CRYSTALLINE STRUCTURE OF SmCo 5 BASED ALLOYS

Transcription

1 CRYSTALLINE STRUCTURE OF SmCo 5 BASED ALLOYS 1 Dr. Ing. V.P. Menushenkov, 1 Dr. Ing. T.A. 1 Sviridova, 1 Ing. E.V. Shelekhov, 2 Dr. Ing. L.M. Belova 1 State Technological University Moscow Steel and Alloys Institute Leninskii prospect 4, Moscow, Russia, menushenkov@gmail.com 2 Dept. Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden belova@mse.kth.se ABSTRACT Microstructure and crystalline structure of SmCo 5 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 SmCo 5 phase in as-cast SmCo 5 based alloys after different heat treatments shows evidence of the Sm enrichment of the SmCo 5 phase. The behavior of the lattice parameters of SmCo 5 phase in Sm-rich alloys when subjected to aging between 1220 o C and 700 o C can be related to the phase transformation of SmCo 5 into SmCo 5-x phases. 1. INTRODUCTION Since its discovery in the later part of the twenty century by Strnat and collaborators [1], SmCo 5 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 SmCo 5 magnets became the new advanced stage of permanent magnets production [2]. The ideal microstructure of SmCo 5 sintered magnets consists of aligned single-domain grains with an ideal SmCo 5 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 (H ci ) was obtained for Sm-rich magnets (SmCo 5-x ). Enrichment of Sm content promotes liquid phase sintering in SmCo 5 magnets and is important for successful post sintering heat treatment (HT). The conventional HT includes slow cooling from 1220 to approx. 900 o C followed by rapid cooling to room temperature [3]. Such HT increases H ci 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

2 SmCo 5 matrix phase, after heat treatment performed at lower temperature. The phase transformation-induced coercivity mechanism [7, 8] suggests that formation of coherent SmCo 5-x phase on the surface of SmCo 5 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 SmCo 5 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 SmCo 5 crystal structure into the Sm-rich Sm 5 Co 19 and Sm 2 Co 7 structure types [10]. The fact that the magnetocrystalline anisotropy depends strongly on the crystalline nature of the SmCo 5 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 SmCo 5±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 Sm y Co 100-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: 1220 o C for 3 h o C for 5 h o C for 10 h o C for 10 h o C 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 SmCo 5 lattice parameters were а = с = Å, (c/a)= 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 Sm, at. % Сo, at. % RESULTS AND DISCUSSION According to the XRD analysis for the heat-treated Sm 16.8 Co 83.2 alloy, only SmCo 5 phase (CaCu 5 type structure) was observed. After aging at 1220 o C the hypostoichiometric alloys (y = ) consisted of the SmCo 5 phase and two crystalline modifications of the Sm 2 Co 17 phase: a high temperature (h) hexagonal phase with Th 2 Ni 17 type structure and а low-temperature (l) rhombohedral phase with Th 2 Zn 17 type structure. As soon as the aging temperature was decreased from 1220 to 700 о С the quantity of Sm 2 Co 17 (h) phase decreased, whereas the quantity of (l) phase increased. After aging at 1220 o C the hyperstoichiometric alloys (y = 16.8, 17.2,

3 ) consisted of the SmCo 5 phase and two crystalline modification of Sm 2 Co 7 phase: a high temperature (h) rhombohedral phase with Er 2 Co 7 type structure and low-temperature (l) hexagonal phase with Ce 2 Ni 7 type structure. When the aging temperature was decreased from 1220 to 700 о С the quantity of Sm 2 Co 7 (h) phase decreased whereas the quantity of (l) phase increased. As opposed to the abovementioned alloys the XRD analysis of the sample 6 (Sm 17.9 Co 82.1 ) indicated presence of the SmCo 5 and Sm 5 Co 19 phases. Namely, after aging at о С the volume of Sm 5 Co 19 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 SmCo 5 phase in Sm y Co 100-y alloys vs Sm concentration y after aging at 1220, 1000, 900 and 700 o C. The dotted line corresponds to stoichiometric SmCo 5. 5,005 4,995 a, Ǻ 4,985 4,975 4,965 4, ,945 12,0 14,0 16,0 18,0 20,0 22,0 % Sm 4, ,995 3, c, Ǻ 3,985 3,98 3,975 3,97 3,965 12,0 14,0 16,0 18,0 20,0 22,0 % Sm 0,81 0,8075 0,805 0, c/a 0,8 0,7975 0,795 0,7925 0,79 12,0 14,0 16,0 18,0 20,0 22,0 % Sm Figure 1 - Lattice parameters and с/a ratio for SmCo 5 phase in Sm y Co 100-y alloys vs Sm content y after aging at different temperatures With decreasing y in Co-rich alloys aged at 1220 o C the c/a value for the SmCo 5+x phase increased up to for Sm 13.2 Co As was shown in [4, 12], the c/a ratio for SmCo 5+x changes linearly with increase of Co content. According to this dependence for Sm 13.2 Co 86.8 alloy, Co content in the SmCo 5+x phase should amount

