Effects of heat treatment and reinforcement size
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I.
INTRODUCTION
D I S C O N T I N U O U S L Y reinforced aluminum (DRA) composites are being increasingly considered for structural applications in the aerospace and automotive industry. Ceramic particulate reinforcements are utilized to produce composite materials with higher strength and elastic modulus compared to the unreinforced matrix alloy. This is often accompanied by a loss in ductility, although recent works }1-4} have shown that secondary processing of both powder metallurgy and cast composites can produce significant improvements in strength and ductility in both Al-based t~'2,3j and Mg-based t4,5} systems. Various aspects of the mechanical behavior of DRA alloy composites have been studied by many authors, 1~-171 and the effects of heat treatment, tr-~l matrix microstructure, }6-~ and volume percent of ceramic particulate reinforcement on the overall mechanical properties t6-161 have been established. However, the role of the reinforcement particles in the overall fracture process is still not very clear. Fracture initiation in particulatereinforced composites is associated with one, or a combination of the following factors: particulate fracture, failure at or near the particulate-matrix interface, or failure initiation in the matrix. Catastrophic fracture depends on the interaction and propagation of these small PREET M. SINGH, Senior Research Associate, and JOHN J. LEWANDOWSKI, Associate Professor, are with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted March 13, 1992. METALLURGICAL TRANSACTIONS A
initial cracks. In general, fracture depends on the characteristics of the reinforcement (e.g., spatial and size distribution) as well as on the matrix properties, i6,7.18}For 7xxx (A1-Zn-Mg-Cu) series DRA materials, analysis of fracture surfaces has indicated that the predominant mode of fracture changes from one of particle fracture in an underaged (UA) heat treatment to one of failure near the reinforcement/matrix interface and failure in the matrix for the overaged (OA) conditions, }6-8'191while preferential void nucleation has been observed in regions of locally high-volume fractions of particulates. I6"7"19"2~ In addition, initial analyses have revealed that particulate fracture occurs preferentially in the large particles as well as those with higher aspect ratios than the average distributions.t6.7.19.2~ The present study focuses on the effects of matrix heat treatment and reinforcement size on the evolution of SiCp fracture, and its role in the deformation and fracture behavior of DRA materials under tensile loading conditions. Continuing work investigating the effects of matrix selection, particulate size, heat treatment, and the effects of superimposed pressure on such features is presented elsewhere, tI~
II.
EXPERIMENTAL PROCEDURES
The powder-processed DRA material used in this study, designated ALCOA* MB78, contains 7 pct Zn, *ALCOA is a trademark of Aluminum Company of America, Pittsburgh, PA.
VOLUME 24A,
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