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The aging characteristics of aluminum alloy A356 and an aluminum alloy A356 containing hollow spherical fly ash particles were studied using optical microscopy, transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, hardness tests, and compressive tests. The variation of hardness and compressive strength as a function of aging time for the composite have been reported. Since the density of the composite is lower than that of the base alloy due to the presence of hollow particles, the composites have a higher specific strength and specific hardness compared to the matrix. Even though the hardness of the as-cast composite was higher than that of the base alloy, no significant change in the aging kinetics was observed, due to the presence of spherical fly ash particles in the matrix. Aging times of the order of 10^sup 4^ to 10^sup 5^ seconds were required to reach the peak hardness (92 HRF) and compressive strength (376 MPa) in both the A356-5 wt pct fly ash composite and the matrix alloy. The possible effects of shape and hollowness of particles, the interface between the matrix and the particles, the low modulus of the particles, and the microcracks formed on the surface of hollow fly ash particles on the kinetics of the age hardening of aluminum alloy A356 are discussed.
I. INTRODUCTION
THE incorporation of reinforcements in a metal matrix provides unique properties, including enhancements in the modulus and strength of metal matrix composites (MMCs). In recent years, cast MMCs have found increasing use in automotive applications, including brake rotors, pistons, and drive shafts, to conserve energy and the environment. There is an increasing interest in composites where the reinforcements are lower in cost and in density than the matrix; an example is the possible use of solid or hollow spherical fly ash particles as fillers or reinforcements in aluminum matrix composites. Recently, cast aluminum-fly ash composites with low density, low cost, and improved hardness and wear resistance have been developed.[1,2]
The mechanical properties of aluminum alloys are influenced by chemical composition, casting process, and microstructures, including grain size, dendrite arm spacing, porosity, and heat treatment.[3] Dislocations and excessive vacancies introduced in the matrix during quenching accelerate precipitation in the matrix during aging. Excessive vacancies enhance the diffusion of solute...