Multiscale characterization of the nucleation and 3D structure of Al3Sc phases using electron microscopy and synchrotron X-ray tomography

Abstract

Scandium (Sc) has been long recognized as one of the most effective grain refining elements for Al alloys because of the Al3Sc phases formed in an Al melt containing Sc. However, there are still lack of comprehensive studies on the exact mechanism of how Al3Sc phases are nucleated in an Al melt and their true 3D structures. In this paper, we used scanning/transmission electron microscopy and synchrotron X-ray tomography to study the nucleation and true 3D structure of primary Al3Sc phases in an Al-2wt%Sc alloy. The multiscale characterization approach revealed that the micrometre α-Al2O3 particles present in the Al melt can facilitate the formation of stacking faults at the α-Al2O3-Al3Sc interface and therefore promote heterogenous nucleation of Al3Sc phases. SEM and tomography clearly revealed that individual primary Al3Sc phases were simple cubes with edge length of 10~35 μm; and majority of them were interconnected to form clusters with accumulative peak volume of ~10,000 μm3. At the interface between an Al matrix and an Al3Sc cube, the Al3Sc phases can also grow into nanometre size particle clusters due to the depletion of Sc.