Microstructural evolutions of Pr13Fe80B7 alloys during solid HDDR process

Article abstract of DOI:10.1016/j.jmmm.2006.10.861

Microstructural evolutions of Pr₁₃Fe₈₀B₇ alloys during solid hydrogenation disproportionation desorption recombination (HDDR) process is systematically investigated. The results show that the early-disproportionated products of Pr₁₃Fe₈₀B₇ alloys are mainly characteristic of rod-like morphology, and the rods are PrH₂ while the matrix is Fe. Moreover, all the PrH₂ rods have the same crystallographic orientation, and grow with a definite orientation related to the Fe matrix. However, it is notable that no iron boride phase except for NdH₂ and Fe is found. With the prolonged disproportionation time, the rod-like disproportionated products coarsen, and the Fe₂B come to form. When the disproportionation time is 17 h, the rod-like disproportionation morphology transforms into sphere, and a large amount of Fe₂B is found. Subsequent investigations for the recombination show that the recombination reactions start at the boundaries between PrH₂ rods and Fe matrix, and the rim-like Pr₂Fe₁₄B is formed on the PrH₂ rods. Moreover, the recombined PrFeB powder of the rod-like microstructure has strong magnetic anisotropy.

Full text of DOI:10.1016/j.jmmm.2006.10.861

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 312 (2007) 337–341
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Microstructural evolutions of Pr Fe B alloys during solid
80 7
13
HDDR process
Ã
Shunquan Liu, Jingzhi Han , Honglin Du, Changsheng Wang,
Haiying chen, Yingchang Yang
School of Physics, Peking University, Beijing 100871, PR China
Received 15 June 2006; received in revised form 12 October 2006
Available online 27 November 2006
Abstract
Microstructural evolutions of Pr13Fe80
process is systematically investigated. The results show that the early-disproportionated products of Pr13Fe80
characteristic of rod-like morphology, and the rods are PrH while the matrix is Fe. Moreover, all the PrH rods have the same
B
7
alloys during solid hydrogenation disproportionation desorption recombination (HDDR)
B
7
alloys are mainly
2
2
crystallographic orientation, and grow with a definite orientation related to the Fe matrix. However, it is notable that no iron boride
phase except for NdH and Fe is found. With the prolonged disproportionation time, the rod-like disproportionated products coarsen,
and the Fe B come to form. When the disproportionation time is 17 h, the rod-like disproportionation morphology transforms into
2
2
sphere, and a large amount of Fe
start at the boundaries between PrH
2
B is found. Subsequent investigations for the recombination show that the recombination reactions
rods and Fe matrix, and the rim-like Pr Fe14B is formed on the PrH rods. Moreover, the
2
2
2
recombined PrFeB powder of the rod-like microstructure has strong magnetic anisotropy.
r 2006 Elsevier B.V. All rights reserved.
PACS: 75.50; 75.30.G; 81.40.R; 81.40
Keywords: Microstructure; Ternary Pr–Fe–B; HDDR
1
. Introduction
but Gutfleisch and Honkura suggest that the well-
aligned Fe B phase existing in the rod-like disproportio-
2
The hydrogenation disproportionation desorption re-
nated morphology, might be related to the origin of
anisotropy [6].
Pr Fe B-type alloys are comparable in terms of their
combination (HDDR) process is well known as an effective
method for producing anisotropic Nd–Fe–B magnetic
powders [1]. However, the mechanism on the inducement
of anisotropy in NdFeB alloys during the HDDR process
is unclear till now. At present, it is commonly accepted that
the formation of anisotropy might be related to the rod-like
or lamella diproportionated products, but how this
disproportionated products lead to the formation of
anisotropy is in dispute [2–6]. For example, Yi advances
that a well-defined crystallographic relation between both
the new intermediate magnetic and lamella phases
and between the tetragonal NdFe-containing phase and
Fe is very important for the formation of anisotropy [3],
2
14
intrinsic magnetic properties and phase relations[7] with
the advantage of a much lower spin reorientation
temperature and possibly a different grain growth behavior
[8], so HDDR process is tried to fabricate anisotropic
Pr–Fe–B magnetic powders recently. At present, aniso-
tropic Pr–Fe–B-type magnetic powder has been obtained
by modifying HDDR process [9–13], but the investigating
results for the microstructure of the disproportionation
products of PrFeB alloys are different. Gutfleisch finds the
metastable Fe B phase in the rod-like disproportionated
3
products of Pr–Fe–B alloys again, but this metastable
phase is not observed by the present authors. In this paper,
microstructural change of ternary Pr Fe B alloys during
Ã
Corresponding author.
E-mail address: hanjingzhi@pku.edu.cn (J. Han).
13 80 7
HDDR process is systematically investigated. Since the
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.10.861

