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Materials Science and Engineering A 527 (2010) 2998–3004

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

he influence of Cu rich intermetallic phases on the microstructure, hardness andensile properties of Al–15% Mg2Si composite

. Emamya,∗, N. Nematib, A. Heidarzadeha

School of Metallurgy and Materials, University of Tehran, North Kargar St., Tehran 11365-4563, IranDepartment of Material Science and Engineering, Sahand University of Technology (SUT), Tabriz, Iran

r t i c l e i n f o

rticle history:eceived 24 November 2009eceived in revised form 13 January 2010ccepted 19 January 2010

a b s t r a c t

This study was undertaken to investigate the effect of different concentrations of copper (0.1, 0.3, 0.5,1.0, 3.0, and 5.0 wt.%) on the microstructure, hardness and tensile properties of an in situ cast composite(Al–15% Mg2Si). The microstructural study of the composite showed both primary and secondary Mg2Siphases in all specimens and intermetallics containing Cu (Q and � phases) were visible at high Cu contents.Hardness and tensile tests demonstrate that the addition of Cu increases both hardness and ultimate

eywords:ardness measurementomposites

ntermetallicslectron microscopy

tensile strength (UTS) values. But that a reduction in elongation occurs with the addition of Cu (≥1%Cu). A study of the specimen’s fracture surfaces via scanning electron microscope (SEM) revealed that allspecimens with large facets of primary Mg2Si particles succumb to brittle fracture. These brittle phasescan initiate cracks, but Cu rich intermetallics phases, produced from the segregation of Cu on eutecticcell boundaries, appear to be the favored path for crack propagation.

astingracture

. Introduction

The growing demand to reduce energy consumption by devel-ping more fuel efficient vehicles is a challenge for the automotivendustry. Recently, a large number of metal matrix compositesMMCs) have been developed for high-performance applications inutomotive industries. The properties of particulate metal matrixomposites (PMMCs), make them ideal candidates to replace steelnd iron in the automotive industry. Al-based composites rein-orced with particulates of Mg2Si have been introduced as a newroup of composites that offer attractive advantages such as lowensity, good wear resistance and good castability [1,2]. They areonsidered new engineering materials, and may be viable replace-ent for the dense materials in automobile engine parts and

irplane components. In situ preparation of Al–Mg2Si compositess one the most effective ways to fabricate these PMMCs sincedvantages like an even distribution of the reinforcing phase, goodarticle wetting, and low costs of production are usually achieved.

Mg2Si is a hard intermetallic compound with a high melting

oint (1085 ◦C). Its low density and low coefficient of thermalxpansion coupled with a reasonably high elastic modulus maket a good choice for use as a reinforcing agent [3].

∗ Corresponding author. Tel.: +98 2182084083; fax: +98 2182084083.E-mail address: emamy@ut.ac.ir (M. Emamy).

921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2010.01.063

© 2010 Elsevier B.V. All rights reserved.

The equilibrium phase diagram (Fig. 1) shows that Mg2Si parti-cles are the primary phase (Mg2SiP) during solidification. Then �-Aland secondary Mg2Si co-solidify from the liquid alloy in the nar-row ternary phase area. According to Eq. (1), this pseudo-eutecticreaction is completed at a temperature of 583.5 ◦C (Fig. 1) [4,5]:

L → L1E:Eutectic

+ Mg2SiP → Mg1SiPP:Primary

+ (Al + Mg2Si)EL1:liquid in two phase region

(1)

Several works have been focused on the modification of thestructure with the addition of various alloying elements, Sr, Ce, Ti,B, Zr and additional quantities of silicon [6–9]. Recently the effectsof Zr, Ti and B on these alloys were investigated and the resultsshowed a substantial change in size and morphology of the Mg2Siparticles [10].

