Exotic eutectic microstructures
Kiran, Aramanda Shanmukha
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Solidification of eutectic systems delivers compelling examples of microstructure formation, which makes the phenomena intriguing to many engineers and scientists. Therefore, eutectic solidification is extensively studied experimentally, theoretically, and numerically. However, some exotic eutectic microstructures are still not understood, particularly the systems that exhibit anisotropy. The main objective of this thesis is to explore the microstructure formation in exotic systems during solidification. As directional solidification is a convenient way (imposes the temperature gradient, solidification velocity accurately and independently) to study the microstructure formation, the first part of the work was directed towards the development of directional solidification configuration. Subsequently, we investigated the microstructure formation in three different exotic eutectic systems. In the first system (Sn-Zn eutectic), we study the formation of two-phase microstructures with endeavors to bring new inferences, as the volume percentage of the (Zn)-phase in the eutectic is less than 10%, so one would expect it to form rods in the matrix of (Sn)-phase; instead, thin lamellae are observed. We claim that the rod-lamellar transition and well-defined lamellar orientation are due to the anisotropy of the free energy of the solid-solid interfaces. We deploy various methods/experiments for confirming the evidence of solid-solid interface anisotropy. We also provide the crystallographic orientation relationships between BCT-(Sn) and HCP-(Zn) in steady-state microstructures. In the second system (SnTe-Te eutectic), we investigate the evolution of complex patterns due to the addition of ternary impurities in the Sn-Te eutectic system that contains trigonal (Te) and an intermetallic SnTe phase with a cubic crystal structure. In this work, we examine the origin of such a microstructure that arises due to a two-phase growth instability induced by impurity addition. The binary eutectics (Sn-Te) and ternary eutectics (Sn-Te with an impurity addition) are directionally solidified at different interfacial velocities in order to study morphological evolution. The binary alloy exhibits a rod-like or an interconnected string of rods morphology, while the addition of a third component leads to a diffusive instability (similar to a Mullins-Sekerka instability) that results in the formation of two-phase colonies. The onset of instability depends on both the growth velocity and impurity concentration, while the growth direction of the cells is normal to the (0001) of (Te) and (111) of SnTe. Through the extensive use of multiple characterization techniques, we have explored the morphological characteristics and crystallography of these colonies. The colonies have a complex internal structure that bears a three-fold symmetry reminiscent of the trigonal symmetry of the (Te) crystal, arising possibly because of strong anisotropy in the solid-liquid interfacial energy or in the kinetics of growth. For the different impurity additions (Ag, Cu, Ge, In, Sb), the internal eutectic morphology of the colony due to the addition of Ag, Cu, Ge, In is different from that observed for the addition of Sb. The latter leads to the formation of lamellae, while a rod-like feature could be observed for impurities Ag, Cu, Ge, In. In the third system (Ag-Cu-Sb), we have investigated the formation of three-phase microstructures of the Ag-Cu-Sb eutectic system that contains two intermetallic compounds, i.e., silver antimonide-Ag3Sb, copper antimonide-Cu2Sb and an antimony rich solid-solution (Sb). 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. The different ternary compositions are morphologically and crystallographically investigated at various velocities. The invariant three-phase eutectic reactions give rise to exotic hollow, 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 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, an exciting 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.