Fracture mechanisms of a 2124 aluminum
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INTRODUCTION
S I C whisker reinforced aluminum (AI-SiCw) composites are generally known to exhibit high elastic modulus and strength combined with light weight, providing a potentially wide range of applications, especially in the aerospace industry, tl,z,31 However, these gains are offset by a dramatic loss in ductility and fracture toughness. Such unfavorable fracture properties severely restrict a major development of A1-SiCw composites for structural applications. A fundamental understanding of the evolution of composite microstructure and its effects on the mechanisms of deformation and failure is essential to developing better composite materials with improved fracture resistance. A number of studies [2,4-61 have been reported concerning the mechanical properties and the fracture toughness of A1-SiCw composites. Tensile elongation of AI-SiCw composites is only 3 to 4 pct and their fracture toughness is about 20 MPa. m 1/2 at best, although there have been considerable efforts to improve these properties. One of the important topics relevant to these efforts is the study of the micromechanisms of fracture that might provide a means for rationalizing the microstructural origin of brittleness. As for 2124 A1-SiCw composites processed by hot pressing above the matrix solidus temperature followed by hot extrusion, major reasons for the brittleness have been suggested as follows: (1) void initiation at whisker ends due to severe stress concentration and resultant localized plastic f l o w ; [7'8'9] (2) decohesion of whisker/matrix interface caused by separation of interface oxide layers; IT,l~ and (3) cracking of load carrying SiC whiskers which were damaged or degraded during composite processing.[12.~31 A few recent w o r k s I14-18] have also emphasized the effect
YOUNG-HWAN KIM, Graduate Student, and SUNGHAK LEE and NACK J. KIM, Associate Professors, are with the Center for Advanced Aerospace Materials, Pohang Institute of Science and Technology, Pohang, 790-600 Korea. Manuscript submitted July 15, 1991. METALLURGICAL TRANSACTIONS A
of matrix microstructure on the deformation and failure behavior of these composites. However, the role of coarse matrix intermetallic particles in fracture processes has not yet been investigated in detail, although their presence in the microstructure on the powder metallurgy (PM) processed 2124 A1-SiCw composites has been reported in several studies. 17,1~ Most of the previous investigations in this field are based on the fine scale transmission electron microscope (TEM) observation of damaged microstructures and, accordingly, provide no further information on the critical events in fracture process and interrelation between two or more different microfracture mechanisms. Recently, the techniques of dynamic fracture observation, which have been used in a few recent fracture studies for other A1-SiC composite systems, ttg,z~ provide more detailed and instructive information on the fracture initiation and propagation processes. Thus, in this study, the fracture processes hav
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