Effect of Mn and cooling rates on α-, β- and δ-Al-Fe-Si intermetallic phase formation in a secondary Al-Si alloy

Secondary Al-Si alloys Al7.1Si(1.5-x(Mn))Fe(x(Mn))Mn with x(Mn) = 0, 0.3, 0.375, 0.6, 0.75 at.% have been solidified with different cooling rates: 0.05 K/s, 1.4 K/s, 11.4 K/s and 200 K/s. In the ternary alloy with x(Mn) = 0 at.%, formation of the primary alpha(h) phase is suppressed upon higher cooling rates at the cost of formation of plate-shaped beta and delta phase particles. In the quaternary alloys, with increasing Mn content, alpha(c)-phase particles with Chinese-script morphology form and replace the plate-shaped intermetallic particles. While the alpha(c) phase forms at intermediate cooling rates only, plate-shaped particles additionally form at low and high cooling rates. The beta phase dominates after solidification with lower cooling rates and the delta phase dominates upon higher cooling rates in the plate-shaped particles. The kinetic effect in terms of solidification rate and the chemical composition effect on the phase selection of Fe-containing intermetallic particles in the alloys along the solidification path have been discussed.