Mechanical behavior and failure micromechanisms of Al/Al 2 O 3 composites under cyclic deformation
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INTRODUCTION
THE tensile properties of Al-base composites reinforced with strong ceramic particulates have been the object of detailed studies during recent years. These materials are usually made up of a cast or wrought A1 alloy reinforced with 10 to 20 pct volume fraction of either A1203 or SiC particulates with an average size between 5 and 25 /zm. The ceramic particulates provide a significant improvement in stiffness (up to 50 pct) and more moderate increments in yield and tensile strength. Conversely, the ductility is dramatically reduced, and hence, the fracture toughness is negatively affected. Research on these materials has been focussed on the changes induced by the presence of ceramic reinforcements in the matrix microstructure and on the mechanisms of plastic deformation and failure. The results of these analyses constitute a fairly coherent body, which explains qualitatively (and sometimes quantitatively) the tensile behavior of commercially available Al-base composites. The studies have been reviewed in various recent monographs. [1-4] A sensible addition to the research activity in this field is to study the mechanical response of these materials under different loading conditions. These new results are very interesting as to check on the range of applicability and on the shortcomings of present-day theories regarding strengthening and fracture in particulate-reinforced metal-matrix composites (MMC), providing a deeper knowledge of the relationship between microstructure and P. POZA, Graduate Research Assistant, and J. LLORCA, Associate Professor, are with the Department of Materials Science, Polytechnic University of Madrid, E.T.S. Ingenieros de Caminos, 28040 Madrid, Spain. This article is based on a presentation made in the symposium entitled "Creep and Fatigue in Metal Matrix Composites" at the 1994 TMS/ASM Spring meeting, held February 28-March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASMMSD Composite Materials Committee. METALLURGICAL AND MATERIALSTRANSACTIONS A
properties. In particular, the behavior under fully reversed cyclic deformation is a first step in this direction, and in addition, the determination of the cyclic stressstrain curve is important because it is a critical property for the design of structural components subjected to lowcycle fatigue. However, most of the research on fatigue of particulate-reinforced MMC has been directed to the analysis of crack propagation (see for instance, the appropriate chapters in References 1 through 4); the mechanical response of particulate-reinforced MMC under cyclic deformation has been addressed in only a few studies.tS-~tl Moreover, data on the complete cyclic stressstrain curve are scarce, and the differences and similitudes in the failure micromechanisms between monotonic and cyclic deformation have not been compared quantitatively. The analysis of the cyclic properties of particulatereinforced MMC is complex and requires selective studies to account separately for the influence of the various f
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