It has also been reported that Cu may have an influence on therelative amount of Mg2Si due to a change in the coexisting equi-librium phase fields [11]. Copper is already used as an importantalloying element in Al alloys, depending on its content, and sev-eral intermetallic compounds have been reported in Al–Mg–Si–Cualloy systems [12]. Q phase is the quaternary intermediate phasewhich has been given different designations and reported withdifferent stoichiometries [12] and it is present in all three tetrahe-

drons phase fields, as shown in Fig. 2. The Q phase has a hexagonalstructure of lattice parameters c = 0.405 nm and a = 1.04 nm, with21 atoms in a unit cell [12,13]. The exact composition of Q phaseis unknown. Nevertheless, it has been described as Al5Cu2Mg8Si6,Al4CuMg5Si4 [14], Al4Cu2Mg8Si7 [13] and Al3Cu2Mg9Si7 [15]. Q

M. Emamy et al. / Materials Science and Engineering A 527 (2010) 2998–3004 2999

Fig. 1. Pseudo-binary phase diagram of Al–Mg2Si [10].

Fig. 2. Line diagram of stable equilibrium phase fields in Al–Mg–Si–Cu system atroom temperature [12].

Table 1Chemical composition of Al–15% Mg2Si (wt.%).

Material Si Mg Fe Ni

Al–15% Mg2Si 5.72 9.82 0.16 0.01

Fig. 4. (a) Cast iron mould and (

Fig. 3. SEM back-scattered electron images of 2014 ingot sample showing the hon-eycomb type structure of the Q phase [12].

phase, formed during solidification, has a complex honeycomb typemorphology as shown in the secondary electron SEM image fora 2014 alloy (Fig. 3). It is important to note that the addition ofCu to Al–Mg–Si alloys not only introduces the Q phase, but it alsocreates � phase (CuAl2) with body centered tetragonal structures[12].

This investigation has been carried out to improve tensile prop-erties of the Al–15% Mg2Si composite by the addition of Cu via acasting technique which is more economical and simple when com-pared to other methods such as rapid solidification and mechanicalalloying [2,16].

2. Experimental procedure

Industrially pure metals (Al, Mg) and Si were used as startingmaterials to prepare Al–15% Mg2Si composite ingots. All materialswere heated in an electrical resistance furnace using a 10 kg SiCcrucible. Table 1 shows the chemical composition of this Al–15%

Zn Mn Cu Ti Cr

0.01 0.01 0.01 0.01 0.01

b) tensile test dimensions.

3000 M. Emamy et al. / Materials Science and Engineering A 527 (2010) 2998–3004

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Fig. 7. SEM back-scattered images of Al–15% Mg2Si–3% Cu composite, showing Cuintermetallics (white colour phase).

F

ig. 5. (a) A typical microstructure of as cast Al–15% Mg2Si composite and (b) mor-hology of primary and pseudo-eutectic Mg2Si.

g2Si composite. The parent ingots were cut in small pieces, withhe approximate dimensions of 40 mm × 30 mm × 20 mm, appro-riate for a 2 kg SiC crucible then the composite was remelted

n another electrical resistance furnace. When the temperatureeached 750 ◦C, pure Cu (99.99%) was added in small increments.en minutes after Cu addition (0.1, 0.3, 0.5, 1.0, 3.0, and 5.0 wt.%)

he solution was hand stirred with a graphite rod for about 1 mino ensure complete mixing. During the melting period continuousddition of flux powder was used to ensure the melt surface wasovered. Degassing was conducted by using dry tablets containing

ig. 6. SEM back-scattered images, showing the microstructures of the Al–Mg2Si composites with different Cu additions: 0.3 wt.%; (b) 1 wt.%; (c) 3 wt.%; and (d) 5 wt.%.

M. Emamy et al. / Materials Science and Engineering A 527 (2010) 2998–3004 3001

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sub-size specimens (Fig. 4b). Tensile tests were carried out on a

ig. 8. SEM back-scattered images of Al–15% Mg2Si–5% Cu composite, showing Cuntermetallics (white colour phase).

