SiC particle cracking in powder metallurgy processed aluminum matrix composite materials
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INTRODUCTION
ALUMINUM matrices reinforced with ceramic particulate additions such as SiC have been of considerable interest for a number of years. In addition to their potential for superior mechanical properties (such as high specific elastic modulus, high specific yield strength, and good wear resistance), this type of metal-matrix composite (MMC) material can be produced with relatively isotropic properties using conventional fabrication techniques. However, the addition of ceramic particulate to aluminum alloys can reduce tensile ductility and fracture toughness to unacceptably low levels. This poor damage tolerance can limit the use of these materials in structural applications. The fracture behavior of a variety of aluminum-matrix, SiC-reinforced composite materials has been studied.[~-21] Due to variations in particle size, particle morphology, particle volume fraction, matrix composition, heat treatment, composite processing, and their combined effects, it is difficult to unambiguously categorize the fracture process in A1/SiC composite materials. However, the presence of SiC reinforcement is typically detrimental to the fracture behavior of a composite material due to the addition of reinforcement fracture, reinforcement/matrix interface decohesion, and matrix failure and/or reinforcement decohesion and/or reinforcement fracture within the clusters to the failure mechanisms of the unreinforced aluminum alloy. (The dominant fracture mechanism contributes most to the deformation and fracture process of a material, even though
B. WANG, Graduate Student, G.M. JANOWSKI, Assistant Professor, and B.R. PATTERSON, Professor, are with the Department of Materials Science and Engineering, The University of Alabama at Birmingham, Birmingham, AL 35294-4461. Manuscript submitted November 18, 1993. METALLURGICALAND MATERIALSTRANSACTIONS A
many failure mechanisms can coexist and are influenced by each other.) For example, SiC particulate cracking has a major influence on the ductility and toughness of SiC/A1 composite materials when the particles are well distributed and strongly bonded, particularly for large SiC particles, t~-3,s-91 However, composite fabrication methods, such as powder metallurgy, casting, and spray forming, can affect SiC particle distribution and particle/matrix interface bond strength, which in turn affects composite material ductility and toughness, t1,2,31Reinforcement particle size also is key in determining the probability of particle cracking and, therefore, the effect of particle cracking on ductility and toughness55-9J Thus, it has been shown that composite fabrication process, reinforcement particle size, and matrix microstructure are major elements in controlling the failure mechanism of composite materials. Quantitative microscopy has been applied to examine the effects of composite fabrication method, reinforcement particle size, and matrix strength on particle fracture in AI/SiC composite materials.[ ~,2,31Lloyd[~] found that the ductility of 6061 reinforced with 10 vol and 20 vol
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