Journal of Magnetism and Magnetic Materials 379 (2015) 58–62

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The phase and microstructure analysis of Alnico magnets with highcoercivity

Y.L. Sun a,b,n, J.T. Zhao a,b, Z. Liu a,b, W.X. Xia a,b, S.M. Zhu a,b, D. Lee a,b, A.R. Yan a,b

a Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Material Technology and Engineering, ChineseAcademy of Sciences, Ningbo 315201, Chinab Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201,China

a r t i c l e i n f o

Article history:Received 27 October 2014Received in revised form4 December 2014Accepted 5 December 2014Available online 9 December 2014

Keywords:Alnico microstructureHigh coercivityHigh magnetic energy

x.doi.org/10.1016/j.jmmm.2014.12.00353/& 2014 Elsevier B.V. All rights reserved.

esponding author at: Zhejiang Province Keynd Application Technology, Ningbo Institutering, Chinese Academy of Sciences, Ningbo 31ail address: yinglisun@nimte.ac.cn (Y.L. Sun).

a b s t r a c t

Alnico magnets with high coercivity and high magnetic energy (Hc about 2020 Oe, (BH)m about9.5 MGOe) are obtained by adjusting chemical compositions and optimizing microstructure. The phaseand microstructure of Alnico magnets with high (Alnico9h) and low coercivity (Alnico9) are analyzedcontrastively. The shape and distribution of FeCo-rich phase (α1) which are needle-like arrangement inAlnico9h are nonuniform, the orientation degree and the volume fraction of α1 phase are less than thatof Alnico9. Energy-dispersive X-ray spectroscopy (EDS) research shows that Fe and Co atoms of α1 phasehave a homogeneous distribution in Alnico9h magnet in which the boundaries between α1 and AlNi-richphases (α2) are clearer than those in Alnico9 magnet. Furthermore, the degree for chemical segregationbetween α1 and α2 phases in Alnico9h is much better than that in Alnico9 magnet.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The development of Alnico dated from 1931 and largely endedin 1970s when rare-earth (RE) magnets with high residual in-duction, or remanence were discovered [1]. Recently, Alnico alloysshow special focus again for its RE-free, strong heat resistanceproperties (up to 600 °C), low temperature dependence of theirmagnetic properties, and excellent corrosion resistance. The mainrestriction of Alnico is its relatively low coercivity. Up to now, thehighest magnetic properties of commercial Alnico magnets withhigh coercivity are BrE8.0 T, HcE2000 Oe, and (BH)max

E6.0 MGOe. In order to further increase the magnetic properties,more research on AlNiCo materials is required.

Alnico magnets are primarily composed of two nano-phasesformed through spinodal decomposition α-α1þα2 during ther-momagnetic treatment: an isolated α1phase and a matrix α2-phase [2,3]. Their magnetic properties depend on the magnetiza-tion difference between the two phases as well as the shape ani-sotropy in the ferromagnetic α1-phase [4,5]. In order to obtainhigh remanence and high coercivity, Alnico alloy must have a goodorientation of columnar crystals and high magnetic anisotropy. Inthis paper, we developed Alnico magnets with high coercivity and

Laboratory of Magnetic Ma-of Material Technology and5201, China.

high magnetic energy by changing the chemical compositions andoptimizing thermomagnetic treatment. The phase and micro-structure of the two different types of Alnico magnets are analyzedcontrastively to elucidate the structure–property relationships andto clarify the way to further increase the coercivity of Alnicomagnets.

2. Experimental details

The Alnico alloys with columnar crystals were prepared byadjusting the alloy composition and optimizing thermomagnetictreatment. The nominal composition of Alnico samples are shownin Table 1. The phase and microstructure of Alnico9 and Alnico9hmagnets were analyzed contrastively. The phase structure wasdetermined by a X-ray diffractometer with Cu Kα radiation. Themicrostructure and chemical analyses were carried out with a FEI-Tecnai F20-XT transmission electron microscope (TEM) operatedat acceleration voltage of 200 kV. The thin foil specimens for TEMobservations were cut to be parallel and perpendicular to the di-rection of the magnetic field applied in the thermomagnetictreatment and prepared using a focused ion-beam to maintainuniform thickness of the specimens. The magnetic properties ofAlnico alloys were measured in a closed magnetic circuit with theB–H apparatus.

