Phase relations in the Nd-Al-Si system at 500°C

Article abstract of DOI:10.1016/S0925-8388(01)01381-0

The partial isothermal section of the phase diagram of the Nd-Al-Si ternary system at 500°C (50 at.% Nd or less) has been investigated by X-ray diffraction analysis, differential thermal analysis, scanning electron microscopy and electron micro-probe analysis. The existence of six binary compounds, αNd₂Si₃, βNdSi_x (x≤1.8≤2.0), NdAl, NdAl₂, NdAl₃ and αNd₃Al₁₁, and three ternary compounds, NdAl₂Si₂ (γ), NdAl_1.75Si_0.25 (δ), NdAl_1.25Si_0.25 (η), was confirmed. The ternary compound NdAl_1.5Si_0.5 was not found in this section. This isothermal section consists of 11 single-phase regions, 21 two-phase regions and 11 three-phase regions. The compound NdAl_1.25Si_0.25 has a large homogeneity range, while the other ternary compounds are non-solubility compounds.

Full text of DOI:10.1016/S0925-8388(01)01381-0

Journal of Alloys and Compounds 325 (2001) 190–193
L
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Phase relations in the Nd–Al–Si system at 5008C
Long Zhilina , , Zhou Yichun , Zhuang Yinghong , Chen Rongzhen , Liu Jingqi , Wang Xianyou
a
b
b
b
a
*
aInstitute of Fundamental Mechanics and Material Engineering, Physics Department, Xiangtan University, Xiangtan, Hunan, 411105, PR China
bThe Industrial Testing and Experimental Centre, Guangxi University, Nanning, Guangxi, 530004, PR China
Received 28 February 2001; accepted 30 March 2001
Abstract
The partial isothermal section of the phase diagram of the Nd–Al–Si ternary system at 5008C (50 at.% Nd or less) has been
investigated by X-ray diffraction analysis, differential thermal analysis, scanning electron microscopy and electron micro-probe analysis.
The existence of six binary compounds, aNd2Si3, bNdSix (x < 1.8 < 2.0), NdAl, NdAl2, NdAl3 and aNd3Al11, and three ternary
compounds, NdAl2Si2 (g ), NdAl1.75Si0.25 (d ), NdAl1.25Si0.25 (h), was confirmed. The ternary compound NdAl1.5Si0.5 was not found in
this section. This isothermal section consists of 11 single-phase regions, 21 two-phase regions and 11 three-phase regions. The compound
NdAl1.25Si0.25 has a large homogeneity range, while the other ternary compounds are non-solubility compounds.
Science B.V. All rights reserved.
2001 Elsevier
Keywords: Rare earth alloys; X-ray diffraction; Isothermal analysis; Phase diagram
1. Introduction
researchers [13–15]. There are six intermetallic com-
pounds, Nd3Al, Nd2Al, NdAl, NdAl2, NdAl3 and NdAl4,
in this system. According to Buschow et al. [15], the
Al-rich compound NdAl4 has been revised as Nd3Al11,
which forms peritectically at 12358C from the melt and
NdAl2 and exists in two allotropic modifications,
aNd3Al11 and bNd3Al11. aNd3Al11 has an orthorhomic
structure, whereas bNd3Al11 has the tetragonal Al-de-
ficient Al4Ba-type structure. The allotropic transformation
occurs at about 9508C (aNd3Al11↔bNd3Al11). Crystal
structures and lattice constants of all Nd–Al phases can be
found in Ref. [16].
The phase diagram and the compounds of the binary
Nd–Si system were reported in Ref. [17]. Six compounds,
Nd5Si3, Nd5Si4, NdSi, Nd3Si4, Nd2Si3 and NdSix exist in
this system. For the Nd–Al–Si ternary system, the ternary
compounds, NdAl1.25Si0.75, NdAl1.5Si0.5, NdAl1.75Si0.25
and NdAl2Si2, in ternary Nd–Al–Si system have been
given by Gschneidner et al. [18].
Al–Si alloys have light weight, low thermal expansion
and good resistance to wear. They are important alloys and
widely used in industries. The improvement of the prop-
erties of these alloys by modifying their microstructure is a
topic for the scientists. The addition of rare elements to
these alloys plays an important role for this purpose and
has been extensively studied by many researchers [1–5]. A
phase diagram can provide useful information to study the
mechanism of the modification of rare earths on Al–Si
alloys. The phase diagrams of the Al–Si–Y [6], Al–Si–Gd
[7] and Al–Si–Ho [8] ternary systems have been reported.
However, the phase diagram of the Nd–Al–Si ternary
system has not been reported. The purpose of the present
investigation is to determine the partial isothermal section
of the phase diagram of the termary system Nd–Al–Si at
5008C (50 at.% Nd or less).
Al–Si is a simple eutectic system [9–12]. No binary
compound exists in this system. The solubility of Si in Al
is 0.8 wt.% (or 0.77 at.%). For the solubility of Al in Si,
Hansen [9] reported that no solubility was found at 5008C,
while Elliott [10] reported that it should be 0.4 at.% at
5778C (eutectic).
2. Experimental details
All alloy samples (260 in total) with a mass of 3 g for
each sample were prepared by high frequency induction
melting or arc melting the appropriate amounts of starting
materials in alumina crucibles under an atmosphere of
ultrapure argon gas. The starting materials for the synthesis
The Nd–Al binary system has been studied by several
*
Corresponding author.
E-mail address: longzhilin@163.net (L. Zhilin).
0925-8388/01/$ – see front matter
PII: S0925-8388(01)01381-0
2001 Elsevier Science B.V. All rights reserved.

