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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 4 to 10 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.

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Cubic GaN was grown on 6H-SiC(0001) by electron-cyclotron resonance plasma-assisted molecular-beam epitaxy. The growth process consisted of first depositing a 20-nm GaN buffer, followed by ten periods of alternating layers of 1 monolayer (ML) Mn and 10 ML GaN, and finally capped with 30 nm GaN. High-resolution transmission electron micrographs of film cross sections were recorded and digital diffractograms were calculated to determine the lattice structures of the different film layers. It was found that the crystal structure in the GaN buffer and capping layer matches the 2H-wurtzite GaN. However, uniform cubic zinc-blende GaN phase was observed in place of the nominal multilayer Mn/GaN region. The density of defects typically observed in GaN films is drastically reduced within the cubic and capping layer, indicating improved film quality possibly due to the surfactant effect of Mn. Based on the one-dimensional Ising model of polytype formation, a mechanism is proposed to explain the growth of cubic GaN in the Mn/GaN region.

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