The role of hafnium in the formation of misoriented defects in Ni-based superalloys: An atom-probe tomographic study

Single-crystal nickel-based superalloys are used for turbine blades in jet engines and power generators. During the solidification of these alloys, misoriented defects, namely freckles, are frequently formed. Freckles can cause degradation in a turbine blade's mechanical properties at high temperatures, and their elimination is of prime technological importance. Atom-probe tomography, transmission electron microscopy and dual-beam focused ion-beam microscopy are employed to analyze the chemical compositions of freckles and their local neighborhoods at several regions of interest, ranging from the millimeter to the nanometer length scales. This combination of techniques enables us to determine the role of minority elements (<500 at. ppm) in these alloys, especially Hf, which has not been addressed to date. It is found that Hf tends to partition to the solid during solidification, and to γ(fcc)-phase during the γ(fcc) → γ′(L1₂) + γ(fcc with a different composition) phase separation, implying that Hf has the potential to promote freckle formation.