2Cl6 (0.3 wt.% of the molten alloy) for about 5 min. After stirringnd cleaning off the dross, alloys with different compositions wereoured into the cast iron mould. The mould was prepared accord-

ng to B108-03a ASTM standard (Fig. 4a). The main advantage ofhis mould is the application of an appropriate uphill filling systemnd feeding design, providing a low turbulence manner of fluid

ow which consequently results in reduced gas entrapment andorosity in cast specimens.

Microstructural studies were made on polished sample surfaceshich were selected from the gauge length portion of the test

ig. 9. (a) SEM back-scattered images of Al–15% Mg2Si–3% Cu composite, (b) enlarged Cu

Fig. 10. Brinell hardness of Al–15% Mg2Si composite as a function of Cu concentra-tion.

bars (6 mm diameter) (Fig. 4b). The cut sections were polished andthen etched by HF (1%) to reveal the structure. Quantitative dataon the microstructures were determined using an optical micro-scope equipped with an image analysis system (Clemex Vision Pro.Ver.3.5.025). The microstructural characteristics of the specimenswere examined by scanning electron microscopy performed in aVega©Tescan SEM equipped with the energy dispersive X-ray anal-ysis (EDX) accessory.

Tensile test bars were machined, according to ASTM B557M-02a

intermetallic and (c) corresponding EDX analysis of Cu-containing intermetallic.

computer controlled MTS tension machine, equipped with a straingauge extensometer, at a constant cross-head speed of 1 mm/min.The fracture surfaces of tensile test specimens were also examinedwith SEM.

3002 M. Emamy et al. / Materials Science and Engineering A 527 (2010) 2998–3004

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Fig. 11. Ultimate tensile strength as a function of Cu addition.

. Results and discussions

.1. Microstructural characterization

Fig. 5(a and b) shows a typical microstructure of Al–15% Mg2Siomposite, which according to the pseudo-binary phase diagramFig. 1) [10], consists of dark faceted particles of primary Mg2Si

ig. 13. SEM back-scattered images, showing the fracture surfaces of Al–Mg2Si composit

Fig. 12. Elongation percent as a function of Cu addition.

and bright �-Al grains in a matrix of Al–Mg2Si eutectic cells.Consequently, primary Mg2Si particles will act naturally as het-erogeneous sites for the nucleation of �-Al in order to decrease theinterfacial energy [10]. This explains why all the Mg2Si particlesare surrounded by a layer of �-Al, as seen in Fig. 5b. Some �-Al

dendrites were also developed at this stage. Then, pseudo-eutecticAl–Mg2Si was formed around the �-Al phase. The pseudo-eutecticMg2Si inside the solidification cell has a fibrous morphology asshown in Fig. 5b. Further microstructural observations on the spec-

es with different Cu contents: (a) 0 wt.%, (b) 1 wt.%, (c) 3 wt.% and (d) 5 wt.%.

M. Emamy et al. / Materials Science and En

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fracture planes of almost all Mg2Si particles create a rapid fracture

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ig. 14. SEM back-scattered images of Al–15% Mg2Si composite, showing fractureaces of primary Mg2Si particles and fine dimples in eutectic region (enlarged areaa)).

mens containing Cu showed the formation of some intermetallicompounds in the Al–Mg2Si composite (bright areas in Figs. 6–9).ig. 6b–d shows that the fraction of continuous intermetallic phasesncreases to some degree with the addition of copper (≥1 wt.%u). The optical micrographs of the Al–Mg2Si composite contain-

ng copper revealed white color phases in pseudo-eutectic area,igs. 7 and 8. The presence of Cu rich intermetallics phases (Qhase with honeycomb morphology and phase with plate-likeorphology) have been reported by some investigators [4,12]. Sim-

lar intermetallics (and Q) were found in the Al–Mg2Si compositeontaining Cu, as seen in Fig. 8. SEM equipped with the energy

ispersive X-ray analysis is used to obtain a more detailed char-cterization of the nature of these intermetallics. As Fig. 9 shows,ome Cu rich phases were found to be similar in morphology of thephase (i.e. complex honeycomb).

