Characterization investigations of a melt-spun ternary

Al–8Si–5.1Cu (in wt.%) alloy

M. Lutfi Ovec�oglu*, Necip Unlu, Niyazi Eruslu, Arda Genc�Department of Metallurgical and Materials Engineering, Faculty of Chemistry–Metallurgy,

Istanbul Technical University, 34469 Maslak-Istanbul, Turkey

Received 7 February 2002; received in revised form 17 December 2002; accepted 7 January 2003

Abstract

A ternary Al–8Si–5.1Cu (in wt.%) alloy was rapidly quenched from the melt at cooling rates between 106 and 107 K/s

using the melt-spinning technique. The resulting ribbons were characterized using optical microscopy, scanning electron

microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and microhardness techniques. On the

basis of the Al peak shifts measured in the XRD scans, a solid solubility extension value of 3.83 at.% Si in Al was determined.

Whereas SEM investigations showed the presence of dendrites rich in Al, TEM investigations revealed nanosized spherically

shaped Si crystals 20–25 nm in size. Both XRD and TEM investigations confirmed the absence of any intermetallic phase

formation. The microhardness value of the melt-spun alloy was measured as 201 kg/mm2.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Melt-spinning; Characterisation methods; Metals; Alloys; Microstructure; Solidification; Ternary Al–Si–Cu alloy

1. Introduction

Rapid solidification processes (RSP) with cooling

rates higher than 103 K/s allow the preparation of

alloys with extraordinary properties; i.e. reduction in

grain sizes, extended solid solution ranges, reduced

levels of segregation and in some cases the formation

of metastable crystalline and amorphous phases [1–

8]. Melt spinning of aluminum alloys at cooling rates

exceeding 106 K/s results in ribbons with superior

mechanical and thermal properties than their conven-

tionally processed counterparts. In the last 2 decades,

research investigations on the rapidly solidified Al

alloys have been focused mainly on their applications

in aerospace and automotive industries. The impor-

tance of Al–Si and Al–Si–X alloys for automotive

applications with some engine parts like connecting

rod, cylinder sleeve, piston and valve retainer and

compressor parts like rotary compressor vane and

shoe disc has been well established [9]. The structural

and mechanical characterizations of rapidly solidified

hypoeutectic and hypereutectic binary Al–Si, Al–Fe

[10–23] and Al–Cu alloys [24] were investigated

previously.

Recently, research efforts have focused on the

effects of a third element addition to Al–Si alloys

such as iron, copper, nickel and other transition

elements [25–29]. These transition elements, when

alloyed in various combinations with aluminum, form

0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0167-577X(03)00051-X

* Corresponding author. Tel.: +90-212-285-3355; fax: +90-212-

285-3427.

E-mail address: ovecoglu@itu.edu.tr (M.L. Ovec�oglu).

www.elsevier.com/locate/matlet

Materials Letters 57 (2003) 3296–3301

fine dispersions of high-modulus second-phase par-

ticles, resulting in increased strength, wear resistance

and thermal stability [30]. Among the ternary Al–Si–

X alloys, the ternary Al–Si–Fe has been extensively

studied using rapid solidification techniques [28,29,

31], including melt-spinning [25]. However, there

exists no characterization work in the literature on

the melt-spun Al–Si–Cu ternary alloys in the hypo-

eutectic composition region of the Al–Si binary. This

study aims to fill this vacuum.

The primary aim of this study is to characterize the

microstructure of a melt-spun alloy belonging to the

ternary Al–Si–Cu system. For this purpose, optical

microscopy, X-ray diffraction (XRD), scanning elec-

tron microscopy (SEM) and transmission electron

microscopy (TEM) investigations were carried out

on the Al–8Si–5.1Cu (in wt.%) alloy ribbons pro-

duced by melt-spinning. The microhardness values of

the ribbons were also measured.

2. Experimental procedure

Chemically pure metals of aluminum (ingot,

>99.95% purity), silicon (powder, >99.995% purity)

and copper (rod, >99.99% purity) were used as start-

ing materials. Technical specifications of the starting

materials including chemical purities are listed in

Table 1. Using these, two master alloys with the

compositions Al–25 wt.% Si and Al–10 wt.% Cu

were prepared by induction melting. This is followed

by stoichiometrically mixing and melting these alloys

to constitute the ternary Al–8.0 wt.% Si–5.1 wt.% Cu

alloy (now hereafter referred to as the Al8Si5Cu

alloy) composition. Remelting of the as-cast alloys

was performed in a graphite crucible using an H65-

type 350 kHz high-frequency furnace during melt-

spinning experiments. Rapidly solidified ribbons were

produced by free jet melt spinning in air by means of

impinging a jet of molten alloy onto the cylindrical

surface of a polished copper wheel with a diameter of

180 mm rotating at 3000 rpm. The temperature of the

melt before ejection was 100 jC above the liquidus

temperature. The dimensions of the as-produced rib-

bons were 6–10 mm in width, 70–1000 mm in length

and 60–80 Am in thickness.

Melt-spun Al8Si5Cu ribbons were characterized

by using optical microscopy, scanning electron micro-

scopy (SEM) and X-ray diffractometry techniques.