Table 1Nominal composition of Alnico9 and Alnico9h (wt%).

Samples Co Cu Al Ti Ni Fe

Alnico9 36 3 7.2 6.05 13.5 BalAlnico9h 40 3 8.4 8.2 14 Bal

Fig. 2. X-ray diffraction (XRD) in vertical direction of Alnico9 and Alnico9h.

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3. Results and discussion

Fig. 1 shows the TEM bright field images in longitudinal view ofAlnico9 and Alnico9h magnets. Typical distribution of spinodaldecomposition phases are observed in Alnico9 and Alnico9hmagnets formed by the effect of applied magnetic field. For Alni-co9, all the precipitated phases are parallel to applied magneticfield, which are shown in Fig. 1(a). As for Alnico9h, the greatmajority of precipitated phase are parallel to applied magneticfield (as shown in Fig. 1(b)) except a small part of latticed (in-dicated by red circles) and angled phase (indicated by red arrows),as can be seen in Fig. 1(c). As a result, the shape and distribution ofα1-phase (dark bars in Fig. 1(b and c)) and α2-phase are non-uniform for Alnico 9h, which leads to worse orientation degreeand less volume fraction of α1-phase than those of Alnico9. It alsocan be seen that the size of α1-phase in width is close to the size ofα2-phase in both Alnico9 and Alnico9h magnets. However, thewidth and length of α1-phase in Alnico9 are larger than those inAlnico9h. The micro-magnetic domains in Alnico9h and Alnico9can be similarly described as bamboo-like and needle-like, re-spectively, and the ratio of length to diameter in Alnico9h issmaller than in Alnico9 (Fig. 1(d and e)).

For Alnico permanent magnet, the coexisting phases are con-tained within individual grains. For a directional solidified Alnico

Fig. 1. TEM bright field images in longitudinal view of Alnico9 (a) and Alnico9h (b). (c) En(d) Alnico9 and (e) Alnico9h. (For interpretation of the references to color in this figure

ingot, the grains are highly ⟨100⟩ textured along the solidificationdirection which is also the applied magnetic field direction. On theother hand, the crystal structures of α1-phase are similar to α2-phase and they share approximated lattice constant because ofspinodal decomposition [6–8]. Fig. 2 shows that Alnico9 andAlnico9h have similar diffraction peaks, however the peaks ofAlnico9h significantly shift to high angle side. According to theresearch results of Yue [9], the orientation degree of Alnico magnetcan be evaluated by the intensity ratio of (100) to (200). TheI(100)/I(200) values are calculated and shown in Table 2. It can beseen that the I(100)/I(200) of Alnico9h is about 0.078 which is

larged image of white box area in (b). Lorentz Fresnel images in longitudinal view of, the reader is referred to the web version of this article.)

Table 2Related parameters of Alnico9 and Alnico9h.

Samples I(100)/I(200)

I(110)/I(200)

Angle correspondingto (200)

a/Å c/Å c/a

Alnico9 0.112 0.086 64.394 2.812 2.885 1.026Alnico9h 0.078 0.289 64.552 2.843 2.891 1.017

Y.L. Sun et al. / Journal of Magnetism and Magnetic Materials 379 (2015) 58–6260

lower than 0.112 for Alnico9, which means that the orientationdegree of Alnico9 magnet is higher than that in Alnico9h magnetwhich has been identified by the result of Fig. 1.

It has been known that α1-phase and α2-phase are formed byspinodal decomposition with elements enriching in specific phasein thermomagnetic field processing. The degree of elements seg-regation inside α1-phase and α2-phases will gradually expandwith muti-stage aging. That spinodal decomposition is uphill dif-fusion [10], the balance between spinodal decomposition andelements downhill diffusion finally lead to the formation of theboundary between α1-phase and α2-phase. For the same crystalstructure and the similar lattice constant between α1-phase andα2-phase, atoms tend to form disordered arrangement in bound-ary area between α1-phase and α2-phase. The more clearness ofboundary between α1-phase and α2-phase, the more accom-plishment of the spinodal decomposition. Meanwhile, Alnicomagnet with the best chemical segregation will achieve excellentmagnetic properties.

