A finite element model of the effects of primary creep in an Al-SiC metal matrix composite
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INTRODUCTION
T H E enhancement of mechanical properties has provided strong motivation for the development of metal matrix composites (MMCs) containing particulates, whiskers, and fibers. Tests performed by numerous investigators have revealed substantial improvements in elastic modulus and increases in ultimate tensile strength by as much as 50 pct for whisker reinforcement. 11-61The discontinuously reinforced composites offer the further advantages of formability using conventional technology and essentially isotropic material properties. For these reasons, they have received the greatest attention for potential applications in the aerospace and automotive industries. In general, there is an increase in composite strength and stiffness with increasing reinforcement volume fraction and aspect ratio. A corresponding reduction in the ductility by as much as an order of magnitude and decrease in coefficient of thermal expansion occurs with the addition of reinforcements to the matrix. Metal matrix composites based on aluminum alloys containing SiC reinforcements have been studied almost exclusively. The qualities of A1 MMCs that make them
STEVEN L. ATKINS, formerly Graduate Research Assistant, University of California-Davis, is a member of the Technical Staff, Microwave Products Division, Watkins-Johnson Company, Palo Alto, CA 94304. JEFFERY C. GIBELING, Associate Professor, is with the Division of Materials Science and Engineering, University of California-Davis, CA 95616. 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/ ASM-MSD Composite Materials Committee.
METALLURGICAL AND MATERIALSTRANSACTIONS A
particularly attractive are the increases in specific stiffness and specific strength. In elevated temperature applications, the creep strength of these materials is also improved compared with the unreinforced matrix material. 17,8.9] Although most investigators agree that the mechanical property enhancements occur when reinforcements are added to a matrix, there is a lack of consensus on the mechanisms that cause these improvements. Some of the theories of strengthening that have been proposed include superposition of precipitation hardening and Orowan bowing, strengthening due to high dislocation densities coupled with a fine distribution of GuinierPreston zones near particulates, subgrain size refinement, and shear-lag transfer of loads from the matrix to the reinforcement. [3,1~ The strength of the bond between matrix and reinforcement plays a key role in determining the mechanical response of the composite. For this reason, proper assumptions regarding the bonding characteristics are crucial to the development of accurate models of mechanical behavior. Aluminum is known to react with SiC and has good wetting properties, t2,6j As a consequence, A1 composites with SiC generally exhibit good bonding if th
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