A phase diagram approach to study liquation cracking in alloy 718

The constitutional liquation of NbC wrought alloy 718 is discussed in terms of a diffusion couple between NbC and the y matrix in a pseudo-ternary of γ-NbC-Laves. Theoretical arguments are presented to show that the diffusion path cannot be a straight line connecting the terminal compositions. Experimental results indicate the existence of the NbC-γ diffusion couple and also support the hypothesis that the diffusion path is not a straight line connecting NbC and γ. Consequently, the temperature at which carbide liquation can occur is below the equilibrium NbC solidus, and hence, carbide liquation is constitutional in nature. The lowest temperature at which carbide liquation initiates depends upon the extent of deviation of the diffusion path from the straight line path. The dependence of the relative volume fractions of carbide and Laves phase in cast alloys on Nb/C ratio is explained using the γ-NbC-Laves pseudo-ternary. The Laves phase present in cast alloys is shown to liquate at the terminal eutectic temperature of the alloy, which could be lower than the carbide liquation temperature.