4 to 85.5 at. %. Aging at 1000 and 900 o C resulted in decomposition of the SmCo 5+x solid solution below the homogeneity limit by precipitation of Sm 2 Co 17 (l) phase. Reduction of the Co content in SmCo 5+x phase resulted in the decrease of the с/a value down to с/а after aging at Т ag = 900 о С and to с/а = after aging at Т ag = 700 o С. The с/a ratio of the SmCo 5 phase in stoichiometric Sm 16.8 Co 83.2 alloy remained constant (с/а = 0.795) during heat treatment in the о С range, but decreased to с/а = after aging at 700 о С. In hyperstoichiometric alloys 5-6 (y = ) the c/a value of the SmCo 5 phase was around independent of the aging temperature. In hyperstoichiometric alloys 7-9 (y = ) aged at 1220 o C the c/a value of the SmCo 5 phase was approx and decreased to с/а = after aging at 900 and 700 о С, which is naturally lower than corresponding values for hypostoichiometric alloys. The comparison of lattice parameters and с/a ratio for the SmCo 5 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 SmCo 5 extending towards both Sm 2 Co 7 and Sm 2 Co 17. The modified Sm-Co diagram [13] indicates SmCo 5+х and Sm 5 Сo 19 in place of SmCo 5 (Fig. 2). The SmCo 5+x phase is stable only at high temperature and decomposes eutectoidly below 1100 o C: SmCo 5+x SmCo 5-x + Sm 2 Co 17. The lowtemperature SmCo 5-x phase is formed at approx o C by peritectoid reaction: SmCo 5+x + Sm 2 Co 7 SmCo 5-x. According to the Sm-Co diagram in Fig. 2, homogeneity range for SmCo 5-x phase widens from SmCo 4.9 to SmCo 4.5. A question arises with regard to the origin of the relatively wide homogeneity region of the SmCo 5-x phase. Figure 2 - Modified Sm-Co phase diagram The crystal structure of the hexagonal SmCo 5 phase (type D2 d ) 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 3 6 -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

5 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 SmCo 5 phase. To produce the shear stacking fault in the (001) basic plane of SmCo 5 phase (with atomic radii ratio R Sm /R Co ~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)α(Bca)β(Bca)β ; removal of the α-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 SmCo 5 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 SmCo 5 based crystalline structures described by the formula: RCo y, where y = 5n+4/n+2, n = 0,1,2, 3 [14]. The crystalline structure of RCo y compound may be described in terms of well-ordered stacking faults with concentration 1 / 3, 1 / 4 and 1 / 5 in the SmCo 3 (n=1), Sm 2 Co 7 (n=2) and Sm 5 Co 19 (n=3) structure types, respectively.

6 Our experimental results (see Fig. 1) showed increase of a and decrease of c parameters of the SmCo 5 phase in hyperstoichiometric alloys when subjected to different aging between 1220 o C and 700 o C. These data suggests that with decrease in aging temperature the SmCo 5 phase is enriched with Sm and is transformed into SmCo 5-x phase. It can be suggested, that in the composition range between SmCo 5 and Sm 5 Co 19 thermodynamically stable SmCo y structures with n 5 might exist. For instance Sm 2 Co 9 (n=10) and Sm 3 Co 14 (n=16) compounds, which composition agree with the border of the homogeneity region for SmCo 5-x phase (Sm 18.2 Co Sm 17.6 Co 82.4 ) in Fig. 2. These hypothetic compounds may be formed by peritectoid reactions in the temperature range of o C. The first stage of the transformation of SmCo 5 into SmCo y 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 SmCo y 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 (Sm 18.2 Co 81.8 ). 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 Sm 2 Co 7 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 Sm 18.2 Co 81.8 alloy starts by formation of the initial grains of the SmCo 5+x phase, which at RT are represented by the dark matrix grains in the microstructure of the alloy. At 1200 o C the intergrain liquid crystallizes as Sm 5 Сo 19 phase via a peritectic reaction: L + SmCo 5+x Sm 5 Сo 19 and after cooling to RT is represented by the lighter intergrain phase. Below 1170 o C the Sm 5 Сo 19 intergrain phase decomposes peritectoidly: Sm 5 Co 19 SmCo 5+x + Sm 2 Co 7, forming a fine mixture of dark and white precipitates. The mechanism of decomposition of the initial grains of SmCo 5+x phase is not quite understood. According to diagram in fig. 2 the phase transformation takes place at 1100 o С. But in nonequilibrium conditions of cooling to RT it is possible that the SmCo 5+x phase decomposes by metastable scheme: SmCo 5+x SmCo 5-x + SmCo 5+2x. According to experimental data in Fig. 1 it occurs between 900 and 700 o C. During the subsequent aging of as-cast alloy in the temperature range of o C the precipitates of Sm 2 Co 7 phase inside the intergrain phase and the precipitates of SmCo 5+2x phase inside the initial grains of the SmCo 5+x phase may transform to SmCo 5-x phase resulting in diffusion of Sm atoms from the first to second phase.

