicted in Fig. 2 (a)–(c). All grains are polycrystalline having equi-axed orientations with no discernible change in the dimension attributable to heat treatment. The grain size varies from 50 to 130 μm. Fig. 2 (a) shows optical micrograph of as-cast alloy. Many inclusions are seen in the form of dark spots. These inclusions are speculated to be oxides and sulfides. Some of these inclusions dissolve during homogenization treatment. The sulfides like iron sulfides dissolve during homogenization and sulfur combines with Ti to form Ti-S rich rods which in turn become permanent part of the alnico cast alloys. Alnico alloys produced by induction melting at industrial scale have fewer inclusions because inclusions are effectively removed in induction melting process. In arc melting small samples are fabricated and it is hard to effectively reduce the inclusions. It is, therefore, expected that alnico magnetic alloys produced through induction casting exhibit better magnetic properties as compared to the alloys fabricated through electric arc melting. The micrograph of the homogenized alloy in Fig. 2(b) has an improved microstructure with fewer inclusions, and the grain boundaries are clearly visible after being etched. The optical micrograph in Fig. 2 (c) is obtained after HT-II. This micrograph has
(a)
(b)
(c)
Inclusions
Grain boundary Inclusions
Fig. 2. Optical micrographs of alnico alloys (a) as-cast, (b) homogenized, (c) fully heat treated. 138
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E
(a) (a)
(b)
B
A
Al-rich phase
F
C
Ti-Zr-Nb rich
D
Fig. 3. SEM micrographs of fractured samples, (a) as-cast, (b) homogenized.
for α2 phase. The coarsened spatial dimension is one reason, in addition to the random orientation, for the low magnetic properties of the alloys. The coarsened and randomly oriented phases confirm the notion that magnetic field treatment at high temperatures is necessary for bias growth of ferromagnetic phase. Fig. 5 (b) shows samples annealed at 840 °C after cooling from 855 °C. The SD phases have further coarsened and the magnetic properties are reduced. The reduction in the Hcj is more prominent which reduces from 1.29 kOe to 1.18 kOe but the reduction in the Br value is only nominal resulting in moderate reduction in energy density (BH)max value. The average dimension of the Fe-Co rich phase is found to be 50 ± 5 nm. Some abnormal growth of SD phases is also observed in the alloys treated at 840 °C for 30 min. Another heat treatment, the continuous cooling in magnetic field, was applied to analyze the effect of magnetic field on SD structure and magnetic properties. The samples were cooled down from 1250 °C to 900 °C without magnetic field, and then magnetic field was applied and sample was allowed to cool down to 810 °C. This sample was initially annealed at 810 °C for 30 min without magnetic field, and further heattreated at 650 °C for 4 h and 560 °C for 15 h. The HRSEM micrograph is given in Fig. 6 (a) where two phase spinodally decomposed structure is clearly visible. The diameter of α1 phase is found out to be 28 ± 2 nm while α2 phase has a diameter of 25 ± 2 nm. The effective temperature range of spinodal decomposition is between 790 and 865 °C [9]. Hence the sample underwent ∼10 s annealing under magnetic field in the effective temperature range of SD. The micrograph in Fig. 6 (b) shows SD structure of sample cooled from 1250 °C to 900 °C, continuously cooled from 900 °C to 840 °C and annealed at 840 °C for 30 min. The phases, especially the Fe-Co rich phase has coarsened as compared to the phases in micrograph in Fig. 6 (a) with diameters increasing from
Table 2 Composition of different areas of the cast alnico 8 alloys. Spectrum
Al
Ti
Fe
Co
Ni
Cu
Nb
Zr
S
A B C D E F
8.58 10.34 5.72 7.34 6.03 6.81
7.02 6.20 7.80 6.32 6.67 6.52
31.73 30.44 32.41 32.28 33.87 33.04
37.09 36.76 37.59 37.97 37.30 37.67
12.43 13.36 13.13 12.80 12.88 12.73
2.62 2.60 2.85 2.79 2.75 2.63
0.45 0.90 0.09 0.10 0.20 0.20
0.1 0.2 0.2 0.3 0.1 0.2
0.1 0.2 0.2 0.2 0.1 0.2
sometimes added to alnico alloys in industry to improve the wetability of the alloys, which eases the casting process. High melting point elements preferably accommodate at grain boundaries and triple junctions of α-grains. Fig. 4 (b) shows Nb-Ti rich phase. While Fig. 4(c) depicts Zr-Nb-Ti rich phase at a triple junction. The composition of this phase is Zr = 21.8, Nb = 4.2, Ti = 6.6, Al = 3.1, Co = 28.1, Fe = 20.2, Ni = 13.0, Cu = 3.02. Micrographs in Fig. 3 (a) and (c) are taken after heat treatment HT-II. Hence two phase structure in the main grains can be observed. Different heat treatment cycles left no obvious impact on the structure and composition of the phases shown in Fig. 4 (a)–(c), which is an indication that high melting point elements do not easily migrate during heat treatment process. Fig. 5 shows HRSEM micrographs of isotropic alnico 8 alloys annealed without applying magnetic field. The spinodally decomposed (SD) structure is seen in the micrograph in Fig. 5 (a). The SD structure depicts a random orientation of the ferromagnetic α1 phase. The moderate magnetic properties of isotropic alloys, shown in Table 1, are attributed to this randomly oriented SD structure. The average diameters of these phases are found to be 40 ± 5 nm for α1 and 35 ± 5 nm
(c)
(b)
(a)
Zr-Nb-Ti rich phase Ti-S rich phase
Nb-Ti rich phase
Fig. 4. SEM micrographs of high melting points elements, (a) Ti-S rich rods, (b) Ti-Nb rich phase, (c) Zr-Nb-Ti rich phase. 139
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(a)
(b) 2
1
2
1
Fig. 5. HRSEM micrographs of isotropic alnico alloys treated at: (a) 810 °C, (b) 840 °C.
