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Restraining effect of strontium on the crystallization of Mg2Si phaseduring solidification in Al/Si/Mg casting alloys and mechanisms
Liao Hengchenga,*, Sun Yu a,b, Sun Guoxionga
a Department of Mechanical Engineering, Southeast University, Si Pai Lou 2, Nanjing 210018, PR Chinab Department of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng 224002, PR China
Received 18 January 2002; received in revised form 24 March 2003
Abstract
The effect of strontium on the crystallization of Mg2Si phase in Al/11.6%Si/0.4%Mg alloys was investigated with optical
microscope and SEM. In the partially modified alloys, the Mg2Si phase grows as network or bamboo-shoot shape. However, very
few and fine Mg2Si particles are isolated at the boundaries of the eutectic cells in the fully modified alloys. Addition of strontium
restrains the crystallization of Mg2Si phase. The Mg2Si phase in Al/Si/Mg casting alloy is thought to nucleate on the surfaces of the
eutectic silicon flakes. This hypothesis was analyzed from the point of view of the lattice match between the Mg2Si and Si crystals.
The restraining effect of strontium is thought to be related to the combination of two effects of strontium, the increase of the amount
of dendritic a phase and the change of the surface characteristic of silicon crystal caused by the addition of strontium.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Al/Si/Mg alloy; Strontium modification; Mg2Si; Crystallization
1. Introduction
Development of modern industry requires products to
be lighter and more resistant. The Al/Si casting alloys
with excellent combination of properties have increas-
ingly wide applications [1]. Hypo-eutectic Al/Si casting
alloys, such as A356, due to excellent strength and
ductility, have been widely used in automotive and
aerospace industries, but the castability (involving the
fluidity, the contractibility and the tendency of shrink-
age micro-void, etc.) is poor compared with that of near-
eutectic Al/Si alloys. However, the applications of thenear-eutectic Al/Si alloys are limited now, because of its
lower strength and ductility, especially in some impor-
tant products. Investigation on improving the strength
and ductility of the near-eutectic Al/Si alloys is scarce
at present. So it is believed that the research and
development of these alloys with excellent properties
will attract increasing attention of the materials re-
searchers and the foundry men.
The structure refinement is one of the most important
methods for improving the strength and ductility of
alloys. For the near-eutectic Al/Si alloys, modifying
treatment during casting has been a basic practice.
Addition of sodium or strontium can cause a transition
of the eutectic silicon phase from coarse flakes to fine
fibers and consequently improve the mechanical proper-
ties, especially the ductility [2/7]. The recovery of
sodium in aluminum melts is poor, and it oxidizes
quickly and the modification effect fades fast too. It hasbeen reported that sodium has the problem of over-
modification. So controlling the modifying treatment
with sodium salt is thought to be difficult [3,6,8,9].
Besides these, sodium modification also results in
environment problems. Strontium has a good modifying
potency, just under sodium, and the modification effect
is capable of retaining a long period. The recovery of it
in aluminum melts is about 90% [5]. It is commonly
thought that strontium modification has not got the
problem of over-modification [3,6/9]. Apparently, the
modifying with strontium can be controlled more easily
than that with sodium salt. A growing body of research
* Corresponding author. Tel.: '/86-25-379-2456; fax: '/86-25-620-
5104.
E-mail address: [email protected] (L. Hengcheng).
Materials Science and Engineering A358 (2003) 164/170
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and studies investigating the modifying effect of stron-
tium, this microalloying addition has found wide
acceptance. The recent work of the authors indicate
that the addition of strontium in near-eutectic Al/Si
alloys results in a considerable increase of the amount of
dendritic a phase and promotes the columnar growth of
dendrites [1,10]. The dendritic a phase has an important
role in improving the mechanical properties of near-
eutectic Al/Si alloys [11]. It has not been reported up till
now whether or not the addition of strontium has an
influence on the crystallization of Mg2Si phase in Al/
Si/Mg casting alloys. As a part of the studies on
improving the strength and ductility of the near-eutectic
Al/Si alloys, it is quite necessary to highlight the effect
of strontium on Mg2Si phase.
