Changing phase equilibria: A method for microstructure optimization and properties improvement in preparing anisotropic Sm2Fe17N3 powders

Article abstract of DOI:10.1016/j.jallcom.2019.01.098

Sm₂Fe₁₇N₃ compound possesses excellent intrinsic permanent magnet properties. Sm₂Fe₁₇N₃ is usually produced by nitridation of the Sm₂Fe₁₇ alloy; however, it is difficult to avoid the formation of α-Fe and SmFe₃ phases in the Sm-Fe system, which may have adverse effects on the magnetic properties of the final magnets. In this study, the three-phase region of SmCu + SmCu₂+Sm₂Fe₁₇ was determined experimentally and a partial isothermal section of the Sm-Fe-Cu phase diagram at 450 °C was established. Based on this newly determined three-phase region, a parent alloy free of α-Fe and SmFe₃ phases was obtained and the effects of Sm-Cu assistant phases on the microstructure and magnetic properties of nitrided powder were investigated. Anisotropic Sm₂Fe₁₇N₃ powders free of α-Fe and having a high (BH)_max exceeding 31 MGOe and H_cj of 13.9 kOe were obtained using a parent alloy containing 3 at.% of Sm-Cu assistant phases. The magnetization reversal process of anisotropic Sm₂Fe₁₇N₃ powders with and without α-Fe was investigated and the underlying mechanisms were analyzed.

Full text of DOI:10.1016/j.jallcom.2019.01.098

Accepted Manuscript
Changing phase equilibria: A method for microstructure optimization and properties
improvement in preparing anisotropic Sm Fe N powders
2
17 3
Cifu Lu, Xiufeng Hong, Xiaoqian Bao, Xuexu Gao, Jie Zhu
PII:
S0925-8388(19)30106-9
DOI:
https://doi.org/10.1016/j.jallcom.2019.01.098
JALCOM 49136
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 6 November 2017
Revised Date: 4 January 2019
Accepted Date: 8 January 2019
Please cite this article as: C. Lu, X. Hong, X. Bao, X. Gao, J. Zhu, Changing phase equilibria: A method
for microstructure optimization and properties improvement in preparing anisotropic Sm Fe
powders, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.01.098.
N
2
17 3
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Changing phase equilibria: a method for microstructure optimization
and properties improvement in preparing anisotropic Sm2Fe17N3
powders
Cifu Lua,b, Xiufeng Hong a, Xiaoqian Bao a, Xuexu Gao a and Jie Zhu a,*
a
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 30 Xue Yuan Road,
Beijing 100083, People's Republic of China
b
Guangdong Research Institute of Rare-Metals, Guangdong Academy of Science, 363 Chang Xing Road, Guangzhou 510651,
People's Republic of China
*Corresponding author: Jie Zhu, Ph.D., Professor, 10008, China, Tel: (+86)62334333
E-mail: jiezhu@ustb.edu.cn
Abstract
Sm2Fe17N3 compound possesses excellent intrinsic permanent magnet properties.
Sm2Fe17N3 is usually produced by nitridation of the Sm2Fe17 alloy; however, it is
difficult to avoid the formation of α-Fe and SmFe3 phases in the Sm-Fe system, which
may have adverse effects on the magnetic properties of the final magnets. In this study,
the three-phase region of SmCu+SmCu2+Sm2Fe17 was determined experimentally and
a partial isothermal section of the Sm-Fe-Cu phase diagram at 450 °C was established.
Based on this newly determined three-phase region, a parent alloy free of α-Fe and
SmFe3 phases was obtained and the effects of Sm-Cu assistant phases on the
microstructure and magnetic properties of nitrided powder were investigated.
Anisotropic Sm2Fe17N3 powders free of α-Fe and having a high (BH)max exceeding 31
MGOe and Hcj of 13.9 kOe were obtained using a parent alloy containing 3 at.% of
Sm-Cu assistant phases. The magnetization reversal process of anisotropic Sm2Fe17N3
powders with and without α-Fe was investigated and the underlying mechanisms were
analyzed.
Key words: Rare-earth permanent magnets; Phase diagram; Sm2Fe17N3
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1. Introduction
Sm2Fe17N3 has a high Curie temperature (470 °C), a huge anisotropy field (21-26
T) and a high saturation magnetization (1.54-1.57 T) comparable to that of Nd2Fe14B
[1-3]. Therefore, Sm2Fe17N3 has been intensively investigated as a promising
candidate to replace (Nd, Dy)-Fe-B permanent magnets for high-temperature
applications [4-8]. Currently, Sm2Fe17N3 can be used to produce high-performance
anisotropic Sm2Fe17N3 bonded magnets [4, 6, 9, 10]. Sm2Fe17N3 compound is usually
produced by nitridation of the Sm2Fe17 alloy. Producing a Sm2Fe17 alloy free of
magnetically soft phases is essential for high-performance Sm2Fe17N3 magnets.
However, owing to the phase equilibria, it is difficult to avoid the precipitation of the
α-Fe and SmFe3 phases in a Sm-Fe binary system.
