METAL 2009 19. – 21. 5 2008 Hradec nad Moravicí

CRYSTALLINE STRUCTURE OF SmCo5 BASED ALLOYS

1Dr. Ing. V.P. Menushenkov, 1Dr. Ing. T.A. 1Sviridova,

1Ing. E.V. Shelekhov, 2Dr. Ing. L.M. Belova

1State Technological University “Moscow Steel and Alloys Institute”

Leninskii prospect 4, 119049 Moscow, Russia, E-mail: menushenkov@gmail.com

2Dept. Materials Science and Engineering, Royal Institute of Technology, Stockholm, 100 44 Sweden E-mail: belova@mse.kth.se

ABSTRACT

Microstructure and crystalline structure of SmCo5 based alloys after various heat treatments were studied using X-ray diffraction and metallographic methods. It was established that complicated microstructure of hyperstoichiometric alloys forms in nonequilibrium conditions during crystallization of the ingots and the subsequent cooling to room temperature. XRD study of the lattice parameters of SmCo5 phase in as-cast SmCo5 based alloys after different heat treatments shows evidence of the Sm enrichment of the SmCo5 phase. The behavior of the lattice parameters of SmCo5 phase in Sm-rich alloys when subjected to aging between 1220oC and 700oC can be related to the phase transformation of SmCo5 into SmCo5-x phases.

1. INTRODUCTION

Since its discovery in the later part of the twenty century by Strnat and collaborators [1], SmCo5 was studied quite extensively due to its being the first intermetallic RE-TM material showing improved magnetocrystalline anisotropy suitable for strong hard magnets. Development of sintered SmCo5 magnets became the new advanced stage of permanent magnets production [2]. The ideal microstructure of SmCo5 sintered magnets consists of aligned single-domain grains with an ideal SmCo5 structure. It is well known that sintered magnets are demagnetized by the domain wall motion, thus the coercive force is determined by the nucleation field of reversed domains. Nucleation of reversed domains takes place in regions with low magnetocrystalline anisotropy, which are concentrated near grain boundaries. The highest coercive force (Hci) was obtained for Sm-rich magnets (SmCo5-x). Enrichment of Sm content promotes liquid phase sintering in SmCo5 magnets and is important for successful post sintering heat treatment (HT). The conventional HT includes slow cooling from 1220 to approx. 900oC followed by rapid cooling to room temperature [3]. Such HT increases Hci of sintered magnets from approx. 1 kOe to more than 40 kOe.

One of the main unsolved questions is the role of HT in development of coercivity. All hypotheses of coercivity increase can be divided into two types depending on whether the magnet has a single-phase or multiphase microstructure. According to the “perfect lattice hypothesis” [4-6], the coercivity increase due to HT is related to the elimination of equilibrium lattice defects from the high temperature

METAL 2009 19. – 21. 5 2008 Hradec nad Moravicí

SmCo5 matrix phase, after heat treatment performed at lower temperature. The “phase transformation-induced coercivity mechanism” [7, 8] suggests that formation of coherent SmCo5-x phase on the surface of SmCo5 grains improves the smooth of the grains surface, decreases the number of reversed domains nuclei and thus increases the coercivity. HRTEM investigations of microstructure of the sintered magnets showed that besides SmCo5 grains with low defect density, also grains with densely packed parallel stacking faults perpendicular to the hexagonal c-axis are observed [9]. Such basal stacking faults correspond to a transformation of the SmCo5 crystal structure into the Sm-rich Sm5Co19 and Sm2Co7 structure types [10]. The fact that the magnetocrystalline anisotropy depends strongly on the crystalline nature of the SmCo5 type phase means that proper understanding of the crystal structure, defects and modification is the key to understanding of the underlying mechanism for the magnetic properties.

In this work, we present results of our investigations of as-cast alloys of the composition range SmCo5±x of the Sm-Co system heat treated at different temperatures using X-ray diffraction and metallographic methods.