2. Experimental procedures
The experimental alloys with a nominal composition
of Al/11.6%Si/0.4%Mg/0.15%Fe were melted in an
electrical resistance furnace using a graphite crucible
with 2.5 kg melt weight per heat. An Al /10%Sr master
alloy was added to the melt at (/730 8C with slight
stirring. After holding for 30 min, SW-RJ-1 flux was
introduced for degassing. Then, the melt was poured at
(/720 8C into a standard tensile sample mold (grey iron,
preheating at 200 8C). According to the recovery of
strontium in the melts measured by ICP Spectrometer,
the strontium content after fluxing are obtained as 0,
0.010, 0.015, 0.020, 0.025, 0.030 and 0.0375%. Metallo-
graphic samples were cut from the gage parts of the
tensile test rods (12 mm diameter, 60 mm gage length).
After etching in a Kellers reagent, microstructures were
observed and recorded using optical microscope
(OLYMPUS, BX-60M). SEM (JEOL-JSM6300 and
HITACHI X-650) was adopted for some deeply etched
samples. X-ray mapping was used to observe the
distributions of magnesium, silicon, strontium, iron
and/or aluminum in constituents.
3. Results
In Al/11.6%Si/0.4%Mg alloys, magnesium present
has no distinct influence on the modification effect of
strontium. The change of the morphology and size of
eutectic silicon phase with varying strontium content is
similar with that in Al/11.6%Si alloys [1]. Because of
the presence of magnesium, some microstructures,
which have not been observed in the alloys without
magnesium, have been found.
3.1. Mg2Si phase in the partially modified alloys
When the strontium content is below or equal to
0.015%, the eutectic silicon phase does not fully change
into fibrous, so these alloys are named the partially
modified alloys (exhibiting unmodified and modified Si
morphologies). When the strontium content is above orequal to 0.020%, all the eutectic silicon crystals exhibit
modified morphology as fine fibers. These alloys are
named the fully modified alloys. Fig. 1 shows the optical
photographs of the as-cast microstructures of Al/
11.6%Si/0.4%Mg alloys. Some coarse network struc-
tures (as arrowed) are observed, which are not typically
found in Al/11.6%Si alloys. In addition to the indivi-
dual network phase isolated in the a phase (as seen in
Fig. 1b), most networks have one or a few heads
connecting to the eutectic silicon flakes. The SEM
photographs clearly show that these networks (as
arrowed in Fig. 2) have a close connection with eutecticsilicon flakes. EDAX result (shown in Fig. 3) indicates
that these structures contain Al, Si, Mg and a trace of Fe
and Sr. There are only a-Al, Si and Mg2Si phases
Fig. 1. Optical micrographs of Al/11.6%Si/0.4%Mg alloys with
partial modification, as-cast: (a) 0%Sr; (b) 0.010%Sr.
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present in the current Al/Si/Mg ternary phase diagram.
Although 0.4%Mg in the alloys is below the solubility of
magnesium in Al/Si alloys (about 0.5/0.6%), ternary
eutectic reaction, L0/a-Al'/Si'/Mg2Si, may occur
under non-equilibrium solidification. So it is rational
to assume that these networks are a'/Si'/Mg2Si ternary
eutectic. Fig. 4a clearly shows a network and it is worth
noting that many tubers (as arrowed) are present on the
surfaces of eutectic silicon flakes. The high magnifica-
tion SEM photograph (as seen in Fig. 4b) shows a
bamboo-shoot shape structure may have a habit plane
in the eutectic silicon flake. The morphology is rather
different from the eutectic silicon flake, without the facet
growth characteristic of silicon crystal. EDAX suggests
that it is Mg2Si phase too [1]. It can be concluded from
the above microstructure observations that Mg2Si phase
may grow on habit planes on the surfaces of the eutectic
silicon flakes in the partially modified alloys, present as
network or bamboo-shoot shape structure.