Previous research on Sm2Fe17N3 materials has been performed mostly on the
basis of the single-phase Sm2Fe17 alloy. This Sm2Fe17 alloy can be produced by ingot
casting, strip casting technique or a reduction-diffusion method [10-16]. In the Sm-Fe
binary system, Sm2Fe17 equilibrates with the α-Fe and SmFe3 phases, and the Sm2Fe17
phase forms through a peritectic reaction between solid iron and a Sm-Fe liquid phase
[17]. The incomplete peritectic reaction results in a presence of considerable amounts
of α-Fe and Sm rich phases in the as-cast ingot, which need to be removed by a
homogenization treatment typically performed at 1000 °C [10, 15, 16]. Although
microstructural heterogeneity can be reduced by the strip casting technique [12],
compositional deviation from the Sm2Fe17 stoichiometry is easy to occur owing to the
easy volatilization of Sm metal. Such composition deviations can also result in the
formation of α-Fe or SmFe3 [17]. Therefore, owing to the phase equilibria in the
Sm-Fe system and the nature of the Sm metal, it is difficult to produce a single-phase
Sm2Fe17 alloy by metal casting. The nitridation of coarse SmFe3 powder below
450 °C is difficult owing to the slow kinetics of forming SmN compound [18, 19].
But the SmFe3 powder still can partially decompose into SmN and α-Fe during
nitridation at temperatures between 250 °C and 450 °C [19].
The coercivity of Sm2Fe17N3 powder is mainly controlled by the nucleation of
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reverse domains [12, 20-22]. Compared with domain nucleation, the propagation of
the reversed domain wall within the Sm2Fe17N3 particle takes place much easier.
Magnetization reversal can occur easily by spontaneous expansion of these nucleated
domain walls. Therefore, the precipitation of α-Fe may have adverse effects on the
magnetic properties of the final magnets and the formation of SmFe3 phase can
decrease the saturation magnetization. Even if high-performance anisotropic
Sm2Fe17N3 powders were produced on the basis of a single phase Sm2Fe17 binary
alloy [9, 12, 23-26], the fluctuation of magnetic properties caused by compositional
deviation and microstructural heterogeneity are unfavorable factors affecting the mass
production and applications of Sm2Fe17N3 magnets.
Moreover, as established in detail for the Nd2Fe14B magnets, the coercivity of
permanent magnets with a nucleation-type coercivity mechanism is extremely
sensitive to their microstructural features. An optimal particle size and smooth,
undamaged particle surfaces are important to reduce nucleation of reversed domains,
and the isolation of grains by a nonmagnetic phase is important to hinder the motion
of reverse domain walls [27, 28]. It is well known that the existence of a
low-melting-point and nonferromagnetic Nd-rich intergranular phase plays a crucial
part in achieving high coercivity through microstructure optimization in
Nd2Fe14B-based magnets [29-37]. However, owing to the phase relations in the
Sm-Fe system, the first step in preparing high-performance Sm2Fe17N3 materials is
obtaining a single-phase Sm2Fe17 alloy. There is no suitable phase to directly decouple
exchange interaction between Sm2Fe17N3 grains. Therefore, it is necessary to find
nonmagnetic phases that can equilibrate with Sm2Fe17 and prevent the formation of
harmful phases. Such information is critically needed for designing Sm2Fe17N3–based
magnets with further improved magnetic properties.
In the Cu-Fe-Nd ternary system [38], nonmagnetic NdCu and NdCu2 phases
equilibrate with Nd2Fe17, and a three-phase region of NdCu+NdCu2+Nd2Fe17 is
present. Based on the similarity of rare earth elements, the existence of a
SmCu+SmCu2+Sm2Fe17 three-phase region is expected but there have been no reports
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on a Cu-Fe-Sm ternary system until now. In the preparation of Sm2Fe17N3 powder, the
nitridation treatment is usually conducted at about 450 °C, and this treatment typically
takes a long time. With the addition of Cu, the parent alloy for the preparation of
Sm2Fe17N3 becomes a multiphase system, so the phase equilibria of Sm-Fe-Cu at
450 °C should be established. Therefore, the temperature of the partial isothermal
section of Sm-Fe-Cu was set at 450 °C.
In this work, the existence of a SmCu+SmCu2+Sm2Fe17 three-phase region at
450 °C in the Cu-Fe-Sm ternary system was experimentally confirmed. Based on this
newly determined phase equilibiria, a strategy is proposed to avoid the formation of
harmful impurity phases in Sm2Fe17N3 powders by locating the composition of the
parent alloy in the three-phase region. The effects of Sm-Cu assistant phases on the
microstructure of the parent alloy, nitrided powder, and associated magnetic properties
were investigated to evaluate the feasibility of this strategy. The microstructure was
characterized using powder X-ray diffraction (XRD) and scanning electron
microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDS). The
underlying mechanisms of the microstructure optimization and properties
improvement are discussed.
