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 single-crystal Ni-based superalloys has been studied over a range of composition with large variations in Re, W, and Ta. Under constant processing conditions, nominally similar experimental alloys containing additions of 0.1 wt pet C exhibited a decreased tendency to develop grain defects, such as freckle chains. The carbon additions resulted in the formation of Ta-rich MC carbides with three distinct morphologies: blocky, nodular, and script. These carbides all precipitate near the liquidus temperature of the alloy. Intentional carbon additions also affected the segregation behavior of the constituent elements. Comparison of experimentally measured distribution coefficients assessed via application of a Scheil-type analysis revealed reduced segregation of Re, W, and Ta in experimental single-crystal alloys containing carbon. The mechanisms by which carbon additions influence freckle formation are considered.