An atom-probe tomographic study of freckle formation in a nickel-based superalloy

Freckles are misoriented defects formed during the solidification of single-crystal (SC) nickel-based superalloys, which are used for turbine blades in jet engines and power generators. Freckles can cause degradation in a turbine blade's mechanical properties at high temperatures, and their elimination is of prime technological importance. Employing atom-probe tomography (APT), analytical transmission electron microscopy and dual-beam focused ion-beam microscopy, the chemical compositions of dendritic cores and inter-dendritic regions for both the freckles and the base material in a multicomponent Ni-based alloy are analyzed. The densities of four liquid-compounds with the same compositions measured using APT are calculated, and it is demonstrated that the inter-dendritic liquids have considerably smaller densities than liquids with compositions the same as the dendritic cores. The latter serves as a quantitative measure for the likelihood of freckle formation.