The phase transition of ZrP induced by Si in Al-Si melts

Article abstract of DOI:10.1016/j.jallcom.2012.03.025

In this article, the reaction process of ZrP in Al-Si melts was investigated, and by the results of XRD, EDS and FESEM, the AlP and (Al, Zr, Si) phases were observed, indicating that the phase transition of ZrP phase has carried out in Al melts induced by Si. Detailed investigation confirmed that the finer particle of (Al, Zr, Si) phase is the type of ZrAl₃ doped with trace Si and the coarser block is the type of ZrSi₂ doped with trace Al. Furthermore, it is found that when the ZrP phase was added into Al-Si melts, Si atoms accumulated around ZrP phase, forming Si deficiency-layer and Si enrichment-layer. The concentration gradient of Si leads to two types of transition process of the ZrP phase in Al melt. Finally, the reaction mechanism of ZrP with Al-Si alloys was analyzed.

Full text of DOI:10.1016/j.jallcom.2012.03.025

Journal of Alloys and Compounds 528 (2012) 45–50
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
The phase transition of ZrP induced by Si in Al–Si melts
Guoju Bao, Dakui Li, Jinfeng Nie, Xiangfa Liu∗
Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jingshi Road 17923, Jinan 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this article, the reaction process of ZrP in Al–Si melts was investigated, and by the results of XRD, EDS
and FESEM, the AlP and (Al, Zr, Si) phases were observed, indicating that the phase transition of ZrP phase
has carried out in Al melts induced by Si. Detailed investigation confirmed that the finer particle of (Al, Zr,
Si) phase is the type of ZrAl3 doped with trace Si and the coarser block is the type of ZrSi2 doped with trace
Al. Furthermore, it is found that when the ZrP phase was added into Al–Si melts, Si atoms accumulated
around ZrP phase, forming Si deficiency-layer and Si enrichment-layer. The concentration gradient of Si
leads to two types of transition process of the ZrP phase in Al melt. Finally, the reaction mechanism of
ZrP with Al–Si alloys was analyzed.
Received 3 January 2012
Received in revised form 3 March 2012
Accepted 5 March 2012
Available online 15 March 2012
Keywords:
ZrP
Al–Si alloy
AlP
(Al
Zr
© 2012 Elsevier B.V. All rights reserved.
Si)
1. Introduction
on near eutectic multicomponent alloys in our previous work [10].
It points out that when Si atom was introduced into the Al–Zr–P
Considerable efforts have been devoted to the development of
near eutectic and hypereutectic Al–Si alloys, due to their excel-
lent wear and corrosion resistance, lower density, higher thermal
stability and outstanding mechanical properties [1–3]. However,
the primary Si in the unmodified near eutectic and hypereutec-
tic Al–Si alloys is usually very coarse and imparts poor properties
to these alloys. Therefore, the primary silicon must be refined and
modified to ensure adequate tensile strength and ductility [4,5].
Many researches have focused on the improvement of refiners to
meet the requirements of environmental protection and industry
applications.
It is generally recognized that primary silicon can be effectively
modified by AlP particles [6,7]. Hence, adding phosphorus is of vital
importance in most casting technologies of Al–Si alloys. At present,
series of Al–P master alloys have been widely applied due to their
stable modification effect without composition contamination [6].
They have higher refining efficiency than other master alloys due
to the fact that AlP particles which can heterogeneously nucle-
ate primary silicon have formed in advance in the master alloys
rather than formed in Al–Si melt through reaction [6,8]. Zuo et al.
have found that the Al–6Zr–2P master alloy with ZrP phase could
refine the hypereutectic A390 alloy effectively [9] and it was con-
firmed that Al–8Zr–2P master alloy has a good modification effect
system, ZrP would react with Si clusters and Al melt to form AlP
and Zr would combine with Si to form ZrSi and ZrSi2 phases. How-
ever, more details about the reaction process and the effect of Si
atom on the reaction process have not been revealed in the refer-
ence, which would have much significance on the control of ZrP
transition process and the refinement effect on Al–Si alloys.
In this article, Al–8Zr–2P master alloy was added into Al–13Si
alloys to observe the reaction process. The initial and final reac-
tion stages were investigated by controlling the reaction time.
Meanwhile, the reaction products with varying morphologies were
confirmed and the effect of Si atom on the reaction mechanism was
revealed.
2. Experimental procedures
The Al–8Zr–2P master alloys were prepared in a high frequency induction fur-
nace with commercial purity Al (99.85%), 99.6% purity Zr and binary Al–3.5P master
alloy supplied by Shandong Al & Mg Melt Technology Co. Ltd (all composition quoted
in this work are in wt.% unless otherwise stated). Firstly, the Al–3.5P master alloy
was re-melted, and then Zr was added into the melt. After holding for about 15 min
above 1500 ◦C, the melt was poured into a “U” type permanent mold to obtain
Al–8Zr–2P specimen. The temperature was measured using the infrared radiation
thermometers.
The Al–13Si alloy ingots were prepared in a clay-bonded graphite crucible
heated by 25 kW medium frequency induction furnace, using 99.85% commercial
purity Al, 98.5% commercial purity crystalline Si. Then the ingots were heated to a
temperature of 780 ◦C in an electric resistance furnace. The temperature was mea-
sured using a digital chromel–alumel thermocouple. Finally, 32% Al–8Zr–2P master
alloys were added into Al–13Si alloys. After the alloys were melted, part of the melt
∗
Corresponding author. Tel.: +86 531 88392006; fax: +86 531 88395414.
E-mail address: xfliu@sdu.edu.cn (X. Liu).
was poured into a permanent mould (70 mm × 35 mm × 20 mm) preheated to 150 ◦
C
0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2012.03.025

