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The formation of two-phase eutectic colonies is often observed in microstructures of directionally solidified ternary alloys. Their formation is driven by microscopic instabilities in a macroscopic planar solidification front, due to impurities of the minor component, diffusing from the two solidifying phases into the liquid. The growth conditions for eutectic colonies, their interactions and their responses to the microstructure during growth are the focus of the current work. Therefore, phase-field simulations based on a grand potential formalism are performed for the high-performance material NiAl-34Cr. To enable the evolution of eutectic colonies in two-dimensional simulations, a concentrationdriven nucleation mechanism is introduced into a multiphase-field framework and is subsequently validated. With this mechanism, two-dimensional largescale phase-field simulations are conducted to study the influence of the applied temperature gradient on the evolving colonies. The patterns are quantitatively analyzed by measuring their number, size and height. Furthermore, the adjustment processes between the eutectic colonies during the directional solidification are investigated. The results demonstrate the ability of the presented phase-field approach with integrated nucleation mechanism for the formation of eutectic colonies in two-dimensional simulations.

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When an Al-droplet is in contact with an Au-solid substrate, the liquid phase reacts with the substrate and an intermetallic layer is formed at the solid-liquid interface due to diffusion and reaction. This phenomenon has been commonly observed in the soldering process and the wetting is termed as reactive wetting, in contrast to the inert wetting where the droplet does not react with the base materials and the substrate is flat. Young's law can be used to interpret the contact angle in the static state, but is not able to predict the dynamics in the process of reactive wetting. In this work, we present a multiphase model including phase transition and fluid dynamics to investigate the effect of the formation of the intermetallic Al 2 Au phase and capillary flow on the reactive wetting in the Al-Au system. In order to capture phase boundaries of solid-, liquid-and intermetallic-vapor, phase-field simulations are performed based on a ternary (Al-Au-X) phase diagram concept and using experimental data. It has been found that the increase of both the liquid-intermetallic interfacial tension as well as the capillary flow lead to an inhibition effect for the growth of the intermetallic phase.