Eutectic microstructures: Role of interfacial energy anisotropy and diffusivities
Khanna, Sumeet Rajesh
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Eutectic alloys find a wide application in industrial cast alloys, solder alloys, and in the emerging field of functional materials. Apart from technological applications, multiphase eutectic alloys have also received keen attention from a scientific viewpoint, where the coupled growth of multiple solid phases offers an exciting problem to study pattern formation in non-equilibrium systems. This thesis explores microstructure evolution in multiphase eutectic alloys grown under directional solidification conditions using the phase-field method. Although numerous thermodynamic and process parameters influence eutectic growth, in this work, we focus on the influence of solid-solid interfacial energy anisotropy and diffusivities on the microstructure evolution. In the first part, we study a two-phase eutectic system with a low volume fraction of the minority phase (mimicking the Sn-Zn eutectic) and explore the role played by solid-solid interfacial energy anisotropy in influencing the rod to lamellar transition. Microstructure evolution from the simulations is compared to that obtained from directional solidification experiments of the Sn-Zn eutectic. Subsequently, we investigate three-phase bulk eutectic growth, where the number of possible morphologies is much larger than those obtained in two-phase eutectic growth, for example, lamellar, brick and hexagonal patterns. We first study the morphological transition pathways that are already available when all the interfaces possess isotropic interfacial energies. Further, we explore how the morphological transition pathways are influenced by introducing different combinations of solid-solid interfacial energy anisotropy and varying the solutal diffusivity contrast in the liquid. Simulations are performed under constrained domain settings to capture mechanistic insights and estimate the stability regimes as well as under extended settings to study morphology selection under conditions comparable to experiments. In the last part, we study pattern formation in thin-film growth, where an infinite number of lamellar patterns with different cycle lengths are possible in the case of three-phase eutectics. Here, we study the stability of the simplest lamellar patterns of the type ABC and BABC to long-wavelength perturbations in the spacing and the competitive growth between these patterns. Pattern selection starting from a random lamellar configuration is also investigated, resulting in the steady-state microstructure exhibiting the occurrence of long mirror-symmetric patterns in addition to the simple ABC and BABC patterns. Moreover, we obtain interesting insights into the role of the solutal diffusivity contrast affecting the stable spacing regime of the patterns and consequently biasing the pattern selection.