28 ± 2 nm to 34 ± 2 nm. No obvious change in magnetic properties was observed in alloys annealed at 810 °C and 840 °C. From these experiments it is apparent that temperature has critical role in forming and coarsening spinodal phases. SD phases grow more rapidly at high temperature, but there is no obvious evidence of further splitting of FeCo rich phases. In the last experiment the samples were treated at 810 °C and 840 °C for various times followed by low temperature annealing at 650 °C for 4 h and 560 °C for 15 h. The alloys treated at 840 °C resulted in better magnetic properties. Fig. 7 (a) shows the HRSEM micrograph of the alloy treated at 840 °C for 5 min. The refined Fe-Co rich α1 and Al-Ni rich α2 phases are visible. Due to relatively long time of magnetic heat treatment below Curie temperature, the ferromagnetic phase has responded to the magnetic field and has elongated parallel to the direction of the field. This biased growth of the ferromagnetic phase is the reason for the better magnetic properties of the alloys. This sample has not been treated for long time at high temperature (> 800 °C), so the spinodal phases are not coarsened. The diameter of the α1 phase is calculated to be 20 ± 3 nm while the α2 phase has an average diameter of 18 ± 3 nm. At this low spatial dimension the ferromagnetic phases behave as single domains [8] which give better magnetic properties. Reducing the dimensions of spinodal phases below 20 nm is practically not viable in bulk alnico alloys, however, in our recent work we reported spatial dimension of the order of 5 nm in alnico nano ribbons [24]. We also find that there is a potential problem with low dimensional SD phases. The SD phases cannot be purified when their dimension is very low [24]. In addition to elongated phase, some
(a)
(b) Branching
2 1
1
2
Splitting
Fig. 7. HRSEM micrographs of alnico alloys treated isothermally in magnetic field, (a) 5 min, (b) 30 min.
branching of ferromagnetic phase is also observed in the micrograph Fig. 7(a). Such areas either undergo poor spinodal decomposition or do not decompose at all, and are called non spinodally decomposed structures. Though Ti and other high melting point elements play important role in heat treatment cycles and magnetic properties of the alloys [10], they hardly migrate through the phases due to large radius. They usually accumulate at grains boundaries, or form rod like structures as shown in Fig. 4(a–c). The high melting point elements are, therefore, speculated to have a role in the branching of spinodal phases. Thus the content of high melting point elements in the alloy is more critical compared to regular elements (Fe-Co-Ni-Al). Fig. 7 (b) shows micrograph of alnico alloy treated at 840 °C for 30 min. The SD phases have apparently coarsened. The magnetic properties are deteriorated due to the coarsening of the phases. It was observed that the optimum
Fig. 6. HRSEM micrographs of anisotropic alnico alloys continuously cooled in magnetic field and treated at: (a) 810 °C, (b) 840 °C for 30 min. 140
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magnetic properties are obtained by annealing in magnetic field for 5 min and no obvious change in magnetic properties was observed by annealing for 7, 10 and 15 min. The magnetic properties deteriorated when samples were annealed for more than 15 min in magnetic field. Heat treatment conditions like temperature range, holding time and cooling rate play significant role in the evolution of microstructures and magnetic properties of alnico magnetic alloys. Furthermore it is suggested, on the basis of different heat treatments, that the spinodal decomposition would not occur effectively in absence of magnetic field during high temperature treatment. Magnetic field has a decisive role in evolution of microstructure and hence in inculcating magnetic properties in the alloys. The spinodal decomposition and coarsening of phases happen during high temperature treatments. During the low temperature treatment only small migration of respective atoms to Fe-Co rich phase and Al-Ni rich phase occurs. As a result the intrinsic coercivity Hcj of the alloys increases during HT-I and HT-II. However, there is no obvious change in the microstructure before and after HT-I and HT-II. The splitting of SD phases and the coarsening of all phases imply that other mechanism like nucleation and growth may also play some role in the structural transformation of the phases besides spinodal decomposition [16]. The splitting shown in Fig. 7 (a) indicates that some elastic strain energy is introduced during coarsening and phases start splitting. It is reported that the splitting of phases occurs between 5 min and 10 min of isothermal annealing in magnetic field [20]. It is also known that splitting of phases cannot occur if SD is the only mechanism and that the phases should coarsen without splitting [20]. The splitting of phases was rarely observed in our investigated alloys, while the coarsening of phases was prominent in all alloys during various treatments depending both on temperature and time. Therefore, it is concluded that spinodal decomposition is the dominant mechanism for two phase formation in alnico alloys, while small effects of nucleation and growth of phases cannot be ruled out.
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4. Conclusion The alnico 8 alloys were fabricated by electric arc melting and annealed by subsequent heat treatments. Investigated systematically are the effects of different heat treatment cycles on microstructures and magnetic properties of the alloys. It is discovered that the high temperature (840 °C) treatment under magnetic field is most effective for the designated composition. The best magnetic properties in the alloys are obtained by the thermomagnetic treatment for five minutes followed by two low temperature draw cycles. High temperature annealing for long time results in coarsening of spinodal phases to deteriorate the magnetic properties. An obvious improvement in Hcj values was observed which indicates that small scale uphill diffusion takes place during HT-I and HT-II, though heat treatment at low temperatures (HT-I and HT-II) has no obvious effects on the microstructure of alnico alloys. Continuous cooling in magnetic field leads to moderate magnetic properties in the alloys. Treatment without magnetic field inculcates isotropic properties in the alloys which are lower than the properties of anisotropic magnets treated in magnetic field. Acknowledgement This work was supported by the National Natural Science
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