Quantitative Evaluation Method for the Micro- and Mesostructure of Irregular Al-Si Eutectics

Quantitative comparison of irregular eutectic microstructures requires descriptors that account for the fragmented, branched, and non-uniform morphology of the Si phase. This study extends an earlier image-analysis method by combining measurements of eutectic fraction, lamella length, spacing, and orientation. The method was demonstrated using one directionally solidified Al–12.6Si specimen and one Al–18Si specimen produced in a four-zone vertical Bridgman furnace. Four image-based descriptors were evaluated: eutectic ratio, a maximum-Feret-diameter-based lamella-length distribution, a perimeter-derived average lamella distance, and the projected two-dimensional orientation of the segmented Si lamellae. In the initial region below 30 mm, the average lamella-distance estimates were 13.1 µm for Al–12.6Si and 7.4 µm for Al–18Si. Between 40 and 80 mm, both values approached 3 µm as the local eutectic-front velocities converged to approximately 0.11 mm s−1. The fitted lamella-length decay coefficient increased from 31.59 to 84.63 mm−1 in Al–12.6Si, whereas values of approximately 44–55 mm−1 were obtained for Al–18Si. The Al–12.6Si specimen exhibited a dominant projected orientation of 15–25°, while a broader orientation distribution was observed for Al–18Si. These results demonstrate that the descriptors distinguish longitudinal microstructural variations within the two examined specimens.