The Growth Morphology and Nucleation Mechanism of Primary Ll<sub>2</sub> Al<sub>3</sub>Sc Particles in Al-Sc Alloys

Abstract

The role of the nucleation mechanism and growth morphology of primary Ll 2 , Al 3 Sc, particles in the grain refinement of Al castings has been investigated using a simple hypereutectic, binary Al-Sc alloy. New evidence suggests a two-stage grain refining mechanism: the primary Al3Sc particles first nucleate on impurities in the melt (possibly Sc 2 O 3 ) before nucleating the α-Al grains. The primary particles grow as faceted dendrites that maintain an overall cubic morphology. High resolution EBSD analysis has shown that α-Al nucleates from Al 3 Sc particles with either an epitaxial, or a twin relationship below the eutectic temperature. α-Al then grows outwards as a single phase, pushing particles that did not act as nucleation centres to the grain boundaries. Some α-Al + Al 3 Sc eutectic is formed at the grain boundaries during the last stages of freezing.