7 METAL 2009 a) c) b) 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.

8 4. CONCLUSIONS XRD study of the lattice parameters of SmCo 5 phase in as-cast SmCo 5 based alloys after different heat treatments shows evidence of enrichment of the SmCo 5 phase with Sm. The behavior of the lattice parameters of SmCo 5 phase in as-cast Sm-rich alloys when subjected to different aging between 1220 o C and 700 o C can be related to the phase transformation of SmCo 5 into SmCo 5-x phases of the Cromer- Larson series (for example, Sm 2 Co 9, Sm 3 Co 14 ) formed by peritectoid reactions below 1100 o C. The phases with disordered stacking faults would not produce supercell reflexes. This is one of the reasons for not having detected the presence of SmCo y phases using X-ray diffraction. It is suggested that in sintered magnets formation of the SmCo y structure during heat treatment starts at the surface of SmCo 5 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 , LITERATURE REFERENCES [1] Strnat K., Hoffer G., Olson J., Ostertag W., Becker J.J., A family of new cobalt- based permanent magnet materials, J. Appl. Phys, 1967, 38, [2] Benz M.G., Martin D.L., Cobalt-samarium permanent magnets prepared by liquid phase sintering, Appl. Phys, Lett., 1970, 17, [3] Weihrauch P.F., Das D.K., The annealing response of Sm-Co magnets and its dependence on composition and processing, Proc. 19 th Annu. AIP Conf. Magn. Magn. Mater., Boston, 1973, Part II, New York, [4] Buschow K.H.J., Goot A.S., Intermetallic compounds in the system samarium-cobalt, J. Less. Comm. Met., 1968, 14, [5] De Campos M.F., Landgraf F.J.G., Saito F.H., Romero S.A., Neiva A.C., Missell F.P., E.de Moras, Gama S., Obrucheva E.V., Jalnin B.V., Chemical composition an coercivity of SmCo 5 magnets, J. Appl. Phys., 1998, 84, [6] Campos M.F. de, Rios P.R., A study of the heat treatment kinetics for SmCo 5 sintered magnets, Proc. 18 th International Workshop on High Performance Magnets and their Applications. Annecy, France, 2004, [7] Menushenkov V.P., The crystal structure and coercive force of SmCo 5 permanent magnets, J. Magn. Magn. Mater., 2005, , [8] Menushenkov V.P., Phase Transformation-induced coercivity mechanism in Rare Earth sintered magnets, J. Appl. Phys., 2006, 99, 08B B [9] Fidler J., Transmission electron microscopy of single phase Co 5 Sm permanent magnets, Phil. Mag., 1982, B 46, [10] Takeda S., Komura Y., Intergrowth and stacking faults in the Sm 5 Co 19 phase, Crystal. Res. & Technol., 1982, 17, [11] Shelekhov E.V., Sviridova T.A., Programs for X-ray analysis of polycrystals, Metal Science and Heat Treatment, 2001, 42, 7, [12] Makarova G.M., Magat L.M., X-ray investigation of decomposition of SmCo 5 solid solution, The Phys. Met. Metallogr., 1977, 43, [13] Khan Y., A contribution to the Sm-Co phase diagram, Acta Cryst., 1974, B30, 4, [14] Cromer D.T., Larson A.C., The crystal structure of Ce 2 Ni 17, Acta Cryst., 1959, 12, 11,