Ellipsometric study of optical properties and oxidation processes of compacted powders based on aluminum alloys

Abstract

The paper presents the results of an ellipsometric study of compacted powders of aluminum-based binary alloys containing 1,5 wt.% of rare earth elements (Sc, La, Ce, Sm) and cast aluminum-silicon alloys with the following compositions: Al–10Si–0,5Mg– 0,3Fe–0,1Ca and Al–12Si–0,6Mg–0,5Fe–0,5Ca–0,45Na. An immersion method was used to determine the optical constants of massive polycrystalline alloys obtained by remelting these powders in vacuum, as well as their oxide films for a wavelength λ = 0,6328 μm. Using the optical constants of these alloys, the dependence of their reflectivity on the surface oxide film thickness was calculated. It was found that an increase in the amount of the alloying component and intermetallic phases in the alloy decreases its reflectivity. In addition, the optical constants were used in the construction of modified Δ–ψ nomograms calculated using the Maxwell-Garnett equation that make it possible to determine the thicknesses of oxide films on particles and the volume fractions of metal in compacted powders, and to study the processes of their oxidation in air. It was shown that oxidation of aluminum ASD-4 powders and Al–1,5% REM binary alloys at 600 °C is described by a simple model where a decrease in the metal fraction leads to an increase in the oxide film thickness. It turned out that the oxidation of aluminum-silicon alloys is much faster and not described by this model, which may be due to the appearance of a liquid phase in the powder. A large number of metal droplets on the surface of particles increase the amount of metal on the studied tablet surface in general. The high oxidation rate of aluminumsilicon alloys in air can be explained by the surface activity of magnesium in relation to liquid aluminum.