Abstract: Controlling the morphology of Al₂Cu intermetallic compounds (IMCs) has been of importance to enhance the properties of Al-based alloys. However, the quantification of Al₂Cu IMCs with diversified morphologies is still lacking, and the morphological evolution of Al₂Cu dendrites remains poorly understood. Using synchrotron X-ray tomography, we have directly quantified the morphological evolution of proeutectic Al₂Cu IMCs in directionally solidified Al-Cu alloys. The three-dimensional (3D) morphologies of Al₂Cu IMCs under different growth rates were quantified using volume, specific surface area, interconnectivity, tortuosity, and Gaussian curvature. The faceted morphology under slow growth rate was divided into three different types, including single hollow prism, irregular prism lacking partial faces, and coalesced prism consisting of two adjacent crystals. The morphological transition from faceted prism to non-faceted algae-like, irregular tree-like, and typically dendritic shapes with increasing growth rates was determined, reflecting the growth modes varied from lateral mode to intermediary and continuous modes. The non-faceted Al₂Cu dendrite had one primary stem, three groups of secondary arms, and a faceted tip. The angles between secondary arms were 120°, and the tip consisted of (011) and ($01\bar{1}$) planes. This work provides a deep understanding of the formation and growth of complex IMCs in metallic alloys.

Key words: Proeutectic Al₂Cu intermetallic, Synchrotron tomography, Directional solidification, 3-D morphology