Optical microscopy observations were conducted in

an Olympus MG model optical microscope. SEM

investigations were carried using a JEOL JSM-T330

scanning electron microscope operated at 25 kV and

linked with an energy dispersive spectrometry (EDS)

attachment. The XRD measurements were carried out

in a Philips PW3710 X-ray diffractometer using

CuKa radiation at 40 kV and 20-mA settings in the

2h range from 20j to 80j. Using standard metallo-

graphic techniques, optical mount specimens were

prepared for optical microscopy and SEM investiga-

tions followed by chemical etching in a 0.5% HF

solution for about 25 s. Transmission electron micro-

scopy (TEM) investigations were conducted on thin

foils prepared by dimpling and jet electropolishing 3-

mm discs drilled directly from the melt-spun ribbons

using the conditions of 30 V and 15–20 mA in an

electrolyte consisting of 25 vol.% of HNO3 and 75%

methanol cooled at � 50 jC. The electropolished foilswere examined in a JEOL 2000EX operated at 200

kV. Microhardness measurements on melt-spun rib-

bons were made with a Vickers diamond indenter in a

Wolpert microhardness tester employing a load of 25

g. Ten indentations were taken from the longitudinal

section of the melt-spun ribbon and the mean value is

taken as the accepted value.

3. Results and discussion

Fig. 1a and b shows X-ray diffractometry (XRD)

patterns taken from the conventionally cast Al8Si5Cu

sample and from the wheel side of the melt-spun

Al8Si5Cu ribbon, respectively. Whereas diffraction

peaks belonging to the Al, Si and Al2Cu phases are

present in the conventionally cast Al8Si5Cu sample

(Fig. 1a), only diffraction peaks of the Al solid

solution are present in the melt-spun Al8Si5Cu ribbon

Table 1

Hardness values for melt-spun ribbon and conventionally cast

Al8Si5Cu samples

Alloy Hardness (kg/mm2)

Melt-spun ribbon Conventionally cast

Al–8Si–5.1Cu 201F 7.4 80F 5.2

M.L. Ovec�oglu et al. / Materials Letters 57 (2003) 3296–3301 3297

(Fig. 1b). As expected, there is no intermetallic phase

formation between Al, Si and Cu in the melt-spun

Al8Si5Cu ribbon. This conforms with those of Unlu

et al. [25] who also reported the absence of any

intermetallic phase formation. Furthermore, Si peaks

are absent in the as-quenched Al8Si5Cu ribbon,

suggesting complete dissolution of Si in Al to form

the solid solution. However, within the limits of

detection by X-ray diffraction (typically about 5

vol.%), the absence of Si and Cu peaks alone cannot

be considered sufficient evidence of complete disso-

lution. First of all, due to the fact that Cu has a low

solubility in Al, Cu dissolution in Al can be neglected.

Secondly, considering the equilibrium solid solubility

of Si, the present results indicate that the Si solubility

in a-Al has been extended by melt spinning. Solid

solubility extension of Si in a-Al matrix as a conse-

quence of rapid solidification has been reported

extensively [9,32–35]. To determine the solid solu-

bility extension of Si in Al for the melt-spun hypo-

eutectic Al8Si5Cu alloy of the present study, the line

shift values of the measured Al(111) and (200) dif-

fraction lines in Fig. 1b were used, which yielded an

Al lattice parameter value of 0.404243 nm. Using the

linear relationship between the lattice parameter and

the atomic fraction of Si given by Bendijk et al. [16], a

solid solubility extension of 3.83 at.% Si in a-Al for

the hypoeutectic melt-spun Al8Si5Cu alloy can be

determined. This value is in agreement with those

given by Van Rooyen et al. [32], Van Mourik et al.

[10] and Timmermans and Froyen [35] for the binary

Al–Si alloy and Unlu et al. [25] for the ternary Al–

Si–Fe alloy who achieved quenching rates similar to

the present study.

Optical microscopy, SEM and TEM techniques

were used to characterize the resultant cross-sectional

microstructure of the melt-spun Al8Si5Cu ribbon

which exhibited typical structural refinement due to

high cooling rates and the morphology, distribution

and nature of the phases in the as-quenched micro-

structure. Fig. 2a displays the respective cross-sec-

tional optical micrograph showing three distinct

adjacent zones: (i) a featureless zone on the wheel

side (chill side), (ii) a transition zone with a columnar

structure and (iii) a zone containing Al-rich dendrites.

The featureless zone, often referred to as zone A, is

unique to rapidly quenched alloys [20,30], and it

indicates good contact between the ribbon and the

wheel. It appears featureless after etching as a result of

grain refinement due to high cooling rates [16,30,36].

The average thickness of the featureless zone varies

between 2 and 3 Am, which is about four times

smaller than that for the melt-spun ternary Al–Si–

Fe alloy; a rather small depth attributed to the higher

thermal conductivity of Cu. On the other hand, the

transition zone between the featureless zone and the

dendritic zone is more evident and has a uniform

depth of about 20 Am and contains columnar grains

oriented almost perpendicular to the wheel contact

surface. The thickness of the dendritic zone is about

40 Am. Fig. 2b displays a representative SEM micro-

graph taken from the cross-section of the melt-spun

Fig. 1. XRD patterns of the Al8Si5Cu alloy taken from: (a) the

conventionally cast sample and (b) the wheel side surface of the

melt-spun ribbon.

