X-ray nano-diffraction study of Sr intermetallic phase during solidification of Al-Sihypoeutectic alloyJeyakumar Manickaraj, Anton Gorny, Zhonghou Cai, and Sumanth Shankar

Citation: Applied Physics Letters 104, 073102 (2014); doi: 10.1063/1.4865496 View online: http://dx.doi.org/10.1063/1.4865496 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A sessile drop setup for the time-resolved synchrotron study of solid-liquid interactions: Application tointermetallic formation in 55%Al-Zn alloys Appl. Phys. Lett. 104, 171608 (2014); 10.1063/1.4874848 Multiscale modeling of the influence of Fe content in a Al–Si–Cu alloy on the size distribution of intermetallicphases and micropores J. Appl. Phys. 107, 061804 (2010); 10.1063/1.3340520 Structural characterization of the Co 2 Fe Z ( Z = Al , Si, Ga, and Ge) Heusler compounds by x-ray diffraction andextended x-ray absorption fine structure spectroscopy Appl. Phys. Lett. 90, 172501 (2007); 10.1063/1.2731314 Experimental Investigation and Numerical Simulation During Backward Extrusion of a SemiSolid AlSiHypoeutectic Alloy AIP Conf. Proc. 907, 620 (2007); 10.1063/1.2729582 The determination of phases formed in AlSiCu/TiN/Ti contact metallization structure of integrated circuits by x-raydiffraction J. Appl. Phys. 83, 132 (1998); 10.1063/1.366710

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X-ray nano-diffraction study of Sr intermetallic phase during solidificationof Al-Si hypoeutectic alloy

Jeyakumar Manickaraj,1 Anton Gorny,1 Zhonghou Cai,2 and Sumanth Shankar1,a)

1Light Metal Casting Research Centre (LMCRC), Department of Mechanical Engineering, McMasterUniversity, 1280 Main Street W, Hamilton, Ontario L8S 4L7, Canada2Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne,Illinois 60439, USA

(Received 28 December 2013; accepted 27 January 2014; published online 18 February 2014)

The evolution of strontium (Sr) containing intermetallic phase in the eutectic reaction of Sr-

modified Al-Si hypoeutectic alloy was studied with high energy synchrotron beam source for nano-

diffraction experiments and x-ray fluorescence elemental mapping. Contrary to popular belief, Sr

does not seem to interfere with the Twin Plane Re-entrant Edge (TPRE) growth mechanism of

eutectic Si, but evolves as the Al2Si2Sr phase during the eutectic reaction at the boundary between

the eutectic Si and Al grains. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865496]

