The isothermal section of the La-Ni-Nb ternary system at 673 K

Article abstract of DOI:10.1016/j.jallcom.2004.06.017

The isothermal section of the phase diagram of the ternary system La-Ni-Nb at 673 K has been investigated by X-ray diffraction, differential thermal analysis, optical microscopy, and electron microscopy techniques. It consists of 14 single-phase regions,

Full text of DOI:10.1016/j.jallcom.2004.06.017

Journal of Alloys and Compounds 386 (2005) 182–184
The isothermal section of the La–Ni–Nb ternary system at 673 K
Huaiying Zhoua,c,∗, Cuiyun Heb, Jingqi Liub, Songliu Yuana, Jialin Yanb, Junxia Lüb
a
Institute of Material Physics and Department of Physics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China
b
Key Laboratory of Nonferrous Metal Materials and New Processing Technology, Guangxi University, Ministry of Education,
Nanning, Guangxi 530004, PR China
Institute of Materials Science, Guilin University of Electronic Technology, Guilin, Guangxi 541004, PR China
c
Received 22 March 2004; received in revised form 4 June 2004; accepted 4 June 2004
Abstract
The isothermal section of the phase diagram of the ternary system La–Ni–Nb at 673 K has been investigated by X-ray diffraction, differential
thermal analysis, optical microscopy, and electron microscopy techniques. It consists of 14 single-phase regions, 25 two-phase regions, and
12 three-phases regions. At 673 K, the maximum solid solubilities of Nb in Ni, LaNi5, La2Ni7, LaNi3, La7Ni16, and La2Ni3 is about 3, 5, 2,
3, 3, and 2 at.% Nb, respectively. The composition range of the Nb in NbNi3 is about from 23.5 to 27 at.% Nb. No ternary compounds were
observed.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Phase diagram; X-ray diffraction; La–Ni–Nb system; Isothermal section
1. Introduction
La3Ni, La7Ni3, LaNi, La2Ni3, LaNi2 (or La7Ni16), LaNi3,
La2Ni7, and LaNi5 have been reported in the La–Ni bi-
nary system in [1–3]. There is a discrepancy between [1–3]
and [10–13] as regards the existence of LaNi2 or La7Ni16.
According to [10–13], La7Ni16 exists instead of LaNi2
and the La3Ni phase was not observed. The La–Nb binary
system phase diagram was reported in [4]. The existence
of any compound was not observed in the La–Nb binary
system.
The alloys of the La–Ni system are hydrogen storage ma-
terials. Now, the LaNi5 based alloys are being used as metal
hydride (MH) electrode for their excellent electrochemical
performance. Multi-component La–Ni alloys with a high
discharge capacity and long cycle lifetime have been widely
investigated.
The phase diagram is an important basis for materials
research and material application. The La–Ni phase dia-
gram is very useful for research in hydrogen storage alloys.
Some phase diagrams of La–Ni–M ternary system were
studied in [10–13], but the phase diagram of La–Ni–Nb
has not been published. In this paper, we will report on
the results of our investigation of the phase relationship in
the 673 K isothermal section of the La–Ni–Nb system. The
Ni–Nb binary phase diagram was reported in [6]. Three
compounds: NbNi8, NbNi3, and Nb7Ni6 were reported in
it. Three compounds: NiNb, ␨-NbNi4, and NbzNi were re-
ported in [5,8,9], two compounds: Nb8Ni92 and Nb15Ni2
were reported in [7]. Eight kinds of La–Ni compound:
2. Experimental details
The present investigation was carried out with 206 sam-
ples having masses of about 2 g. The purity of the starting
elements were 99.9% La, 99.99% Ni, and 99.99% Nb, re-
spectively. The samples were prepared by arc melting on a
water-cooled copper crucible with a non-consumable tung-
sten electrode under pure argon atmosphere. The samples
were re-melted three times for homogeneity. For these sam-
ples, the weight loss is less than 1% after melting. Therefore,
no chemical analyses were carried out.
After melting, the samples were sealed in an evacu-
ated quartz tube for homogenization annealing. The heat
treatment temperature was chosen according to the phase
∗
Corresponding author.
E-mail address: zhuang@gxu.edu.cn (H. Zhou).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.06.017

