The effect of cooling rate on the morphology of primary Al 3Sc intermetallic particles in Al–Sc alloys

Abstract

The precipitation of Al 3Sc primary particles in aluminium alloy melts is of considerable technological significance due to their strong grain refining effect on cast structures. The nucleation behaviour and growth morphology of the Al 3Sc primary particles formed in a slightly hypereutectic Al–0.7Sc alloy have been studied for a range of cooling rates in a FEG-SEM, using extracted particles, and in the TEM. It has been found that the Al 3Sc particles nucleate heterogeneously on oxides within the melt. At slow cooling rates (∼1 K s −1), the particles exhibit faceted growth, forming {100} faceted cubic shaped particles and larger particles comprised of several attached cubes. At higher cooling rates (100–1000 K s −1), the particles' interface becomes unstable along the cube edges and corners, forming growth perturbations. This leads to the formation of particles, which still adhere to an overall cubic morphology, but have cellular–dendritic sub-structures.