High-Temperature Fatigue of a Hybrid Aluminum Metal Matrix Composite
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RODUCTION AND BACKGROUND
METAL matrix composites (MMCs) typically consist of a ductile metal matrix (e.g., Al, Mg, Cu, Ti) and a high modulus ceramic reinforcement such as SiC, Al2O3, or TiC. Combining multiple material classes results in a composite having advantageous properties from each component. The matrix generally provides ductility and high thermal and electrical conductivity. The high modulus ceramic reinforcement acts to stiffen and strengthen the composite, which can restrict the composite thermal expansion. MMCs are often classified by the reinforcement morphology, the most common forms being fibers and particles. In most cases, only one reinforcement morphology is used, but there are situations in which utilizing multiple reinforcement types is advantageous. Composites with multiple reinforcement morphologies are known as hybrid composites, and their study has been limited.[1] To date, most investigations have been at room temperature, although it is expected that hybrid composites will be used at elevated service temperatures. In MMCs with large reinforcing particles (>15 lm), the composite is strengthened by load transfer from the matrix to the particles, which then carry a significant portion of the load. These large particles can increase the wear resistance over the monolithic material.[2] The load transfer strengthening mechanism is not as effective as dispersion strengthening in which smaller (1 lm) J.T. CLARK, formerly Graduate Student with the Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI, is now Metallurgist with Benteler Aluminium Systems Michigan Inc., Holland, MI. P.G. SANDERS, Assistant Professor, is with the Department of Materials Science and Engineering, Michigan Technological University. Contact e-mail: sanders@ mtu.edu Manuscript submitted January 17, 2013. Article published online September 7, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
particles provide barriers to dislocation motion. Fibers act to strengthen the composite in a manner similar to large particles, in which the load is transferred to the fibers through the fiber–matrix interface. Fibers can introduce anisotropy to the composite as the shear stress induced by the load is transferred along the length of the fiber parallel to the loading direction. A transverse fiber provides less strengthening, as the short distance over which the fiber is loaded is not great enough to reach the fiber fracture strength. Generally, fatigue performance of a material is dependent on the ease of crack initiation and growth, with emphasis on initiation in high-cycle fatigue and growth in low-cycle fatigue. For many systems, the addition of SiC particles has been found to increase the fatigue performance over a monolithic matrix metal.[3] Additions of alumina fibers can improve the fatigue performance of both Al and Mg composites.[4] Classic fatigue crack features related to initiation, growth, and final failure are present on the fracture surface of both particle and short fiber-reinforced MMCs.[5,6
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