Materials Letters 79 (2012) 232–234

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Materials Letters

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Microstructure evolution and precipitation of SiC particle reinforced Al–20Sicomposite solidified under high pressures

Pan Ma, Chunming Zou, Hongwei Wang, Jinchuan Jie, Yandong Jia, Binguo Fu, Zunjie Wei ⁎School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

⁎ Corresponding author. Tel.: +86 451 86403150; faE-mail address: weizj@hit.edu.cn (Z. Wei).

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.03.061

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 February 2012Accepted 15 March 2012Available online 23 March 2012

Keywords:Metal matrix compositesHigh pressure solidificationMicrostructureGP zonesPrecipitation

Evolution of microstructure and precipitation process of 40%SiCp/Al–20Si composite solidified under highpressures were investigated. The results demonstrate that undissolved Si phase under 1 GPa is much small-er than 0.02 GPa, and disappears under 2 GPa. Primary α-Al phase appears at 1 GPa, and with increasedpressure, more primary α-Al phase emerges. Eutectic silicon phase is modified effectively under high pres-sures. A supersaturated Al(Si) solid solution is obtained. GP zones form in primary α-Al phase when solid-ified under 3 GPa and nature aging for 400 days. Dispersed and fine Si phase precipitates by manual aging at209 °C.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

SiC reinforced Al metal matrix composites (Al-MMCs) have re-ceived considerable attention in automotive, electrical and aerospaceindustries due to their excellent properties, such as low weight, highthermal conductivity, low coefficient of thermal expansion, and wellwear resistance [1,2].

Al-MMCs reinforced with SiC are mainly prepared by powder met-allurgy (PM) and casting technique. The advantages of PM are gooddistribution and no limitation on the volume fraction of the reinforce-ment. However, this technique is complex and needs secondaryprocessing [3]. The casting process tends to offer more flexibility,whereas, it is difficult to synthesize Al-MMCs due to the non-uniform distribution of reinforcement, poor wettability as well asundesirable chemical reactions at the interface [4]. In order to solvethe above mentioned disadvantages, high pressure solidificationmethod has gained favorable attention due to the unique featuressuch as refined microstructures, increased uniformity and extensionof solid solubility [5–7].

The purpose of this paper is to investigate the microstructure evo-lution and precipitation behavior of SiC particle reinforced Al–20Sicomposite solidified under high pressures.

2. Experimental procedures

Al powders and Si powders (more than 99.7% pure, particle size10–30 μm), SiC powders (particle size 5–15 μm)were used as starting

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materials, and the nominal composition was 40 vol.% SiCp/Al–20Si.The powders were thoroughly homogenized and uniaxially coldpressed in a cylindrical die at 300 MPa to form green compacts(Φ20×18 mm). Samples were heated in a graphite furnace in thehigh-pressure equipment and the pressures were 1 GPa, 2 GPa and3 GPa, respectively. A detailed description of the high-pressure exper-imental procedures can be found elsewhere [8]. For comparison, com-posite solidified at 0.02 GPa was also prepared.

The phases were characterized by a Rigaku D/max-RB X-ray dif-fractometer (XRD) with monochromatic Cu-Kα radiation. Scanningelectron microscope (SEM), Philips CM12 transmission electron mi-croscopy (TEM), high-resolution transmission electron microscopy(HRTEM) and differential scanning calorimetry (DSC) measurementswere employed to study the morphology and precipitation behaviorof the composite. An X-ray stress analyzer was employed to measurestresses in the composite using the sin2Ψ technique.

3. Results and discussion

Fig. 1 illustrates the microstructures of 40%SiCp/Al–20Si compositesolidified under different pressures. Undissolved Si phase and eutecticstructure are found in the matrix at 0.02 GPa and the matrix is com-posed of undissolved Si phase, eutectic structure as well as α-Alphase under 1 GPa. The Si phase disappears and a large formation ofα-Al phase occurs when the pressure is 2 GPa and 3 GPa. This resultimplies that hypereutectic Al–20Si alloy can exhibit hypoeutecticstructure due to the application of high pressure. In spite of the exis-tent of undissolved Si phase under 0.02 GPa and 1 GPa, the morphol-ogy changes obviously from a coarse lamellar to a refined lamellar. Inaddition, the structure of eutectic silicon varies significantly. The

Fig. 1. Microstructures of 40%SiCp/Al–20Si composite solidified under different pressures. (a) 0.02 GPa, (b) 1 GPa, (c) 2 GPa, and (d) 3 GPa.

233P. Ma et al. / Materials Letters 79 (2012) 232–234

eutectic silicon obtained at 0.02 GPa shows lamellar structure, where-as it becomes rod-like and fibrous morphologies at the pressures of1 GPa and 2 GPa. The eutectic silicon transforms thoroughly to finefibrous structure at 3 GPa.