3.2. Mg2Si phase in the fully modified alloys
When the alloy is fully modified, the network or
bamboo-shoot shape structure as present in the partially
modified alloys, disappears now, but some bright
particles are found at the boundaries of the eutectic
cells, the size of which is larger apparently than that ofthe fibers in the eutectic cells, as shown in Fig. 5a. These
particles had not been found in Al/11.6%Si alloys yet,
so the authors suspect it might be another Mg2Si phase
morphology. Fig. 5b shows the high magnification
microstructure of the eutectic cell boundary zone. The
result of X-ray mapping in the eutectic zone is shown in
Fig. 6. It indicates there is no definite proof for the
presence of Mg2Si phase. EDAX result of the large
particles at the eutectic cell boundaries suggests it is still
silicon crystals. But a very small particle is found at the
boundary of eutectic cells too, as arrowed in Fig. 5b.
The energy dispersive X-ray tracing of it, shown in Fig.7, suggests it may be Mg2Si crystal. Apparently, it is not
observed that this fine Mg2Si particle phase has any
growth relation to the silicon crystals. The morphology
of it is also completely different from that of the
partially modified alloys. The size of it is only about 1
mm, and the amount is rather low. From the results
above, it is suggested that the nucleation of Mg2Si phase
is restrained severely in the fully modified alloys.
4. Discussions
The crystallization of Mg2Si phase in the partiallymodified alloys is completely different from that in the
fully modified alloys. It demonstrates that strontium
present in Al/Si/Mg casting alloys has an important
influence on the crystallization of Mg2Si phase.
The first reason as to why these changes occur is
thought to be related to the increase of the amount of
the dendritic a phase with the addition of strontium.
The break of crystallization of Mg2Si phase is coincident
with the abrupt increase of the amount of the dendritic a
phase caused by the addition of strontium. The amount
of the dendritic a phase with varying strontium content
in Al/11.6%Si/0.4%Mg alloys are shown in Fig. 8.When the strontium content in Al/11.6%Si/0.4%Mg
alloys increases from 0.015 (partial modification) to
0.020% (full modification), the amount of the dendritic
a phase increases from 21.06 to 41.24 by 95.8% [1,10/
12]. The increase of the amount of the dendritic a phase
results in the increase of the amount of magnesium
dissolving in the a solution, so the amount of magne-
sium consumed in the crystallization of Mg2Si phase
decreases sharply.
The second reason is related to the change of the
surface characteristic of the eutectic silicon crystals due
to the addition of strontium. During the solidification of
Fig. 2. Micrographs of Al/11.6%Si/0.4%Mg alloys with partial
modification, as-cast: (a) 0%Sr; (b) 0.015%Sr.
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Al/Si/Mg alloys, the binary eutectic reaction L0/a'/
Si, which is the main eutectic reaction, has taken place
before the ternary eutectic reaction (L0/a'/Si'/Mg2Si)
occurs. Although the space groups of Mg2Si and Si
crystals are different, Fm3m and Fd3m; respectively,
both of them belong to face center cubic lattices [13].
The Si crystal has A4 structure, 4 Si atoms occupying
the (000; 12
12
12; ) positions, and the other 4 Si atoms
inside the cubic (/14
14
14; 3
434
14; ). The Mg2Si crystal has C1
structure, in which the occupation of 4 Si atoms is also
(000;/12
12
12; ). Four of 8 Mg atoms within the cubic are in
place of 4 Si atoms as in the Si lattice and the other 4 Mg
atoms are at the positions (/34
34
34; 1
414
34; 3
414
14; 1
434
14): The
schematic lattices of Mg2Si and Si crystals are shown in
Fig. 9. In unmodified Al/Si alloys, the eutectic silicon
phase is present as flakes. The surfaces of the silicon
flakes are the low energy {111} planes [3,4,14/16]. The
{111} are also the low index planes of Mg2Si crystal. It
is seen from Fig. 9 that the Si atom layer at {111} faces
of the eutectic silicon flakes can directly act as a part of
the Mg2Si crystal and the Mg atom layer at {111} planes
of Mg2Si crystal can also directly be accommodated by
the Si atom layer at {111} planes of Si crystal. So it is
rational to consider that the {111} faces of the eutectic
silicon flakes, crystallizing prior to the Mg2Si phase,
may have the potency of affording the epitaxial sub-
strates for Mg2Si crystallizing. However, it needs further
microstructural investigation by TEM. SAD pattern at
the interface of Mg2Si and Si crystals in the partially
modified alloys should open out the crystalline orienta-
tions of them, which should further substantiate the
above hypothesis.
