Effect of particle size on the microstructure of rapidly solidified Al–20Si–5Fe–2X (X = Cu, Ni, Cr) powder

The effects of particle size and alloying elements on the size and morphology of secondary/intermetallic phases in gas atomized Al–20Si–5Fe–2X (X = Cu, Ni, Cr) powders were studied. The microstructure composed of particle-like primary Si with average diameter of 0.5–2.7 μm, δ-Al4FeSi2 intermetallic compound containing small amount of dissolved alloying elements with the size of 1.4–7.7 μm, and fine Al–Si eutectic matrix with 0.25–0.4 μm spacing. As the powder particle size decreases, the primary silicon becomes finer while its morphology is not affected. For instance, 58–75% reduction in the size of primary silicon was observed when decreasing the average particle size from 89 to 18 μm. A significant refinement in the size of δ-Al4FeSi2 intermetallic, i.e. 52–79% refinement, was also observed. This particle size effect on the microstructure of Al–20Si–5Fe alloy was found to be influenced by the presence of chromium. While Cu and Ni exhibited only marginal influence on the microstructural refinement, the effect of Cr was observed to be significant. In this case, the morphology of δ-intermetallic compound was changed from acicular to particle-like when the size of particles is <38 μm. The microstructural refinement by controlling the particle size leads to an increase in the hardness of the powder particles in the amount of 7–17% dependent on the composition.