2. EXPERIMENTAL

Ingots with nominal composition of SmyCo100-y , as showed in Table 1, were prepared by induction melting in Ar atmosphere followed by casting in an iron mould. The samples were aged in a vacuum furnace in series: 1220oC for 3 h + 1000oC for 5 h + 900oC for 10 h + 850oC for 10 h + 700oC for 20 h. After each step of aging the samples were cooled inside the furnace to room temperature (RT). Phase identification was carried out by X-ray diffraction (XRD) using Cu-Kα radiation. Rietveld refinement was used for quantitative phase analysis [11]. The experimental errors of determination of SmCo5 lattice parameters were ∆а = ∆с = 0.003 Å, ∆(c/a)=0.001. Scanning electron microscopy (SEM) and Elemental Dispersion Spectroscopy (EDS) analyses were conducted on the Nova600 NanoLab DualBeam system (FEI Company) and were used to characterize the phase changes in as-cast and heat treated Sm-Co alloys.

Table 1. Chemical composition of as-cast Sm-Co alloys

№ 1 2 3 4 5 6 7 8 9

Sm, at. % 13.2 15.5 16.3 16.8 17.2 17.9 18.2 20.1 21.1

Сo, at. % 86.8 84.5 83.7 83.2 82.8 82.1 81.8 79.9 78.9

3. RESULTS AND DISCUSSION

According to the XRD analysis for the heat-treated Sm16.8Co83.2 alloy, only SmCo5 phase (CaCu5 type structure) was observed. After aging at 1220oC the hypostoichiometric alloys (y = 13.2 - 16.3) consisted of the SmCo5 phase and two crystalline modifications of the Sm2Co17 phase: a high temperature (h) hexagonal phase with Th2Ni17 type structure and а low-temperature (l) rhombohedral phase with Th2Zn17 type structure. As soon as the aging temperature was decreased from 1220 to 700оС the quantity of Sm2Co17 (h) phase decreased, whereas the quantity of (l) phase increased. After aging at 1220oC the hyperstoichiometric alloys (y = 16.8, 17.2,

METAL 2009 19. – 21. 5 2008 Hradec nad Moravicí

18.2 - 21.1) consisted of the SmCo5 phase and two crystalline modification of Sm2Co7 phase: a high temperature (h) rhombohedral phase with Er2Co7 type structure and low-temperature (l) hexagonal phase with Ce2Ni7 type structure. When the aging temperature was decreased from 1220 to 700оС the quantity of Sm2Co7 (h) phase decreased whereas the quantity of (l) phase increased. As opposed to the abovementioned alloys the XRD analysis of the sample № 6 (Sm17.9Co82.1) indicated presence of the SmCo5 and Sm5Co19 phases. Namely, after aging at 1220-700оС the volume of Sm5Co19 phase was approx. 30 % and its amount did not decrease with decrease in temperature. It is important to note however, that decoding of diffraction patterns of the samples № 5-7 using Rietveld method needs accuracy refinement and additional testing because of low intensity and superposition of diffraction peaks.

Fig. 1 shows lattice parameters and с/a ratio for the SmCo5 phase in SmyCo100-y alloys vs Sm concentration y after aging at 1220, 1000, 900 and 700oC. The dotted line corresponds to stoichiometric SmCo5.

4,945

4,955

4,965

4,975

4,985

4,995

5,005

12,0 14,0 16,0 18,0 20,0 22,0

% Sm

a,

Ǻ

90010001200700

3,965

3,97

3,975

3,98

3,985

3,99

3,995

4

4,005

12,0 14,0 16,0 18,0 20,0 22,0

% Sm

c, Ǻ

90010001220700

0,79

0,7925

0,795

0,7975

0,8

0,8025

0,805

0,8075

0,81

12,0 14,0 16,0 18,0 20,0 22,0

% Sm

c/a

90010001220700

Figure 1 - Lattice parameters and с/a ratio for SmCo5 phase in SmyCo100-y

alloys vs Sm content y after aging at different temperatures With decreasing y in Co-rich alloys aged at 1220oC the c/a value for the