Strontium (Sr) is generally added as a trace element to

the Al-Si hypoeutectic alloy to enable significant refinement

(modification) of the Si phase and Al grains, alike.1 The

addition of Sr increases the strength and ductility of the final

product by changing the eutectic Si from coarse plate-like

morphology into fine fibrous-like structure.2 There are two

schools of thought in explaining the mechanism of modifica-

tion of the eutectic phases by Sr addition: the poisoning of

the Twin Plane Re-entrant Edge (TPRE) in the growth of the

eutectic Si phase by Sr atoms3–6 and recently, the role of Sr

in significantly altering the nucleation of intermetallic and

eutectic phases during solidification by altering the atomic

structure of the inter-dendritic liquid alloy.7–9

The mechanism of Sr poisoning the TPRE growth mode

of the eutectic Si phase relies on trace elemental additions to

the Al-Si alloy melt such that the ideal atomic radius ratio

between the modifier element and silicon is around 1.65.5

However, this mechanism did not explain the effectiveness

of several elements that fit this radius ratio and vice-versa,

for example, Yb (ratio of 1.66) was not able to modify Si

while Na (ratio of 1.59) was able to effectively modify Si

under similar conditions.10 Further, it was reported that the

mechanism of interference with the TPRE growth mecha-

nism warranted a significantly higher twin density in the

eutectic Si phases;5 however, a few literature sources11,12

have shown that Si could be modified even without any

appreciable change in twin density of the modified eutectic

Si phase. The poisoning of the TPRE growth mechanism

suggests that Sr would need to segregate exclusively on the

eutectic Si phase during its growth and poison existing

favored crystallographic growth ledges and promote growth

by significantly higher twinning of the Si phase to attain a

refined morphology of the solidified phase. In 1988, Atomic

Absorption Spectroscopy (AAS) study conducted by

Clapham and Smith13 showed Sr segregation on eutectic Si,

while in 2006, the micro X-ray fluorescence (l-XRF) analy-

sis conducted by Nogita and coworkers14 showed Sr as

distributed, relatively homogeneous, in the modified Si

phase. In their continued effort in 2010,15 the micro X-ray

fluorescence results showed a Sr intermetallic phase on the

eutectic Si phase and there was no segregation of Sr in the

primary and eutectic Al. According to Cho et al.,9 the pre-

eutectic Al2Si2Sr phase formed on the Aluminium phosphide

(AlP) particle and isolated near the primary Al

dendrite-liquid interface and not on the eutectic Si; AlP was

presented as an impurity phase universally found in Al

alloys. In 2010, Zarif et al.16 studied the sample prepared by

melt spinning using Differential Scanning Calorimetry

(DSC) at a cooling rate of 10 �C min�1 and observed that

Al2Si2Sr phase forming immediately after the eutectic for-

mation in the solidification, also it was observed that there is

no effect on Sr levels (100 to 3000 ppm) in the alloy on the

formation of the Al2Si2Sr phase as a function of temperature.

Recently, in 2012, Timpel et al.17 studied Sr distribution

in eutectic Si using atom probe tomography and transmission

electron microscopy (TEM) with nanometer resolution and

observed Sr segregation on eutectic Si in the form of AlSiSr

intermetallic phases, also their recent analysis18 showed the

AlSiSr intermetallic phases as rod shaped morphology exist-

ing with Al rich locations inside the eutectic Si. Their

study17,18 confirmed that two types of Sr containing interme-

tallic phases co-existed along grain boundaries of the eutec-

tic Si phase; one phase (about 20% of all Sr atoms) was

found inside the eutectic Si as a rod shape and attributed to

that promoting the twin formation and the other one (about

80% of all Sr atoms) existed near the eutectic Al/Si interface

which restricted the growth of eutectic Si in the preferred

crystallographic directions. Additionally, the study17,18 con-

firmed that there was no elemental Sr in existence in or

around the eutectic Si phase, which contradicts the earlier

theory of elemental Sr poisoning the TPRE growth mecha-

nism of the Si phase.14

In summary, the literature on the role of Sr in Al-Si

alloys is fairly polarized with one faction suggesting that the

Sr element directly poisons the TPRE growth mechanism of

the eutectic Si phase and the other promotes the idea of Sr

playing a more complicated role in interfering with the nucle-

ation of the various secondary phases during solidification

a)Author to whom correspondence should be addressed. Electronic mail:

shankar@mcmaster.ca. Tel.: þ1 905 525 9140 ext. 26473. Fax: þ1 905

572 7944

0003-6951/2014/104(7)/073102/4/$30.00 VC 2014 AIP Publishing LLC104, 073102-1

APPLIED PHYSICS LETTERS 104, 073102 (2014)

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while finally evolving as the AlSiSr intermetallic phase along

with the eutectic phases during solidification. The latter

school of thought seems to be far more substantiated with ex-

perimental evidences and mechanistic theories. In our recent

efforts,7,8,19–21 several research outcome have been published

to substantiate that Sr addition significantly alters the nuclea-

tion environment of several secondary phases such as the Fe

bearing intermetallic and eutectic phases during solidifica-

tion; in this publication, the AlSiSr intermetallic phase has

been analyzed using the nano-diffraction technique with a

high energy synchrotron beam source coupled with x-ray flu-

orescence imaging system to show that the Sr only evolves as

AlSiSr intermetallic phase at the eutectic temperature, pre-

dominantly as the Al2Si2Sr phase. In this publication, all the

elemental levels in the alloys mentioned are in weight per-

centage unless otherwise mentioned.

Since, the level of Fe in the alloy is closely linked to the

effectiveness of the Sr addition to the Al-Si alloys,19 and that

Fe is an unavoidable impurity in aluminium alloys,22 the two

alloys investigated in this study were: Al-7Si-0.05Fe-0.03Sr

and Al-7Si-0.25Fe-0.03Sr; all the alloys were prepared using

99.999% purity Al, 99.9999% purity Si, Al-25 wt. %Fe, and

Al-10 wt. %Sr master alloys as stock materials. Electric re-

sistance furnace was used to melt them and each sample was

prepared using a clean high purity alumina crucible. Alloys

were cast into steel mold with a cooling rate of 50 K s�1 in

liquid phase and the compositions of the final alloys were

evaluated by the Glow Discharge Optical Emission

Spectroscopy (GDOES) technique. A JEOL 7000 Scanning

Electron Microscope (SEM) with a cold Field Emission Gun

(FEG) and equipped with an Oxford instruments Energy

Dispersive X-Ray Spectrometer (EDX) model 7558 was

used; the sample foils for the TEM study were prepared by

Focused Ion Beam (FIB) milling from preferred section in

the sample microstructure using a LEO 1530 dual and cross

beam SEM, and the analysis was carried out using a JEOL

2010 TEM (200 keV incident beam voltage) equipped with

two high resolution digital cameras, Scanning TEM (STEM)

detector and Oxford Instruments Link Pentaflet EDX (model

6494) system.

