Phase-field study of eutectic colony formation in NiAl-34Cr

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 concentration-driven nucleation mechanism is introduced into a multiphase-field framework and is subsequently validated. With this mechanism, two-dimensional large-scale 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.