EFFECT OF COOLING RATE AND FRONT VELOCITY ON SOLIDIFICATION MICROSTRUCTURE SELECTION IN AL-3.5WT-PERCENT-FE-O TO 8.5WT-PERCENT-SI ALLOYS

The aim of this work was to study the effect of solidification conditi ons 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 direct ional solidification. Wedge casting gave cooling rates in the range 10 (0)-10(3) K s(-1), whilst Bridgman solidification was carried out at f ront velocities between 10(-2) and 2 mm s(-1), and a temperature gradi ent of about 15 K mm(-1). Far the first three alloy compositions, the first phase to appear for the slower cooled regimes was the equilibriu m primary phase of Al13Fe4. Material which solidified in wedge casting s at cooling rates greater than about 150 K s(-1) did so without forma tion of primary intermetallic and had microstructures of alpha-Al dend rites with interdendritic eutectic. Primary intermetallic was also sup pressed in the Bridgman solidified material al front velocities greate r than approximately 1 mm s(-1). The as-solidified microstructure of t he ahoy containing 8.5 wt.% Si at lower cooling rates in the wedge and at all front velocities in Bridgman solidified material contained pri mary beta-AlFeSi intermetallic in preference to the expected alpha-typ e AlFeSi ternary intermetallic, suggesting a shift in the liquidus sur face phase boundaries. (C) 1997 Elsevier Science S.A.