High Energy Synchrotron beam source at the Line D of

Sector 223 in the Advanced Photon Source (APS) at the

Argonne National Labs, Argonne, IL, USA was used for the

nano-diffraction experiments; the energy of the beam was

10.1 keV with a wavelength of 0.12275 nm. In addition to a

goniometer equipped with a Charge-Coupled Device (CCD)

camera detector for the diffracted beam intensities, an XRF

detector was used to obtain elemental map from the micro-

structure of the solidified specimen surface. The X-ray fluo-

rescence elemental maps were obtained for an area of 21 �21 pixel grid from Al-7Si-0.05Fe-0.03Sr sample and 18 �18 pixel grid from Al-7Si-0.25Fe-0.03Sr sample; each pixel

grid represents 2 � 2 lm2 region (4 lm2 area) and X-Ray dif-

fraction (XRD) pattern was obtained from each pixel grid.

Typical microstructures of the eutectic region of Al-7Si-

0.05Fe-0.03Sr and Al-7Si-0.25Fe-0.03Sr alloys are shown in

Figures 1(a) and 1(b), respectively; the white phase repre-

sents Al-Si-Fe intermetallic phase, light grey area represents

Al-Si eutectic regions, and darker grey area represents pri-

mary Al dendrites; it was observed that increasing the Fe

composition increased the amount and size of Fe intermetal-

lic phases.

TEM sample was prepared from the eutectic area adja-

cent to a primary Al dendrite in the Al-7Si-0.05Fe-0.03Sr

alloy. Figure 2(a) shows typical STEM micrographs of the

sample and the magnified region from Figure 2(a) are shown

in Figure 2(b) where small Al2Si2Sr phases adjacent to eutec-

tic Si have been identified and demarcated.

Nano-diffraction was carried out in the eutectic region

using high energy synchrotron beam source. Elemental dis-

tribution obtained by using XRF for Al-7Si-0.05Fe-0.03Sr

and Al-7Si-0.25Fe-0.03Sr alloys are shown in Figures 3(a)

and 3(c), respectively, and the XRF map was used to identify

the eutectic region. The shaded grids in Figures 3(b) and

3(d) are analyzed for the Al-7Si-0.05Fe-0.03Sr and Al-7Si-

0.25Fe-0.03Sr alloys, respectively. “x” denotes the region

where Sr containing intermetallic phase is observed and it

should be noted that this phase was not distributed homoge-

neously in the eutectic area.

Each pixel grid in Figure 3 will be represented by (u,v)

where u and v are the row and column, respectively, for each

grid. A typical XRD spectrum obtained from Al-7Si-0.05Fe-

0.03Sr alloy at (3,8) location in Figure 3(a) is shown in

Figure 4. The images of diffraction rings were obtained with

constant exposure time of 20 s at three fixed inclinations of

CCD camera (28�, 43�, and 58�), then integrated and proc-

essed to obtain the resultant spectrum (intensity versus 2h).23

The diffraction result confirmed that Sr evolved along with

Si only as the Al2Si2Sr phase, which was indexed using the

Joint Committee on Powder Diffraction Standards (JCPDS)

data file No. 361334 (Hexagonal, p-3m1, a¼ 4.1872 A,

c¼ 7.427 A).24 It is notable that the result of the XRD

FIG. 1. Typical SEM-Backscattered

electrons (BSE) micrographs of (a) Al-

7Si-0.05Fe-0.03Sr alloy and (b) Al-

7Si-0.25Fe-0.03Sr alloy cast at a cool-

ing rate of 50 K s�1.

073102-2 Manickaraj et al. Appl. Phys. Lett. 104, 073102 (2014)

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analysis on all the grid locations marked by “x” in Figures

3(b) and 3(d), alike showed identical results as in Figure 4,

except that the relative heights of the peak intensity margin-

ally varied amongst them. Additionally, no XRD peak

related to elemental Sr was observed in any grid location

shown in Figure 3; the resolution of the experimental analy-

sis is in nanometer scale.

The results shown in Figures 3 and 4 confirm that stron-

tium was observed only in the form of the intermetallic

phase with Al and Si, predominantly in the eutectic region.

