The effect of matrix microstructure on the tensile and fatigue behavior of SiC particle-reinforced 2080 Al matrix compos
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NTRODUCTION
THE mechanical behavior of metal matrix composites (MMCs) has been shown to be very dependent on matrix microstructure, which is determined by a combination of the matrix alloy, reinforcement, and thermomechanical processing history of the material.[1–5] Processing-related defects in the form of intermetallic inclusions or particle clusters are also part of the matrix microstructure and play a role in fatigue strength.[6–9] These defects act as stress concentrators that increase the local stress intensity and promote easy crack nucleation. In MMCs, the presence of the reinforcement significantly affects the nature of the matrix microstructure. The addition of reinforcement has been shown to result in decreased grain size, accelerated aging, and changes in precipitate size and distribution.[10,11, 12] There is also a high density of dislocations near the reinforcement/matrix interface, which is caused by the mismatch in the coefficients of thermal expansion (CTE) between the particle and the matrix. The higher thermal expansion in the matrix produces plastic deformation during cooling and, therefore, the observed increase in dislocation density.[13,14,15] These effects can be attributed in N. CHAWLA, formerly Research Fellow, Department of Materials Science and Engineering, University of Michigan, is Assistant Professor, Department of Chemical, Bio, and Materials Engineering, Arizona State University, Tempe, AZ 85287-6006 [email protected] . U. HABEL, formerly Research Fellow, Department of Materials Science and Engineering, University of Michigan, is Senior Research Engineer, Crucible Research Co., Pittsburgh, PA 15205-1022. Y.-L. SHEN, Assistant Professor, is with the Department of Mechanical Engineering, University of New Mexico, Albuquerque, NM 87131. C. ANDRES, formerly Research Fellow, Department of Materials Science and Engineering, University of Michigan, is at Jochim-Sahling-Weg 63, Hamburg 22549, Germany. J.W. JONES, Associate Dean for Undergraduate Education, is with the College of Engineering, University of Michigan, Ann Arbor, MI 48109. J.E. ALLISON, Senior Staff Technical Specialist, is with the Scientific Research Laboratory, Ford Motor Company, Dearborn, MI 48124. Manuscript submitted January 29, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
part to nonhomogeneous deformation of the matrix, which arises from the significantly greater thermal expansion coefficient of the aluminum matrix compared to that of the SiC reinforcement. Cooling from elevated temperatures produces tensile strains in the matrix near the particles, which is accommodated by increased dislocation density. The inhomogeneous dislocation distribution then affects subsequent plastic flow, yield strength, and work hardening, as well as the precipitation kinetics during aging.[16,17] Strengthening of composites in this manner is an example of “indirect strengthening,” i.e., strengthening that is due to a change in matrix microstructure as a result of the addition of the reinforcement. Thus, comparison of properties between
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