Effect of cooling rate and front velocity on solidification micro structure selection in Al-3.5wt.%Fe-0 to 8.5wt.%Si alloys

The aim of this work was to study the effect of solidification conditions on microstructure selection in Al-3.5Fe-0 to 8.5Si (wt.%) alloys cooled under non-equilibrium conditions. The alloys studied were Al-3.5Fe, Al-3.5Fe-1Si, Al-3.5Fe-2,8Si, Al-3.5Fe-7Si and Al-3.5Fe-8.5Si (wt.%), which were solidified by wedge chill casting and Bridgman directional solidification. Wedge casting gave cooling rates in the range 100–103 K −1, whilst Bridgman solidification was carried out at front velocities between 10−2 and 2 mm s−1, and a temperature gradient of about 15 K mm−1. For the first three alloy compositions, the first phase to appear for the slower cooled regimes was the equilibrium primary phase of Al13Fe4. Material which solidified in wedge castings at cooling rates greater than about 150 K s−1 did so without formation of primary intermetallic and had microstructures of α-Al dendrites with interdendritic eutectic. Primary intermetallic was also suppressed in the Bridgman solidified material at front velocities greater than approximately 1 mm s−1. The as-solidified microstructure of the alloy containing 8.5 wt.% Si at lower cooling rates in the wedge and at all front velocities in Bridgman solidified material contained primary β-AlFeSi intermetallic in preference to the expected a-type AlFeSi ternary intermetallic, suggesting a shift in the liquidus surface phase boundaries.