As the microstructure directly influences the macroscopic properties of a material, the development of a tailored microstructure is crucial for the applicability of high performance materials. In order to control the arising microstructure during directional solidification, it is important to understand the underlying mechanisms controlling the pattern evolution. Therefore, directional solidification processes of the high temperature material NiAl-9Mo are computationally investigated by applying a phase-field model based on the grand potential approach.
During the eutectic solidification of NiAl-9Mo, Mo-rich fibers evolve embedded in a NiAl-rich matrix. Regarding experimental results, square or rectangular cross sectional areas are observed for the molybdenum fibers.
To reproduce these fiber shapes in the simulations, strongly anisotropic interfacial energies between the solid phases are introduced. To compare the complexity and computational effort, the simulations are performed by using two different approaches: A full ternary and a pseudo-binary approach. In the pseudo-binary case, the elements Ni and Al are modeled as one combined component NiAl, whereas in the ternary system all three elements are considered as individual components. Each system requires an individual thermodynamic modeling from the Gibbs energies provided by Calphad databases.
For both systems, first two-dimensional phase-field simulations are performed to validate the thermodynamic modeling. Based on these results three-dimensional simulations are conducted to investigate the shape forming process of the Mo-fibers due to the applied anisotropy. For all 2D and 3D simulations, Jackson-Hunt analyses are presented and their correlations with reported experimental results like fiber volume fractions, lamellar spacings and rod sizes, are discussed. Finally a parametric study on the impact of different anisotropy models on the solidification behavior, the undercooling and the lamellar spacing is performed.