SmCo5+x phase increased up to 0.809 for Sm13.2Co86.8. As was shown in [4, 12], the c/a ratio for SmCo5+x changes linearly with increase of Co content. According to this dependence for Sm13.2Co86.8 alloy, Co content in the SmCo5+x phase should amount

METAL 2009 19. – 21. 5 2008 Hradec nad Moravicí

to ≈ 85.5 at. %. Aging at 1000 and 900oC resulted in decomposition of the SmCo5+x solid solution below the homogeneity limit by precipitation of Sm2Co17 (l) phase. Reduction of the Co content in SmCo5+x phase resulted in the decrease of the с/a value down to с/а ≤ 0.795 after aging at Тag = 900оС and to с/а = 0.794 after aging at Тag = 700o

С. The с/a ratio of the SmCo5 phase in stoichiometric Sm16.8Co83.2 alloy remained

constant (с/а = 0.795) during heat treatment in the 1220-900оС range, but decreased to с/а = 0.794 after aging at 700оС. In hyperstoichiometric alloys № 5-6 (y = 17.2 - 17.9) the c/a value of the SmCo5 phase was around 0.793 independent of the aging temperature. In hyperstoichiometric alloys № 7-9 (y = 18.2 - 21.1) aged at 1220oC the c/a value of the SmCo5 phase was approx. 0.794 and decreased to с/а = 0.793 after aging at 900 and 700оС, which is naturally lower than corresponding values for hypostoichiometric alloys. The comparison of lattice parameters and с/a ratio for the SmCo5 phase in Sm-rich alloys and stoichiometric alloy shows that the lower value of the с/a ratio for hyperstoichiometric alloys is related with lower value of the parameter c ≤ 3,968 Å and higher value of the parameter a ≥ 5,002 Å.

The Sm-Co phase diagram developed by K.H.J. Buschow and A.S. Goot [4] indicates wide high-temperature solubility for SmCo5 extending towards both Sm2Co7 and Sm2Co17. The modified Sm-Co diagram [13] indicates SmCo5+х and Sm5Сo19 in place of SmCo5 (Fig. 2). The SmCo5+x phase is stable only at high temperature and decomposes eutectoidly below 1100oC: SmCo5+x ⇔ SmCo5-x + Sm2Co17. The low-temperature SmCo5-x phase is formed at approx. 1150oC by peritectoid reaction: SmCo5+x + Sm2Co7 ⇔ SmCo5-x. According to the Sm-Co diagram in Fig. 2, homogeneity range for SmCo5-x phase widens from SmCo4.9 to SmCo4.5. A question arises with regard to the origin of the relatively wide homogeneity region of the SmCo5-x phase.

Figure 2 - Modified Sm-Co phase diagram The crystal structure of the hexagonal SmCo5 phase (type D2d) can be

described as a sequence of (Аbc)α blocks stacked without shift in the (001) plane. The mixed Sm-Co layer (Abc) consists of three 36-nets: A composed of Sm-atoms, b and c constituted by Co-atoms. The b and c nets are displaced with regard to A-net

METAL 2009 19. – 21. 5 2008 Hradec nad Moravicí

by vectors t = 1/3 (a – b) and –t, respectively. The A, b and c nets are depicted in Fig. 3a as empty large, empty small and hatched small circles, respectively. The α-layer (with holes in A-positions) consists of Co-atoms only, which form the 6363-net (see Fig. 3 b). The initial block (Аbc)α when shifted by vectors t and –t turns into (Bca)β and (Сab)γ correspondingly.

a) b)

Figure 3 - Structure of (Abc)-layer (a) and α-layer (b) in SmCo5 phase.