Due to the mismatch in the coefficients of thermal expansionbetween SiC and the matrix, internal stresses are introduced whencomposite is cooled down from the fabrication temperature. Fig. 2illustrates the residual stress results of SiC (2110) plane at about 134°with Cu-Kα radiation. As can be seen, the values of residual stress (σ)in SiC vary slightly, accordingly, the change of residual stress can beneglected compared with variation of applied pressures. Consequently,high pressure technology plays a key role in determining microstruc-tures of composite during solidification.

Fig. 3 displays the XRD patterns of 40%SiCp/Al–20Si composite so-lidified under different pressures. The peaks of XRD are completelyattributed to the Al, Si and SiC phases. The peaks of Al phase shift to

Fig. 2. Measurements of residual stresses in SiC solidified under different pressures.(a) 0.02 GPa, (b) 1 GPa, (c) 2 GPa, and (d) 3 GPa.

the higher angles, indicating that the lattice parameter of Al phase de-creases with increasing pressure as a result of variation of Si contentinα-Al solid solution. According to Vegard's law [9], the change in lat-tice parameter corresponds to Si solubility of approximately 7.93 at.%under 3 GPa, which increases greatly compared with the equilibriumdiagram of the Al–Si binary system. A possible explanation would bethat the decrease in diffusion coefficient and the solute segregationduring solidification give rise to the formation of supersaturatedsolid solution [10].

Fig. 4 presents the microstructure of GP zones precipitated in α-Alphase. Some dark precipitates can be observed in Fig. 4(a). Fig. 4(b)shows HRTEM image taken with the incident beam parallel to the[011]m axis and the insert is the responding FFT pattern. The GP zonesare fully coherent with α-Al phase. Fig. 4(c) exhibits the EDS analysisof the GP zones. The results cannot be used to show the exact chemicalcomposition due to the small size of the GP zones. However, it is

Fig. 3. XRD patterns of 40%SiCp/Al–20Si composite solidified under different pressures.(a) 0.02 GPa, (b) 1 GPa, (c) 2 GPa, (d) 3 GPa and (e) 3 GPa, then aging at 209 °C.

Fig. 4. GP zones precipitates in α-Al solidified under 3 GPa and then nature aging for 400 days. (a) Bright field image, (b) HRTEM image and (c) EDS analysis.

Fig. 5. (a) DSC curves of 40%SiCp/Al–20Si composite solidified under 0.02 GPa and 3 GPa and (b) microstructure of composite solidified under 3 GPa and then annealed at 209 °C.

234 P. Ma et al. / Materials Letters 79 (2012) 232–234

acceptable from the EDS result that GP zones are composed of Al atomsand Si atoms. The volume fraction of GP zones is analyzed by ImageProplus software with several bright field images. Nearly 15.5 vol.% GPzones by calculation precipitate in α-Al phase. High pressure can in-crease solid solubility of Si in α-Al during solidification, leading toprecipitation of GP zones.

The DSC curves of the 40%SiCp/Al–20Si composite solidified underdifferent pressures are shown in Fig. 5(a). No thermal peaks can beobserved under 0.02 GPa, whereas the curve shows two consecutiveexothermic peaks at approximately 209 °C and 366 °C under 3 GPa,respectively. The formation of the initial large exothermic peak maybe attributed to precipitation of Si phase from the supersaturatedα-Al phase. The second exothermic peak is found to be connectedwith silicon phase coarsening [11]. No obvious thermal peaks corre-sponding to the dissolution of GP zones can be found due to their lim-ited size. Based on the results of thermal analysis curve and earlierinvestigation [12], the precipitation sequences in supersaturatedα-Al phase can be given as:

α SSSð Þ→GP zones→θ Sið Þ

where α (SSS) represents the supersaturated solid solution; GP zonesare aggregates of solute atoms in α-Al phase; θ (Si) is an equilibriumSi phase.

Fig. 5(b) shows the Si phase precipitates from α-Al phase afteraging at 209 °C. As can be seen from the XRD pattern of the corre-sponding composite (Fig. 3(e)), the peaks of the α-Al phase shift tolower angles, strong diffraction lines of the Si phase are observed. It

is further confirmed that the fine and white granules in Fig. 5(b) areSi phase by manual aging.

4. Conclusions

Microstructures of 40%SiCp/Al–20Si composite solidified underdifferent pressures vary obviously. The undissolved Si phase existsat 0.02 GPa and 1 GPa, accompanied with the morphology changesfrom a coarse lamellar to a refined lamellar. Primary α-Al phaseemerges under 1 GPa, and a large formation occurs with increasedpressure. A supersaturated Al(Si) solid solution is obtained and solidsolubility extends up to 7.93 at.% under 3 GPa. GP zones precipitatewith a fully coherent relationship with primary α-Al phase solidifiedunder 3 GPa after natural aging for 400 days. Dispersed and fine Siphase precipitates when manual aging at 209 °C.

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