Aluminum alloy A356 matrix syntactic foams filled with SiC hollow particles (SiCHP) are studied in the present work. Two compositions of syntactic foams are studied for quasi-static and high strain rate compression. In addition, dynamic mechanical analysis is conducted to study the temperature dependent energy dissipation and damping capabilities of these materials. The thermal characterization includes study of the coefficient of thermal expansion (CTE). A356/SiCHP syntactic foams are not strain rate sensitive as the compressive strength displayed little variation between the tested strain rates of 0.001-2100 s −1. Microscopic analysis of the high strain rate compression tested specimens showed that the fracture is initiated by the failure of hollow particles at the onset of the plastic deformation region. This is followed by plastic deformation of the matrix material and further crushing of particles. The syntactic foams showed decrease in storage modulus with increasing temperature and the trend was nearly linear up to 500 °C. The alloy shows a similar behavior at low temperature but the decrease in storage modulus increases sharply over 375 °C. The loss modulus is very small for the tested materials OPEN ACCESSMetals 2014, 4 531 because of lack of viscoelasticity in metallic materials. The trend in the loss modulus is opposite, where the matrix alloy has lower loss modulus than syntactic foams at low temperature. However, over 250 °C the matrix loss modulus starts to increase rapidly and attains a peak around 460 °C. Syntactic foams have higher damping parameter at low temperatures than the matrix alloy. Incorporation of SiCHP helps in decreasing CTE. Compared to the CTE of the matrix alloy, 23.4 × 10 −6 °C −1 , syntactic foams showed CTE values as low as 11.67 × 10 −6 °C −1 .

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The use of γ Fe2O3 as a magnetic recording pigment is well known. Most commonly it is prepared by dehydration of the ∝ FeOOH to ∝ Fe2O3, reduction of ∝ Fe2O3 in hydrogen or other reducing gas to Fe3O4 and finally oxidation of the Fe3O4 to γ Fe2O3.Because of the known influence of grain boundaries on magnetic domains, a knowledge of the microstructure of the individual magnetic particles is important in theoretical considerations of switching mechanisms. Literature references disagree on the microstructure of γ Fe2O3. Some investigators claim the individual particles are essentially single crystalline ; others claim the particles are polycrystalline. In the present work, we have found no evidence that sub-grains are nucleated during the structural transformations of ∝ Fe2O3 to γ Fe2O3. Rather we have found that the single crystalline character of the starting ∝ FeOOH is preserved through the conversion to γ Fe2O3.