Morphology evolution and growth mechanism of primary Al3Sc and eutectic Al3Sc in Al–Sc alloys

Abstract

Al–Sc master alloys are crucial raw materials for the preparation of high-strength and high-toughness Al alloys. The primary particles in these alloys are Al3Sc, which has been proved to have strong grain refining and strengthening effects. The material properties are related to the morphology of the primary particles. However, the mechanism of morphological changes of primary Al3Sc particles at different cooling rates during industrial production is not clear. To address this, we conducted a study where we controlled the solidification cooling medium and obtained the primary Al3Sc particles with different morphologies. Also, the three-dimensional (3D) morphology of the eutectic Al3Sc phase was obtained by holding at eutectic temperature for 72 h. The results indicate that the cooling rates have a strong impact on the morphology of primary Al3Sc particles due to their high anisotropy and constitutional supercooling in the melt. The primary Al3Sc particles have a multilayer structure of "Al3Sc+α-Al + Al3Sc+ … ", which can be explained in terms of solidification sequence. The eutectic Al3Sc phase is fibrous and perpendicular to the surface of the matrix or the primary Al3Sc particle, and grows in a spiral pattern.