Analysis and Control of Microstructure in Binary Alloys
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When metallic alloys solidify, various microstructures form inside the alloys. Most solidified alloys have a polycrystalline structure, which is an assembly of crystalline grains with boundaries between any two grains. Each grain is a single crystal with a unique crystalline orientation. Many physical properties of polycrystalline alloys are determined by the arrangement of these grains and grain boundaries. During solidification of a single crystal, microstructures with even smaller microscopic lengthscales form, such as dendritic and eutectic structures. The physical properties of single crystal alloys are largely influenced by the lengthscales of these structures. Therefore, the understanding and control of microstructure formation in solidification is important in order to achieve desired properties.
Microstructures form while the system is not in equilibrium. What microstructures form is not based on minimization of free energy of the system, but depends on the dynamics of the solidification process, which is the focus of our study. We used an alloy model system, Succinonitrile-Coumarin152, to experimentally investigate dynamic selection and control of grain boundary structures and dendritic structures in binary alloys. We found that in a temperature gradient the grain boundaries drift toward the high temperature region in addition to the migration due to grain coarsening. We show how we can control grain boundary orientations by generating local temperature gradient through UV or laser heatings. We show that perturbations also permit accurate control of the microstructure within a single crystal during the directional solidification process. Dendritic patterns can be controlled either by guiding the initial formation of the pattern or by triggering subcritical transitions between stable microstructures. We also investigated the role of surface tension anisotropy on the stability of cellular/dendritic arrays using three crystals of different growth orientations with respect to the surface tension anisotropy. We found that the surface tension anisotropy affects the spacing between dendrites and stability via the surface tension perpendicular to the growth direction.