2. Experimental procedure
(Sm10.5Fe89.5)90(Sm100-xCux)10 (x = 30, 56, 70) (at. %) alloy ingots were
synthesized to determine the phase relations by induction melting of 99.95% pure
elements in a corundum crucible under the protection of an argon atmosphere. An
excess of 5 wt. % Sm was used to compensate for the weight losses that occurred
during the melting owing to the evaporation of Sm. To determine the phase relations
at 450 °C, (Sm10.5Fe89.5)90(Sm100-xCux)10 (x = 30, 56, 70) ingots were sealed in an
argon atmosphere-protected quartz tube, annealed at 1000 °C for 10 h and
subsequently annealed at 450 °C for 48 h. Based on the obtained phase relations, an
alloy with a nominal composition of (Sm10.5Fe89.5)97(Sm45Cu55)3 (at.%) was designed
and produced by the strip casting technique and annealing at 1000 °C for 20 min (this
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alloy is denoted as SmFeCu throughout the text). The strip casting cooling roller
speed was 2 - 5 m/s. The annealed SmFeCu strips were mechanically crushed into a
coarse powder and nitrided at 420 °C for 35 h under a high-purity N2 atmosphere
at atmospheric pressure (this nitrided powder is denoted as SmFeCuN throughout the
text). For comparison, a commercially available Sm2Fe17N3 coarse powder (denoted
as SmFeN throughout the text, the Sm2Fe17 alloy was produced by strip casting) was
used as a reference sample prepared without Cu addition. SmFeCuN and SmFeN
powders were further refined by ball milling in heptane used as a medium and oleic
acid as the surfactant. The weight ratio of the powder to the balls was 1:10 [26]. To
prepare aligned samples for the magnetic testing, fine Sm2Fe17N3 powders were
mixed with hot paraffin and aligned in a magnetic field of 15 kOe until the paraffin
solidified.
Powder XRD (Cu-Kα, Rigaku Ultima IV) was used to determine the phases
present in the specimens. The morphology and chemical composition analyses of
phases were carried out using SEM/EDS (Supra55). The melting points of Sm-Cu
phases were measured by DTA (Netzsch STA 449F3) at a heating rate of 20 °C /min.
The nitrogen contents in SmFeCuN and SmFeN powders were determined using an
oxygen and nitrogen analyzer (TC500, Leco Co.). The room-temperature magnetic
properties of the aligned samples were measured using a vibrating sample
magnetometer (VSM, PPSM-9, Quantum Design Co.) with a maximum field of 50
kOe; no demagnetization field correction was applied. To calculate the (BH)max of
SmFeN and SmFeCuN powders, density values of 7.68 g/cm3 and 7.69 g/cm3,
respectively, were used. The room-temperature magnetic properties for the bulk
bonded magnets were measured using a magnetic measurement device (NIM-200C,
National Institute of Metrology, P. R. China).
3. Results and Discussion
3.1 The partial isothermal section of Sm-Fe-Cu at 450 °C
Buschow determined the first Sm-Fe binary phase diagram [17]. The existence,
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crystal structure and composition of three Sm-Fe intermetallic compounds were
reported. They were Sm2Fe17 (Th2Zn17 type structure), SmFe3 (PuNi3 type structure),
and SmFe2 (MgCu2 type structure). The Sm-Cu phase diagram was reported by
several investigators [39-44]. The existence of SmCu6 (CeCu6 type structure), SmCu5
(CaCu5 type structure), Sm2Cu9, SmCu2 (CeCu2 type structure) and SmCu (CsCl type
structure) was determined in the Sm-Cu binary system. A Sm-Cu diagram containing
Sm2Cu9 was first reported by Derkaoui et al [43] but to the best of our knowledge, the
crystallographic parameters were not determined. The Fe-Cu binary system has been
investigated many times, and no intermetallic compounds were found [45-49]. The
solubility of Fe in (Cu) and Cu in α-Fe is less than 2.0 at.% below 900 °C. The
crystallographic parameters of the binary compounds mentioned above are given in
Table 1.
Table 1. Crystallographic parameters of the binary compounds in the Sm-Fe and
Sm-Cu systems.
Composition Structure
type
Space
group
R-3m
R-3m
Fd-3m
Pnma
-
Unit-cell parameters (Å)
Reference
a
b
c
Sm2Fe17
SmFe3
SmFe2
SmCu6
Sm2Cu9
SmCu5
SmCu2
SmCu
Th2Zn17
PuNi3
MgCu2
CeCu6
-
8.570
5.187
7.417
8.050
12.440
24.910
[17]
[17]
[17]
[50]
[43]
[51]
[52]
[27]
5.037
10.072
CaCu5
CeCu2
CsCl
P6/mmm 5.074(5)
4.099(5)
Imma
4.360(5) 6.925(5) 7.375(5)
3.530(3)
Pm-3m
Fig. 1 shows the back-scattered electron (BSE) micrographs and XRD patterns of
thermally equilibrated (Sm10.5Fe89.5)90(Sm100-xCux)10 (x = 30, 56, 70) alloys. Based on
the SEM-EDS and XRD results, three phases were identified in all the thermally
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equilibrated alloys. The BSE micrograph of the alloy with x=30 is presented in Fig.
