ERIF Final Report The Impact of Carbides on the Permeability of Dendrites: Characterization and Modeling

The formation of macroscopic grain defects during the directional solidification of single crystal Ni-base superalloys can have a significant impact on resulting fatigue and creep properties of critical structural components used in a variety of aerospace applications and gas turbines for power generation. Common casting defects in Ni-base superalloys include freckles, Figure 1, which consist of channels comprised of small equiaxed grains or dendrite fragments that are aligned parallel to the solidification direction. Each of the individual grains in a freckle chain is misoriented with respect to the <001> direction. Additionally, the composition of the equiaxed grains present in the freckle chains is enriched with elements rejected to the interdendritic regions (Ta and Al) and significantly different from the bulk crystal(s). Because of the adverse effects of high angle grain boundaries on mechanical properties, the presence of either of these unacceptable casting defects in single crystal components results in rejection of the component and significantly decreases overall manufacturing yields.