M.L. Ovec�oglu et al. / Materials Letters 57 (2003) 3296–33013298

hypoeutectic Al8Si5Cu ribbon showing a fine and

homogeneous dendritic structure. A series of EDS

analyses carried out on the dendrites revealed that

they are rich in aluminum, indicating that the primary

Al-rich phase in Al–Si–Cu system of this study grew

dendritically. It is possible to estimate the cooling rate

of the melt-spinning process by measuring the den-

drite arm spacing which depends on the degree of

supercooling and thus on the rate of cooling. The

average dendrite arm spacing on the dendrites shown

in Fig. 2b is measured approximately as 0.36 Am,

which is about 30 times smaller than that measured for

the conventionally cast counterpart (f 11.42 Am) of

the same alloy. Using the relationship introduced by

Matyja et al. [37], the dendrite arm spacing of 0.36

Am corresponds to a cooling rate of 1.14� 107 K/s for

the melt-spun hypoeutectic Al8Si5Cu alloy. Fig. 2c is

a bright-field (BF) TEM micrograph taken from the

featureless zone of the hypoeutectic Al8Si5Cu alloy

ribbon. Fine distribution of nanosized particles spher-

ical in shape and ranging from 20 to 25 nm in size is

revealed in the microstructure. Selected area diffrac-

tion pattern (SADP) taken from this region (Fig. 2d) is

indexed as arising from two phases. Whereas the spot

pattern is indexed as the a-Al matrix phase, the ring

pattern corresponds to the Si phase which has a

Fig. 2. (a) Representative optical micrograph taken from the cross-section of the melt-spun Al8Si5Cu ribbon. (b) SEM micrograph of the cross-

section of the melt-spun Al8SiCu ribbon. (c) Bright-field TEM micrograph taken from the featureless zone of the melt-spun Al8Si5Cu ribbon

revealing nanosized silicon particles in the a-Al matrix, (d) corresponding selected area diffraction pattern. Zone axis is from Refs. [1–12].

M.L. Ovec�oglu et al. / Materials Letters 57 (2003) 3296–3301 3299

diamond cubic structure with the lattice parameter

a = 0.5431 nm. From the double diffraction informa-

tion shown in Fig. 2d, it can be inferred that nanosized

discrete Si particles are crowded inside a-Al grains in

the featureless zone. The average size of these discrete

Si particles is close to those detected by Birol [20] in

the featureless zone and smaller than those detected

by Apaydin [23] of the binary melt-spun Al–Si alloys

at the eutectic composition.

Table 1 lists the Vickers microhardness value of the

melt-spun Al8Si5Cu ribbon and its conventionally

cast alloy having the same composition. It is evident

from Table 1 that the microhardness of the as-

quenched hypoeutectic alloy is approximately 2.5

times more than its conventionally cast counterpart.

Furthermore, these values are significantly higher than

those for the binary melt-spun Al–Si alloys having

similar chemical composition to the alloys of the

present investigation, i.e. 201 vs. 195 kg/mm2 for

the melt-spun Al–12 wt.% Si [20]. Although quench-

ing rates similar to those reported were employed in

the present investigation, we believe that the hardness

improvement for the hypoeutectic Al8Si5Cu alloy is

due to the solid solution strengthening effect of Cu in

the a-Al matrix.

4. Conclusions

On the basis of the results reported in this study,

the following conclusions can be drawn:

(1) XRD patterns revealed only the a-Al solid

solution peaks for the hypoeutectic Al–8Si–

5.1Cu alloy ribbon. There are no intermetallic

phase formations between Al, Si and Cu. Based

on the peak shift measurements on the XRD

scans, a solid solubility extension value of 3.83

at.% Si in Al was determined for the hypoeutectic

Al–8Si–5.1Cu alloy ribbon.

(2) The cross-sections of the melt-spun Al8Si5Cu

ribbon exhibited featureless, transition and den-

dritic zones. Based on the measurements of the

dendrite arm spacings, an average cooling rate of

1.14� 107 K/s was predicted.

(3) TEM investigations taken from the featureless

zone of the melt-spun Al8Si5Fe alloy revealed

the presence of nanosized (20–25 nm) spheri-

cally shaped Si particles inside the grains of the

a-Al matrix.

(4) Microhardness value of the melt-spun hypoeu-

tectic Al–Si–Cu alloy is 201 kg/mm2, which

corresponds to a hardness improvement of about

2.5 times than that of the conventionally cast

alloy having the same composition.

Acknowledgements

The authors wish to thank Ms. Nurten Dinc�er forher help and contributions during the SEM inves-

tigations of this study. The initial phase of this

investigation was funded by the State Planning

Organization (DPT) under the project title ‘‘Develop-

ment of Al alloys by Osprey Process’’ and this is

gratefully acknowledged.

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