The STEM images in longitudinal view (Fig. 3) show that pre-cipitated phases in Alnico are parallel to the directions of appliedmagnetic fields. Owing to spinodal decomposition, magneticatoms mainly gather in α1-phase and non-magnetic atoms tend to

Fig. 3. High-angle annular dark-field (HAADF) scanning TEM (STEM) images and EDS liorientation. (For interpretation of the references to color in this figure legend, the read

gather in α2-phase. By contrast, the spinodal phase boundariesbetween α1-phase and α2-phase in Alnico9h are more clear thanthose in Alnico9. It has been shown by Zhou et al. that in AlniCo8 and 9 α1-phases are commonly faceted on their {110} planes[13], therefore the TEM specimen was tilted to make the electronbeam parallel to the ⟨110⟩ direction to observe the boundary andcomposition difference between two phases. The component dis-tributions across phases are given by line profiles in Fig. 3. Themain elements of Alnico alloy are periodically distributed acrossthe spinodal phases for both Alnico9 and Alnico9h. The Co and Feelements are more inclined to distribute in α1 phases, which aredifferent from Alnico 5–7 and Alnico 8 [11]. For Alnico9, the Fe, Coelements have sharp “peak” and “valley” across the center of α1and α2 phases. In contrast, the compositional profiles show moreplateaus for Fe and Co elements in Alnico9h. From the distributionof Fe and Co elements in α1-phases, it also can be seen that thedegree of chemical segregation between α1-phase and α2-phasein Alnico9h is much larger than that in Alnico9. Cu peak appearingin α2-phase and being stronger in α1-phase in Fig. 3(b) means Cu-rich particles appear in Alnico9h. As we know that Cu segregatesin α2-phase and Cu-rich particles lie along the corners of two{110} facets of α1-phase [12,13]. The appearance of Cu peaks inα1-phase could be due to the overlap of Cu precipitates and α1-phases because thin foil specimen for TEM observation was ran-domly cut and only parallel to the direction of the magnetic field.

The bright-field TEM image in transverse view of Alnico (Fig. 4)shows that isolated α1-phases are round shape and uniformlydispersed in the continuous α2 phases. The boundaries of spinodalphases in Alnico9h are much more clear than those in Alnico9.Table 1 shows that Alnico9 contains 68.98% (FeþCo)%, while the

ne scan results of (a) Alnico9 and (b) Alnico9h. The red arrows indicate the crystaler is referred to the web version of this article.)

Fig. 4. TEM images in transverse view of alnico9 (a) and alnico9h (b). (c) and (d) are magnified views of (a) and (b), respectively.

Table 3Elemental compositions (at%) obtained by EDS in the regions shown in Fig. 4.

Samples Positions Fe Co Al Ti Cu Ni

Alnico9 A 39.22 34.87 5.82 2.28 3.42 6.19B 13.64 27.84 19.21 9.21 7.75 19.18C 15.57 22.38 15.21 5.58 4.28 16.68

Alnico9h A 34.59 36.33 5.96 2.07 2.23 6.82B 15.8 32.24 14.96 10.55 5.49 16.36C 23.07 29.95 12.21 5.49 8.61 10.09

Fig. 5. Magnetic hysteresis loop of Alnico9 and Alnico9h.

Y.L. Sun et al. / Journal of Magnetism and Magnetic Materials 379 (2015) 58–62 61

Alnico9h contains 64.97 (FeþCo)%. It means that the volume of α1phase content in Alnico9h is less than that in Alnico9. In addition,for Alnico9h, α1 phase nodules (3–8 nm) between two big α1patches are observed and indicated by the red arrows in Fig. 4(b),which can be interpreted by the result of the rich Fe–Co phasesstability bifurcation in thermomagnetic treatment [14]. In order toconfirm the area of Cu-rich particles inclined to lie, we selectedthree regions (A, B, and C in Fig. 4(c) and (d)) and took spot scan byEDS. Table 3 shows the composition results of A, B and C positionsin Alnico9 and Alnico9h. As shown in Fig. 3, the Cu atoms tend to

gather in α2-phases. According to Table 3, for Alnico9h, the Cuatoms tend to emerge in the places between two close α1-phasesand form Cu-rich particles. The Cu-rich particles distributing along

Table 4Magnetic properties of Alnico9 and Alnico9h.

