Irregular eutectic growth in the copper-boron alloy during directional solidification

Many industrially important alloys belong to the class of irregular eutectics, which has motivated extensive and ongoing research on their solidification behavior. However, the mechanisms governing morphological and microstructural selection, the occurrence of coupled and decoupled growth modes, and the transitions between these regimes are not yet fully understood. The Cu–B irregular eutectic alloy is particularly unique because it exhibits microstructural characteristics analogous to both Al–Si and Fe–C eutectic systems. Despite this relevance, a systematic microstructural characterization of the Cu–B eutectic has not been reported to date. In this work, Cu–B is employed as a model irregular eutectic alloy and investigated using a Bridgman-type directional solidification setup to systematically characterize nonfaceted–faceted solidification microstructures in this class of alloys. The results show that the solidification velocity (V) is the decisive parameter in the evolution of boron morphologies. Three shape descriptors were simultaneously employed to be able to quantify the boron morphologies. Quantitative analyses reveal that the massive boron morphology dominates the microstructure, exhibiting the largest area fraction compared with flake and fiber morphologies at low V, while increasing the velocity favors the development of flake and fiber morphologies. The morphological transitions occur gradually, and at an intermediate velocity of V = 2 µm/s, all three morphologies coexist with comparable area fractions. In addition, the initial transient of irregular eutectic growth in the copper–boron alloy, various irregular eutectic microstructures, three-dimensional eutectic boron morphologies obtained through deep-etching, and several notable two-phase patterns are reported.