L. Zhilin et al. / Journal of Alloys and Compounds 325 (2001) 190–193
191
of the alloys were from commercially available metals of
high purity (neodymium, 99.9%; alumium, 99.99%;
silicon, 99.999%). Excesses of 2% Nd were added to
compensate for mass loss due to the evaporation of the rare
earth elements during melting. To ensure homogeneity, the
sample was remelted three times. The weight losses during
melting were less than 2 wt.% for a total 260 specimens.
The samples were sealed in quartz capsules in vacuum
and homogenized. The temperature for homogenization
was chosen on the basis of the three binary phase diagrams
and the results of differential thermal analysis (DTA) of
some typical ternary alloy samples. The alloy samples
containing more than 20 at.% Nd were annealed at 7508C
for 90 days, those containing 20–10 at.% Nd at 6008C for
110 days and those containing less than 10 at.% Nd at
5208C for 140 days. After homogenization treatment, all
samples were cooled down to 5008C at a rate of about
38C/h and kept there for 7 days, then quenched into
ice–water. The samples for X-ray diffraction analysis were
powdered by hammering the brittle ingots or chiseling the
ductile ingots. The powder was annealed at 5008C658C
for 10 days and subsequently quenched into liquid nitro-
gen.
Fig. 1. The microstructure of the Nd-33.3 at.% Si-16.7% Al alloy located
in the two-phase region (d 1h). Magnification 6003. h (lip), d (sub-
strate).
The phase identification of the samples was carried out
by X-ray power diffraction, using a Rigaku 3015 diffrac-
tometer, Cu-radiation and Ni-filters. High-purity Si powder
was used as an internal standard for measurements of the
lattice parameter. By comparing and analyzing the X-ray
diffraction patterns of the samples annealed for different
periods of time, it was shown that the equilibrium state of
the sample at 5008C was retained by the above treatment.
The diffraction lines were coherent and sharp. Second
phases were easily detected when the compositions of the
alloys deviated from the phase boundary by 1 at.%. Thus
the phase diagram can be determined by X-ray diffraction.
The phase boundaries were determined by X-ray diffrac-
tion analysis and were checked by electron probe micro-
analysis and scanning electron microscopy. Metallographic
samples were etched by using solution of HNO3 1HCl1
H3PO4 1HAc mixture. These are given in the figure
captions.
5008C by XRD analysis. The single-phase samples were
obtained at the respective nominal composition. XRD also
shows that the compound Nd2Si3 exists with its low
temperature form (aNd2Si3) while NdSix is in its high
temperature phase (bNdSix). The existence of NdAl1.5Si0.5
reported in Ref. [18] could not be confirmed in this work.
In our experiment, the XRD pattern for the sample with
3. Result and discussion
3.1. Phase analysis
The results obtained from S.E.M. examination of the
samples in the two-phase and three-phase regions agree
with those obtained by X-ray diffraction analysis. The
microstructure of some samples are shown in Figs. 1 and
2.
The existence of six binary compounds, Nd2Si3, NdSix
(x<1.8<2.0), NdAl, NdAl2, NdAl3 and Nd3Al11, and
Fig. 2. The microstructure of the Nd-6 at.% Si-50 at.% Al alloy located
in the three region (Al1Si1g ). Magnification 1003. Si (black, lip), g
(white), Al1Si (substrate).
three ternary compounds, NdAl2Si2, NdAl1.75Si0.25
,
NdAl1.25Si0.75, were confirmed in this isothermal section at

