CRYSTALLINE STRUCTURE OF SmCo5 BASED ALLOYS 1Dr. Ing

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CRYSTALLINE STRUCTURE OF SmCo5 BASED ALLOYS 1Dr. Ing
19. – 21. 5 2008 Hradec nad Moravicí
METAL 2009
CRYSTALLINE STRUCTURE OF SmCo5 BASED ALLOYS
1
Dr. Ing. V.P. Menushenkov, 1Dr. Ing. T.A. 1Sviridova,
1
Ing. E.V. Shelekhov, 2Dr. Ing. L.M. Belova
1
State Technological University “Moscow Steel and Alloys Institute”
Leninskii prospect 4, 119049 Moscow, Russia,
E-mail: [email protected]
2
Dept. Materials Science and Engineering, Royal Institute of Technology,
Stockholm, 100 44 Sweden
E-mail: [email protected]
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
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METAL 2009
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,
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METAL 2009
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.
5,005
4,995
a, Ǻ
4,985
900
1000
1200
700
4,975
4,965
4,955
4,945
12,0
14,0
16,0
18,0
20,0
22,0
% Sm
4,005
4
900
1000
1220
700
3,995
c, Ǻ
3,99
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
900
1000
1220
700
0,8075
0,805
c/a
0,8025
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 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
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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 = 700oС.
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 lowtemperature 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
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METAL 2009
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 αlayer (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)α
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 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.
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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 1100oС. 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.
19. – 21. 5 2008 Hradec nad Moravicí
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.
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 CromerLarson 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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