2B lu üç fazlı ötektik dizilimlerin katılaştırma dinamikleri

Materials select particular growth patterns during solidification depending on the crystalline nature of phases as well as external parameters such as cooling rate, composition, and sample dimensions. Although one cannot change the nature of phases, solidification dynamics can be altered by changing the external parameters in order to understand and control the microstructural features of solidified materials. Controlling these features is highly critical since the material properties including mechanical, physical, and electrical properties are extremely dependent on the microstructure of the materials. Among these solidification patterns, alloys forming eutectic patterns are used extensively in industry in a wide range of processes such as casting, welding, and soldering of engineered components due to their favorable properties resulting from their micro-composite structure. The majority of the eutectic solidification studies in the literature focus on two-phased eutectic systems where there is only one possible arrangement of phases (the ABAB order), due to their simplicity compared to higher-order systems. However, in pursuit of new generations of materials, alloys obtained using ternary, quaternary, and even higher order systems, have been extensively used in the industry. With the advent of the third phase, the number of possible arrangements of phases and spacing adjustment mechanisms increases tremendously. In this thesis, the growth dynamics of In-Bi-Sn three-phased eutectic system is investigated experimentally using real-time directional solidification technique and 2D samples at the ternary eutectic composition. The stable microstructure formed in three-phased eutectics is found to grow with ABAC stacking order, where A, B, and C stand for individual lamella of the In2Bi, β-In, and γ-Sn phases, respectively. This ABAC order is not only stable in the basic state, but also in tilted and asymmetric patterns. At a given growth velocity V, this pattern is observed to be stable over a finite range of eutectic spacing, similar to binary eutectic microstructures. When the stable ABAC pattern is perturbed enough by changing the growth velocity, it is forced to go beyond the stability limits and as a result, the system employs spacing adjustment mechanisms like elimination, oscillation, and branching to go back to the stable regime. Beyond the upper limit of the stability, branching takes place, which is systematically preceded by the amplification of a period-preserving oscillatory mode (1λO) that has been predicted by phase-field simulations of a three-phased eutectic system [1]. However, in this study, oscillations were never observed to grow at steady-state, but they were transients that are either amplified or damped. Beyond the lower stability limit, the system eliminates a complete ABAC pattern where at two-phased eutectics only A or B phase is eliminated at spacing values below the lower limit. Additionally, the elimination observed in the two-phased eutectics is Eckhaus instability, whereas for three-phased eutectics the requirements of Eckhaus instability are found to be invalid. The lower stability limit for three-phased eutectics is measured to be substantially lower than the minimum undercooling spacing which is consistent with binary eutectic systems. The scaling constant [2] is found as λ_JH^2 V = 135 ± 35 μm3/s. The upper and lower limits of stability for ABAC pattern are found as Λbr = 1.9 ± 0.5 and 0.6 < Λc < 1.1, respectively, where Λ is normalized eutectic spacing with respect to minimum undercooling spacing. Finally, some observations of more complex patterns, tilted patterns, thickness-asymmetry in In2Bi phases, and large-scale superstructures are reported.