192
L. Zhilin et al. / Journal of Alloys and Compounds 325 (2001) 190–193
Fig. 3. The partial isothermal section (5008C) of the phase diagram of the Nd–Al–Si (50 at.% Nd or less) ternary system: d, single-phase region; s,
two-phase region; s, three-phase region; d(Nd Al1.75Si0.25), g(NdAl2Si2 ), h(NdAl11ySi12y 0<y<0.4).
composition of NdAl1.5Si0.5 shows
a
mixture of
bNdSix (1.8<x<2.0). aNd2Si3, NdAl1.75Si0.25 (d ),
NdAl2Si2 (g ) and NdAl11ySi12y (h0 < x < 0.4). The two-
phase regions are NdAl1NdAl2, NdAl2 1NdAl3, NdAl3 1
aNd3Al11, aNd3Al11 1Al, Al1Si, Si1bNdSix, bNdSix 1
aNd2Si3, d 1NdAl, d 1NdAl2, d 1NdAl3, d 1h, g 1Al,
g 1Si, g 1bNdSix, g 1h, h1NdAl, h1NdAl3, h1
aNd3Al11, h1Al, h1bNdSix and h1aNd2Si3. The three-
phase regions are NdAl1NdAl2 1d, NdAl1d 1h,
NdAl2 1NdAl3 1d, d 1NdAl3 1h, NdAl3 1aNd3Al11 1h,
h1aNd3Al11 1Al, Al1g 1h, Al1Si1g, Si1bNdSix 1g,
bNdSix 1g 1h and bNdSix 1aNd2Si3 1h.
NdAl1.75Si0.25 and NdAl1.25Si0.75 phases. Therefore, this
sample consists of two phases rather than a single phase.
This is also confirmed by S.E.M. observation (as shown in
Fig. 1).
3.2. Solid solubility
The range of solid solubility for each phase at 5008C has
been determined by XRD, viz. from the appearance of the
reflections of the adjacent phase as well as from the
variation of the lattice parameters with composition,
together with S.E.M. observation and electron micro-probe
analysis. The solid solubility of Si in Al determined by
lattice-parameter measurements is 0.82 at.%, however,
there is not any detectable solid solubility of Al in Si at
5008C. The results are convincing due to the high-purity Al
and Si used in the investigation and the long enough
annealing time to reach phase equilibrium. The solubilities
of Nd in Al and Si are not detectable by X-ray diffraction.
The compound bNdSix has a homogeneity range extending
from about 64 to 67 at.% Si. The compound NdAl1.25Si0.75
has a rather wide composition range and its diffraction
pattern shifted rather continuously as the composition is
changed. It may be expressed as NdAl11ySi12y, with
0 < y < 0.4.
Acknowledgements
This work was supported by the National Natural
Science Foundation of China under Contract No.
10072052.
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