1(a), clearly showing the presence of three different phase regions. The compositions
of the dark, gray, and white regions were Sm11.8Fe86.6Cu1.6, Sm26.1Fe72.3Cu1.6, and
Sm49.2Cu45.0Fe5.8, respectively. Based on the EDS analysis and XRD patterns, these
phases were identified to be Sm2Fe17, SmFe3, SmCu. Fig. 1(b) clearly shows that
Sm2Fe17, SmCu, and SmCu2 phases coexist in the alloy with x = 56. Two Sm-Cu
phases were clearly distinguished based on the differences in their chemical
composition. Three phases, Sm2Fe17, SmCu2, and α-Fe coexisted in the alloy with x =
70, as shown in Fig. 1(c). The characteristic peaks of the three thermally equilibrated
phases in all alloys could be clearly distinguished in the XRD patterns, as shown in
Fig. 1(d). Therefore, all the phases and phase relations shown in the BSE micrographs
were confirmed by the XRD analysis. Notably, no ternary compounds could be
detected in these alloys. The data concerning the phases being in equilibrium and their
chemical compositions detected in the (Sm10.5Fe89.5)90(Sm100-xCux)10 (x=30, 56, 70)
alloys are summarized in Table 2. The solubility of the third element in binary
compounds Sm2Fe17, SmFe3, and SmCu2 was less than 1.6 at.%, and the solubility of
Cu in α-Fe was also very low. However, the SmCu phase could accommodate 5.8 at.%
of Fe.
Table 2. The equilibrium phases and their chemical compositions in
(Sm10.5Fe89.5)90(Sm100-xCux)10 (x = 30, 56, 70) alloys equilibrated at 450 °C.
Alloy
Phase
Composition (at.%)
composition
Cu
45.0
1.6
Fe
5.8
Sm
SmCu
SmFe3
Sm2Fe17
SmCu
49.2
26.1
11.8
49.2
34.4
11.8
34.4
(Sm10.5Fe89.5)90(Sm70Cu30)10
72.3
86.6
5.8
1.6
45.0
63.4
1.1
(Sm10.5Fe89.5)90(Sm44Cu56)10
SmCu2
Sm2Fe17
SmCu2
1.2
86.6
1.2
63.4
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(Sm10.5Fe89.5)90(Sm30Cu70)10
Sm2Fe17
1.6
1.4
86.6
98.6
11.8
—
α-Fe
Based on the experimental results above and the phase relations in the Sm-Cu,
Sm-Fe and Cu-Fe binary systems[17, 39-44], a partial isothermal section of the
Cu-Fe-Sm system at 450 °C was constructed and is shown in Fig. 2(a). The phase
equilibrium relationships in the Sm2Fe17-related regions can be clearly seen in this
partial ternary phase diagram. The diagram is comprised of three different three-phase
regions ꢀ (Ι) α-Fe+Sm2Fe17+SmCu2, (ΙΙ) Sm2Fe17+SmCu2+SmCu, and (ΙΙΙ)
Sm2Fe17+SmCu +SmFe3. Thus, the existence of the Sm2Fe17+SmCu2+SmCu
three-phase region is confirmed. Fig. 2(b) presents the enlarged partial isothermal
section of the Sm-Fe-Cu phase diagram, showing that (Sm10.5Fe89.5)90(Sm100-xCux)10
(x=30, 56, 70) alloys are located in regions of (Ι), (ΙΙ), and (ΙΙΙ), respectively.
SmCu and SmCu2 are paramagnetic phases [53, 54], while α-Fe, Sm2Fe17, and
SmFe3 are ferromagnetic phases at room temperature. Thus regions (Ι) and (ΙΙΙ)
contain two ferromagnetic phases, whereas region (ΙΙ) contains only one
ferromagnetic phase, Sm2Fe17. Therefore, a single ferromagnetic phase composition
region (ΙΙ) can be created by the addition of Cu. Thus, α-Fe and SmFe3 are not present
in an equilibrium alloy whose composition is located in region (ΙΙ).
3.2 The stability of the Sm2Fe17 phase in the Sm-Fe-Cu ternary system
In the Nd-Fe binary system, the Nd2Fe17 phase decomposes into solid γ-Fe and
Nd-Fe liquid phase at 1205 °C [55], but the decomposition temperature of Nd2Fe17
phase in the Nd-Fe-Cu ternary system can be reduced to 831 °C [56]. Therefore, it
was necessary to investigate the temperature stability of the Sm2Fe17 phase in the
Sm-Fe-Cu ternary system. Fig. 3(a) shows the DTA curves of
(Sm10.5Fe89.5)90(Sm100-xCu56)10 (x = 56, 70) alloys. In the temperature range of 400 -
1150 °C only one endothermic peak was seen in each case, located at 733 °C and
849 °C for x= 56 and 70, respectively. In the Sm-Cu binary system, the melting points
of SmCu and SmCu2 were 735 °C and 862 °C, respectively [44]. The endothermic
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peaks observed at 733 °C and 849 °C should correspond to the melting points of
SmCu+SmCu2 and SmCu2, respectively. No other peaks related to the decomposition
of Sm2Fe17 were observed in the temperature range of 400 - 1150 °C, revealing the
high temperature stability of the Sm2Fe17 phase in the Sm-Fe-Cu ternary system.