46
G. Bao et al. / Journal of Alloys and Compounds 528 (2012) 45–50
Fig. 1. FESEM image and TEM analysis of ZrP: (a and b) FESEM images; (c and d) TEM analysis.
immediately (marked as M1). And the remained was casted in the same mould after
holding for 30 min (marked as M2).
diffuses gradually to other regions (revealed in Fig. 2b). After hold-
ing for 30 min, the reaction has finished and the products with
varies of morphologies and sizes gathering around the correspond-
ing site of the original ZrP particle as illustrated in Fig. 2c. The
gray phase exhibits two kinds of morphologies, block and spher-
ical. Fig. 2d is a magnification image of the marked part in Fig. 2c.
It’s worth noting that the spherical phase contains a kind of black
particles inside, and compared with the block phase, the average
size decreases to a large extent.
Extraction technology was used to investigate the three-dimensional mor-
phology of ZrP phase in Al–8Zr–2P master alloy. Specimens for metallographic
microstructure observation were cut from the same position of Al–Si alloys added
with Al–8Zr–2P. Metallographic specimens were polished in the usual manner and
final polishing was carried out with fine magnesia powder by hand. The microstruc-
tures were observed by using transmission electron microscope (TEM, JEM-2100),
scanning electron microscope (SEM, JSM-6380LA) and field emission scanning elec-
tron microscope (FESEM, SU-70, Japan). X-ray diffraction (XRD, Rigaku D/max-rB)
was carried out to identify the phase constitution of Al–8Zr–2P master alloy.
Fig. 3 shows the XRD patterns and microanalysis of M1 alloy.
It indicates that Al, AlP (Si) and remained ZrP phases are obtained
in the microstructure in Fig. 3a. The AlP phase cannot be identified
by the method of XRD analysis in the presence of Si due to the
fact that they have the identical diffraction peaks, and therefore
it need to be confirmed by further analysis. From Fig. 3b, it can
be seen that the reaction has generated and the reaction products
distribute around the boundary of the remained ZrP phase with
two layers: finer particles of the inner layer adhering to ZrP phase
and large blocky phase of the outer layer. Both the large block and
the finer particle phase contain Al, Zr and Si elements. However,
the composition varies with the morphologies. The blocky phase
contains 7.2 at.% Al, 34.5 at.% Zr and 58.3 at.% Si while the finer one
contains 61.8 at.% Al, 27.8 at.% Zr and 10.4 at.% Si. The phase cannot
be identified by XRD due to the low content at the initial stage of
the reaction. Besides, a kind of black phase was also found on the
matrix and inside of the finer particles at the initial reaction stage as
shown in Fig. 3c and d, which probably thought to be the oxidative
AlP phase.
3. Results and discussion
3.1. Morphology of ZrP phase in Al–8Zr–2P master alloy
The ZrP phase has two kinds of crystal structures: dense-
hexagonal (h) and face-center (c). It is reported that the structure
of ZrP will change from dense-hexagonal crystal into face-center-
cubic structure above 1425 ◦C [11]. Fig. 1 shows the FESEM images
and TEM analysis for the ZrP phase in Al–8Zr–2P master alloy pre-
pared above 1500 ◦C. As shown in Fig. 1a and b, the phase exhibits
regular blocky morphology with about 4 ␮m in two dimension
and presents a cubic three-dimensional morphology. EDS analysis
showed that it contained Zr and P element and can be deduced to
be cubic ZrP. Fig. 1c and d shows the TEM analysis of ZrP phase
extracted from the Al–8Zr–2P master alloy. The corresponding
selected-area electron diffraction (SAED) pattern of face-centered
cubic ZrP [0 0 1] zone axis is shown in Fig. 1d.
Fig. 4a–g shows the XRD and EDS analysis of M2 alloy. Compared
to Fig. 3a, it indicates that besides Al and AlP (Si) remained, the
ZrP diffraction peaks disappeared while a new kind of (Al, Zr, Si)
phase generated. Therefore, it can be deduced that the reaction of
ZrP with Al–Si alloy has completed after holding for 30 min. The
block phase contains 6.9 at.% Al, 33.2 at.% Zr and 59.9 at.% Si, while
the finer particle contains 70.1 at.% Al, 17.9 at.% Zr and 12.0 at.% Si
as shown in Fig. 4b, c and d, e, respectively. It can be found that
the compositions were similar with those finer and block particles
3.2. Reaction between ZrP phase with Al–Si alloys
The FESEM images of ZrP phase, together with M1 and M2 spec-
imens are shown in Fig. 2. In comparison with the microstructure
of ZrP phase in Al–8Zr–2P master alloy as shown in Fig. 2a, the
reaction can be observed to generate in M1 and M2 alloys as illus-
trated in Fig. 2b–d. The reaction of ZrP particle with Al–Si alloy
begins from the boundary as well as the interior with infects, then