To produce the shear stacking fault in the (001) basic plane of SmCo5 phase

(with atomic radii ratio RSm/RCo~1.4) the shift of the neighboring blocks by vector t should be accompanied by a partial removal of Co atoms with attendant composition change and lattice parameters accommodation. The following rearrangements lead to stacking fault formation: • displacement of (Abc)α-block in combination with overlying part of the lattice • by vector t, which brings the layers sequence in the vicinity of the stacking fault to

…(Аbc)α(Аbc)αααα(Bca)β(Bca)β…; • removal of the αααα-layer in the stacking fault plane; • withdrawal of three Co nets in the fault-adjacent blocks, namely b, c and a, the

residual c-net being displaced into mid-height position of the two former с-layers; • shift of Sm-layers A and B towards each other to a short distance along <001>

direction. Thus the layer succession nearby the stacking fault assumes the form

...(Аbc)αАсBβ(Bca)β… . The Sm-layers A and B rapprochement results in reduction of the lattice

parameter c or of the average block thickness. Moreover, the neighboring Sm atoms in layers A and B prove to be too close to each other, whereas the main projection of the interatomic vector lies in the (001) plane. Therefore, extension of lattice in the basic plane, i.e. lattice parameter a increase, is needed. Thus insertion of randomly distributed stacking faults in SmCo5 phase results in Sm enrichment, increase of a and decrease of c lattice parameters.

It is well known that the some Co-rich intermetallic compounds of the R-Co systems form Cromer-Larson family of SmCo5 based crystalline structures described by the formula: RCoy, where y = 5n+4/n+2, n = 0,1,2, 3 [14]. The crystalline structure of RCoy compound may be described in terms of well-ordered stacking faults with concentration 1/3, 1/4 and 1/5 in the SmCo3 (n=1), Sm2Co7 (n=2) and Sm5Co19 (n=3) structure types, respectively.

METAL 2009 19. – 21. 5 2008 Hradec nad Moravicí

Our experimental results (see Fig. 1) showed increase of a and decrease of c parameters of the SmCo5 phase in hyperstoichiometric alloys when subjected to different aging between 1220oC and 700oC. These data suggests that with decrease in aging temperature the SmCo5 phase is enriched with Sm and is transformed into SmCo5-x phase. It can be suggested, that in the composition range between SmCo5 and Sm5Co19 thermodynamically stable SmCoy structures with n≥5 might exist. For instance Sm2Co9 (n=10) and Sm3Co14 (n=16) compounds, which composition agree with the border of the homogeneity region for SmCo5-x phase (Sm18.2Co81.8 - Sm17.6Co82.4) in Fig. 2. These hypothetic compounds may be formed by peritectoid reactions in the temperature range of 1150–950oC. The first stage of the transformation of SmCo5 into SmCoy structures occurs via formation of disordered stacking faults. It must be noted that XRD analysis only gives weak evidence of these transformations. The disordered stacking faults do not produce supercell reflexes and can be only detected by broadening of peaks with well-defined indices provided the stacking faults concentration is high enough. This is one of the reasons for not having detected the presence of SmCoy phases using X-ray diffraction.

Experimental evidence of heterogeneous microstructure of the hyperstoichiometric alloys was obtained when Sm-Co samples where studied by metallographic methods. The scanning electron micrographs in backscattered mode (Fig. 4) show microstructure of as-cast alloy № 7 (Sm18.2Co81.8). These micrographs reveal two phases characterized by the dark matrix grains and lighter intergrain phase. The more detailed higher resolution SEM micrographs show that both of these phases decomposed into two types of precipitates, the shape and dimensions of which are distinct in different parts of the sample. EDS analysis showed (Fig. 4c) that on an average the white intergrain phase is enriched by Sm by more than 2.5 at. % as compared to the dark matrix grains, where composition is close to stoichiometric (16.6 at % Sm). In our microstructural studies the presence of the initial precipitates of Sm2Co7 phase in hyperstoichiometric alloys was not detected.