To further investigate the temperature stability of Sm2Fe17 in the Sm-Fe-Cu
ternary system, a (Sm10.5Fe89.5)90(Sm44Cu56)10 alloy ingot was annealed at 1000 °C for
16 h and quenched into cold water. The BSE image of this sample is shown in Fig.
3(b). From SEM/ EDS data, the dark and gray areas represent the Sm2Fe17 and SmCu2
phases, respectively. The SmCu phase disappeared owing to the evaporation of Sm
during the long-time annealing. No α-Fe or other phase was observed, indicating that
the Sm2Fe17 phase remained stable during annealing at 1000 °C.
During the formation of the Sm2Fe17 binary alloy through metal melting, the
temperature of the molten alloy becomes higher than the peritectic reaction
temperature of 1280 °C. At this temperature, the saturation vapor pressure of pure Sm
metal exceeds 5 kPa. Owing to the easy volatilization of Sm metal and
oxidation by residual oxygen, the content of Sm in an alloy may easily deviate from
the intended value. Fig. 4(a) shows the content of impurity phases (α–Fe or SmFe3)
caused by the variation of Sm content in an off-stoichiometric Sm10.52+yFe89.48-y alloy.
The impurity phase contents was calculated based on the Sm-Fe phase diagram. As
the content of Sm in Sm2Fe17 was only 10.52 at.%, the phase composition of the
Sm2Fe17 binary alloy was sensitive to Sm content. In addition, the impurity phase
content (α–Fe or SmFe3) increased rapidly with the changes in Sm content.
Furthermore, the inhomogeneities of microstructure and chemical composition
formed in the peritectic reaction suggest that, it would be difficult to completely avoid
the formation of magnetic impurity phases (α–Fe or SmFe3) in a Sm-Fe alloy.
On the contrary, α-Fe and SmFe3 cannot exist in an equilibrium alloy if the
composition remains in the SmCu+SmCu2+Sm2Fe17 three-phase region. This
three-phase region creates an enlarged compositional range, in which the variation of
Sm content only causes the formation of Sm-Cu phases, avoiding the formation of
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magnetic impurity phases (SmFe2, SmFe3 or α-Fe). When the content of Cu is fixed,
the composition interval of the Sm2Fe17+SmCu2+SmCu three-phase region is constant
and can be measured by the content interval of Sm. As accordingly shown in Fig. 4(b),
the content interval of Sm (composition interval of the three-phase region) increases
with the content of Cu. Moreover, low-melting-point Sm-Cu phases should tend to
form along the grain boundaries during solidification and act as channels for Sm
atoms. Thus, the Sm-Cu phase should be helpful for reducing composition
inhomogeneity.
In summary, in the Sm-Fe-Cu ternary system, the Sm2Fe17 phase retains its
high-temperature stability, and its compositional stability limits can be broadened
through the formation of a SmCu+SmCu2+Sm2Fe17 three-phase region. Thus, with the
addition of Sm-Cu assistant phases, it should be much easier to completely avoid the
formation of α-Fe and SmFe3 phases during the synthesis of Sm2Fe17.
3.3 Microstructure of parent alloy and as-nitrided powder
Since the presence of magnetically soft phases can result in coercivity decrease,
avoiding the precipitation of α-Fe in nitrided powder is the first step in the synthesis
of high-performance Sm2Fe17N3 magnets. To avoid the precipitation of α-Fe and
SmFe3 without an obvious reduction in magnetization, a SmFeCu alloy containing 3
at.% of Sm-Cu phases was designed and prepared by strip casting and annealing.
The nominal composition of SmFeCu alloy is (Sm10.5Fe89.5)97(Sm45Cu55)3, marked as
alloy 4 in Fig. 2(b), which is located in the SmCu+SmCu2+Sm2Fe17 three-phase
region in the Cu-Fe-Sm phase diagram.
Fig. 5 shows the BSE micrographs of the cross-section of the strip-cast SmFeCu
alloy. As can be seen from the micrographs of the enlarged regions, the plate like
cast alloy has a homogeneous and fine scaled microstructure, which is comprised of
principal Sm2Fe17 phase and Sm-Cu intergranular phases. The interlamellar spacing
between the Sm-Cu intergranular phases is approximately 3 ꢀm. There is no
indication of α-Fe dendrites or SmFe3 phases in the micrographs and XRD data (not
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shown here).
