On the Effect of Interphase Boundary Energy Anisotropy on Morphologies: A New Type of Eutectic Grain Observed in a Three-Phase Eutectic System

Eutectic microstructures are dramatically affected by the anisotropy in interphase boundary energy. Depending on this anisotropy function, different eutectic grains may grow simultaneously at the same experimental conditions. In all reported quasi-isotropic and anisotropic two-phase and three-phase eutectic grains in thin samples, lamellar morphology is observed and the microstructure is essentially two dimensional (2D), since the interphase boundaries are perpendicular to the sample walls. Using the beta(In)-In2Bi-gamma(Sn) system and real-time solidification experiments in thin samples, we introduce a unique and new type of anisotropic three-phase eutectic grain, entitled here as "Laminated Matrix with Rods (LMR)." In this grain, due to the anisotropy in In2Bi/gamma(Sn) interphase boundary, the evolving phases, and hence, the microstructures observed through the two glass plates of the thin sample are completely different, despite the strong confinement effect. During rotating directional solidification (RDS) experiments, the morphology or the aspect ratio of all phases changes periodically and drastically. Specifically, In2Bi, beta(In), and gamma(Sn) phases evolve from all being lamellar perpendicular to the sample walls to the matrix, elongated/trapezoidal rods, and a lamella parallel to the sample walls, respectively. Our experimental results show that these morphological transitions are due to change in the interphase boundary orientation with respect to the growth direction.