Microstructural investigations of the hyperstoichiometric alloy allows assuming that its complicated microstructure forms in nonequilibrium conditions during crystallization of the ingot followed by cooling down to RT. According to the Sm-Co phase diagram in Fig. 2 crystallization of Sm18.2Co81.8 alloy starts by formation of the initial grains of the SmCo5+x phase, which at RT are represented by the dark matrix grains in the microstructure of the alloy. At 1200oC the intergrain liquid crystallizes as Sm5Сo19 phase via a peritectic reaction: L + SmCo5+x ⇔ Sm5Сo19 and after cooling to RT is represented by the lighter intergrain phase. Below 1170oC the Sm5Сo19 intergrain phase decomposes peritectoidly: Sm5Co19 ⇔ SmCo5+x + Sm2Co7, forming a fine mixture of dark and white precipitates. The mechanism of decomposition of the initial grains of SmCo5+x phase is not quite understood. According to diagram in fig. 2 the phase transformation takes place at 1100o

С. But in nonequilibrium conditions of cooling to RT it is possible that the SmCo5+x phase decomposes by metastable scheme: SmCo5+x ⇔ SmCo5-x + SmCo5+2x. According to experimental data in Fig. 1 it occurs between 900 and 700oC. During the subsequent aging of as-cast alloy in the temperature range of 1220 – 700oC the precipitates of Sm2Co7 phase inside the intergrain phase and the precipitates of SmCo5+2x phase inside the initial grains of the SmCo5+x phase may transform to SmCo5-x phase resulting in diffusion of Sm atoms from the first to second phase.

METAL 2009 19. – 21. 5 2008 Hradec nad Moravicí

a)

b)

c)

d)

Figure 4 – SEM micrographs of the as-cast alloy № 7 (Sm18.2Co81.8). Line profiles in fig. 4с indicate change in concentrations of Co (curve 1), Sm (2), С (3) and O (4)

It is well-known that processing of commercial sintered magnets includes melting of hyperstoichiometric alloy, ball milling, pressing of the powder in presence of magnetic field, sintering and heat treatment. Usually composition of commercial alloys is approximately close to the composition of alloy № 7 (Sm18.2Co81.8). Hence, after ball milling the powder of this alloy might consists of the particles of above listed phases (see Fig. 4). After sintering, the microstructure of magnets consists of SmCo5 grains and small quantity of Sm-rich phases which are concentrated in the intergranular area. According to numerous microstructural investigations the post sintering heat treatment doesn’t change the microstructure of the sintered magnets. However, XRD data show changes in the lattice parameters of the SmCo5 phase (see Fig. 1). It can be concluded, that during HT the transformation of the SmCo5 into SmCoy crystal structure most likely starts preferentially at the surface of the SmCo5 grains. As it was suggested earlier [7, 8] this transformation improves smoothness of the grain surface of the principal phase and eliminates regions with low magnetocrystalline anisotropy. It decreases the number of reversed domains nuclei and thus increases coercivity of the magnets.

METAL 2009 19. – 21. 5 2008 Hradec nad Moravicí

4. CONCLUSIONS

XRD study of the lattice parameters of SmCo5 phase in as-cast SmCo5 based alloys after different heat treatments shows evidence of enrichment of the SmCo5 phase with Sm. The behavior of the lattice parameters of SmCo5 phase in as-cast Sm-rich alloys when subjected to different aging between 1220oC and 700oC can be related to the phase transformation of SmCo5 into SmCo5-x phases of the Cromer-Larson series (for example, Sm2Co9, Sm3Co14) formed by peritectoid reactions below 1100oC. The phases with disordered stacking faults would not produce supercell reflexes. This is one of the reasons for not having detected the presence of SmCoy phases using X-ray diffraction.

It is suggested that in sintered magnets formation of the SmCoy structure during heat treatment starts at the surface of SmCo5 grains. This transformation improves the smoothness of the grains surface and eliminates regions with low magnetocrystalline anisotropy. Both of these structural changes decrease the number of reversed domains nuclei and increase coercivity of the magnets.

ACKNOWLEDGEMENTS

This work was financial supported by Rosnauka grant GК № 02.513.11.3385, 2008.

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