Fig. 6(a) presents the BSE micrograph of the cross section of the strip cast
SmFeCu parent alloy after annealing at 1000 °C for 20 min. The annealed alloy
consists of coarse Sm2Fe17 grains and a small fraction of Sm-Cu phases. Most of the
Sm2Fe17 grains are larger than 10 ꢀm. After nitridation, as shown in Fig. 6(b), the
interface between the Sm2Fe17N3 main phase and nitrided Sm-Cu phases was still
smooth and clear, and no α-Fe or other new phase could be detected even in the high
magnification BSE image. On the contrary, the BSE image of the cross section for the
SmFeN reference sample shown in Fig. 6(c) indicates that, along with the coarse
Sm2Fe17N3 main phase, there exists a light grey region, which should originate from
the SmFe3 phase, according to EDS analysis. Fig. 6(d) shows the XRD patterns of the
SmFeCu alloy, the nitride SmFeCuN, and the SmFeN reference sample. Only peaks
contributed by the Th2Zn17-type Sm2Fe17 and Sm2Fe17N3 phases were detected in the
XRD patterns of the SmFeCu alloy and its nitride SmFeCuN. The diffraction peaks of
the SmFeCuN shifted to lower angles, indicating the interstitial modification of the
Sm2Fe17 by nitrogen atoms. By comparison, the XRD pattern of the SmFeN reference
sample clearly contains a small peak corresponding to the α-Fe phase. Based on the
microstructure analysis, the α-Fe phase in the SmFeN reference sample appears to
originate from the nitridation of SmFe3. According to the nitrogen content determined
using a nitrogen analyzer, the nominal chemical formulas of the SmFeCuN and
SmFeN samples can be expressed as Sm2Fe17N3.7(SmCu1.2)0.26 and Sm2Fe17N3.3,
respectively.
The XRD and BSE analysis of SmFeCuN indicates that α-Fe precipitates in the
SmFeCuN powder were largely avoided, which is a significantly favorable factor for
the synthesis of high performance anisotropic Sm2Fe17N3 powders. More importantly,
the Sm-Cu phases were potentially suitable for magnetic isolation of Sm2Fe17N3
grains.
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3.4 Magnetic properties of anisotropic Sm2Fe17N3 powders
Coarse Sm2Fe17N3 powder normally has low coercivity, and further milling is
usually used to reduce the size of Sm2Fe17N3 particles [57], since the coercivity
increases with the reduction of powder size [20, 58]. Therefore, the coarse SmFeCuN
powder was milled into fine powder to improve the coercivity.
The magnetic hysteresis loops of SmFeCuN powders milled for 4 - 12 h and their
secondary electron SEM images are shown in Fig. 7. The detailed magnetic property
parameters of the SmFeCuN powders are listed in Table 2. It is worth noting that no
step was observed in the demagnetization curves. The particle sizes of the powders
were measured from the SEM images. The average particle sizes were 2.30 ꢀm, 0.88
ꢀm, and 0.76 ꢀm for the powders milled for 4 h, 8 h, and 12 h, respectively. The
coercivity increases from 6.53 kOe to 12.99 kOe when the particle size decreases. The
magnetization and remanence slightly decrease with increasing milling time, these
changes were attributed to oxidation and microstructure damage [12, 59]. The
Sm2Fe17N3 particles were reported to be surrounded by an oxygen-rich amorphous
layer of about 10 nm in thickness [59, 60]. As milling time increases, the oxidation
and microstructure damage also intensify, leading to the decreases in the
magnetization and remanence ratio. As the particle size decreased, the maximum
energy product (BH)max increased from 25.80 MGOe to 31.93 MGOe. However,
because of the decreased remanence, the (BH)max of the 12 h-milled power decreased
slightly to 31.55 MGOe. The SmFeN reference sample was also milled for 4 h to
prepare a fine powder with an average particle size of 1.0 ꢀm. The detailed magnetic
property parameters of SmFeN powders are listed in Table 3.
Table 3. Magnetic properties for aligned SmFeCuN and SmFeN powder samples.
Sample
Particle size Ms @ 50 kOe
Mr
(kGs) (kOe) (%)
13.22 6.53 92.16 42.19
Hcj
Mr/Ms Hk/Hcj (BH)max
(ꢀm)
(kGs)
14.35
(%)
(MGOe)
25.80
SmFeCuN
2.30
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SmFeCuN
SmFeCuN
SmFeN
0.88
0.76
1.0
14.24
14.21
14.97
12.89 11.47 90.52 38.90
31.93
31.55
28.64
12.48 12.99 87.82 39.70
12.75 10.80 85.17 28.29
The most obvious difference between the SmFeCuN and SmFeN powders was
that the remanence ratio (Mr/Ms) and squareness (Hk/Hcj) of the SmFeCuN powder
were markedly superior to those of SmFeN. Compared with the pure SmFeN powder,
the magnetization (Ms @ 50 kOe) of the SmFeCuN powder was reduced with the
addition of 4.9 wt.% paramagnetic Sm-Cu phase. Even so, the remanence of the
SmFeCuN powder milled for 4 h was still higher than that of SmFeN, owing to the
high remanence ratio. As a result, when the coercivities are roughly equal, the
maximum energy product of SmFeCuN is higher than that of SmFeN.