G. Bao et al. / Journal of Alloys and Compounds 528 (2012) 45–50
47
Fig. 2. Microstructure analysis at different reaction stages: (a) ZrP phase in Al–8Zr–2P master alloy; (b) microstructure of M1 alloy; (c and d) microstructure of M2 alloy.
Fig. 3. XRD patterns and microanalysis of M1 alloy: (a) XRD patterns; (b–d) microanalysis.

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G. Bao et al. / Journal of Alloys and Compounds 528 (2012) 45–50
Fig. 4. XRD, FESEM images and EDS analysis of M2 alloys: (a) XRD patterns; (b–e) FESEM image and EDS analysis of (Al, Zr, Si) phase; (f and g) microanalysis of AlP phase.
in M1 alloy. So it is obvious that both of the block and the finer
particles in M1 and M2 alloys are considered as (Al, Zr, Si) phase in
XRD analysis though different in composition, morphologies and
sizes.
Just as mentioned above, there exist a kind of oxygen-rich com-
pound on the matrix and inside of the finer (Al, Zr, Si) phase. When
the reaction has finished, the compounds were found at the cor-
responding sites as shown in Fig. 4f and g. Detailed investigation
found that the oxygen-rich compound contains Al and P besides
O element. It has been demonstrated that the distribution of oxy-
gen is likely to correspond with the phosphorus distribution, which
appears to be a rapid-oxidation event of the P-rich particle [12].
Therefore, it is reasonable to deduce that the black intermetallic
compound on the matrix and inside of the finer (Al, Zr, Si) phase
is AlP phase. However, it is worth noting that the size of the AlP
phase inside of the finer (Al, Zr, Si) phase is much smaller than that

G. Bao et al. / Journal of Alloys and Compounds 528 (2012) 45–50
49
Fig. 5. Microanalysis of element distribution along ZrP phase: (a) SEI image; (b–e) line-scanning analysis.
on the matrix. Through the analysis of the reaction products at the
initial and final reaction stage, it can be confirmed that the phase
transition of ZrP phase has carried out in Al melts induced by Si
when the Al–8Zr–2P master alloy was added into Al–13Si alloys.
The reaction can be described as Eq. (1).
just as it was easy to replace Al by Si to obtain DO22 (ꢀ1), it is also
feasible to have Si replaced by Al to form the superstructure of ZrSi2
phase. In our work, it can be noted that the stoichiometry is close
to ZrSi2 doped with trace Al from the composition listed in Table 1.
Therefore, it can be reasonable to deduce that the (Al, Zr, Si) phase
with finer grain and large block morphologies was formed by the
mechanism of Al replaced by Si in ZrAl3 phase and Si replaced by
ZrP(s) + [Si] + Al(l) → (Al, Zr, Si)(s) + AlP(s)
(1)
Up to now, there are few reports on the (Al, Zr, Si) phase and it
is still not clear about the atomic ratio of it. The composition of the
(Al, Zr, Si) phase with two different morphologies is summarized in
Table 1. According to the atomic ratio of the finer (Al, Zr, Si) phase
illustrated in Table 1, the chemical formula is close to Al5SiZr2 stoi-
chiometry, which is referred to as the ꢀ1 phase reported by Raman
and Schubert [13]. The ꢀ1 phase has been observed to appear in
isothermal sections at 700 and 1200 ◦C, around 7.5–10 at.% Si and
25 at.% Zr (with Zr(Al1−ꢁSiꢁ)3, 0.1 ≤ ꢁ ≤ 0.14) [14,15]. Furthermore,
it confirmed that the Al5SiZr2 phase was Si substituted ZrAl3 parti-
cles by EDAX analysis [16]. Besides, the author also suggested that,
Table 1
Analysis of element composition of (Al, Zr, Si) phase.
Phase
Element content (at. %)
Doping type
Morphologies
Al
Zr
Si
59.4 29.3
60.7 26.9
61.0 26.9
11.3
12.3
12.1
59.9
59.7
59.2
ZrAl3 doped with
trace Si
Fine grain
Block
(Al, Zr, Si)
6.9
6.4
6.6
33.2
34.0
34.3
ZrSi2 doped with
trace Al

