Efecto Del Sr en Mg2Si

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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

www.elsevier.com/locate/msea

0921-5093/03/$ - see front matter# 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0921-5093(03)00276-4
mailto:[email protected]:[email protected]


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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.

L. Hengcheng et al. / Materials Science and Engineering A358 (2003) 164/170 165


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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.

L. Hengcheng et al. / Materials Science and Engineering A358 (2003) 164/170166


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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

content in Al/11.6%Si/0.4%Mg alloys.

Fig. 9. Schematics lattices of: (a) Mg2Si; and (b) Si crystals.


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Efecto Del Sr en Mg2Si