3.5 The mechanism of the improvements of the magnetic properties
The domain nucleation was deemed to be the dominant magnetic hardening
mechanism for Sm2Fe17N3 powders [12, 20, 22]. As coarse Sm2Fe17N3 particles have
more easy nucleation sites and the propagation of reverse domain walls occurs easily
within Sm2Fe17N3 grains [27], coarse Sm2Fe17N3 powder normally has very low
coercivity. However, fine Sm2Fe17N3 powders with particle sizes smaller than 4 ꢀm
exhibit characteristics of single-domain magnetic particles and have a much higher
nucleation field [59]. Even if the fine Sm2Fe17N3 powder is sintered into fully dense
bulk magnets, the magnetic properties of the Sm2Fe17N3 powder still can be
maintained [61, 62]. This fact reveals that the reversed domain wall cannot move
easily across the grain boundaries in dense Sm2Fe17N3 magnets, otherwise the
propagation of the reversed domain wall within the whole Sm2Fe17N3 magnet would
drastically reduce the squareness and coercivity. Hence, the reversal of fully
magnetized Sm2Fe17N3 particles in anisotropic Sm2Fe17N3 powder should tend to
proceed relatively independently. When the fully magnetized powder is exposed to an
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external demagnetization field, the reversal of Sm2Fe17N3 powder should not proceed
uniformly, according to the nucleation field of the Sm2Fe17N3 particles. Namely, the
squareness of an aligned Sm2Fe17N3 magnet should reflect the distribution of
nucleation fields, which is influenced by various microstructural features of the
Sm2Fe17N3 particles.
Fig. 8(a) shows the XRD patterns of the 4 h-milled SmFeN reference sample and
8 h-milled SmFeCuN powder. There was no evidence of α-Fe in 8 h-milled SmFeCuN
powder, whereas the XRD pattern of 4 h-milled SmFeN powder clearly contained a
small peak corresponding to the α-Fe phase.
As shown in Fig. 7(b-d), the powders consisted of Sm2Fe17N3 grains with
different particle sizes, and in most cases the Sm2Fe17N3 particles contained defects
and ridges on the surface. Hence, the nucleation fields of Sm2Fe17N3 particles were
inhomogeneous. Fig. 8(b) shows the normalized hysteresis curves of the 4 h-milled
SmFeN and 8 h-milled SmFeCuN powders. The normalized magnetization of
SmFeCuN reduced slowly when the external magnetic field is near zero, indicating
that the Sm2Fe17N3 particles with low coercivity had a very low proportion. In
contrast, the magnetic hysteresis curve of 4 h-milled SmFeN powder contained a clear
step even before the external magnetic field was reduced to zero. The SmFeN powder
contained some magnetically soft α-Fe phases, which were thought to be the origin of
the coercivity decrease in a part of Sm2Fe17N3 particles. As a result, these Sm2Fe17N3
particles could reverse even under a low external demagnetization field. Thus, the
decrease of the remanence ratio for the 4 h-milled SmFeN powder should also be
attributed to the presence of α-Fe, which increases the proportion of Sm2Fe17N3
particles having low coercivity.
Fig. 8(c) shows the normalized demagnetization curves of 4 h-milled SmFeN and
8 h-milled SmFeCuN powders. Although the coercivity of SmFeCuN was only
slightly larger than that of the SmFeN powder, the normalized magnetization of
SmFeCuN reduced slower with the external demagnetization field than it did for
SmFeN. As a result, as shown in Table 2, the squareness and (BH)max of SmFeCuN
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were higher than those of SmFeN, even though the SmFeCuN powders had smaller
remanence than SmFeN.
Fig. 8(d) shows the normalized demagnetization curves for bonded magnets made
from SmFeN powders milled for 4 h and SmFeCuN powders milled for 4 - 12 h. The
magnetic property of bonded magnets was tested in a closed-loop mode, free of
internal demagnetization field (NeffM). The normalized demagnetization curve of
SmFeN bonded magnets also contained a step. With a low squareness of 35%, the
normalized magnetization declined markedly even though the external
demagnetization field was very low. By comparison, the demagnetization curves of
bonded magnets made of SmFeCuN powders contained no step. Notably, with a large
squareness exceeding 52%, the demagnetization curves contained a “platform” whose
range extends with increasing coercivity. As most of the Sm2Fe17N3 fine powders
produced by ball milling still contain defects and ridges on the surface, the squareness
of Sm2Fe17N3 bonded magnets was not as large as that of the sintered Nd-Fe-B
magnets.
In summary, along with the α-Fe phase removed from Sm2Fe17N3 powders, the
proportion of Sm2Fe17N3 particles with low coercivity could be reduced, leading to
the improved and superior remanence ratio and squareness. The improvement of the
squareness in Sm2Fe17N3 powders helps to improve the energy product and
enables the formation of a linear B-H curve, which has advantages in device design
and dynamic operation. Hence, the addition of Sm-Cu assistant phases to the Sm2Fe17
alloy is necessary and may make significant contributions to the mass production and
applications of Sm2Fe17N3 magnets.
4. Conclusions
A partial isothermal section of the Sm-Fe-Cu ternary phase diagram at 450 °C has
been established by investigating the phase equilibria in the thermally-equilibrated
Sm-Fe-Cu alloys. This partial isothermal section contained the following three
different three-phase regions: α-Fe+Sm2Fe17+SmCu2, Sm2Fe17+SmCu2+SmCu, and
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Sm2Fe17+SmCu+SmFe3.
The Sm2Fe17 phase still shows a high-temperature stability in the Sm-Fe-Cu
ternary system. The alloy composition range free of α-Fe and SmFe3 was broadened
with the formation of a SmCu+SmCu2+Sm2Fe17 three-phase region. With the addition
of Sm-Cu assistant phases, it is much easier to completely avoid the formation of α-Fe
and SmFe3 phases in the production of the Sm2Fe17 alloy.