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G. Bao et al. / Journal of Alloys and Compounds 528 (2012) 45–50
Al in ZrSi2 phase, respectively. However, in the XRD patterns, no
apparent ZrAl3 and ZrSi2 diffraction peaks appeared, which prob-
ably due to the diffraction peaks migrate. The generation of (Al,
Zr, Si) phase with two kinds of doping way probably predicts the
variation in transition process of ZrP phase.
melt to form AlP phase. The reaction process including three steps
was listed in Eqs. (4)–(6).
[Zr] + [Si] → ZrSi2
ZrSi2 + Al(l) → (Al, Zr, Si)
[P] + Al(l) → AlP
(4)
(5)
(6)
3.3. Mechanism of the reaction
The ZrSi2 phase doped with trace Al grows rapidly once formed due
to the fact that it inclines to grow to be larger size [17]. When the
holding time was prolonged to 30 min at the same temperature,
the reaction has completed and the reaction products aggregated
on the matrix at the corresponding sites of the original ZrP due to
incomplete diffusion.
When the Al–8Zr–2P master alloy was added into Al–13Si alloys,
the Si atom was introduced into Al–Zr–P system, so it can be
deduced that the Si atom probably has effect on the transition pro-
cess of ZrP phase. Fig. 5 illustrates the element distribution along
ZrP phase. It can be seen from Fig. 5b that the Si element infil-
trate into the ZrP phase and generate a concentration gradient along
the ZrP phase, namely Si deficiency-layer and Si enrichment-layer.
It is known that the difference in chemical potential among vari-
ous components is the driving force of every physical process. The
atoms would transfer from the phase with higher chemical poten-
tial to the one with lower chemical potential due to the interaction
between homogeneous and heterogeneous atom groups and the
injection of entropy of mixing. When ZrP was added into the melt,
the potential of Zr and P in ZrP compound was far higher than that
in the melt. Besides, the infiltration of Si atoms into ZrP phase has
an improvement effect on the instability of ZrP. Therefore ZrP dis-
sociated because of the thermal motion of the melt, thus signifying
4. Conclusions
(1) The reaction products of ZrP with Al–Si alloy were AlP and (Al,
Zr, Si) phase. The finer grain and the coarser block of (Al, Zr,
Si) phase were the types of ZrAl3 doped with trace Si and ZrSi2
doped with trace Al, respectively.
(2) The infiltration of Si into ZrP phase had an improvement effect
on the dissociation of ZrP and the concentration gradient of Si
leads to two types of the ZrP transition process which confirmed
by the (Al, Zr, Si) phase with two kinds of doping way.
Si
Acknowledgments
that a reaction ZrP(s)−→ [Zr] + [P] took place and then forming a
dissolved zone around the ZrP phase. Subsequently, the Zr and P
atom diffused to the melt. Meanwhile, Si atoms diffused to the
ZrP phase according to the positive concentration gradient, forming
Si deficiency-layer and Si enrichment-layer around the ZrP phase
(shown in Fig. 5b). This concentration gradient of Si leads to two
types of the ZrP transition process.
The authors gratefully acknowledge the support of the National
Natural Science Foundation of China under Projects 51071097 and
51001065, “Taishan Scholar” Programs Foundation of Shandong
Province in China and Independent Innovation Foundation of Shan-
dong University (2010TS081).
At the first type of transition, the Zr and P atoms diffused to
the Si deficiency-layer and combined with Al to form ZrAl3 and AlP
phase. Then the ZrAl3 phase doped with trace Si to form (Al, Zr, Si)
phase. The reaction process including two steps was listed in Eqs.
(2) and (3).
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[Zr] + [P] + Al(l) → ZrAl3 + AlP
ZrAl3 + [Si] → (Al, Zr, Si)
(2)
(3)
The ZrAl3 phase doped with trace Si nucleated heterogeneously on
parts of the AlP phase. Thus the growth of AlP particles inside of
the finer (Al, Zr, Si) phase was limited and compared with the AlP
phase on the matrix, the size is much smaller just as mentioned
above. Besides, the ZrAl3 phase doped with trace Si exhibits regu-
lar morphology and small size due to the fact that large amounts
of particles formed and diffused slowly, both of which limited its
growth rate.
The Zr and P atom diffused outside continually, then to the Si
enrichment-layer and carried out the second type of transition. The
Zr combine with Si to form ZrSi2 phase then doped with trace Al
forming (Al, Zr, Si) phase, while the P atom combine with Al in the