H. Zhou et al. / Journal of Alloys and Compounds 386 (2005) 182–184
183
diagram of the binary La–Ni and Nb–Ni system. The
samples in the La-rich part and the Ni-rich part were an-
nealed at 773 K for 720 h. The others were annealed at
1073 K for 480 h. Then, all these samples were cooled
down to 673 K at a rate of 0.15 K/min and kept at 673 K for
240 h. In the end, the samples were quenched into liquid
nitrogen.
All these homogenized alloy powders or buttons were in-
vestigated by X-ray diffraction, which was carried out on a
Rigaku D/Max 2500v powder diffractometer equipped with
Cu K␣ radiation and graphite monochromator which was
operated at 40 kV and 200 mA. The analysis software is
Jade5.0 [8] and the 2θ scan range was from 20 to 60◦.
Some typical alloys were analyzed in an S-570 scanning
electron microscope (SEM), by electron probe microanal-
ysis (EPMA) or differential thermal analysis (DTA). From
all these data, the phase relations in the La–Ni–Nb system
were determined.
Fig. 1. The isothermal section of the La–Ni–Nb ternary system at 673 K.
3. Results and discussion
3, 3, and 2 at.% Nb, respectively. The concentration of Nb in
NbNi3 ranges from 23.5 to 27 at.% Nb. The solid solubility
in the other single-phase regions is too small to be obser-
ved.
3.1. Binary system
3.1.1. La–Ni system
In our investigation, seven compounds: La7Ni3, LaNi,
La2Ni3, La7Ni16, LaNi3, La2Ni7, and LaNi5 were con-
firmed. We did not observe La3Ni. The XRD pattern of
the sample of “La3Ni” in our experiment consisted of the
patterns of La7Ni3 and La.
3.3. Isothermal sections
By comparing and analyzing the X-ray diffraction patterns
of 206 samples, the isothermal section of the La–Ni–Nb
ternary system at 673 K was determined, as shown in Fig. 1.
The 673 K isothermal section consists of 14 single-phase
regions, 25 two-phase regions, and 12 three-phase regions.
The 14 single-phase region are La, La7Ni3, LaNi, La2Ni3,
La7Ni16, LaNi3, La2Ni7, LaNi5, Ni, NbNi8, NbNi3, NbNi,
Nb15Ni2, and Nb. Details of the three-phase regions are
given in Table 1. No ternary compound was discovered in
the La–Ni–Nb system.
3.1.2. Ni–Nb system
Four kinds of Ni–Nb compounds: NbNi8, NbNi3, NbNi,
and Nb15Ni2 were confirmed in our experiment. The NbzNi
compound (z = 5
x, 96F, Fd3m, a = 1.1642 nm, NiTi2
structure type) was first reported in [5] and it was later
¯
expressed as Nb15Ni2 (cF96, Fd3m, a = 1.1642 nm, NiTi2
structure type) in [7]. The Nb15Ni2 compound was not
reported in the Nb–Ni binary phase diagram [6], but we
observed the pattern of Nb15Ni2 (NbzNi) for 88.2% Nb,
11.8% Ni, and several other corresponding regions in our
experiment. So we consider that the Nb15Ni2 compound
exist under our experimental conditions. Otherwise, we did
not observe the Nb8Ni92 and the ␨-NbNi4 compounds in
our investigation. The patterns of “␨-NbNi4” and several
alloys near “␨-NbNi4” consisted of NbNi8 and NbNi3.
Table 1
Three-phase region of the La–Ni–Nb ternary system
Region number
Phase 1
Phase 2
Phase 3
1
2
3
4
5
6
7
8
9
Ni
NbNi8
NbNi8
La2Ni7
La2Ni7
LaNi3
La2Ni3
La2Ni3
La2Ni3
La2Ni3
Nb15Ni2
La7Ni3
La7Ni3
LaNi5
LaNi5
LaNi5
LaNi3
La7Ni16
La7Ni16
NbNi
Nb15Ni2
Nb15Ni2
Nb
NbNi3
NbNi3
NbNi3
NbNi3
NbNi3
NbNi3
NbNi
LaNi
LaNi
LaNi
La
3.1.3. La–Nb system
By comparing and analyzing the X-ray diffraction patterns
of the samples in the La–Nb system, no binary compound
was observed.
3.2. Solid solubility
10
11
12
Nb
Nb
At 673 K, the maximum solid solubility of Nb in Ni,
LaNi5, La2Ni7, LaNi3, La7Ni16, and La2Ni3 is about 3, 5, 2,

184
H. Zhou et al. / Journal of Alloys and Compounds 386 (2005) 182–184
[4] H. Okamoto, Binary Alloy Phase Diagrams, second ed., ASM Inter-
Acknowledgements
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This work was jointly supported by the National Nat-
ural Science Foundation of China (authorized number:
50001002) and by the Natural Science Foundation of
Guangxi (authorized number: 0135005).
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