The Effect of Matrix Microstructure on the Tensile and Fatigue Behavior of SiC Particle–Reinforced 2080 Al Matrix Composites N. CHAWLA, U. HABEL, Y.-L. SHEN, C. ANDRES, J.W. JONES, and J.E. ALLISON The effect of matrix microstructure on the stress-controlled fatigue behavior of a 2080 Al alloy reinforced with 30 pct SiC particles was investigated. A thermomechanical heat treatment (T8) produced a fine and homogeneous distribution of S' precipitates, while a thermal heat treatment (T6) resulted in coarser and inhomogeneously distributed S' precipitates. The cyclic and monotonic strength, as well as the cyclic stress-strain response, were found to be significantly affected by the microstructure of the matrix. Because of the finer and more-closely spaced precipitates, the composite given the T8 treatment exhibited higher yield strengths than the T6 materials. Despite its lower yield strength, the T6 matrix composite exhibited higher fatigue resistance than the T8 matrix composite. The cyclic deformation behavior of the composites is compared to monotonic deformation behavior and is explained in terms of microstructural instabilities that cause cyclic hardening or softening. The effect of precipitate spacing and size has a significant effect on fatigue behavior and is discussed. The interactive role of matrix strength and SiC reinforcement on stress within “rogue” inclusions was quantified using a finite-element analysis (FEA) unit-cell model. I. INTRODUCTION part to nonhomogeneous deformation of the matrix, which arises from the significantly greater thermal expansion coef- THE mechanical behavior of metal matrix composites ficient of the aluminum matrix compared to that of the SiC (MMCs) has been shown to be very dependent on matrix reinforcement. Cooling from elevated temperatures produces microstructure, which is determined by a combination of tensile strains in the matrix near the particles, which is the matrix alloy, reinforcement, and thermomechanical pro- accommodated by increased dislocation density. The inho- cessing history of the material. [1–5] Processing-related mogeneous dislocation distribution then affects subsequent defects in the form of intermetallic inclusions or particle plastic flow, yield strength, and work hardening, as well as clusters are also part of the matrix microstructure and play the precipitation kinetics during aging. [16,17] Strengthening a role in fatigue strength. [6–9] These defects act as stress of composites in this manner is an example of “indirect concentrators that increase the local stress intensity and pro- strengthening,” i.e., strengthening that is due to a change mote easy crack nucleation. in matrix microstructure as a result of the addition of the In MMCs, the presence of the reinforcement significantly reinforcement. Thus, comparison of properties between affects the nature of the matrix microstructure. The addition unreinforced and reinforced materials with dissimilar micro- of reinforcement has been shown to result in decreased grain structures presents a problem. size, accelerated aging, and changes in precipitate size and Since the fatigue properties of Al alloys are known to be distribution. [10,11, 12] There is also a high density of disloca- very sensitive to microstructure, [18,19,20] a main concern of tions near the reinforcement/matrix interface, which is this work was to vary the matrix microstructure by utilizing caused by the mismatch in the coefficients of thermal expan- thermal and thermomechanical heat treatments, while keep- sion (CTE) between the particle and the matrix. The higher ing the reinforcement volume fraction and size constant. thermal expansion in the matrix produces plastic deforma- The behavior of the composites under monotonic and cyclic tion during cooling and, therefore, the observed increase in conditions is compared. Microstructure and mechanical dislocation density. [13,14,15] These effects can be attributed in behavior resulting from different heat treatments are com- pared and interpreted, with a focus on fatigue properties. A finite-element analysis (FEA) unit-cell model was used to N. CHAWLA, formerly Research Fellow, Department of Materials Sci- ence and Engineering, University of Michigan, is Assistant Professor, further elucidate the effect of matrix strength on the stress Department of Chemical, Bio, and Materials Engineering, Arizona State concentration occurring within the near-surface “rogue” University, Tempe, AZ 85287-6006 nchawla@asu.edu . U. HABEL, for- inclusions, which have been associated with initiation of merly Research Fellow, Department of Materials Science and Engineering, fatigue cracks. [21] 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, Depart- II. MATERIALS AND EXPERIMENTAL ment of Materials Science and Engineering, University of Michigan, is at PROCEDURE Jochim-Sahling-Weg 63, Hamburg 22549, Germany. J.W. JONES, Associ- ate Dean for Undergraduate Education, is with the College of Engineering, A 2080 Al alloy and a 2080 Al alloy reinforced with SiC University of Michigan, Ann Arbor, MI 48109. J.E. ALLISON, Senior particles were processed by powder metallurgy and extruded Staff Technical Specialist, is with the Scientific Research Laboratory, Ford (Alcoa Technical Center, Alcoa, PA). The composition of Motor Company, Dearborn, MI 48124. Manuscript submitted January 29, 1999. both the base alloy and the matrix of the composite was 3.6 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 31A, FEBRUARY 2000—531