Stabilization of thermosolutal convective instabilities in Ni-based single-crystal superalloys: Carbon additions and freckle formation

The effect of carbon additions on the solidification characteristics of sin gle-crystal Ni-based superalloys has been studied over a range of compositi on with large variations in Re, W, and Ta. Under constant processing condit ions, nominally similar experimental alloys containing additions of 0.1 wt pet C exhibited a decreased tendency to develop grain defects, such as frec kle chains. The carbon additions resulted in the formation of Ta-rich MC ca rbides with three distinct morphologies: blocky, nodular, and script. These carbides all precipitate near the liquidus temperature of the alloy. Inten tional carbon additions also affected the segregation behavior of the const ituent elements. Comparison of experimentally measured distribution coeffic ients assessed via application of a Scheil-type analysis revealed reduced s egregation of Re, W, and Ta in experimental single-crystal alloys containin g carbon. The mechanisms by which carbon additions influence freckle format ion are considered.