Stabilization of thermosolutal convective instabilities in Ni-based single-crystal superalloys: Carbide precipitation and rayleigh numbers

The influence of carbide precipitation on grain-defect formation during unidirectional solidification of experimental single-crystal Ni-based superalloys has been assessed over a wide range of compositions with large variations in Re, W, and Ta. In all instances, carbon additions of up to 0.15 wt pct were determined to be statistically significant with respect to stabilizing against the formation of grain defects, such as freckle chains, during solidification. Assessment of the segregation behavior of the constituent alloying additions via a Scheil-type analysis enabled estimation of critical Rayleigh numbers denoting the onset of thermosolutal convection. Precipitation of Ta-rich MC carbides near the liquidus temperature of the alloy was found to interact strongly with the mechanisms associated with freckle formation. Segregation analyses and phase-transformation temperature measurements were used to assess the corresponding Rayleigh numbers for the experimental alloys and to modify the Rayleigh criterion to account for carbide precipitation. Mechanisms pertaining to the interaction of carbides with the onset of thermosolutal convection are discussed.