Fabrication of a new SiC/2024Al co-continuous composite with lamellar microstructure and high mechanical properties
Introduction
Ceramic–metal composites have attracted great interests in the past decades due to the combined effects of metallic and ceramic materials. Among them, SiC reinforced aluminum matrix composites offer excellent mechanical and thermo-mechanical properties and are thus applied in many fields, such as automotive, aerospace and packaging and thermal management in electronic devices [1], [2], [3]. In these composites, discontinuous SiC reinforcements are always ceramic particles, whiskers and short fibers, while continuous reinforcements are fibers [4], [5], [6], [7], [8]. In recent years, there has been renewed interest in co-continuous composites in which both matrix and reinforcement are continuous in three-dimensional (3-D), resulting in an interpenetrating microstructure [9], [10], [11]. One typical method to achieve such microstructure is the infiltration of molten metal into porous ceramic preform with open pores under external pressure. Accordingly, if the preforms were prepared with controlled porosities and pore structures, the content and microstructure of reinforcement in the final co-continuous composites can be tailored to a particular engineering application.
Few studies had been done on the SiC/Al co-continuous composites, for example, Herzog had fabricated SiC–Al MMC by the direct-squeeze-casting technique using porous wood-derived biomorphic-SiC as a preform [12]. While Cree had produced SiC/Al interpenetrating composites from commercial SiC Ultrafoam™ and A356 aluminum alloy [13]. The shape and spatial arrangement of the reinforcement in metal matrix composites are key parameters in determining their mechanical behavior [14]. However, the effects of microstructure for the SiC preform on the properties of composites had not been investigated in these studies. Consequently, properties of SiC/Al co-continuous composites prepared from porous SiC preforms with new microstructure are still necessary to be further studied. Previous studies had confirmed that porous ceramics with controlled porosities and pore structures across several length scales can be produced by freeze casting using ceramic aqueous slurry [15], [16]. The porous SiC prepared by this method can be designed to possess lamellar or cellular pore structure [17]. Therefore, infiltrating molten aluminum alloy into an open porous SiC preform produced by freeze casting, SiC/Al co-continuous composites with new continuous architectural reinforcement are expected to be fabricated. As a result, different mechanical performance will be obtained. As a method for infiltration, squeeze casting is an ideal processing to obtain dense composites at low temperature and cost, also has the advantage of overcoming infiltration problem derive from bad wettability between Al alloy and SiC ceramics. Therefore, commercial Al alloy can be directly used as the metal phase in the co-continuous composites.
In the present work, porous SiC preforms with controlled porosities and pore structures were fabricated by freeze casting. Then they were used for producing SiC/2024Al alloy co-continuous composites with new microstructure by squeeze casting. In the envisaged role for the current material, the SiC is to provide increased strength and stiffness of the composite, while the 2024Al alloy phase is preferred for its metallic performance, like high toughness. The microstructural characteristics and mechanical properties for the composites are also clarified in relation to the porosities and pore structure in the ceramic preforms.
Section snippets
Materials
α-SiC powder with 6H polytype (98% grade, d50 = 3.2 μm, Kaihua SiC Co., Ltd. China), Al2O3 (grade A16SG, Alcoa), Y2O3 (99.99% grade, Rare Metallic Co., Ltd., Japan) were used as the starting materials to fabricate porous SiC preforms. 90 wt% α-SiC and 10 wt% Al2O3 + Y2O3 (molar ratio of Al2O3 and Y2O3 was 5:3) were wet-milled in methanol for 24 h using Si3N4 balls. The slurry was dried and sieved through a 120 μm screen. Commercial 2024Al alloy was used as the metallic phase in the composites, and its
Results
Fig. 2(a and b) shows the typical porous SiC preform and SiC/Al co-continuous composite prepared from this preform respectively. At the first glance, the dimension and shape of the preform had not changed after squeeze casting. The result illustrates that the preform has enough strength to withstand the pressure during the squeeze casting.
Fig. 3 shows the SEM images of the SiC preforms prepared from suspensions with different solid loading. These images reveal that the SiC preforms have
Discussions
In freeze casting, open porosities in porous SiC preforms were obtained by sublimation of ice crystals. Because of the anisotropic growth kinetics of ice crystals, the preforms exhibit a hierarchical lamellar structure. The results have revealed that the porosity and microstructure of porous SiC preforms produced by freeze casting can be well controlled through tuning the parameters of the freeze casting, such as initial solid loading and freezing temperature. Consequently, the content and
Conclusions
New SiC/2024Al co-continuous composites with lamellar microstructure have been successfully fabricated via squeezing molten 2024Al alloy into porous SiC preforms produced by freeze casting. These new co-continuous composites exhibit lamellar microstructure composed of alternating ceramic and metallic lamellar. The porosity and microstructure of porous SiC preform can be well controlled through tuning the initial solid loading or freezing temperature during freeze casting. Consequently, the
Acknowledgment
This work was financial supported by the National Natural Science Foundation of China (No. 51021002).
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