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S. Liu et al. / Journal of Magnetism and Magnetic Materials 312 (2007) 337–341
338
obtainment of anisotropy is found to be sensitive to the
disproportionation time [14], effects of the disproportiona-
tion time on the microstructures of Pr Fe B alloys are
1
3
80 7
mainly examined.
2
. Experimental procedure
The alloys with a nominal composition of Pr Fe B are
melted under an Ar atmosphere in a vacuum induction
1
3
80 7
furnace. The alloy ingots are homogenized at 1050 1C for
2
4 h and crushed into powders with a particle diameter size
of ꢀ0.2 mm. The above as-cast powders are heated in
hydrogen at 820 1C for 0.5–17 h with a partial pressure of
0
.1 MPa, followed by a slow desorption treatment in low
vacuum at 850 1C and then a quick desorption in high
vacuum. The microstructure of the orientation sample is
analyzed by X-ray diffraction (XRD). Microstructure
changes are investigated by transmission electron micro-
scopy (TEM) and high-resolution TEM.
3
. Results and discussion
Fig. 1 shows XRD patterns for the disproportionated
products of Pr Fe B alloy. It is seen that the dispro-
1
3
80 7
portionation rate of Pr Fe B alloy is very rapid. For
1
3
80 7
example, when the disproportionation time is only 5 min,
the diffraction peaks of original Pr Fe B alloy has nearly
disappeared. In addition, it seems that Fe B is not found in
1
3
80 7
2
the early disproportionation products, and come to appear
with the prolonged disproportionation time. This indicates
that the component of the early disproportiontation
products might differ from that of the final disproportio-
nated products. Whereas, it is notable that no other phase
except for PrH , Fe and Fe B are found by the present
Fig. 1. X-ray powder diffraction patterns for the disproportionated
products of Pr13Fe80 alloy. The powder specimens are obtained by
annealing in 0.1 MPa H for 5 min, 30 min, 3 h and 17 h at 800 1C, and
2
2
B
7
XRD examinations, and this differs from the Gutfleisch’s
observation for the disproportionated products [11].
Subsequently, microstructural changes of the dispropor-
tionation of Pr Fe B alloys are examined by TEM.
2
immediately quenched to room temperature (a) and (b) is the close-up of (a).
1
3
80 7
Fig. 2 shows the morphology of Pr Fe B alloys
1
3
80 7
disproportionated for 30 min at 820 1C. The results show
that the early disproportionation product is mainly a
characteristic of rod-like morphology, and the rod is the
PrH while the matrix is Fe. Moreover, all the PH rods are
Subsequently, the 3 and 17 h disproportionated products
are also investigated. The results show that with the
prolonged disproportionation time, the rod-like morphol-
ogy coarsens gradually, and Fe B come to appear. When
2
2
2
aligned parallel, and grow with a definite orientation
related to the Fe matrix (ð1 0 0Þ
the disproportionation time reaches 17 h, the early rod-like
morphology completely transforms into sphere (seen in
Fig. 4), and a large amount of Fe B is formed. In additions,
no crystallographic orientation relationship is found
among the spherical disproportionated products.
=ð1 0 0ÞaꢁFe, h1 1 0iPrH2
=
PrH2
h1 1 0i ), However, no Fe B is still found.
a
In order to further investigate whether the Fe B phase
ꢁFe
2
2
2
exists in the early disproportionated products, high-
resolution TEM is used. The results are shown in Fig. 3.
It is seen that the boundaries between PrH and Fe is very
In the end, recombined microstructure of Pr Fe B
80 7
1
3
alloys during HDDR process is investigated. Since the rod-
like disproportionation morphology is beneficial to the
formation of anisotropy [14], the recombination of this
morphology is mainly investigated. It is found that the
recombination reactions start at the boundaries between
PrH rods and Fe matrix, and the rim-like Pr Fe B is
2
clear, and no Fe B phase or other iron boride phase is
2
found. This indicates that the formation of Fe B is delayed
2
compared with that of PrH and Fe. Recently, we propose
2
that the atom B might be enriched in the PrH rods of the
2
rod-like disproportionated products, and this is reported
elsewhere [13]
2
2
14
formed on the PrH rods (seen in Fig. 5). In addition, no
2

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Fig. 2. TEM bright-field micrographs show the solid-disproportionation states of Pr13Fe80B alloys at 30 min disproportionation at 800 1C. The images (a)
and (b) are taken from different areas of the same powder specimen, the image (c) is the diffraction pattern taken from the matrix of the rod-like
disproportionated products, and the image (d) is the diffraction pattern of the rod-like disproportionated products.
crystallographic orientation relationship between the re-
combined grain and the disproportionated products is
found.
According to XRD observations for the magnetic
aligned sample of recombined Pr Fe B powders, it is
1
3
80 7
found that the intensity of the (0 0 6) diffraction peak is
very strong for the recombined Pr–Fe–B powders of the
rod-like disproportionated products while that is very weak
in the case of the recombined powders of spherical
disproportionated products, as seen in Fig. 6. This
indicates that the rod-like microstructure is essential to
obtain magnetic anisotropy while the spherical morphol-
ogy leads to isotropy.
4. Conclusions
The early-disproportionated product of ternary Pr-Fe-B
alloys is mainly characteristic of rod-like morphology.
Moreover, the rods are PrH while the matrix is Fe, and the
2
formation of Fe B is delayed compared with that of PrH
2
2
and Fe. When the disproportionation time is prolonged,
the rod-like disproportionated products coarsen, and
finally transform into spherical morphology with the
Fig. 3. High-resolution TEM micrograph for the rod-like disproportio-
nated products of Pr13Fe80 alloy.
B
7

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Fig. 4. TEM bright-field micrographs show the solid-disproportionation states of Pr13Fe80B alloys at 3 h (a) and 17 h (b) at 800 1C
Fig. 5. (a) TEM bright-field micrographs of the partially recombined state of the rod-like disproportionation morphology at 15 min desorption at 800 1C,
b) is the close-up of (a).
(
appearance of Fe B. The recombination reactions start at
2
the boundaries between PrH rods and Fe matrix, the rim-
2
like Pr Fe B is formed on the PrH rods, and the
2
14
2
recombined Pr–Fe–B powder of the rod-like microstruc-
ture has strong magnetic anisotropy
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