Üç fazlı sistemde ötektik büyüme dinamikleri

Solidification is involved in a variety of manufacturing processes such as casting, welding, soldering, and additive manufacturing. Solidification microstructures, which are the traces left in the solid by the propagating solid/liquid interface, evolve depending on the inherent characteristics of the materials as well as the process parameters. The properties of materials are significantly dependent on the microstructure and therefore, understanding the pattern formation phenomena is necessary to control and optimize the material properties. Because many industrially used materials are multi-phase, in recent years, an extensive research effort is dedicated to understanding how different microstructures form during solidification. However, this is a challenging task due to the fact that the multiplicity of phases and interphase boundaries lead to formation of highly complex microstructures. One of the examples of multi-phase materials is three-phase eutectic systems in which three crystalline phases are spontaneously growing from a homogenous liquid at the non-variant eutectic composition. Due to not having a mushy-zone and having well-ordered patterns, low melting temperatures, and excellent fluidity during casting, eutectic systems are used extensively in the industry. The research effort explained in this thesis aims to understand the fundamental and crucial, but currently unknown, aspects of three-phase eutectic growth by performing systematic experiments on In-Bi-Sn ternary system. The employed methodology is based on real-time investigation of solidification front dynamics from both sides of the quasi-2D samples using a novel optical microscopy system along with directional solidification techniques, namely directional solidification and rotating directional solidification setups. In the first part of the thesis, the effect of sample thicknesses on the stability of the ABAC-type growth pattern, which is the basic state of the three-phase eutectic system in absence of interphase energy anisotropy, is investigated. It is found that by increasing sample thickness, the stability region of the ABAC pattern shrinks and this effect is correlated with the mechanisms of the instabilities, namely lamellae elimination and branching. The dynamics and features of these instabilities are characterized thanks to the double-sided microscope by observing the phenomena simultaneously from top and bottom of the samples. Two key pattern recovery mechanisms are identified, namely phase invasion and phase exchange, which assist the system to spontaneously recover the isotropic ABAC pattern if the stacking faults are formed in the arrangement. In the second part, different three-phase eutectic grains are identified as a function of interphase anisotropy. Depending on the 2D-Wulff plots of the interphase boundaries, four distinct eutectic grain types are established in three-phase systems, namely isotropic, nearly-locked, fully-locked, and separated eutectic grains. While the isotropic eutectic grains have orientation-independent interphase boundary energies, non-singular and singular minima in certain angular ranges of the interphases' energies lead to formation of the nearly-locked and fully-locked eutectic grains, respectively. In these grains, the lamellar structures are inclined into the low-energy direction of the interphase boundary energies. In the last type of the three-phase eutectic grains called separated eutectic grain, not only the microstructure is deviated from regular ABAC pattern, but also the morphologies of the phases are totally altered due to the interphase anisotropy. Two distinguishable separated microstructures are identified and possible origins of each are proposed. Finally, other types of microstructures that are observed within this study are classified in terms of sample thickness dependent and anisotropy-driven microstructures.