After nitridation, the interface between the Sm2Fe17N3 main phase and Sm-Cu
phases was still smooth and clear, and no α-Fe or other new phase could be detected.
This indicates that, the precipitation of α-Fe in Sm2Fe17N3 powder was avoided
completely.
A SmFeCu alloy without α-Fe or SmFe3 phases present was produced with the
addition of 3 at.% Sm-Cu assistant phases. Anisotropic Sm2Fe17N3 powders without
α-Fe were prepared through nitridation and ball milling. This anisotropic powder had
a high coercivity of 13.9 kOe, high (BH)max
exceeding 31 MGOe, and high
squareness exceeding 52%. On the contrary, the anisotropic Sm2Fe17N3 powder
prepared using binary Sm2Fe17 alloy contained an α-Fe phase. Thus, the squareness
was reduced to 35% by α-Fe. The higher squareness should be attributed to the
reduced proportion of Sm2Fe17N3 particles with easy nucleation of reversed domains.
Based on the newly-determined Sm2Fe17+SmCu2+SmCu phase equilibria, the
precipitation of α-Fe in Sm2Fe17N3 powder and the related deterioration in
magnetism can be avoided completely. The addition of the Sm-Cu assistant phases to
the Sm2Fe17 alloy is necessary and may make significant contributions to further
improvement of magnetic properties, mass production, and applications of Sm2Fe17N3
magnets.
Acknowledgments
This work was supported by the State Key Laboratory for Advanced Metals and
Materials [Grant No. 2016Z-21].
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Figure Captions
Fig. 1. The back-scattered electron (BSE) micrographs of Sm-Fe-Cu alloys
equilibrated at 450 °Cꢀ
(a) (Sm10.5Fe89.5)90(Sm70Cu30)10 alloy, (b) (Sm10.5Fe89.5)90(Sm44Cu56)10 alloy, (c)
(Sm10.5Fe89.5)90(Sm30Cu70)10 alloy. (d) The XRD patterns of the thermally-equilibrated
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(Sm10.5Fe89.5)90(Sm100-xCux)10 (x = 30, 56, 70) alloys.
Fig.2. (a) The partial isothermal section of the Sm-Fe-Cu phase diagram at 450 °C. (b)
The enlarged partial isothermal section of the Sm-Fe-Cu phase diagram superimposed
with the nominal compositions of 1 - 4 alloys. (alloy 1: (Sm10.5Fe89.5)90(Sm70Cu30)10
alloy, 2: (Sm10.5Fe89.5)90(Sm44Cu56)10 alloy, 3: (Sm10.5Fe89.5)90(Sm30Cu70)10 alloy, 4:
(Sm10.5Fe89.5)97(Sm45Cu55)3 ) alloy.
Fig. 3. (a) The DTA curves of the (Sm10.5Fe89.5)90(Sm100-xCux)10 (x= 56, 70) alloys. (b)
The BSE image of the (Sm10.5Fe89.5)90(Sm44Cu56)10 ingot, annealed at 1000ꢁ for 16
h and quenched in water.
Fig. 4. (a) The impurity phases content (α–Fe or SmFe3) caused by the variation of
Sm content in an off-stoichiometric Sm10.52+yFe89.48-y alloy. (b) The composition
interval of the Sm2Fe17+SmCu2+SmCu three-phase region can be measured by the
content interval of Sm. The content interval of Sm (composition interval of the
three-phase region) increases with the content of Cu.
Fig. 5. The BSE micrographs of (a) the cross section of the casted SmFeCu parent
alloy; (b)-(d) the enlarged region 1 - 3, respectively; showing the distribution of the
Sm2Fe17 phase (gray), and Sm-Cu rich phase (white).
Fig. 6. The BSE micrographs of the cross section of (a) strip casted SmFeCu parent
alloy after annealing at 1000ꢁ; (b) as-nitrided SmFeCuN particle, and (c) as-nitrided
SmFeN reference sample. (d) The XRD patterns of the SmFeCu parent alloy,
SmFeCuN, and SmFeN reference sample.
Fig. 7. (a) The magnetic hysteresis loops of the aligned SmFeCuN powder samples at
room temperature. The secondary electron SEM images of the SmFeCuN powders
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milled for (a) 4 h, (b) 8 h, (c) 12 h.
Fig. 8. (a) The XRD patterns of the 4 h-milled SmFeN and 8 h-milled SmFeCuN
powders. (b-c) the normalized magnetic hysteresis curves and demagnetization curves
of 4 h-milled SmFeN and 8 h-milled SmFeCuN powders, respectively. (d) The
normalized demagnetization curves of bonded magnets made from 4 h-milled SmFeN
and 4 -12 h-milled SmFeCuN powders.
Table Captions
Table 1 The crystallographic parameters of the binary compounds in the Sm-Fe and
Sm-Cu systems.
Table 2 The equilibrium phases and their chemical compositions in
(Sm10.5Fe89.5)90(Sm100-xCux)10 (x= 30, 56, 70) alloys equilibrated at 450 °C.
Table 3. Magnetic properties for the aligned SmFeCuN and SmFeN powder samples.
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