ig. 15. SEM back-scattered images of (a) a typical fracture surface of Al–15% Mg2Si–3articles indicating a brittle mode of failure.

gineering A 527 (2010) 2998–3004 3003

3.2. Hardness and tensile properties

Fig. 10 shows the variation of hardness with Cu in Al–15% Mg2Sicomposite. It is seen that by adding small amounts of copper (<3%)to the composite, its hardness gradually improves. With largeramounts (>3%) of Cu the hardness enhancement is significant. Theformation of intermetallics in the microstructure is theorized to bea dominant factor on hardness improvement.

Figs. 11 and 12 show ultimate tensile strength and elongationpercentage data for the MMCs with increasing Cu content. It can beseen that the addition of Cu improves UTS values for higher concen-trations (≥1 wt.% Cu), but Cu content at this range has detrimentaleffect on elongation. The drop in elongation of the composite withCu addition at these Cu levels is probably due to the formation ofintermetallic compounds which form a rather continuous phase inthe last stage of the solidification of the MMC. In fact Cu has showna low solid solubility in Al–Si–Mg alloy systems, and during solidi-fication, it can be pushed into intercellular regions by the advanceof the freezing front. Although the optical microstructure of Q and� phases appear as fine eutectic structures [16], their continuousstructures at grain boundaries (or intercellular regions, Figs. 7–9)may cause initiation of cracks and fast intergranular crack propa-gation.

3.3. Fractography

Fig. 13a–d exhibit typical fracture surfaces of Al–Mg2Si com-posite with and without Cu. Figs. 14 and 15 also show the fracturesurfaces of a low Cu (0.5 wt.%) and a high Cu (5 wt.%) composite,respectively. When stress is applied, the matrix plastically deforms,gradually transferring stress to the particles. The behavior of theparticle depends on the relative strength of the interface and thematrix. When the interface is strong enough, the load is trans-ferred to Mg2Si primary particles and fracture occurs as soon asthe threshold stress is reached. Examinations of several fracturedsurfaces of MMCs show broken primary Mg2Si intermetallics. The

due to their intrinsic brittleness and precracked structure. Fig. 15exhibits a typical fracture surface of Al–Mg2Si composite contain-ing Cu which is composed of large facets of primary Mg2Si particles(Fig. 15c), suggesting a brittle mode of failure. From Fig. 15b, it is

% Cu composite, (b) intermetallics with honeycomb appearance and (c) fractured

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lso seen that some intermetallics with honeycomb appearance (Qhase) are formed in the fracture surfaces. It is concluded that atigh Cu concentrations (i.e. 5% Cu) the presence of Q phase in inter-ellular regions plays an important role in changing the fractureehavior from ductile to brittle.

. Conclusion

The effect of different quantities of copper on the microstruc-ural and mechanical properties of Al–15% Mg2Si alloy was studied.he following conclusions can be drawn:

. The addition of high concentrations of Cu (1–5 wt.%) to theAl–15% Mg2Si composite introduces Cu rich intermetallics,mainly Q and � phases.

. The formation of intermetallics in the microstructure increasesthe hardness of the composite, enhances the UTS, but reducesthe elongation before failure. The reduction in elongation isattributed to the effect of Cu rich intermetallic compounds seg-regated at the eutectic cells.

. Fracture micrographs of all tensile test specimens reveal anapparently brittle fracture with large facets of Mg2Si particles.

Large primary Mg2Si particles appear to be favored sites for cracknucleation but in composites with higher Cu concentrations(>1 wt.%), propagation of cracks may occur along the eutectic cellboundaries which are rich in intermetallic compounds (� and Qphases).

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gineering A 527 (2010) 2998–3004

Acknowledgement

The authors would like to thank University of Tehran for finan-cial support of this work.

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