Samples Br/kGs Hcb/Oe (BH)m/MGOe Hk/Hcj

Alnico9 10.8 1570 11.31 0.89Alnico9h 8.55 2020 9.5 0.59

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the corners of two α1-phases may play an important role in de-termining the coercivity of the Alnico alloys.

The hysteresis loops and magnetic properties of Alnico9 andAlnico9h are shown in Fig. 5 and Table 4, respectively. The re-manence (Br) and intrinsic coercivity (Hci) are marked different.For Alnico9h, the lower remanence could be due to lower volumefraction of α1-phase,the worse orientation degree of α1-phasecomparatively lead to the lower Hk/Hcj. Meanwhile, the higherintrinsic coercivity could be due to the more clear boundary be-tween α1-phase and α2-phase, and the homogeneous distributionof Fe and Co atoms in magnetic α1-phase. The inhomogeneousdistribution of Cu-rich particles in α2-phase would also be bene-ficial for coercivity.

Acknowledgments

The project was supported by following foundations: NaturalScience Foundation of Zhejiang Province for Youth of China No.LQ13E010004, National Natural Science Foundation of China No.51301190, the National Basic Research Program of China (973program, Grant no. 2014CB643702).

References

[1] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, John Wiley andSons, Inc., Hoboken, New Jersey, 2009 (p. 485.).

[2] E.G. Povolotskii, YaM. Dovgalevskii, V.K. Baitina, Effectiveness of magnetic fieldin thermomagnetic treatment of Alnico-type alloys, Metalloved. Term. Obrab.Metallov 11 (1963) 10–14.

[3] D.A. Laptei, V.E. Shcherbakov, A.I. Drokin, Magnetics tructural properties inAlNiCo alloys under quilibrium conditions and under magenetization, Russ.Phys. J.18 (1975) 1118–1122.

[4] P. Campbell, IEEE Trans. Magn. 18 (1982) 898.[5] D.J. Craik, R. Lane, J. Phys. D 2 (1969) 33.[6] J.J. Mason, D.W. Ashall, A.V. Dean, The structure of Ni–Co–Al–Ti–Fe permanent

magnets, Cobalt 46 (1970) 23–28.[7] J.J. Mason, D.W. Ashall, A.V. Dean, The structure of Ni–Co–Al–Ti–Fe permanent

magnets, IEEE Trans. Magn.MAG-6 (1970) 191–194.[8] P. Pashkov, A. Fridman, E. Granovsky, V. Sergeyev, R. Larichkina, Some aspects

of precipitation and magnetic properties of Alnico alloys, J. Appl. Phys. 40(1969) 1308.

[9] Xu Yue, Sun Kai Liang, Chu Wei Guo, Fei Wei Dong, Microstructure of quen-ched Alnico8 alloy, Chin. J. Spectrosc. Lab. 19 (5) (2002) 580–583.

[10] J.W. Cahn, Spinodal decomposition, Trans. Metal. Soc. AIME 242 (1986)166–174.

[11] Q. Xing, M.K. Miller, L. Zhou, et al., Phase and elemental distributions in Alnicomagnetic materials, IEEE Trans. Magn. 49 (7) (2013) 3314–3317.

[12] Lin Zhou, M.K. Miller, H. Dillon, et al., Role of the applied magnetic field on themicrostructural evolution in Alnico 8 alloys, Metall. Mater. Trans. E 1 (1)(2014) 27–35.

[13] Lin Zhou, M.K. Miller, Ping Lu, et al., Architecture and magnetism of Alnico,Acta Mater. 74 (2014) 224–233.

[14] Chu Weiguo, Fei Weidong, Yang Dezhuang, Stability bifurcation of the mi-crostructure coarsening of AlNiCo8 alloy thermomagnetically treated at hightemperature[J], Acta Metall. Sinica 35(12), 1999, 1253–1258.