Experimental and Modeling of the Coupled Influences of Variously Sized Particles on the Tensile Ductility of SiC p /Al M
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INTRODUCTION
THE aluminum-alloy-based metal matrix composites reinforced with SiC particles are widely used in aerospace, military, and civil manufacturing industries, because of their high strength, modulus, wear resistance, and fatigue resistance. Usually, the introduction of the SiC particles increases the elastic modulus and yield stress but decreases the tensile ductility and fracture toughness of the materials. Optimizing the mechanical properties of the SiCp/Al composites has been of continued interest[1–5] during the last several decades. The work includes both experimental studies and mechanical modeling. The Eshelby-type model, the shear lag model, and the modified shear lag model[3–5] have been successfully used to predict the yield stress and elastic modulus of the SiC/Al composites. However, few models exist that deal with the tensile ductility and fracture toughness. The reasons may include the difficulty in quantifying the relationship between the ductility and toughness, and the complicated microstructures (including variously sized particles such as SiC particles, constituents, dispersoids, and precipitates) of SiC/Al composites. MIN SONG, Postdoctoral Researcher, is with the State Key Laboratory of Power Metallurgy, Central South University, Changsha 410083, P.R. China, and the Key Laboratory of Materials Physics, Ministry of Education, Zhengzhou University, Zhengzhou 450052, China. Contact e-mail: [email protected] DAWEI HUANG, Engineer, is with the Concrete Machinery Company, Product Research Institute, Zoomlion Heavy Industry, Science and Technology Development Cd. Ltd, Changsha 410013, P.R. China. Manuscript submitted April 16, 2007. Article published online July 25, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
It is widely accepted that three types of in-situ secondphase particles with various sizes (range from nanoscale to microscale) and shapes exist in heat-treatable aluminum alloys.[6–8] Coarse ellipse/sphere-shaped constituents are ~1 to 10 lm in diameter, intermediate sphere-shaped dispersoids are ~50 to 500 nm, and fine disk/plate-shaped (for a 2XXX aluminum alloy), rod/ needle-shaped (for a 6XXX aluminum alloy), or sphereshaped (for a 7XXX aluminum alloy) precipitates are tens and hundreds of nanometers in diameter or length.[9] Previous studies[9–12] indicated that all three of these types of in-situ second-phase particles have important influences on the tensile ductility and fracture toughness of the aluminum alloys. The coarse constituents are brittle and initiate microcracks at low strain during deformation. The microcracks will grow in size up to coalescence, at which point the fracture is triggered if the deformation continues to increase. At the same time, the incompatibility in shape change between the unreinforced matrials and dispersoids and precipitates leads to geometrically-necessary dislocations introduced to make up for the discrepancy.[13] Thus, to model the ductility and toughness of an aluminum alloy, the influences of these three types of in-situ sec
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