Exotic three-phase microstructures in the ternary Ag-Cu-Sb eutectic system

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Eutectic solidification in ternary systems is a complex process involving the interaction of several material and process parameters resulting in the formation of intriguing patterns. Recently, we have observed the evolution of complex patterns in the ternary Ag-Cu-Sb eutectic system consisting of two intermetallic phases (Ag3Sb, Cu2Sb), along with the Sb-rich solid-solution. A vast range of microstructures in this system arise due to the possibilities of both invariant reactions giving rise to three-phase eutectic growth as well univariant reactions that are amenable to diffusive instabilities giving rise to microstructures involving two-phase colonies along with three-phase eutectic morphologies. Additionally, the combination of interfaces influenced by anisotropy in either the solid-solid and solid-liquid interfaces leads to the formation of exotic patterns. This paper investigates the origin of such three-phase microstructures along with two-phase growth instabilities that arise in the vicinity of three-phase eutectic composition. The different ternary compositions are directionally solidified at various velocities and the solidified samples are morphologically and crystallographically characterized using a combination of Scanning and Transmission electron microscopes along with high energy X-ray tomography in order to understand the microstructure evolution. The invariant three-phase eutectic reactions give rise to hollow/enclosed, dog bone and fibrous (Sb) crystals along with lamellar/rod type morphologies of the Cu2Sb phase while the Ag3Sb has a continuous morphology. The different microstructures have an underlying crystallographic basis and emerge as a result of different orientation relationships between the crystals as well as distinct growth mechanisms that are influenced by the crystal orientation with the imposed temperature gradient. Among the compositions that give rise to two-phase colonies, a particularly interesting structure emerges in the Ag3Sb-(Sb) two-phase colonies that exhibit a complex 3-fold fish skeleton structure reminiscent of the rhombohedral (Sb) crystals. Similarly, the Ag3Sb-Cu2Sb colonies exhibit a complex plate morphology influenced by the anisotropic nature of the Ag3Sb-Cu2Sb interface.