5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

The properties of a material are mainly defined by their chemical composition and by the underlying microstructure. Depending on the process conditions during the directional solidification and the applied material system different microstructures evolve, which are suitable for the demands of specific applications. Instable growth conditions during the directional solidification can lead to an increase of microstructure regions with irregular phase arrangements. These areas can compromise the resulting properties of the material and hence their applicability. A result of instabilities is the growth of eutectic colonies in the microstructure. Eutectic colonies are mainly observed in ternary systems for alloy compositions in the vicinity of a binary eutectic reaction. The formation of colonies is driven by instabilities in the planar solidification front, which are caused by the ternary impurities diffusing from the two solidifying phases into the liquid. To investigate the formation process of eutectic colonies in their complex spatial arrangement, two- and three-dimensional large-scale phase-field simulations based on a Grand potential formalism are conducted. The formation of eutectic colonies is observed for two independent systems in different manners. In a first study, increasing amounts of silver are added to the isothermal undercooled binary eutectic system Al-Cu, to study the influence of the third component concentration on the colony formation. Due to the different compositions, colonies in different shapes and sizes form. In an additional study, the composition of the high-performance material system NiAl-34Cr is set fixed, but the solidification velocity is systematically varied beyond the stability range of the microstructure to initiate the growth of colonies. Based on this the stability of the evolved eutectic colonies is analyzed by a systematic variation of the applied temperature gradient. The focus of both studies is the investigation of the underlying mechanism of colony formation and of the correlation between the colony arrangement and the eutectic structure inside each colony. Furthermore the interactions in the contact zones between the colonies are studied as these are indicated to be the weak points for the applicability.