The working hypothesis that explains the mechanism of the

Sr addition to these alloys suggests that Sr play a critical role

in the alloy melt at high temperatures by altering the cluster-

ing tendencies of the Fe and Si atoms in the liquid that alters

the nucleation of the secondary phases during solidifica-

tion.7,25 In the absence of Sr, the eutectic phases evolve with

the Si phase nucleating on the s6–Al9Fe2Si2 intermetallic

phase on the boundary between the primary Al phase and the

eutectic liquid, followed by the evolution of the eutectic Al

epitaxially on the primary Al or through nucleation on the

eutectic Si phase with a plate morphology.19,20 The addition

of Sr alters the Al-Fe-Si intermetallic phase to one that is

FIG. 2. Typical STEM micrographs of

Al-7Si-0.05Fe-0.03Sr alloy (a) low

magnification view of the sample and

(b) magnified view of the area marked

with white square in (a).

FIG. 3. Typical X-ray fluorescence

maps of Si elemental distribution; (a)

in Al-7Si-0.05Fe-0.03Sr alloy, (b) rep-

resentation of the pixel grids in (a), (c)

in Al-7Si-0.25Fe-0.03Sr alloy, and (d)

representation of pixel grids in (c). The

purple to white gradation of colours in

(a) and (c) denotes negligible to high

Si level in a pixel grid, respectively.

073102-3 Manickaraj et al. Appl. Phys. Lett. 104, 073102 (2014)

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unfavourable for the nucleation of eutectic Si, thereby, intro-

ducing a pronounced undercooling of the eutectic tempera-

ture during which the primary Al phase continues to grow

and enrich the eutectic liquid with super saturation of Si.19,21

When the super saturation of the eutectic liquid with Si

reaches the maximum allowable limit, the Si phase is forced

to crystallize on the boundary of the primary Al phase and

the liquid and renders the liquid back to eutectic composition

and further, prevent any contact between the Al phase and

the liquid.19 This leads to an undercooled eutectic liquid

leading to the a high rate of nucleation of the eutectic Al

phase on the Si and the growth of copious number of highly

grain refined eutectic Al grains.19 The growth of the eutectic

Si phase is subsequently forced through the tortuous

inter-granular regions of the eutectic Al phase; the ability of

the Si to twin with ease enables the growth of a significantly

refined fibrous morphology of the phase.19 During the

growth of the Si phase in the liquid present in the

inter-granular regions of the solidified (highly refined) eutec-

tic Al phase, the Sr is segregated to the regions enveloped by

the growing Si, solidified eutectic Al grain, and remaining

eutectic liquid; wherein, they nucleate as the Al2Si2Sr inter-

metallic phase along with the Al and Si from the remaining

eutectic liquid; this is shown by the conclusive evidence pre-

sented in Figures 2–4. Further, recent conclusive evidences

presented by Timpel et al.17,18 shows that 20% of the total Sr

atoms in the alloy evolve as rod shaped nano-sized Al-Si-Sr

intermetallic phase in the eutectic Si while the remaining

80% evolve as micro sized Al-Si-Sr intermetallic phase at

the boundary of the eutectic Al and Si phases. This study

confirms that elemental Sr does not exist in the solidified

Al-Si alloys with trace levels of Sr addition and that the Sr

predominantly evolves as the Al2Si2Sr phase during the

eutectic reaction as normally predicted by fundamental ther-

modynamics and kinetics. There may be possibility of the

nano-sized rod shaped Al-Si-Sr intermetallic phase further

promoting the ability of the Si to grow in the narrow re-

stricted path present in the eutectic region, in the

inter-granular liquid regions of the highly grain refined

eutectic Al phase.

The authors wish to extend their sincere gratitude to the

Ontario Research Fund for the financial assistance through

the Initiative for Automotive Manufacturing Innovation

(IAMI) at McMaster University. The authors also want to

recognize the use of the synchrotron beam line 2-ID-D at the

Advanced Photon Source, an Office of Science by Argonne

National Laboratory, supported by the U.S. DOE under

Contract No. DE-AC02-06CH11357.

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FIG. 4. Typical XRD spectrum obtained from Al-7Si-0.05Fe-0.03Sr alloy at

(3,8) grid location in Figure 3(a), showing the indexed peaks of Al, Si, and

Al2Si2Sr24 phase. The unmarked peaks are from the Fe containing interme-

tallic phases.

073102-4 Manickaraj et al. Appl. Phys. Lett. 104, 073102 (2014)

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