According to this supposition, the nucleation of
Mg2Si crystal is thought to be easy in the partially
modified alloys. Mg2Si phase can grow on the surfacesof the eutectic silicon flakes, presenting as network or
bamboo-shoot shape structures. Although a gradual
change of the morphology of the eutectic silicon crystals
with increase in the addition of strontium has been
observed in the partially modified alloys [1], the facet
characteristic of flake silicon crystals has not completely
been changed. The surface of the silicon flakes as the
nucleation sites of Mg2Si phase is still abundant, so the
nucleation and growth of Mg2Si crystals have not been
changed. But in the fully modified alloys, the eutectic
silicon crystals present as fibrous and the surfaces of the
silicon crystals are not single {111} face. High-density
Fig. 3. Energy dispersive X-ray tracing of the network structure in Fig. 2b.
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twins and stacking faults have been demonstrated to be
present at the surface of the eutectic silicon fibers
[3,4,14/16] and the accumulation of strontium atoms
in the liquid in front of the growth front of silicon
crystal forces atoms of strontium into the lattice of
silicon crystal [4]. These changes of the surface char-
acteristic of the silicon fibers may poison the habit
planes of Mg2Si crystal and hence the surfaces of the
fibrous silicon crystals have no longer the potency as the
nucleating sites of Mg2Si phase. So the nucleation of
Mg2Si crystal is restrained severely and magnesium is
forced to retain in the a solution during solidification of
casting. The mechanism of the restraining effect of
strontium on the crystallization of Mg2Si phase may
be the combination of the two effects above of strontium
in Al/Si/Mg casting alloys. It is anticipated that the
highlight on the effect of strontium on the crystallization
of Mg2Si crystal will afford some instructions for the
casting practice of the Al/Si/Mg casting alloys.
5. Conclusion
In the partially modified alloys, the Mg2Si crystals,
present as the network structure or bamboo-shoot shape
structure, grow on habit planes on the surfaces of the
eutectic silicon flakes. But, in the fully modified alloys,
the nucleation of the Mg2Si crystal is restrained severely
and very few and fine Mg2Si particles are isolated at the
boundaries of the eutectic cells.
The restraining effect of strontium on the crystal-
lization of Mg2Si phase is thought to be related to the
effects of strontium on the amount of the dendritic a
phase and/or the change of the surface characteristic of
the eutectic silicon crystals.
Fig. 4. SEM micrograph of Al/11.6%Si/0.4%Mg with 0.010%Sr
modification: (a) network structure; (b) bamboo-shoot shape structure.
Fig. 5. SEM micrograph of the eutectic cell boundary zone in Al/
11.6%Si/0.4%Mg alloys with 0.0375%Sr Sr modification: (a) low
magnification; (b) high magnification.
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Fig. 6. X-ray mapping of the eutectic zone in Al/11.6%Si/0.4%Mg alloy modified with 0.0375%Sr.
Fig. 7. Energy dispersive X-ray tracing of the fine particle in Fig. 5b (as arrowed).
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Fig. 8. The amount of the dendritic a phase with varying strontium
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Fig. 9. Schematics lattices of: (a) Mg2Si; and (b) Si crystals.
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