Phase-Field Simulations of Growth in Eutectic Colonies


Scientists all over the world are looking for new hightech materials. The hunt for taylormade alloys with improved properties like ultrahigh strenght and durability makes it neccesary to understand and predict the solidification of alloys on the microscopic scale. The microstructure of cold crucible solidified eutectic TiFe shows colonies of lamellar growth, where the lamellae orientation is roughly aligned along a tetrahedral symmetry around randomly distributed and oriented nuclei. The lamellae during eutectic solidification can - to some extent - alter their width and direction of growth depending on the local cooling rate and composition. More than just the Jackson Hunt law between the lamellae-width and the local undercooling for ideal reduced models, this non-equilibrium growth of lamellae has to fit the geometric situation and the local concentration fields and shows nonlinearities for large cooling rates. Hence the lamellae turn out to be only roughly aligned with branches and other phenomenae. Therefore, full dynamic simulations are required in order to predict the formation of these lamellae and the resulting microstructure, which in turn determines the macroscopic mechanical properties. For this purpose, the model of directed lamellar growth with periodic boundary conditions shall be generalized to a model of free volume eutectic solidification. This model shall be adapted and compared to experimental results for eutectic TiFe. The effect of the thermal gradient and the local undercooling shall be studied to understand the occurrence of almost uniquely oriented lamellae in arc melt ingots. An extended parameter studies and detailed examinations will be pursued in order to validate the method and obtain an in-depth understanding of eutectic free-volume solidification at different cooling rates and gradients.